20 General utilities library [utilities]

template<class T>
  constexpr void swap(T& a, T& b) noexcept(see below);

Constraints: is_move_constructible_v<T> is true and is_move_assignable_v<T> is true.

Preconditions: Type T meets the Cpp17MoveConstructible (Table 28) and Cpp17MoveAssignable (Table 30) requirements.

Effects: Exchanges values stored in two locations.

Remarks: This function is a designated customization point ([namespace.std]).

The expression inside noexcept is equivalent to:

is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>

template<class T, size_t N>
  constexpr void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable_v<T>);

Constraints: is_swappable_v<T> is true.

Preconditions: a[i] is swappable with ([swappable.requirements]) b[i] for all i in the range [0, N).

Effects: As if by swap_ranges(a, a + N, b).

20.2.3 exchange [utility.exchange]

template<class T, class U = T>
  constexpr T exchange(T& obj, U&& new_val);

Effects: Equivalent to:

T old_val = std::move(obj);
obj = std::forward<U>(new_val);
return old_val;

20.2.4 Forward/move helpers [forward]

The library provides templated helper functions to simplify applying move semantics to an lvalue and to simplify the implementation of forwarding functions.

All functions specified in this subclause are signal-safe .

template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept;
template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;

Mandates: For the second overload, is_lvalue_reference_v<T> is false.

Returns: static_cast<T&&>(t).

template<class T, class A1, class A2>
shared_ptr<T> factory(A1&& a1, A2&& a2) {
  return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
}

struct A {
  A(int&, const double&);
};

void g() {
  shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int&
  int i = 2;
  shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
}

In the first call to factory, A1 is deduced as int, so 2 is forwarded to A's constructor as an rvalue.

In the second call to factory, A1 is deduced as int&, so i is forwarded to A's constructor as an lvalue.

In both cases, A2 is deduced as double, so 1.414 is forwarded to A's constructor as an rvalue.

— end example

]

template<class T> constexpr remove_reference_t<T>&& move(T&& t) noexcept;

Returns: static_cast<remove_reference_t<T>&&>(t).

template<class T, class A1>
shared_ptr<T> factory(A1&& a1) {
  return shared_ptr<T>(new T(std::forward<A1>(a1)));
}

struct A {
  A();
  A(const A&);      // copies from lvalues
  A(A&&);           // moves from rvalues
};

void g() {
  A a;
  shared_ptr<A> sp1 = factory<A>(a);                // “a” binds to A(const A&)
  shared_ptr<A> sp1 = factory<A>(std::move(a));     // “a” binds to A(A&&)
}

In the first call to factory, A1 is deduced as A&, so a is forwarded as a non-const lvalue.

This binds to the constructor A(const A&), which copies the value from a.

In the second call to factory, because of the call std::move(a), A1 is deduced as A, so a is forwarded as an rvalue.

This binds to the constructor A(A&&), which moves the value from a.

— end example

]

template<class T> constexpr conditional_t<
    !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&>
  move_if_noexcept(T& x) noexcept;

Returns: std::move(x).

20.2.5 Function template as_const [utility.as.const]

template<class T> constexpr add_const_t<T>& as_const(T& t) noexcept;

Returns: t.

20.2.6 Function template declval [declval]

The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands .

template<class T> add_rvalue_reference_t<T> declval() noexcept;    // as unevaluated operand

Mandates: This function is not odr-used ([basic.def.odr]).

Remarks: The template parameter T of declval may be an incomplete type.

template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&);

declares a function template convert which only participates in overloading if the type From can be explicitly converted to type To.

For another example see class template common_type ([meta.trans.other]).

— end example

]

20.2.7 Integer comparison functions [utility.intcmp]

template<class T, class U>
  constexpr bool cmp_equal(T t, U u) noexcept;

Mandates: Both T and U are standard integer types or extended integer types ([basic.fundamental]).

Effects: Equivalent to:

using UT = make_unsigned_t<T>;
using UU = make_unsigned_t<U>;
if constexpr (is_signed_v<T> == is_signed_v<U>)
  return t == u;
else if constexpr (is_signed_v<T>)
  return t < 0 ? false : UT(t) == u;
else
  return u < 0 ? false : t == UU(u);

template<class T, class U>
  constexpr bool cmp_not_equal(T t, U u) noexcept;

Effects: Equivalent to: return !cmp_equal(t, u);

template<class T, class U>
  constexpr bool cmp_less(T t, U u) noexcept;

Mandates: Both T and U are standard integer types or extended integer types ([basic.fundamental]).

Effects: Equivalent to:

using UT = make_unsigned_t<T>;
using UU = make_unsigned_t<U>;
if constexpr (is_signed_v<T> == is_signed_v<U>)
  return t < u;
else if constexpr (is_signed_v<T>)
  return t < 0 ? true : UT(t) < u;
else
  return u < 0 ? false : t < UU(u);

template<class T, class U>
  constexpr bool cmp_greater(T t, U u) noexcept;

Effects: Equivalent to: return cmp_less(u, t);

template<class T, class U>
  constexpr bool cmp_less_equal(T t, U u) noexcept;

Effects: Equivalent to: return !cmp_greater(t, u);

template<class T, class U>
  constexpr bool cmp_greater_equal(T t, U u) noexcept;

Effects: Equivalent to: return !cmp_less(t, u);

template<class R, class T>
  constexpr bool in_range(T t) noexcept;

Mandates: Both T and R are standard integer types or extended integer types ([basic.fundamental]).

Effects: Equivalent to:

return cmp_greater_equal(t, numeric_limits<R>::min()) &&
       cmp_less_equal(t, numeric_limits<R>::max());

These function templates cannot be used to compare byte, char, char8_t, char16_t, char32_t, wchar_t, and bool.

— end note

]

20.3 Compile-time integer sequences [intseq]

20.3.1 In general [intseq.general]

The library provides a class template that can represent an integer sequence.

When used as an argument to a function template the template parameter pack defining the sequence can be deduced and used in a pack expansion.

The index_sequence alias template is provided for the common case of an integer sequence of type size_t; see also [tuple.apply].

— end note

]

20.3.2 Class template integer_sequence [intseq.intseq]

namespace std {
  template<class T, T... I> struct integer_sequence {
    using value_type = T;
    static constexpr size_t size() noexcept { return sizeof...(I); }
  };
}

Mandates: T is an integer type.

20.3.3 Alias template make_integer_sequence [intseq.make]

template<class T, T N>
  using make_integer_sequence = integer_sequence<T, see below>;

Mandates:

N \geq 0

The alias template make_integer_sequence denotes a specialization of integer_sequence with N non-type template arguments.

The type make_integer_sequence<T, N> is an alias for the type integer_sequence<T, 0, 1, ..., N-1>.

make_integer_sequence<int, 0> is an alias for the type integer_sequence<int>.

— end note

]

20.4 Pairs [pairs]

20.4.1 In general [pairs.general]

The library provides a template for heterogeneous pairs of values.

The library also provides a matching function template to simplify their construction and several templates that provide access to pair objects as if they were tuple objects (see [tuple.helper] and [tuple.elem]).

20.4.2 Class template pair [pairs.pair]

namespace std {
  template<class T1, class T2>
  struct pair {
    using first_type  = T1;
    using second_type = T2;

    T1 first;
    T2 second;

    pair(const pair&) = default;
    pair(pair&&) = default;
    constexpr explicit(see below) pair();
    constexpr explicit(see below) pair(const T1& x, const T2& y);
    template<class U1, class U2>
      constexpr explicit(see below) pair(U1&& x, U2&& y);
    template<class U1, class U2>
      constexpr explicit(see below) pair(const pair<U1, U2>& p);
    template<class U1, class U2>
      constexpr explicit(see below) pair(pair<U1, U2>&& p);
    template<class... Args1, class... Args2>
      constexpr pair(piecewise_construct_t,
                     tuple<Args1...> first_args, tuple<Args2...> second_args);

    constexpr pair& operator=(const pair& p);
    template<class U1, class U2>
      constexpr pair& operator=(const pair<U1, U2>& p);
    constexpr pair& operator=(pair&& p) noexcept(see below);
    template<class U1, class U2>
      constexpr pair& operator=(pair<U1, U2>&& p);

    constexpr void swap(pair& p) noexcept(see below);
  };

  template<class T1, class T2>
    pair(T1, T2) -> pair<T1, T2>;
}

Constructors and member functions of pair do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.

The defaulted move and copy constructor, respectively, of pair is a constexpr function if and only if all required element-wise initializations for move and copy, respectively, would satisfy the requirements for a constexpr function.

If (is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>) is true, then the destructor of pair is trivial.

pair<T, U> is a structural type ([temp.param]) if T and U are both structural types.

Two values p1 and p2 of type pair<T, U> are template-argument-equivalent ([temp.type]) if and only if p1.first and p2.first are template-argument-equivalent and p1.second and p2.second are template-argument-equivalent.

constexpr explicit(see below) pair();

Constraints:

(5.1)
is_default_constructible_v<first_type> is true and
(5.2)
is_default_constructible_v<second_type> is true.

Effects: Value-initializes first and second.

Remarks: The expression inside explicit evaluates to true if and only if either first_type or second_type is not implicitly default-constructible.

This behavior can be implemented with a trait that checks whether a const first_type& or a const second_type& can be initialized with {}.

— end note

]

constexpr explicit(see below) pair(const T1& x, const T2& y);

Constraints:

(8.1)
is_copy_constructible_v<first_type> is true and
(8.2)
is_copy_constructible_v<second_type> is true.

Effects: Initializes first with x and second with y.

Remarks: The expression inside explicit is equivalent to:

!is_convertible_v<const first_type&, first_type> ||
  !is_convertible_v<const second_type&, second_type>

template<class U1, class U2> constexpr explicit(see below) pair(U1&& x, U2&& y);

Constraints:

(11.1)
is_constructible_v<first_type, U1> is true and
(11.2)
is_constructible_v<second_type, U2> is true.

Effects: Initializes first with std::forward<U1>(x) and second with std::forward<U2>(y).

Remarks: The expression inside explicit is equivalent to:

!is_convertible_v<U1, first_type> || !is_convertible_v<U2, second_type>

template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p);

Constraints:

(14.1)
is_constructible_v<first_type, const U1&> is true and
(14.2)
is_constructible_v<second_type, const U2&> is true.

Effects: Initializes members from the corresponding members of the argument.

Remarks: The expression inside explicit is equivalent to:

!is_convertible_v<const U1&, first_type> || !is_convertible_v<const U2&, second_type>

template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>&& p);

Constraints:

(17.1)
is_constructible_v<first_type, U1> is true and
(17.2)
is_constructible_v<second_type, U2> is true.

Effects: Initializes first with std::forward<U1>(p.first) and second with std::forward<U2>(p.second).

Remarks: The expression inside explicit is equivalent to:

!is_convertible_v<U1, first_type> || !is_convertible_v<U2, second_type>

template<class... Args1, class... Args2>
  constexpr pair(piecewise_construct_t,
                 tuple<Args1...> first_args, tuple<Args2...> second_args);

Mandates:

(20.1)
is_constructible_v<first_type, Args1...> is true and
(20.2)
is_constructible_v<second_type, Args2...> is true.

Effects: Initializes first with arguments of types Args1... obtained by forwarding the elements of first_args and initializes second with arguments of types Args2... obtained by forwarding the elements of second_args.

(Here, forwarding an element x of type U within a tuple object means calling std::forward(x).)

This form of construction, whereby constructor arguments for first and second are each provided in a separate tuple object, is called piecewise construction.

constexpr pair& operator=(const pair& p);

Effects: Assigns p.first to first and p.second to second.

Remarks: This operator is defined as deleted unless is_copy_assignable_v<first_type> is true and is_copy_assignable_v<second_type> is true.

Returns: *this.

template<class U1, class U2> constexpr pair& operator=(const pair<U1, U2>& p);

Constraints:

(25.1)
is_assignable_v<first_type&, const U1&> is true and
(25.2)
is_assignable_v<second_type&, const U2&> is true.

Effects: Assigns p.first to first and p.second to second.

Returns: *this.

constexpr pair& operator=(pair&& p) noexcept(see below);

Constraints:

(28.1)
is_move_assignable_v<first_type> is true and
(28.2)
is_move_assignable_v<second_type> is true.

Effects: Assigns to first with std::forward<first_type>(p.first) and to second with

std::forward<second_type>(p.second).

Returns: *this.

Remarks: The expression inside noexcept is equivalent to:

is_nothrow_move_assignable_v<T1> && is_nothrow_move_assignable_v<T2>

template<class U1, class U2> constexpr pair& operator=(pair<U1, U2>&& p);

Constraints:

(32.1)
is_assignable_v<first_type&, U1> is true and
(32.2)
is_assignable_v<second_type&, U2> is true.

Effects: Assigns to first with std::forward<U1>(p.first) and to second with

std::forward<U2>(p.second).

Returns: *this.

constexpr void swap(pair& p) noexcept(see below);

Preconditions: first is swappable with ([swappable.requirements]) p.first and second is swappable with p.second.

Effects: Swaps first with p.first and second with p.second.

Remarks: The expression inside noexcept is equivalent to:

is_nothrow_swappable_v<first_type> && is_nothrow_swappable_v<second_type>

20.4.3 Specialized algorithms [pairs.spec]

template<class T1, class T2>
  constexpr bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y);

Returns: x.first == y.first && x.second == y.second.

template<class T1, class T2>
  constexpr common_comparison_category_t<synth-three-way-result<T1>,
                                         synth-three-way-result<T2>>
    operator<=>(const pair<T1, T2>& x, const pair<T1, T2>& y);

Effects: Equivalent to:

if (auto c = synth-three-way(x.first, y.first); c != 0) return c;
return synth-three-way(x.second, y.second);

template<class T1, class T2>
  constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_swappable_v<T1> is true and is_swappable_v<T2> is true.

Effects: Equivalent to x.swap(y).

template<class T1, class T2>
  constexpr pair<unwrap_ref_decay_t<T1>, unwrap_ref_decay_t<T2>> make_pair(T1&& x, T2&& y);

Returns:

pair<unwrap_ref_decay_t<T1>,
     unwrap_ref_decay_t<T2>>(std::forward<T1>(x), std::forward<T2>(y))

In place of:

return pair<int, double>(5, 3.1415926);     // explicit types

a C++ program may contain:

return make_pair(5, 3.1415926);             // types are deduced

— end example

]

20.4.4 Tuple-like access to pair [pair.astuple]

template<class T1, class T2>
  struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> { };

template<size_t I, class T1, class T2>
  struct tuple_element<I, pair<T1, T2>> {
    using type = see below ;
  };

Mandates:

I < 2

Type: The type T1 if I is 0, otherwise the type T2.

template<size_t I, class T1, class T2>
  constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>& p) noexcept;
template<size_t I, class T1, class T2>
  constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>& p) noexcept;
template<size_t I, class T1, class T2>
  constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&& p) noexcept;
template<size_t I, class T1, class T2>
  constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&& p) noexcept;

Mandates:

I < 2

Returns:

(4.1)
If I is 0, returns a reference to p.first.
(4.2)
If I is 1, returns a reference to p.second.

template<class T1, class T2>
  constexpr T1& get(pair<T1, T2>& p) noexcept;
template<class T1, class T2>
  constexpr const T1& get(const pair<T1, T2>& p) noexcept;
template<class T1, class T2>
  constexpr T1&& get(pair<T1, T2>&& p) noexcept;
template<class T1, class T2>
  constexpr const T1&& get(const pair<T1, T2>&& p) noexcept;

Mandates: T1 and T2 are distinct types.

Returns: A reference to p.first.

template<class T2, class T1>
  constexpr T2& get(pair<T1, T2>& p) noexcept;
template<class T2, class T1>
  constexpr const T2& get(const pair<T1, T2>& p) noexcept;
template<class T2, class T1>
  constexpr T2&& get(pair<T1, T2>&& p) noexcept;
template<class T2, class T1>
  constexpr const T2&& get(const pair<T1, T2>&& p) noexcept;

Mandates: T1 and T2 are distinct types.

Returns: A reference to p.second.

20.4.5 Piecewise construction [pair.piecewise]

struct piecewise_construct_t {
  explicit piecewise_construct_t() = default;
};
inline constexpr piecewise_construct_t piecewise_construct{};

The struct piecewise_construct_t is an empty class type used as a unique type to disambiguate constructor and function overloading.

Specifically, pair has a constructor with piecewise_construct_t as the first argument, immediately followed by two tuple arguments used for piecewise construction of the elements of the pair object.

20.5 Tuples [tuple]

20.5.1 In general [tuple.general]

Subclause [tuple] describes the tuple library that provides a tuple type as the class template tuple that can be instantiated with any number of arguments.

Each template argument specifies the type of an element in the tuple.

Consequently, tuples are heterogeneous, fixed-size collections of values.

An instantiation of tuple with two arguments is similar to an instantiation of pair with the same two arguments.

See [pairs].

20.5.2 Header <tuple> synopsis [tuple.syn]

#include <compare>              // see [compare.syn]

namespace std {
  // [tuple.tuple], class template tuple
  template<class... Types>
    class tuple;

  // [tuple.creation], tuple creation functions
  inline constexpr unspecified ignore;

  template<class... TTypes>
    constexpr tuple<unwrap_ref_decay_t<TTypes>...> make_tuple(TTypes&&...);

  template<class... TTypes>
    constexpr tuple<TTypes&&...> forward_as_tuple(TTypes&&...) noexcept;

  template<class... TTypes>
    constexpr tuple<TTypes&...> tie(TTypes&...) noexcept;

  template<class... Tuples>
    constexpr tuple<CTypes...> tuple_cat(Tuples&&...);

  // [tuple.apply], calling a function with a tuple of arguments
  template<class F, class Tuple>
    constexpr decltype(auto) apply(F&& f, Tuple&& t);

  template<class T, class Tuple>
    constexpr T make_from_tuple(Tuple&& t);

  // [tuple.helper], tuple helper classes
  template<class T> struct tuple_size;                  // not defined
  template<class T> struct tuple_size<const T>;

  template<class... Types> struct tuple_size<tuple<Types...>>;

  template<size_t I, class T> struct tuple_element;     // not defined
  template<size_t I, class T> struct tuple_element<I, const T>;

  template<size_t I, class... Types>
    struct tuple_element<I, tuple<Types...>>;

  template<size_t I, class T>
    using tuple_element_t = typename tuple_element<I, T>::type;

  // [tuple.elem], element access
  template<size_t I, class... Types>
    constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>&) noexcept;
  template<size_t I, class... Types>
    constexpr tuple_element_t<I, tuple<Types...>>&& get(tuple<Types...>&&) noexcept;
  template<size_t I, class... Types>
    constexpr const tuple_element_t<I, tuple<Types...>>& get(const tuple<Types...>&) noexcept;
  template<size_t I, class... Types>
    constexpr const tuple_element_t<I, tuple<Types...>>&& get(const tuple<Types...>&&) noexcept;
  template<class T, class... Types>
    constexpr T& get(tuple<Types...>& t) noexcept;
  template<class T, class... Types>
    constexpr T&& get(tuple<Types...>&& t) noexcept;
  template<class T, class... Types>
    constexpr const T& get(const tuple<Types...>& t) noexcept;
  template<class T, class... Types>
    constexpr const T&& get(const tuple<Types...>&& t) noexcept;

  // [tuple.rel], relational operators
  template<class... TTypes, class... UTypes>
    constexpr bool operator==(const tuple<TTypes...>&, const tuple<UTypes...>&);
  template<class... TTypes, class... UTypes>
    constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...>
      operator<=>(const tuple<TTypes...>&, const tuple<UTypes...>&);

  // [tuple.traits], allocator-related traits
  template<class... Types, class Alloc>
    struct uses_allocator<tuple<Types...>, Alloc>;

  // [tuple.special], specialized algorithms
  template<class... Types>
    constexpr void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

  // [tuple.helper], tuple helper classes
  template<class T>
    inline constexpr size_t tuple_size_v = tuple_size<T>::value;
}

20.5.3 Class template tuple [tuple.tuple]

namespace std {
  template<class... Types>
  class tuple {
  public:
    // [tuple.cnstr], tuple construction
    constexpr explicit(see below) tuple();
    constexpr explicit(see below) tuple(const Types&...);         // only if sizeof...(Types) >= 1
    template<class... UTypes>
      constexpr explicit(see below) tuple(UTypes&&...);           // only if sizeof...(Types) >= 1

    tuple(const tuple&) = default;
    tuple(tuple&&) = default;

    template<class... UTypes>
      constexpr explicit(see below) tuple(const tuple<UTypes...>&);
    template<class... UTypes>
      constexpr explicit(see below) tuple(tuple<UTypes...>&&);

    template<class U1, class U2>
      constexpr explicit(see below) tuple(const pair<U1, U2>&);   // only if sizeof...(Types) == 2
    template<class U1, class U2>
      constexpr explicit(see below) tuple(pair<U1, U2>&&);        // only if sizeof...(Types) == 2

    // allocator-extended constructors
    template<class Alloc>
      constexpr explicit(see below)
        tuple(allocator_arg_t, const Alloc& a);
    template<class Alloc>
      constexpr explicit(see below)
        tuple(allocator_arg_t, const Alloc& a, const Types&...);
    template<class Alloc, class... UTypes>
      constexpr explicit(see below)
        tuple(allocator_arg_t, const Alloc& a, UTypes&&...);
    template<class Alloc>
      constexpr tuple(allocator_arg_t, const Alloc& a, const tuple&);
    template<class Alloc>
      constexpr tuple(allocator_arg_t, const Alloc& a, tuple&&);
    template<class Alloc, class... UTypes>
      constexpr explicit(see below)
        tuple(allocator_arg_t, const Alloc& a, const tuple<UTypes...>&);
    template<class Alloc, class... UTypes>
      constexpr explicit(see below)
        tuple(allocator_arg_t, const Alloc& a, tuple<UTypes...>&&);
    template<class Alloc, class U1, class U2>
      constexpr explicit(see below)
        tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&);
    template<class Alloc, class U1, class U2>
      constexpr explicit(see below)
        tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);

    // [tuple.assign], tuple assignment
    constexpr tuple& operator=(const tuple&);
    constexpr tuple& operator=(tuple&&) noexcept(see below);

    template<class... UTypes>
      constexpr tuple& operator=(const tuple<UTypes...>&);
    template<class... UTypes>
      constexpr tuple& operator=(tuple<UTypes...>&&);

    template<class U1, class U2>
      constexpr tuple& operator=(const pair<U1, U2>&);          // only if sizeof...(Types) == 2
    template<class U1, class U2>
      constexpr tuple& operator=(pair<U1, U2>&&);               // only if sizeof...(Types) == 2

    // [tuple.swap], tuple swap
    constexpr void swap(tuple&) noexcept(see below);
  };

  template<class... UTypes>
    tuple(UTypes...) -> tuple<UTypes...>;
  template<class T1, class T2>
    tuple(pair<T1, T2>) -> tuple<T1, T2>;
  template<class Alloc, class... UTypes>
    tuple(allocator_arg_t, Alloc, UTypes...) -> tuple<UTypes...>;
  template<class Alloc, class T1, class T2>
    tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>;
  template<class Alloc, class... UTypes>
    tuple(allocator_arg_t, Alloc, tuple<UTypes...>) -> tuple<UTypes...>;
}

20.5.3.1 Construction [tuple.cnstr]

In the descriptions that follow, let i be in the range [0, sizeof...(Types)) in order,

T_{i}

be the

i^{th}

type in Types, and

U_{i}

be the

i^{th}

type in a template parameter pack named UTypes, where indexing is zero-based.

For each tuple constructor, an exception is thrown only if the construction of one of the types in Types throws an exception.

The defaulted move and copy constructor, respectively, of tuple is a constexpr function if and only if all required element-wise initializations for move and copy, respectively, would satisfy the requirements for a constexpr function.

The defaulted move and copy constructor of tuple<> are constexpr functions.

If is_trivially_destructible_v<

T_{i}

> is true for all

T_{i}

, then the destructor of tuple is trivial.

constexpr explicit(see below) tuple();

Constraints: is_default_constructible_v<

T_{i}

> is true for all i.

Effects: Value-initializes each element.

Remarks: The expression inside explicit evaluates to true if and only if

T_{i}

is not copy-list-initializable from an empty list for at least one i.

This behavior can be implemented with a trait that checks whether a const

T_{i}

& can be initialized with {}.

— end note

]

constexpr explicit(see below) tuple(const Types&...);

Constraints:

sizeof...(Types) \geq 1

and is_copy_constructible_v<

T_{i}

> is true for all i.

Effects: Initializes each element with the value of the corresponding parameter.

Remarks: The expression inside explicit is equivalent to:

!conjunction_v<is_convertible<const Types&, Types>...>

template<class... UTypes> constexpr explicit(see below) tuple(UTypes&&... u);

Constraints: sizeof...(Types) equals sizeof...(UTypes) and

sizeof...(Types) \geq 1

and is_constructible_v<

T_{i}

U_{i}

> is true for all i.

Effects: Initializes the elements in the tuple with the corresponding value in std::forward<UTypes>(u).

Remarks: The expression inside explicit is equivalent to:

!conjunction_v<is_convertible<UTypes, Types>...>

tuple(const tuple& u) = default;

Mandates: is_copy_constructible_v<

T_{i}

> is true for all i.

Effects: Initializes each element of *this with the corresponding element of u.

tuple(tuple&& u) = default;

Constraints: is_move_constructible_v<

T_{i}

> is true for all i.

Effects: For all i, initializes the

i^{th}

element of *this with std::forward<

T_{i}

>(get(u)).

template<class... UTypes> constexpr explicit(see below) tuple(const tuple<UTypes...>& u);

Constraints:

(18.1)
sizeof...(Types) equals sizeof...(UTypes) and
(18.2)
is_constructible_v< $T_{i}$ , const $U_{i}$ &> is true for all i, and
(18.3)
either sizeof...(Types) is not 1, or (when Types... expands to T and UTypes... expands to U) is_convertible_v<const tuple&, T>, is_constructible_v<T, const tuple&>, and is_same_v<T, U> are all false.

Effects: Initializes each element of *this with the corresponding element of u.

Remarks: The expression inside explicit is equivalent to:

!conjunction_v<is_convertible<const UTypes&, Types>...>

template<class... UTypes> constexpr explicit(see below) tuple(tuple<UTypes...>&& u);

Constraints:

(21.1)
sizeof...(Types) equals sizeof...(UTypes), and
(21.2)
is_constructible_v< $T_{i}$ , $U_{i}$ > is true for all i, and
(21.3)
either sizeof...(Types) is not 1, or (when Types... expands to T and UTypes... expands to U) is_convertible_v<tuple, T>, is_constructible_v<T, tuple>, and is_same_v<T, U> are all false.

Effects: For all i, initializes the

i^{th}

element of *this with std::forward<

U_{i}

>(get(u)).

Remarks: The expression inside explicit is equivalent to:

!conjunction_v<is_convertible<UTypes, Types>...>

template<class U1, class U2> constexpr explicit(see below) tuple(const pair<U1, U2>& u);

Constraints:

(24.1)
sizeof...(Types) is 2,
(24.2)
is_constructible_v< $T_{0}$ , const U1&> is true, and
(24.3)
is_constructible_v< $T_{1}$ , const U2&> is true.

Effects: Initializes the first element with u.first and the second element with u.second.

The expression inside explicit is equivalent to:

!is_convertible_v<const U1&,  $T_{0}$ > || !is_convertible_v<const U2&,  $T_{1}$ >

template<class U1, class U2> constexpr explicit(see below) tuple(pair<U1, U2>&& u);

Constraints:

(27.1)
sizeof...(Types) is 2,
(27.2)
is_constructible_v< $T_{0}$ , U1> is true, and
(27.3)
is_constructible_v< $T_{1}$ , U2> is true.

Effects: Initializes the first element with std::forward<U1>(u.first) and the second element with std::forward<U2>(u.second).

The expression inside explicit is equivalent to:

!is_convertible_v<U1,  $T_{0}$ > || !is_convertible_v<U2,  $T_{1}$ >

template<class Alloc>
  constexpr explicit(see below)
    tuple(allocator_arg_t, const Alloc& a);
template<class Alloc>
  constexpr explicit(see below)
    tuple(allocator_arg_t, const Alloc& a, const Types&...);
template<class Alloc, class... UTypes>
  constexpr explicit(see below)
    tuple(allocator_arg_t, const Alloc& a, UTypes&&...);
template<class Alloc>
  constexpr tuple(allocator_arg_t, const Alloc& a, const tuple&);
template<class Alloc>
  constexpr tuple(allocator_arg_t, const Alloc& a, tuple&&);
template<class Alloc, class... UTypes>
  constexpr explicit(see below)
    tuple(allocator_arg_t, const Alloc& a, const tuple<UTypes...>&);
template<class Alloc, class... UTypes>
  constexpr explicit(see below)
    tuple(allocator_arg_t, const Alloc& a, tuple<UTypes...>&&);
template<class Alloc, class U1, class U2>
  constexpr explicit(see below)
    tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&);
template<class Alloc, class U1, class U2>
  constexpr explicit(see below)
    tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);

Preconditions: Alloc meets the Cpp17Allocator requirements (Table 36).

Effects: Equivalent to the preceding constructors except that each element is constructed with uses-allocator construction .

20.5.3.2 Assignment [tuple.assign]

For each tuple assignment operator, an exception is thrown only if the assignment of one of the types in Types throws an exception.

In the function descriptions that follow, let i be in the range [0, sizeof...(Types)) in order,

T_{i}

be the

i^{th}

type in Types, and

U_{i}

be the

i^{th}

type in a template parameter pack named UTypes, where indexing is zero-based.

constexpr tuple& operator=(const tuple& u);

Effects: Assigns each element of u to the corresponding element of *this.

Remarks: This operator is defined as deleted unless is_copy_assignable_v<

T_{i}

> is true for all i.

Returns: *this.

constexpr tuple& operator=(tuple&& u) noexcept(see below);

Constraints: is_move_assignable_v<

T_{i}

> is true for all i.

Effects: For all i, assigns std::forward<

T_{i}

>(get(u)) to get(*this).

Remarks: The expression inside noexcept is equivalent to the logical and of the following expressions:

is_nothrow_move_assignable_v< $T_{i}$ >

where

T_{i}

is the

i^{th}

type in Types.

Returns: *this.

template<class... UTypes> constexpr tuple& operator=(const tuple<UTypes...>& u);

Constraints:

(9.1)
sizeof...(Types) equals sizeof...(UTypes) and
(9.2)
is_assignable_v< $T_{i}$ &, const $U_{i}$ &> is true for all i.

Effects: Assigns each element of u to the corresponding element of *this.

Returns: *this.

template<class... UTypes> constexpr tuple& operator=(tuple<UTypes...>&& u);

Constraints:

(12.1)
sizeof...(Types) equals sizeof...(UTypes) and
(12.2)
is_assignable_v< $T_{i}$ &, $U_{i}$ > is true for all i.

Effects: For all i, assigns std::forward<

U_{i}

>(get(u)) to get(*this).

Returns: *this.

template<class U1, class U2> constexpr tuple& operator=(const pair<U1, U2>& u);

Constraints:

(15.1)
sizeof...(Types) is 2 and
(15.2)
is_assignable_v< $T_{0}$ &, const U1&> is true, and
(15.3)
is_assignable_v< $T_{1}$ &, const U2&> is true.

Effects: Assigns u.first to the first element of *this and u.second to the second element of *this.

Returns: *this.

template<class U1, class U2> constexpr tuple& operator=(pair<U1, U2>&& u);

Constraints:

(18.1)
sizeof...(Types) is 2 and
(18.2)
is_assignable_v< $T_{0}$ &, U1> is true, and
(18.3)
is_assignable_v< $T_{1}$ &, U2> is true.

Effects: Assigns std::forward<U1>(u.first) to the first element of *this and

std::forward<U2>(u.second) to the second element of *this.

Returns: *this.

20.5.3.3 swap [tuple.swap]

constexpr void swap(tuple& rhs) noexcept(see below);

Preconditions: Each element in *this is swappable with ([swappable.requirements]) the corresponding element in rhs.

Effects: Calls swap for each element in *this and its corresponding element in rhs.

Remarks: The expression inside noexcept is equivalent to the logical and of the following expressions:

is_nothrow_swappable_v< $T_{i}$ >

where

T_{i}

is the

i^{th}

type in Types.

Throws: Nothing unless one of the element-wise swap calls throws an exception.

20.5.4 Tuple creation functions [tuple.creation]

In the function descriptions that follow, the members of a template parameter pack XTypes are denoted by X

_{i}

for i in [0, sizeof...(XTypes)) in order, where indexing is zero-based.

template<class... TTypes>
  constexpr tuple<unwrap_ref_decay_t<TTypes>...> make_tuple(TTypes&&... t);

Returns: tuple<unwrap_ref_decay_t<TTypes>...>(std::forward<TTypes>(t)...).

int i; float j;
make_tuple(1, ref(i), cref(j))

creates a tuple of type tuple<int, int&, const float&>.

— end example

]

template<class... TTypes>
  constexpr tuple<TTypes&&...> forward_as_tuple(TTypes&&... t) noexcept;

Effects: Constructs a tuple of references to the arguments in t suitable for forwarding as arguments to a function.

Because the result may contain references to temporary objects, a program shall ensure that the return value of this function does not outlive any of its arguments (e.g., the program should typically not store the result in a named variable).

Returns: tuple<TTypes&&...>(std::forward<TTypes>(t)...).

template<class... TTypes>
  constexpr tuple<TTypes&...> tie(TTypes&... t) noexcept;

Returns: tuple<TTypes&...>(t...).

When an argument in t is ignore, assigning any value to the corresponding tuple element has no effect.

tie functions allow one to create tuples that unpack tuples into variables.

ignore can be used for elements that are not needed:

int i; std::string s;
tie(i, ignore, s) = make_tuple(42, 3.14, "C++");
// i == 42, s == "C++"

— end example

]

template<class... Tuples>
  constexpr tuple<CTypes...> tuple_cat(Tuples&&... tpls);

In the following paragraphs, let

T_{i}

be the

i^{th}

type in Tuples,

U_{i}

be remove_reference_t<T

_{i}

>, and

{tp}_{i}

be the

i^{th}

parameter in the function parameter pack tpls, where all indexing is zero-based.

Preconditions: For all i,

U_{i}

is the type

{c v}_{i}

tuple<

{Args}_{i}

...>, where

{c v}_{i}

is the (possibly empty)

i^{th}

cv-qualifier-seq and

{Args}_{i}

is the template parameter pack representing the element types in

U_{i}

Let

A_{i k}

be the

k^{th}

type in

{Args}_{i}

For all

A_{i k}

the following requirements are met:

(9.1)
If $T_{i}$ is deduced as an lvalue reference type, then is_constructible_v< $A_{i k}$ , $c v_{i} A_{i k}$ &> == true, otherwise
(9.2)
is_constructible_v< $A_{i k}$ , $c v_{i} A_{i k}$ &&> == true.

Remarks: The types in CTypes are equal to the ordered sequence of the extended types

{Args}_{0}

...,

{Args}_{1}

..., …,

{Args}_{n - 1}

..., where n is equal to sizeof...(Tuples).

Let

e_{i}

... be the

i^{th}

ordered sequence of tuple elements of the resulting tuple object corresponding to the type sequence

{Args}_{i}

Returns: A tuple object constructed by initializing the

{k_{i}}^{th}

type element

e_{i k}

e_{i}

... with

get< $k_{i}$ >(std::forward< $T_{i}$ >( ${tp}_{i}$ ))

for each valid

k_{i}

and each group

e_{i}

in order.

An implementation may support additional types in the template parameter pack Tuples that support the tuple-like protocol, such as pair and array.

— end note

]

20.5.5 Calling a function with a tuple of arguments [tuple.apply]

template<class F, class Tuple>
  constexpr decltype(auto) apply(F&& f, Tuple&& t);

Effects: Given the exposition-only function:

template<class F, class Tuple, size_t... I>
constexpr decltype(auto) apply-impl(F&& f, Tuple&& t, index_sequence<I...>) {
                                                                        // exposition only
  return INVOKE(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...);  // see [func.require]
}

Equivalent to:

return apply-impl(std::forward<F>(f), std::forward<Tuple>(t),
                  make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});

template<class T, class Tuple>
  constexpr T make_from_tuple(Tuple&& t);

Effects: Given the exposition-only function:

template<class T, class Tuple, size_t... I>
constexpr T make-from-tuple-impl(Tuple&& t, index_sequence<I...>) {     // exposition only
  return T(get<I>(std::forward<Tuple>(t))...);
}

Equivalent to:

return make-from-tuple-impl<T>(
           forward<Tuple>(t),
           make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});

: The type of T must be supplied as an explicit template parameter, as it cannot be deduced from the argument list. — end note

]

20.5.6 Tuple helper classes [tuple.helper]

template<class T> struct tuple_size;

All specializations of tuple_size meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, N> for some N.

template<class... Types>
  struct tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> { };

template<size_t I, class... Types>
  struct tuple_element<I, tuple<Types...>> {
    using type = TI;
  };

Mandates:

I < sizeof...(Types)

Type: TI is the type of the

I^{th}

element of Types, where indexing is zero-based.

template<class T> struct tuple_size<const T>;

Let TS denote tuple_size<T> of the cv-unqualified type T.

If the expression TS::value is well-formed when treated as an unevaluated operand, then each specialization of the template meets the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of

integral_constant<size_t, TS::value>

Otherwise, it has no member value.

Access checking is performed as if in a context unrelated to TS and T.

Only the validity of the immediate context of the expression is considered.

The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.

Such side effects are not in the “immediate context” and can result in the program being ill-formed.

— end note

]

In addition to being available via inclusion of the <tuple> header, the template is available when any of the headers <array> ([array.syn]), <ranges> ([ranges.syn]), or <utility> ([utility.syn]) are included.

template<size_t I, class T> struct tuple_element<I, const T>;

Let TE denote tuple_element_t<I, T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17TransformationTrait requirements ([meta.rqmts]) with a member typedef type that names the type add_const_t<TE>.

20.5.7 Element access [tuple.elem]

template<size_t I, class... Types>
  constexpr tuple_element_t<I, tuple<Types...>>&
    get(tuple<Types...>& t) noexcept;
template<size_t I, class... Types>
  constexpr tuple_element_t<I, tuple<Types...>>&&
    get(tuple<Types...>&& t) noexcept;        // Note A
template<size_t I, class... Types>
  constexpr const tuple_element_t<I, tuple<Types...>>&
    get(const tuple<Types...>& t) noexcept;   // Note B
template<size_t I, class... Types>
  constexpr const tuple_element_t<I, tuple<Types...>>&& get(const tuple<Types...>&& t) noexcept;

Mandates:

I < sizeof...(Types)

Returns: A reference to the

I^{th}

element of t, where indexing is zero-based.

[Note A

If a type T in Types is some reference type X&, the return type is X&, not X&&.

However, if the element type is a non-reference type T, the return type is T&&.

— end note

]

[Note B

Constness is shallow.

If a type T in Types is some reference type X&, the return type is X&, not const X&.

However, if the element type is a non-reference type T, the return type is const T&.

This is consistent with how constness is defined to work for member variables of reference type.

— end note

]

template<class T, class... Types>
  constexpr T& get(tuple<Types...>& t) noexcept;
template<class T, class... Types>
  constexpr T&& get(tuple<Types...>&& t) noexcept;
template<class T, class... Types>
  constexpr const T& get(const tuple<Types...>& t) noexcept;
template<class T, class... Types>
  constexpr const T&& get(const tuple<Types...>&& t) noexcept;

Mandates: The type T occurs exactly once in Types.

Returns: A reference to the element of t corresponding to the type T in Types.

const tuple<int, const int, double, double> t(1, 2, 3.4, 5.6);
const int& i1 = get<int>(t);                    // OK, i1 has value 1
const int& i2 = get<const int>(t);              // OK, i2 has value 2
const double& d = get<double>(t);               // error: type double is not unique within t

— end example

]

The reason get is a non-member function is that if this functionality had been provided as a member function, code where the type depended on a template parameter would have required using the template keyword.

— end note

]

20.5.8 Relational operators [tuple.rel]

template<class... TTypes, class... UTypes>
  constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Mandates: For all i, where

0 \leq i < sizeof...(TTypes)

, get(t) == get(u) is a valid expression returning a type that is convertible to bool.

sizeof...(TTypes) equals sizeof...(UTypes).

Returns: true if get(t) == get(u) for all i, otherwise false.

For any two zero-length tuples e and f, e == f returns true.

Effects: The elementary comparisons are performed in order from the zeroth index upwards.

No comparisons or element accesses are performed after the first equality comparison that evaluates to false.

template<class... TTypes, class... UTypes>
  constexpr common_comparison_category_t<synth-three-way-result<TTypes, UTypes>...>
    operator<=>(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Effects: Performs a lexicographical comparison between t and u.

For any two zero-length tuples t and u, t <=> u returns strong_ordering::equal.

Otherwise, equivalent to:

if (auto c = synth-three-way(get<0>(t), get<0>(u)); c != 0) return c;
return  $t_{t a i l}$  <=>  $u_{t a i l}$ ;

where

r_{t a i l}

for some tuple r is a tuple containing all but the first element of r.

The above definition does not require t

_{t a i l}

(or u

_{t a i l}

) to be constructed.

It may not even be possible, as t and u are not required to be copy constructible.

Also, all comparison functions are short circuited; they do not perform element accesses beyond what is required to determine the result of the comparison.

— end note

]

20.5.9 Tuple traits [tuple.traits]

template<class... Types, class Alloc>
  struct uses_allocator<tuple<Types...>, Alloc> : true_type { };

Preconditions: Alloc meets the Cpp17Allocator requirements (Table 36).

Specialization of this trait informs other library components that tuple can be constructed with an allocator, even though it does not have a nested allocator_type.

— end note

]

20.5.10 Tuple specialized algorithms [tuple.special]

template<class... Types>
  constexpr void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

Constraints: is_swappable_v<T> is true for every type T in Types.

Remarks: The expression inside noexcept is equivalent to:

noexcept(x.swap(y))

Effects: As if by x.swap(y).

20.6 Optional objects [optional]

20.6.1 In general [optional.general]

Subclause [optional] describes class template optional that represents optional objects.

An optional object is an object that contains the storage for another object and manages the lifetime of this contained object, if any.

The contained object may be initialized after the optional object has been initialized, and may be destroyed before the optional object has been destroyed.

The initialization state of the contained object is tracked by the optional object.

20.6.2 Header <optional> synopsis [optional.syn]

#include <compare>              // see [compare.syn]

namespace std {
  // [optional.optional], class template optional
  template<class T>
    class optional;

  // [optional.nullopt], no-value state indicator
  struct nullopt_t{see below};
  inline constexpr nullopt_t nullopt(unspecified);

  // [optional.bad.access], class bad_optional_access
  class bad_optional_access;

  // [optional.relops], relational operators
  template<class T, class U>
    constexpr bool operator==(const optional<T>&, const optional<U>&);
  template<class T, class U>
    constexpr bool operator!=(const optional<T>&, const optional<U>&);
  template<class T, class U>
    constexpr bool operator<(const optional<T>&, const optional<U>&);
  template<class T, class U>
    constexpr bool operator>(const optional<T>&, const optional<U>&);
  template<class T, class U>
    constexpr bool operator<=(const optional<T>&, const optional<U>&);
  template<class T, class U>
    constexpr bool operator>=(const optional<T>&, const optional<U>&);
  template<class T, three_way_comparable_with<T> U>
    constexpr compare_three_way_result_t<T,U>
      operator<=>(const optional<T>&, const optional<U>&);

  // [optional.nullops], comparison with nullopt
  template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
  template<class T>
    constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;

  // [optional.comp.with.t], comparison with T
  template<class T, class U> constexpr bool operator==(const optional<T>&, const U&);
  template<class T, class U> constexpr bool operator==(const T&, const optional<U>&);
  template<class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
  template<class T, class U> constexpr bool operator!=(const T&, const optional<U>&);
  template<class T, class U> constexpr bool operator<(const optional<T>&, const U&);
  template<class T, class U> constexpr bool operator<(const T&, const optional<U>&);
  template<class T, class U> constexpr bool operator>(const optional<T>&, const U&);
  template<class T, class U> constexpr bool operator>(const T&, const optional<U>&);
  template<class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
  template<class T, class U> constexpr bool operator<=(const T&, const optional<U>&);
  template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
  template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
  template<class T, three_way_comparable_with<T> U>
    constexpr compare_three_way_result_t<T,U>
      operator<=>(const optional<T>&, const U&);

  // [optional.specalg], specialized algorithms
  template<class T>
    void swap(optional<T>&, optional<T>&) noexcept(see below);

  template<class T>
    constexpr optional<see below> make_optional(T&&);
  template<class T, class... Args>
    constexpr optional<T> make_optional(Args&&... args);
  template<class T, class U, class... Args>
    constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);

  // [optional.hash], hash support
  template<class T> struct hash;
  template<class T> struct hash<optional<T>>;
}

20.6.3 Class template optional [optional.optional]

namespace std {
  template<class T>
  class optional {
  public:
    using value_type = T;

    // [optional.ctor], constructors
    constexpr optional() noexcept;
    constexpr optional(nullopt_t) noexcept;
    constexpr optional(const optional&);
    constexpr optional(optional&&) noexcept(see below);
    template<class... Args>
      constexpr explicit optional(in_place_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);
    template<class U = T>
      constexpr explicit(see below) optional(U&&);
    template<class U>
      explicit(see below) optional(const optional<U>&);
    template<class U>
      explicit(see below) optional(optional<U>&&);

    // [optional.dtor], destructor
    ~optional();

    // [optional.assign], assignment
    optional& operator=(nullopt_t) noexcept;
    constexpr optional& operator=(const optional&);
    constexpr optional& operator=(optional&&) noexcept(see below);
    template<class U = T> optional& operator=(U&&);
    template<class U> optional& operator=(const optional<U>&);
    template<class U> optional& operator=(optional<U>&&);
    template<class... Args> T& emplace(Args&&...);
    template<class U, class... Args> T& emplace(initializer_list<U>, Args&&...);

    // [optional.swap], swap
    void swap(optional&) noexcept(see below);

    // [optional.observe], observers
    constexpr const T* operator->() const;
    constexpr T* operator->();
    constexpr const T& operator*() const&;
    constexpr T& operator*() &;
    constexpr T&& operator*() &&;
    constexpr const T&& operator*() const&&;
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr const T& value() const&;
    constexpr T& value() &;
    constexpr T&& value() &&;
    constexpr const T&& value() const&&;
    template<class U> constexpr T value_or(U&&) const&;
    template<class U> constexpr T value_or(U&&) &&;

    // [optional.mod], modifiers
    void reset() noexcept;

  private:
    T *val;         // exposition only
  };

  template<class T>
    optional(T) -> optional<T>;
}

Any instance of optional<T> at any given time either contains a value or does not contain a value.

When an instance of optional<T> contains a value, it means that an object of type T, referred to as the optional object's contained value, is allocated within the storage of the optional object.

Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.

The contained value shall be allocated in a region of the optional<T> storage suitably aligned for the type T.

When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value; otherwise the conversion returns false.

Member val is provided for exposition only.

When an optional<T> object contains a value, val points to the contained value.

T shall be a type other than cv in_place_t or cv nullopt_t that meets the Cpp17Destructible requirements (Table 32).

20.6.3.1 Constructors [optional.ctor]

constexpr optional() noexcept;
constexpr optional(nullopt_t) noexcept;

Postconditions: *this does not contain a value.

Remarks: No contained value is initialized.

For every object type T these constructors are constexpr constructors ([dcl.constexpr]).

constexpr optional(const optional& rhs);

Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T.

Remarks: This constructor is defined as deleted unless is_copy_constructible_v<T> is true.

If is_trivially_copy_constructible_v<T> is true, this constructor is trivial.

constexpr optional(optional&& rhs) noexcept(see below);

Constraints: is_move_constructible_v<T> is true.

Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::move(*rhs).

bool(rhs) is unchanged.

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T.

Remarks: The expression inside noexcept is equivalent to is_nothrow_move_constructible_v<T>.

If is_trivially_move_constructible_v<T> is true, this constructor is trivial.

template<class... Args> constexpr explicit optional(in_place_t, Args&&... args);

Constraints: is_constructible_v<T, Args...> is true.

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std::forward<Args>(args)....

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected constructor of T.

Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.

template<class U, class... Args>
  constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);

Constraints: is_constructible_v<T, initializer_list&, Args...> is true.

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std::forward<Args>(args)....

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected constructor of T.

Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor.

template<class U = T> constexpr explicit(see below) optional(U&& v);

Constraints: is_constructible_v<T, U> is true, is_same_v<remove_cvref_t, in_place_t> is false, and is_same_v<remove_cvref_t, optional> is false.

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::forward(v).

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected constructor of T.

Remarks: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor.

The expression inside explicit is equivalent to:

!is_convertible_v<U, T>

template<class U> explicit(see below) optional(const optional<U>& rhs);

Constraints:

(27.1)
is_constructible_v<T, const U&> is true,
(27.2)
is_constructible_v<T, optional&> is false,
(27.3)
is_constructible_v<T, optional&&> is false,
(27.4)
is_constructible_v<T, const optional&> is false,
(27.5)
is_constructible_v<T, const optional&&> is false,
(27.6)
is_convertible_v<optional&, T> is false,
(27.7)
is_convertible_v<optional&&, T> is false,
(27.8)
is_convertible_v<const optional&, T> is false, and
(27.9)
is_convertible_v<const optional&&, T> is false.

Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T.

Remarks: The expression inside explicit is equivalent to:

!is_convertible_v<const U&, T>

template<class U> explicit(see below) optional(optional<U>&& rhs);

Constraints:

(32.1)
is_constructible_v<T, U> is true,
(32.2)
is_constructible_v<T, optional&> is false,
(32.3)
is_constructible_v<T, optional&&> is false,
(32.4)
is_constructible_v<T, const optional&> is false,
(32.5)
is_constructible_v<T, const optional&&> is false,
(32.6)
is_convertible_v<optional&, T> is false,
(32.7)
is_convertible_v<optional&&, T> is false,
(32.8)
is_convertible_v<const optional&, T> is false, and
(32.9)
is_convertible_v<const optional&&, T> is false.

Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::move(*rhs).

bool(rhs) is unchanged.

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T.

Remarks: The expression inside explicit is equivalent to:

!is_convertible_v<U, T>

20.6.3.2 Destructor [optional.dtor]

~optional();

Effects: If is_trivially_destructible_v<T> != true and *this contains a value, calls

val->T::~T()

Remarks: If is_trivially_destructible_v<T> is true, then this destructor is trivial.

20.6.3.3 Assignment [optional.assign]

optional<T>& operator=(nullopt_t) noexcept;

Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.

Postconditions: *this does not contain a value.

Returns: *this.

constexpr optional<T>& operator=(const optional& rhs);

Effects: See Table 42 .

Table 42: optional::operator=(const optional&) effects [tab:optional.assign.copy]

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value as if direct-non-list-initializing an object of type T with *rhs
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.

If an exception is thrown during the call to T's copy constructor, no effect.

If an exception is thrown during the call to T's copy assignment, the state of its contained value is as defined by the exception safety guarantee of T's copy assignment.

This operator is defined as deleted unless is_copy_constructible_v<T> is true and is_copy_assignable_v<T> is true.

If is_trivially_copy_constructible_v<T> && is_trivially_copy_assignable_v<T> && is_trivially_destructible_v<T> is true, this assignment operator is trivial.

constexpr optional& operator=(optional&& rhs) noexcept(see below);

Constraints: is_move_constructible_v<T> is true and is_move_assignable_v<T> is true.

Effects: See Table 43 .

The result of the expression bool(rhs) remains unchanged.

Table 43: optional::operator=(optional&&) effects [tab:optional.assign.move]

	*this contains a value	*this does not contain a value
rhs contains a value	assigns std::move(*rhs) to the contained value	initializes the contained value as if direct-non-list-initializing an object of type T with std::move(*rhs)
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: The expression inside noexcept is equivalent to:

is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>

If any exception is thrown, the result of the expression bool(*this) remains unchanged.

If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor.

If an exception is thrown during the call to T's move assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment.

If is_trivially_move_constructible_v<T> && is_trivially_move_assignable_v<T> && is_trivially_destructible_v<T> is true, this assignment operator is trivial.

template<class U = T> optional<T>& operator=(U&& v);

Constraints: is_same_v<remove_cvref_t, optional> is false, conjunction_v<is_scalar<T>, is_same<T, decay_t>> is false, is_constructible_v<T, U> is true, and is_assignable_v<T&, U> is true.

Effects: If *this contains a value, assigns std::forward(v) to the contained value; otherwise initializes the contained value as if direct-non-list-initializing object of type T with std::forward(v).

Postconditions: *this contains a value.

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.

If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor.

If an exception is thrown during the call to T's assignment, the state of *val and v is determined by the exception safety guarantee of T's assignment.

template<class U> optional<T>& operator=(const optional<U>& rhs);

Constraints:

(19.1)
is_constructible_v<T, const U&> is true,
(19.2)
is_assignable_v<T&, const U&> is true,
(19.3)
is_constructible_v<T, optional&> is false,
(19.4)
is_constructible_v<T, optional&&> is false,
(19.5)
is_constructible_v<T, const optional&> is false,
(19.6)
is_constructible_v<T, const optional&&> is false,
(19.7)
is_convertible_v<optional&, T> is false,
(19.8)
is_convertible_v<optional&&, T> is false,
(19.9)
is_convertible_v<const optional&, T> is false,
(19.10)
is_convertible_v<const optional&&, T> is false,
(19.11)
is_assignable_v<T&, optional&> is false,
(19.12)
is_assignable_v<T&, optional&&> is false,
(19.13)
is_assignable_v<T&, const optional&> is false, and
(19.14)
is_assignable_v<T&, const optional&&> is false.

Effects: See Table 44 .

Table 44: optional::operator=(const optional&) effects [tab:optional.assign.copy.templ]

	*this contains a value	*this does not contain a value
rhs contains a value	assigns *rhs to the contained value	initializes the contained value as if direct-non-list-initializing an object of type T with *rhs
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.

If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor.

If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.

template<class U> optional<T>& operator=(optional<U>&& rhs);

Constraints:

(24.1)
is_constructible_v<T, U> is true,
(24.2)
is_assignable_v<T&, U> is true,
(24.3)
is_constructible_v<T, optional&> is false,
(24.4)
is_constructible_v<T, optional&&> is false,
(24.5)
is_constructible_v<T, const optional&> is false,
(24.6)
is_constructible_v<T, const optional&&> is false,
(24.7)
is_convertible_v<optional&, T> is false,
(24.8)
is_convertible_v<optional&&, T> is false,
(24.9)
is_convertible_v<const optional&, T> is false,
(24.10)
is_convertible_v<const optional&&, T> is false,
(24.11)
is_assignable_v<T&, optional&> is false,
(24.12)
is_assignable_v<T&, optional&&> is false,
(24.13)
is_assignable_v<T&, const optional&> is false, and
(24.14)
is_assignable_v<T&, const optional&&> is false.

Effects: See Table 45 .

The result of the expression bool(rhs) remains unchanged.

Table 45: optional::operator=(optional&&) effects [tab:optional.assign.move.templ]

	*this contains a value	*this does not contain a value
rhs contains a value	assigns std::move(*rhs) to the contained value	initializes the contained value as if direct-non-list-initializing an object of type T with std::move(*rhs)
rhs does not contain a value	destroys the contained value by calling val->T::~T()	no effect

Postconditions: bool(rhs) == bool(*this).

Returns: *this.

Remarks: If any exception is thrown, the result of the expression bool(*this) remains unchanged.

If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor.

If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment.

template<class... Args> T& emplace(Args&&... args);

Mandates: is_constructible_v<T, Args...> is true.

Effects: Calls *this = nullopt.

Then initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std::forward<Args>(args)....

Postconditions: *this contains a value.

Returns: A reference to the new contained value.

Throws: Any exception thrown by the selected constructor of T.

Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.

template<class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);

Constraints: is_constructible_v<T, initializer_list&, Args...> is true.

Effects: Calls *this = nullopt.

Then initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std::forward<Args>(args)....

Postconditions: *this contains a value.

Returns: A reference to the new contained value.

Throws: Any exception thrown by the selected constructor of T.

Remarks: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed.

20.6.3.4 Swap [optional.swap]

void swap(optional& rhs) noexcept(see below);

Mandates: is_move_constructible_v<T> is true.

Preconditions: Lvalues of type T are swappable.

Effects: See Table 46 .

Table 46: optional::swap(optional&) effects [tab:optional.swap]

	*this contains a value	*this does not contain a value
rhs contains a value	calls swap((this), *rhs)	initializes the contained value of this as if direct-non-list-initializing an object of type T with the expression std::move(rhs), followed by rhs.val->T::~T(); postcondition is that *this contains a value and rhs does not contain a value
rhs does not contain a value	initializes the contained value of rhs as if direct-non-list-initializing an object of type T with the expression std::move((this)), followed by val->T::~T(); postcondition is that *this does not contain a value and rhs contains a value	no effect

Throws: Any exceptions thrown by the operations in the relevant part of Table 46 .

Remarks: The expression inside noexcept is equivalent to:

is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>

If any exception is thrown, the results of the expressions bool(*this) and bool(rhs) remain unchanged.

If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T.

If an exception is thrown during the call to T's move constructor, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move constructor.

20.6.3.5 Observers [optional.observe]

constexpr const T* operator->() const;
constexpr T* operator->();

Preconditions: *this contains a value.

Returns: val.

Throws: Nothing.

Remarks: These functions are constexpr functions.

constexpr const T& operator*() const&;
constexpr T& operator*() &;

Preconditions: *this contains a value.

Returns: *val.

Throws: Nothing.

Remarks: These functions are constexpr functions.

constexpr T&& operator*() &&;
constexpr const T&& operator*() const&&;

Preconditions: *this contains a value.

Effects: Equivalent to: return std::move(*val);

constexpr explicit operator bool() const noexcept;

Returns: true if and only if *this contains a value.

Remarks: This function is a constexpr function.

constexpr bool has_value() const noexcept;

Returns: true if and only if *this contains a value.

Remarks: This function is a constexpr function.

constexpr const T& value() const&;
constexpr T& value() &;

Effects: Equivalent to:

return bool(*this) ? *val : throw bad_optional_access();

constexpr T&& value() &&;
constexpr const T&& value() const&&;

Effects: Equivalent to:

return bool(*this) ? std::move(*val) : throw bad_optional_access();

template<class U> constexpr T value_or(U&& v) const&;

Mandates: is_copy_constructible_v<T> && is_convertible_v<U&&, T> is true.

Effects: Equivalent to:

return bool(*this) ? **this : static_cast<T>(std::forward<U>(v));

template<class U> constexpr T value_or(U&& v) &&;

Mandates: is_move_constructible_v<T> && is_convertible_v<U&&, T> is true.

Effects: Equivalent to:

return bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(v));

20.6.3.6 Modifiers [optional.mod]

void reset() noexcept;

Effects: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect.

Postconditions: *this does not contain a value.

20.6.4 No-value state indicator [optional.nullopt]

struct nullopt_t{see below};
inline constexpr nullopt_t nullopt(unspecified);

The struct nullopt_t is an empty class type used as a unique type to indicate the state of not containing a value for optional objects.

In particular, optional<T> has a constructor with nullopt_t as a single argument; this indicates that an optional object not containing a value shall be constructed.

Type nullopt_t shall not have a default constructor or an initializer-list constructor, and shall not be an aggregate.

20.6.5 Class bad_optional_access [optional.bad.access]

class bad_optional_access : public exception {
public:
  // see [exception] for the specification of the special member functions
  const char* what() const noexcept override;
};

The class bad_optional_access defines the type of objects thrown as exceptions to report the situation where an attempt is made to access the value of an optional object that does not contain a value.

const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

20.6.6 Relational operators [optional.relops]

template<class T, class U> constexpr bool operator==(const optional<T>& x, const optional<U>& y);

Mandates: The expression *x == *y is well-formed and its result is convertible to bool.

T need not be Cpp17EqualityComparable.

— end note

]

Returns: If bool(x) != bool(y), false; otherwise if bool(x) == false, true; otherwise *x == *y.

Remarks: Specializations of this function template for which *x == *y is a core constant expression are constexpr functions.

template<class T, class U> constexpr bool operator!=(const optional<T>& x, const optional<U>& y);

Mandates: The expression *x != *y is well-formed and its result is convertible to bool.

Returns: If bool(x) != bool(y), true; otherwise, if bool(x) == false, false; otherwise *x != *y.

Remarks: Specializations of this function template for which *x != *y is a core constant expression are constexpr functions.

template<class T, class U> constexpr bool operator<(const optional<T>& x, const optional<U>& y);

Mandates: *x < *y is well-formed and its result is convertible to bool.

Returns: If !y, false; otherwise, if !x, true; otherwise *x < *y.

Remarks: Specializations of this function template for which *x < *y is a core constant expression are constexpr functions.

template<class T, class U> constexpr bool operator>(const optional<T>& x, const optional<U>& y);

Mandates: The expression *x > *y is well-formed and its result is convertible to bool.

Returns: If !x, false; otherwise, if !y, true; otherwise *x > *y.

Remarks: Specializations of this function template for which *x > *y is a core constant expression are constexpr functions.

template<class T, class U> constexpr bool operator<=(const optional<T>& x, const optional<U>& y);

Mandates: The expression *x <= *y is well-formed and its result is convertible to bool.

Returns: If !x, true; otherwise, if !y, false; otherwise *x <= *y.

Remarks: Specializations of this function template for which *x <= *y is a core constant expression are constexpr functions.

template<class T, class U> constexpr bool operator>=(const optional<T>& x, const optional<U>& y);

Mandates: The expression *x >= *y is well-formed and its result is convertible to bool.

Returns: If !y, true; otherwise, if !x, false; otherwise *x >= *y.

Remarks: Specializations of this function template for which *x >= *y is a core constant expression are constexpr functions.

template<class T, three_way_comparable_with<T> U>
  constexpr compare_three_way_result_t<T,U>
    operator<=>(const optional<T>& x, const optional<U>& y);

Returns: If x && y, *x <=> *y; otherwise bool(x) <=> bool(y).

Remarks: Specializations of this function template for which *x <=> *y is a core constant expression are constexpr functions.

20.6.7 Comparison with nullopt [optional.nullops]

template<class T> constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept;

Returns: !x.

template<class T> constexpr strong_ordering operator<=>(const optional<T>& x, nullopt_t) noexcept;

Returns: bool(x) <=> false.

20.6.8 Comparison with T [optional.comp.with.t]

template<class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);

Mandates: The expression *x == v is well-formed and its result is convertible to bool.

T need not be Cpp17EqualityComparable.

— end note

]

Effects: Equivalent to: return bool(x) ? *x == v : false;

template<class T, class U> constexpr bool operator==(const T& v, const optional<U>& x);

Mandates: The expression v == *x is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? v == *x : false;

template<class T, class U> constexpr bool operator!=(const optional<T>& x, const U& v);

Mandates: The expression *x != v is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? *x != v : true;

template<class T, class U> constexpr bool operator!=(const T& v, const optional<U>& x);

Mandates: The expression v != *x is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? v != *x : true;

template<class T, class U> constexpr bool operator<(const optional<T>& x, const U& v);

Mandates: The expression *x < v is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? *x < v : true;

template<class T, class U> constexpr bool operator<(const T& v, const optional<U>& x);

Mandates: The expression v < *x is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? v < *x : false;

template<class T, class U> constexpr bool operator>(const optional<T>& x, const U& v);

Mandates: The expression *x > v is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? *x > v : false;

template<class T, class U> constexpr bool operator>(const T& v, const optional<U>& x);

Mandates: The expression v > *x is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? v > *x : true;

template<class T, class U> constexpr bool operator<=(const optional<T>& x, const U& v);

Mandates: The expression *x <= v is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? *x <= v : true;

template<class T, class U> constexpr bool operator<=(const T& v, const optional<U>& x);

Mandates: The expression v <= *x is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? v <= *x : false;

template<class T, class U> constexpr bool operator>=(const optional<T>& x, const U& v);

Mandates: The expression *x >= v is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? *x >= v : false;

template<class T, class U> constexpr bool operator>=(const T& v, const optional<U>& x);

Mandates: The expression v >= *x is well-formed and its result is convertible to bool.

Effects: Equivalent to: return bool(x) ? v >= *x : true;

template<class T, three_way_comparable_with<T> U>
  constexpr compare_three_way_result_t<T,U>
    operator<=>(const optional<T>& x, const U& v);

Effects: Equivalent to: return bool(x) ? *x <=> v : strong_ordering::less;

20.6.9 Specialized algorithms [optional.specalg]

template<class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));

Constraints: is_move_constructible_v<T> is true and is_swappable_v<T> is true.

Effects: Calls x.swap(y).

template<class T> constexpr optional<decay_t<T>> make_optional(T&& v);

Returns: optional<decay_t<T>>(std::forward<T>(v)).

template<class T, class...Args>
  constexpr optional<T> make_optional(Args&&... args);

Effects: Equivalent to: return optional<T>(in_place, std::forward<Args>(args)...);

template<class T, class U, class... Args>
  constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);

Effects: Equivalent to: return optional<T>(in_place, il, std::forward<Args>(args)...);

20.6.10 Hash support [optional.hash]

template<class T> struct hash<optional<T>>;

The specialization hash<optional<T>> is enabled ([unord.hash]) if and only if hash<remove_const_t<T>> is enabled.

When enabled, for an object o of type optional<T>, if bool(o) == true, then hash<optional<T>>()(o) evaluates to the same value as hash<remove_const_t<T>>()(*o); otherwise it evaluates to an unspecified value.

The member functions are not guaranteed to be noexcept.

20.7 Variants [variant]

20.7.1 In general [variant.general]

A variant object holds and manages the lifetime of a value.

If the variant holds a value, that value's type has to be one of the template argument types given to variant.

These template arguments are called alternatives.

20.7.2 Header <variant> synopsis [variant.syn]

#include <compare>              // see [compare.syn]

namespace std {
  // [variant.variant], class template variant
  template<class... Types>
    class variant;

  // [variant.helper], variant helper classes
  template<class T> struct variant_size;                        // not defined
  template<class T> struct variant_size<const T>;
  template<class T>
    inline constexpr size_t variant_size_v = variant_size<T>::value;

  template<class... Types>
    struct variant_size<variant<Types...>>;

  template<size_t I, class T> struct variant_alternative;       // not defined
  template<size_t I, class T> struct variant_alternative<I, const T>;
  template<size_t I, class T>
    using variant_alternative_t = typename variant_alternative<I, T>::type;

  template<size_t I, class... Types>
    struct variant_alternative<I, variant<Types...>>;

  inline constexpr size_t variant_npos = -1;

  // [variant.get], value access
  template<class T, class... Types>
    constexpr bool holds_alternative(const variant<Types...>&) noexcept;

  template<size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
  template<size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&);
  template<size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>&);
  template<size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&&);

  template<class T, class... Types>
    constexpr T& get(variant<Types...>&);
  template<class T, class... Types>
    constexpr T&& get(variant<Types...>&&);
  template<class T, class... Types>
    constexpr const T& get(const variant<Types...>&);
  template<class T, class... Types>
    constexpr const T&& get(const variant<Types...>&&);

  template<size_t I, class... Types>
    constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
      get_if(variant<Types...>*) noexcept;
  template<size_t I, class... Types>
    constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
      get_if(const variant<Types...>*) noexcept;

  template<class T, class... Types>
    constexpr add_pointer_t<T>
      get_if(variant<Types...>*) noexcept;
  template<class T, class... Types>
    constexpr add_pointer_t<const T>
      get_if(const variant<Types...>*) noexcept;

  // [variant.relops], relational operators
  template<class... Types>
    constexpr bool operator==(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator<(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator>(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types> requires (three_way_comparable<Types> && ...)
    constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
      operator<=>(const variant<Types...>&, const variant<Types...>&);

  // [variant.visit], visitation
  template<class Visitor, class... Variants>
    constexpr see below visit(Visitor&&, Variants&&...);
  template<class R, class Visitor, class... Variants>
    constexpr R visit(Visitor&&, Variants&&...);

  // [variant.monostate], class monostate
  struct monostate;

  // [variant.monostate.relops], monostate relational operators
  constexpr bool operator==(monostate, monostate) noexcept;
  constexpr strong_ordering operator<=>(monostate, monostate) noexcept;

  // [variant.specalg], specialized algorithms
  template<class... Types>
    void swap(variant<Types...>&, variant<Types...>&) noexcept(see below);

  // [variant.bad.access], class bad_variant_access
  class bad_variant_access;

  // [variant.hash], hash support
  template<class T> struct hash;
  template<class... Types> struct hash<variant<Types...>>;
  template<> struct hash<monostate>;
}

20.7.3 Class template variant [variant.variant]

namespace std {
  template<class... Types>
  class variant {
  public:
    // [variant.ctor], constructors
    constexpr variant() noexcept(see below);
    constexpr variant(const variant&);
    constexpr variant(variant&&) noexcept(see below);

    template<class T>
      constexpr variant(T&&) noexcept(see below);

    template<class T, class... Args>
      constexpr explicit variant(in_place_type_t<T>, Args&&...);
    template<class T, class U, class... Args>
      constexpr explicit variant(in_place_type_t<T>, initializer_list<U>, Args&&...);

    template<size_t I, class... Args>
      constexpr explicit variant(in_place_index_t<I>, Args&&...);
    template<size_t I, class U, class... Args>
      constexpr explicit variant(in_place_index_t<I>, initializer_list<U>, Args&&...);

    // [variant.dtor], destructor
    ~variant();

    // [variant.assign], assignment
    constexpr variant& operator=(const variant&);
    constexpr variant& operator=(variant&&) noexcept(see below);

    template<class T> variant& operator=(T&&) noexcept(see below);

    // [variant.mod], modifiers
    template<class T, class... Args>
      T& emplace(Args&&...);
    template<class T, class U, class... Args>
      T& emplace(initializer_list<U>, Args&&...);
    template<size_t I, class... Args>
      variant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
    template<size_t I, class U, class... Args>
      variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U>, Args&&...);

    // [variant.status], value status
    constexpr bool valueless_by_exception() const noexcept;
    constexpr size_t index() const noexcept;

    // [variant.swap], swap
    void swap(variant&) noexcept(see below);
  };
}

Any instance of variant at any given time either holds a value of one of its alternative types or holds no value.

When an instance of variant holds a value of alternative type T, it means that a value of type T, referred to as the variant object's contained value, is allocated within the storage of the variant object.

Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.

The contained value shall be allocated in a region of the variant storage suitably aligned for all types in Types.

All types in Types shall meet the Cpp17Destructible requirements (Table 32).

A program that instantiates the definition of variant with no template arguments is ill-formed.

20.7.3.1 Constructors [variant.ctor]

In the descriptions that follow, let i be in the range [0, sizeof...(Types)), and

T_{i}

be the

i^{th}

type in Types.

constexpr variant() noexcept(see below);

Constraints: is_default_constructible_v<

T_{0}

> is true.

Effects: Constructs a variant holding a value-initialized value of type

T_{0}

Postconditions: valueless_by_exception() is false and index() is 0.

Throws: Any exception thrown by the value-initialization of

T_{0}

Remarks: This function is constexpr if and only if the value-initialization of the alternative type

T_{0}

would satisfy the requirements for a constexpr function.

The expression inside noexcept is equivalent to is_nothrow_default_constructible_v<

T_{0}

20.7.3.2 Destructor [variant.dtor]

~variant();

Effects: If valueless_by_exception() is false, destroys the currently contained value.

Remarks: If is_trivially_destructible_v<

T_{i}

> is true for all

T_{i}

, then this destructor is trivial.

20.7.3.3 Assignment [variant.assign]

constexpr variant& operator=(const variant& rhs);

Let j be rhs.index().

Effects:

(2.1)
If neither *this nor rhs holds a value, there is no effect.
(2.2)
Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
(2.3)
Otherwise, if index() == j, assigns the value contained in rhs to the value contained in *this.
(2.4)
Otherwise, if either is_nothrow_copy_constructible_v< $T_{j}$ > is true or is_nothrow_move_constructible_v< $T_{j}$ > is false, equivalent to emplace<j>(get<j>(rhs)).
(2.5)
Otherwise, equivalent to operator=(variant(rhs)).

Returns: *this.

Postconditions: index() == rhs.index().

Remarks: This operator is defined as deleted unless is_copy_constructible_v<

T_{i}

> && is_copy_assignable_v<

T_{i}

> is true for all i.

If is_trivially_copy_constructible_v<

T_{i}

> && is_trivially_copy_assignable_v<

T_{i}

> && is_trivially_destructible_v<

T_{i}

> is true for all i, this assignment operator is trivial.

constexpr variant& operator=(variant&& rhs) noexcept(see below);

Let j be rhs.index().

Constraints: is_move_constructible_v<

T_{i}

> && is_move_assignable_v<

T_{i}

> is true for all i.

Effects:

(8.1)
If neither *this nor rhs holds a value, there is no effect.
(8.2)
Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
(8.3)
Otherwise, if index() == j, assigns get<j>(std::move(rhs)) to the value contained in *this.
(8.4)
Otherwise, equivalent to emplace<j>(get<j>(std::move(rhs))).

Returns: *this.

Remarks: If is_trivially_move_constructible_v<

T_{i}

> && is_trivially_move_assignable_v<

T_{i}

> && is_trivially_destructible_v<

T_{i}

> is true for all i, this assignment operator is trivial.

The expression inside noexcept is equivalent to: is_nothrow_move_constructible_v<

T_{i}

> && is_nothrow_move_assignable_v<

T_{i}

> for all i.

(10.1)
If an exception is thrown during the call to $T_{j}$ 's move construction (with j being rhs.index()), the variant will hold no value.
(10.2)
If an exception is thrown during the call to $T_{j}$ 's move assignment, the state of the contained value is as defined by the exception safety guarantee of $T_{j}$ 's move assignment; index() will be j.

template<class T> variant& operator=(T&& t) noexcept(see below);

Let

T_{j}

be a type that is determined as follows: build an imaginary function FUN(

T_{i}

) for each alternative type

T_{i}

for which

T_{i}

x[] = {std::forward<T>(t)}; is well-formed for some invented variable x.

The overload FUN(

T_{j}

) selected by overload resolution for the expression FUN(std::forward<T>(t)) defines the alternative

T_{j}

which is the type of the contained value after assignment.

Constraints:

(12.1)
is_same_v<remove_cvref_t<T>, variant> is false,
(12.2)
is_assignable_v< $T_{j}$ &, T> && is_constructible_v< $T_{j}$ , T> is true, and
(12.3)
the expression FUN(std::forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well-formed.
[Note
:
```
variant<string, string> v;
v = "abc";
```
is ill-formed, as both alternative types have an equally viable constructor for the argument. — end note
]

Effects:

(13.1)
If *this holds a $T_{j}$ , assigns std::forward<T>(t) to the value contained in *this.
(13.2)
Otherwise, if is_nothrow_constructible_v< $T_{j}$ , T> || !is_nothrow_move_constructible_v< $T_{j}$ > is true, equivalent to emplace<j>(std::forward<T>(t)).
(13.3)
Otherwise, equivalent to operator=(variant(std::forward<T>(t))).

Postconditions: holds_alternative<

T_{j}

>(*this) is true, with

T_{j}

selected by the imaginary function overload resolution described above.

Returns: *this.

Remarks: The expression inside noexcept is equivalent to:

is_nothrow_assignable_v<T $_{j}$ &, T> && is_nothrow_constructible_v<T $_{j}$ , T>

(16.1)
If an exception is thrown during the assignment of std::forward<T>(t) to the value contained in *this, the state of the contained value and t are as defined by the exception safety guarantee of the assignment expression; valueless_by_exception() will be false.
(16.2)
If an exception is thrown during the initialization of the contained value, the variant object might not hold a value.

20.7.3.4 Modifiers [variant.mod]

template<class T, class... Args> T& emplace(Args&&... args);

Constraints: is_constructible_v<T, Args...> is true, and T occurs exactly once in Types.

Effects: Equivalent to:

return emplace<I>(std::forward<Args>(args)...);

where I is the zero-based index of T in Types.

template<class T, class U, class... Args> T& emplace(initializer_list<U> il, Args&&... args);

Constraints: is_constructible_v<T, initializer_list&, Args...> is true, and T occurs exactly once in Types.

Effects: Equivalent to:

return emplace<I>(il, std::forward<Args>(args)...);

where I is the zero-based index of T in Types.

template<size_t I, class... Args>
  variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);

Mandates:

I < sizeof...(Types)

Constraints: is_constructible_v<

T_{I}

, Args...> is true.

Effects: Destroys the currently contained value if valueless_by_exception() is false.

Then initializes the contained value as if direct-non-list-initializing a value of type

T_{I}

with the arguments std::forward<Args>(args)....

Postconditions: index() is I.

Returns: A reference to the new contained value.

Throws: Any exception thrown during the initialization of the contained value.

Remarks: If an exception is thrown during the initialization of the contained value, the variant might not hold a value.

template<size_t I, class U, class... Args>
  variant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);

Mandates:

I < sizeof...(Types)

Constraints: is_constructible_v<

T_{I}

, initializer_list&, Args...> is true.

Effects: Destroys the currently contained value if valueless_by_exception() is false.

Then initializes the contained value as if direct-non-list-initializing a value of type

T_{I}

with the arguments il, std::forward<Args>(args)....

Postconditions: index() is I.

Returns: A reference to the new contained value.

Throws: Any exception thrown during the initialization of the contained value.

Remarks: If an exception is thrown during the initialization of the contained value, the variant might not hold a value.

20.7.3.5 Value status [variant.status]

constexpr bool valueless_by_exception() const noexcept;

Effects: Returns false if and only if the variant holds a value.

A variant might not hold a value if an exception is thrown during a type-changing assignment or emplacement.

The latter means that even a variant<float, int> can become valueless_by_exception(), for instance by

struct S { operator int() { throw 42; }};
variant<float, int> v{12.f};
v.emplace<1>(S());

— end note

]

constexpr size_t index() const noexcept;

Effects: If valueless_by_exception() is true, returns variant_npos.

Otherwise, returns the zero-based index of the alternative of the contained value.

20.7.3.6 Swap [variant.swap]

void swap(variant& rhs) noexcept(see below);

Mandates: is_move_constructible_v<

T_{i}

> is true for all i.

Preconditions: Lvalues of type

T_{i}

are swappable ([swappable.requirements]).

Effects:

(3.1)
If valueless_by_exception() && rhs.valueless_by_exception() no effect.
(3.2)
Otherwise, if index() == rhs.index(), calls swap(get(*this), get(rhs)) where i is index().
(3.3)
Otherwise, exchanges values of rhs and *this.

Throws: If index() == rhs.index(), any exception thrown by swap(get(*this), get(rhs)) with i being index().

Otherwise, any exception thrown by the move constructor of

T_{i}

T_{j}

with i being index() and j being rhs.index().

Remarks: If an exception is thrown during the call to function swap(get(*this), get(rhs)), the states of the contained values of *this and of rhs are determined by the exception safety guarantee of swap for lvalues of

T_{i}

with i being index().

If an exception is thrown during the exchange of the values of *this and rhs, the states of the values of *this and of rhs are determined by the exception safety guarantee of variant's move constructor.

The expression inside noexcept is equivalent to the logical AND of is_nothrow_move_constructible_v<

T_{i}

> && is_nothrow_swappable_v<

T_{i}

> for all i.

20.7.4 variant helper classes [variant.helper]

template<class T> struct variant_size;

All specializations of variant_size meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, N> for some N.

template<class T> class variant_size<const T>;

Let VS denote variant_size<T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, VS::value>.

template<class... Types>
  struct variant_size<variant<Types...>> : integral_constant<size_t, sizeof...(Types)> { };

template<size_t I, class T> class variant_alternative<I, const T>;

Let VA denote variant_alternative<I, T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17TransformationTrait requirements ([meta.rqmts]) with a member typedef type that names the type add_const_t<VA::type>.

variant_alternative<I, variant<Types...>>::type

Mandates:

I < sizeof...(Types)

Type: The type

T_{I}

20.7.5 Value access [variant.get]

template<class T, class... Types>
  constexpr bool holds_alternative(const variant<Types...>& v) noexcept;

Mandates: The type T occurs exactly once in Types.

Returns: true if index() is equal to the zero-based index of T in Types.

template<size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>& v);
template<size_t I, class... Types>
  constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&& v);
template<size_t I, class... Types>
  constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>& v);
template<size_t I, class... Types>
  constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&& v);

Mandates:

I < sizeof...(Types)

Effects: If v.index() is I, returns a reference to the object stored in the variant.

Otherwise, throws an exception of type bad_variant_access.

template<class T, class... Types> constexpr T& get(variant<Types...>& v);
template<class T, class... Types> constexpr T&& get(variant<Types...>&& v);
template<class T, class... Types> constexpr const T& get(const variant<Types...>& v);
template<class T, class... Types> constexpr const T&& get(const variant<Types...>&& v);

Mandates: The type T occurs exactly once in Types.

Effects: If v holds a value of type T, returns a reference to that value.

Otherwise, throws an exception of type bad_variant_access.

template<size_t I, class... Types>
  constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
    get_if(variant<Types...>* v) noexcept;
template<size_t I, class... Types>
  constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
    get_if(const variant<Types...>* v) noexcept;

Mandates:

I < sizeof...(Types)

Returns: A pointer to the value stored in the variant, if v != nullptr and v->index() == I.

Otherwise, returns nullptr.

template<class T, class... Types>
  constexpr add_pointer_t<T>
    get_if(variant<Types...>* v) noexcept;
template<class T, class... Types>
  constexpr add_pointer_t<const T>
    get_if(const variant<Types...>* v) noexcept;

Mandates: The type T occurs exactly once in Types.

Effects: Equivalent to: return get_if(v); with i being the zero-based index of T in Types.

20.7.6 Relational operators [variant.relops]

template<class... Types>
  constexpr bool operator==(const variant<Types...>& v, const variant<Types...>& w);

Mandates: get(v) == get(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.index() != w.index(), false; otherwise if v.valueless_by_exception(), true; otherwise get(v) == get(w) with i being v.index().

template<class... Types>
  constexpr bool operator!=(const variant<Types...>& v, const variant<Types...>& w);

Mandates: get(v) != get(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.index() != w.index(), true; otherwise if v.valueless_by_exception(), false; otherwise get(v) != get(w) with i being v.index().

template<class... Types>
  constexpr bool operator<(const variant<Types...>& v, const variant<Types...>& w);

Mandates: get(v) < get(w) is a valid expression that is convertible to bool, for all i.

Returns: If w.valueless_by_exception(), false; otherwise if v.valueless_by_exception(), true; otherwise, if v.index() < w.index(), true; otherwise if v.index() > w.index(), false; otherwise get(v) < get(w) with i being v.index().

template<class... Types>
  constexpr bool operator>(const variant<Types...>& v, const variant<Types...>& w);

Mandates: get(v) > get(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.valueless_by_exception(), false; otherwise if w.valueless_by_exception(), true; otherwise, if v.index() > w.index(), true; otherwise if v.index() < w.index(), false; otherwise get(v) > get(w) with i being v.index().

template<class... Types>
  constexpr bool operator<=(const variant<Types...>& v, const variant<Types...>& w);

Mandates: get(v) <= get(w) is a valid expression that is convertible to bool, for all i.

Returns: If v.valueless_by_exception(), true; otherwise if w.valueless_by_exception(), false; otherwise, if v.index() < w.index(), true; otherwise if v.index() > w.index(), false; otherwise get(v) <= get(w) with i being v.index().

template<class... Types>
  constexpr bool operator>=(const variant<Types...>& v, const variant<Types...>& w);

Mandates: get(v) >= get(w) is a valid expression that is convertible to bool, for all i.

Returns: If w.valueless_by_exception(), true; otherwise if v.valueless_by_exception(), false; otherwise, if v.index() > w.index(), true; otherwise if v.index() < w.index(), false; otherwise get(v) >= get(w) with i being v.index().

template<class... Types> requires (three_way_comparable<Types> && ...)
  constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
    operator<=>(const variant<Types...>& v, const variant<Types...>& w);

Effects: Equivalent to:

if (v.valueless_by_exception() && w.valueless_by_exception())
  return strong_ordering::equal;
if (v.valueless_by_exception()) return strong_ordering::less;
if (w.valueless_by_exception()) return strong_ordering::greater;
if (auto c = v.index() <=> w.index(); c != 0) return c;
return get<i>(v) <=> get<i>(w);

with i being v.index().

20.7.7 Visitation [variant.visit]

template<class Visitor, class... Variants>
  constexpr see below visit(Visitor&& vis, Variants&&... vars);
template<class R, class Visitor, class... Variants>
  constexpr R visit(Visitor&& vis, Variants&&... vars);

Let n be sizeof...(Variants).

Let m be a pack of n values of type size_t.

Such a pack is called valid if

0 \leq m_{i} < {variant_size_v<remove_reference_t<Variants}_{i} >>

for all

0 \leq i < n

For each valid pack m, let e(m) denote the expression:

INVOKE(std::forward<Visitor>(vis), get<m>(std::forward<Variants>(vars))...) // see [func.require]

for the first form and

INVOKE<R>(std::forward<Visitor>(vis), get<m>(std::forward<Variants>(vars))...) // see [func.require]

for the second form.

Mandates: For each valid pack m, e(m) is a valid expression.

All such expressions are of the same type and value category.

Returns: e(m), where m is the pack for which

m_{i}

is vars

_{i}

.index() for all

0 \leq i < n

The return type is decltype(e(m)) for the first form.

Throws: bad_variant_access if any variant in vars is valueless_by_exception().

Complexity: For

n \leq 1

, the invocation of the callable object is implemented in constant time, i.e., for

n = 1

, it does not depend on the number of alternative types of

{Variants}_{0}

For

n > 1

, the invocation of the callable object has no complexity requirements.

20.7.8 Class monostate [variant.monostate]

struct monostate{};

The class monostate can serve as a first alternative type for a variant to make the variant type default constructible.

20.7.9 monostate relational operators [variant.monostate.relops]

constexpr bool operator==(monostate, monostate) noexcept { return true; }
constexpr strong_ordering operator<=>(monostate, monostate) noexcept
{ return strong_ordering::equal; }

monostate objects have only a single state; they thus always compare equal.

— end note

]

20.7.10 Specialized algorithms [variant.specalg]

template<class... Types>
  void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);

Constraints: is_move_constructible_v<

T_{i}

> && is_swappable_v<

T_{i}

> is true for all i.

Effects: Equivalent to v.swap(w).

Remarks: The expression inside noexcept is equivalent to noexcept(v.swap(w)).

20.7.11 Class bad_variant_access [variant.bad.access]

class bad_variant_access : public exception {
public:
  // see [exception] for the specification of the special member functions
  const char* what() const noexcept override;
};

Objects of type bad_variant_access are thrown to report invalid accesses to the value of a variant object.

const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

20.7.12 Hash support [variant.hash]

template<class... Types> struct hash<variant<Types...>>;

The specialization hash<variant<Types...>> is enabled ([unord.hash]) if and only if every specialization in hash<remove_const_t<Types>>... is enabled.

The member functions are not guaranteed to be noexcept.

template<> struct hash<monostate>;

The specialization is enabled ([unord.hash]).

20.8 Storage for any type [any]

This subclause describes components that C++ programs may use to perform operations on objects of a discriminated type.

The discriminated type may contain values of different types but does not attempt conversion between them, i.e., 5 is held strictly as an int and is not implicitly convertible either to "5" or to 5.0.

This indifference to interpretation but awareness of type effectively allows safe, generic containers of single values, with no scope for surprises from ambiguous conversions.

— end note

]

20.8.1 Header <any> synopsis [any.synop]

namespace std {
  // [any.bad.any.cast], class bad_any_cast
  class bad_any_cast;

  // [any.class], class any
  class any;

  // [any.nonmembers], non-member functions
  void swap(any& x, any& y) noexcept;

  template<class T, class... Args>
    any make_any(Args&&... args);
  template<class T, class U, class... Args>
    any make_any(initializer_list<U> il, Args&&... args);

  template<class T>
    T any_cast(const any& operand);
  template<class T>
    T any_cast(any& operand);
  template<class T>
    T any_cast(any&& operand);

  template<class T>
    const T* any_cast(const any* operand) noexcept;
  template<class T>
    T* any_cast(any* operand) noexcept;
}

20.8.2 Class bad_any_cast [any.bad.any.cast]

class bad_any_cast : public bad_cast {
public:
  // see [exception] for the specification of the special member functions
  const char* what() const noexcept override;
};

Objects of type bad_any_cast are thrown by a failed any_cast .

const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

20.8.3 Class any [any.class]

namespace std {
  class any {
  public:
    // [any.cons], construction and destruction
    constexpr any() noexcept;

    any(const any& other);
    any(any&& other) noexcept;

    template<class T>
      any(T&& value);

    template<class T, class... Args>
      explicit any(in_place_type_t<T>, Args&&...);
    template<class T, class U, class... Args>
      explicit any(in_place_type_t<T>, initializer_list<U>, Args&&...);

    ~any();

    // [any.assign], assignments
    any& operator=(const any& rhs);
    any& operator=(any&& rhs) noexcept;

    template<class T>
      any& operator=(T&& rhs);

    // [any.modifiers], modifiers
    template<class T, class... Args>
      decay_t<T>& emplace(Args&&...);
    template<class T, class U, class... Args>
      decay_t<T>& emplace(initializer_list<U>, Args&&...);
    void reset() noexcept;
    void swap(any& rhs) noexcept;

    // [any.observers], observers
    bool has_value() const noexcept;
    const type_info& type() const noexcept;
  };
}

An object of class any stores an instance of any type that meets the constructor requirements or it has no value, and this is referred to as the state of the class any object.

The stored instance is called the contained value.

Two states are equivalent if either they both have no value, or they both have a value and the contained values are equivalent.

The non-member any_cast functions provide type-safe access to the contained value.

Implementations should avoid the use of dynamically allocated memory for a small contained value.

However, any such small-object optimization shall only be applied to types T for which is_nothrow_move_constructible_v<T> is true.

A contained value of type int could be stored in an internal buffer, not in separately-allocated memory.

— end example

]

20.8.3.1 Construction and destruction [any.cons]

constexpr any() noexcept;

Postconditions: has_value() is false.

any(const any& other);

Effects: If other.has_value() is false, constructs an object that has no value.

Otherwise, equivalent to any(in_place_type<T>, any_cast<const T&>(other)) where T is the type of the contained value.

Throws: Any exceptions arising from calling the selected constructor for the contained value.

any(any&& other) noexcept;

Effects: If other.has_value() is false, constructs an object that has no value.

Otherwise, constructs an object of type any that contains either the contained value of other, or contains an object of the same type constructed from the contained value of other considering that contained value as an rvalue.

template<class T>
  any(T&& value);

Let VT be decay_t<T>.

Constraints: VT is not the same type as any, VT is not a specialization of in_place_type_t, and is_copy_constructible_v<VT> is true.

Preconditions: VT meets the Cpp17CopyConstructible requirements.

Effects: Constructs an object of type any that contains an object of type VT direct-initialized with std::forward<T>(value).

Throws: Any exception thrown by the selected constructor of VT.

template<class T, class... Args>
  explicit any(in_place_type_t<T>, Args&&... args);

Let VT be decay_t<T>.

Constraints: is_copy_constructible_v<VT> is true and is_constructible_v<VT, Args...> is true.

Preconditions: VT meets the Cpp17CopyConstructible requirements.

Effects: Initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments std::forward<Args>(args)....

Postconditions: *this contains a value of type VT.

Throws: Any exception thrown by the selected constructor of VT.

template<class T, class U, class... Args>
  explicit any(in_place_type_t<T>, initializer_list<U> il, Args&&... args);

Let VT be decay_t<T>.

Constraints: is_copy_constructible_v<VT> is true and is_constructible_v<VT, initializer_list&, Args...> is true.

Preconditions: VT meets the Cpp17CopyConstructible requirements.

Effects: Initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments il, std::forward<Args>(args)....

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected constructor of VT.

~any();

Effects: As if by reset().

20.8.3.2 Assignment [any.assign]

any& operator=(const any& rhs);

Effects: As if by any(rhs).swap(*this).

No effects if an exception is thrown.

Returns: *this.

Throws: Any exceptions arising from the copy constructor for the contained value.

any& operator=(any&& rhs) noexcept;

Effects: As if by any(std::move(rhs)).swap(*this).

Returns: *this.

Postconditions: The state of *this is equivalent to the original state of rhs.

template<class T>
  any& operator=(T&& rhs);

Let VT be decay_t<T>.

Constraints: VT is not the same type as any and is_copy_constructible_v<VT> is true.

Preconditions: VT meets the Cpp17CopyConstructible requirements.

Effects: Constructs an object tmp of type any that contains an object of type VT direct-initialized with std::forward<T>(rhs), and tmp.swap(*this).

No effects if an exception is thrown.

Returns: *this.

Throws: Any exception thrown by the selected constructor of VT.

20.8.3.3 Modifiers [any.modifiers]

template<class T, class... Args>
  decay_t<T>& emplace(Args&&... args);

Let VT be decay_t<T>.

Constraints: is_copy_constructible_v<VT> is true and is_constructible_v<VT, Args...> is true.

Preconditions: VT meets the Cpp17CopyConstructible requirements.

Effects: Calls reset().

Then initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments std::forward<Args>(args)....

Postconditions: *this contains a value.

Returns: A reference to the new contained value.

Throws: Any exception thrown by the selected constructor of VT.

Remarks: If an exception is thrown during the call to VT's constructor, *this does not contain a value, and any previously contained value has been destroyed.

template<class T, class U, class... Args>
  decay_t<T>& emplace(initializer_list<U> il, Args&&... args);

Let VT be decay_t<T>.

Constraints: is_copy_constructible_v<VT> is true and is_constructible_v<VT, initializer_list&, Args...> is true.

Preconditions: VT meets the Cpp17CopyConstructible requirements.

Effects: Calls reset().

Then initializes the contained value as if direct-non-list-initializing an object of type VT with the arguments il, std::forward<Args>(args)....

Postconditions: *this contains a value.

Returns: A reference to the new contained value.

Throws: Any exception thrown by the selected constructor of VT.

Remarks: If an exception is thrown during the call to VT's constructor, *this does not contain a value, and any previously contained value has been destroyed.

void reset() noexcept;

Effects: If has_value() is true, destroys the contained value.

Postconditions: has_value() is false.

void swap(any& rhs) noexcept;

Effects: Exchanges the states of *this and rhs.

20.8.3.4 Observers [any.observers]

bool has_value() const noexcept;

Returns: true if *this contains an object, otherwise false.

const type_info& type() const noexcept;

Returns: typeid(T) if *this has a contained value of type T, otherwise typeid(void).

Useful for querying against types known either at compile time or only at runtime.

— end note

]

20.8.4 Non-member functions [any.nonmembers]

void swap(any& x, any& y) noexcept;

Effects: Equivalent to x.swap(y).

template<class T, class... Args>
  any make_any(Args&&... args);

Effects: Equivalent to: return any(in_place_type<T>, std::forward<Args>(args)...);

template<class T, class U, class... Args>
  any make_any(initializer_list<U> il, Args&&... args);

Effects: Equivalent to: return any(in_place_type<T>, il, std::forward<Args>(args)...);

template<class T>
  T any_cast(const any& operand);
template<class T>
  T any_cast(any& operand);
template<class T>
  T any_cast(any&& operand);

Let U be the type remove_cvref_t<T>.

Mandates: For the first overload, is_constructible_v<T, const U&> is true.

For the second overload, is_constructible_v<T, U&> is true.

For the third overload, is_constructible_v<T, U> is true.

Returns: For the first and second overload, static_cast<T>(*any_cast(&operand)).

For the third overload, static_cast<T>(std::move(*any_cast(&operand))).

Throws: bad_any_cast if operand.type() != typeid(remove_reference_t<T>).

any x(5);                                   // x holds int
assert(any_cast<int>(x) == 5);              // cast to value
any_cast<int&>(x) = 10;                     // cast to reference
assert(any_cast<int>(x) == 10);

x = "Meow";                                 // x holds const char*
assert(strcmp(any_cast<const char*>(x), "Meow") == 0);
any_cast<const char*&>(x) = "Harry";
assert(strcmp(any_cast<const char*>(x), "Harry") == 0);

x = string("Meow");                         // x holds string
string s, s2("Jane");
s = move(any_cast<string&>(x));             // move from any
assert(s == "Meow");
any_cast<string&>(x) = move(s2);            // move to any
assert(any_cast<const string&>(x) == "Jane");

string cat("Meow");
const any y(cat);                           // const y holds string
assert(any_cast<const string&>(y) == cat);

any_cast<string&>(y);                       // error: cannot any_cast away const

— end example

]

template<class T>
  const T* any_cast(const any* operand) noexcept;
template<class T>
  T* any_cast(any* operand) noexcept;

Returns: If operand != nullptr && operand->type() == typeid(T), a pointer to the object contained by operand; otherwise, nullptr.

bool is_string(const any& operand) {
  return any_cast<string>(&operand) != nullptr;
}

— end example

]

20.9 Bitsets [bitset]

20.9.1 Header <bitset> synopsis [bitset.syn]

The header <bitset> defines a class template and several related functions for representing and manipulating fixed-size sequences of bits.

#include <string>
#include <iosfwd>   // for istream, ostream, see [iosfwd.syn]

namespace std {
  template<size_t N> class bitset;

  // [bitset.operators], bitset operators
  template<size_t N>
    bitset<N> operator&(const bitset<N>&, const bitset<N>&) noexcept;
  template<size_t N>
    bitset<N> operator|(const bitset<N>&, const bitset<N>&) noexcept;
  template<size_t N>
    bitset<N> operator^(const bitset<N>&, const bitset<N>&) noexcept;
  template<class charT, class traits, size_t N>
    basic_istream<charT, traits>&
      operator>>(basic_istream<charT, traits>& is, bitset<N>& x);
  template<class charT, class traits, size_t N>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& os, const bitset<N>& x);
}

20.9.2 Class template bitset [template.bitset]

namespace std {
  template<size_t N> class bitset {
  public:
    // bit reference
    class reference {
      friend class bitset;
      reference() noexcept;

    public:
      reference(const reference&) = default;
      ~reference();
      reference& operator=(bool x) noexcept;            // for b[i] = x;
      reference& operator=(const reference&) noexcept;  // for b[i] = b[j];
      bool operator~() const noexcept;                  // flips the bit
      operator bool() const noexcept;                   // for x = b[i];
      reference& flip() noexcept;                       // for b[i].flip();
    };

    // [bitset.cons], constructors
    constexpr bitset() noexcept;
    constexpr bitset(unsigned long long val) noexcept;
    template<class charT, class traits, class Allocator>
      explicit bitset(
        const basic_string<charT, traits, Allocator>& str,
        typename basic_string<charT, traits, Allocator>::size_type pos = 0,
        typename basic_string<charT, traits, Allocator>::size_type n
          = basic_string<charT, traits, Allocator>::npos,
        charT zero = charT('0'),
        charT one = charT('1'));
    template<class charT>
      explicit bitset(
        const charT* str,
        typename basic_string<charT>::size_type n = basic_string<charT>::npos,
        charT zero = charT('0'),
        charT one = charT('1'));

    // [bitset.members], bitset operations
    bitset<N>& operator&=(const bitset<N>& rhs) noexcept;
    bitset<N>& operator|=(const bitset<N>& rhs) noexcept;
    bitset<N>& operator^=(const bitset<N>& rhs) noexcept;
    bitset<N>& operator<<=(size_t pos) noexcept;
    bitset<N>& operator>>=(size_t pos) noexcept;
    bitset<N>& set() noexcept;
    bitset<N>& set(size_t pos, bool val = true);
    bitset<N>& reset() noexcept;
    bitset<N>& reset(size_t pos);
    bitset<N>  operator~() const noexcept;
    bitset<N>& flip() noexcept;
    bitset<N>& flip(size_t pos);

    // element access
    constexpr bool operator[](size_t pos) const;        // for b[i];
    reference operator[](size_t pos);                   // for b[i];

    unsigned long to_ulong() const;
    unsigned long long to_ullong() const;
    template<class charT = char,
             class traits = char_traits<charT>,
             class Allocator = allocator<charT>>
      basic_string<charT, traits, Allocator>
        to_string(charT zero = charT('0'), charT one = charT('1')) const;

    size_t count() const noexcept;
    constexpr size_t size() const noexcept;
    bool operator==(const bitset<N>& rhs) const noexcept;
    bool test(size_t pos) const;
    bool all() const noexcept;
    bool any() const noexcept;
    bool none() const noexcept;
    bitset<N> operator<<(size_t pos) const noexcept;
    bitset<N> operator>>(size_t pos) const noexcept;
  };

  // [bitset.hash], hash support
  template<class T> struct hash;
  template<size_t N> struct hash<bitset<N>>;
}

The class template bitset<N> describes an object that can store a sequence consisting of a fixed number of bits, N.

Each bit represents either the value zero (reset) or one (set).

To toggle a bit is to change the value zero to one, or the value one to zero.

Each bit has a non-negative position pos.

When converting between an object of class bitset<N> and a value of some integral type, bit position pos corresponds to the bit value 1 << pos.

The integral value corresponding to two or more bits is the sum of their bit values.

The functions described in this subclause can report three kinds of errors, each associated with a distinct exception:

(3.1)
an invalid-argument error is associated with exceptions of type invalid_argument ([invalid.argument]);
(3.2)
an out-of-range error is associated with exceptions of type out_of_range ([out.of.range]);
(3.3)
an overflow error is associated with exceptions of type overflow_error ([overflow.error]).

20.9.2.1 Constructors [bitset.cons]

constexpr bitset() noexcept;

Effects: Initializes all bits in *this to zero.

constexpr bitset(unsigned long long val) noexcept;

Effects: Initializes the first M bit positions to the corresponding bit values in val.

M is the smaller of N and the number of bits in the value representation of unsigned long long.

If M < N, the remaining bit positions are initialized to zero.

template<class charT, class traits, class Allocator>
  explicit bitset(
    const basic_string<charT, traits, Allocator>& str,
    typename basic_string<charT, traits, Allocator>::size_type pos = 0,
    typename basic_string<charT, traits, Allocator>::size_type n
      = basic_string<charT, traits, Allocator>::npos,
    charT zero = charT('0'),
    charT one = charT('1'));

Effects: Determines the effective length rlen of the initializing string as the smaller of n and str.size() - pos.

Initializes the first M bit positions to values determined from the corresponding characters in the string str.

M is the smaller of N and rlen.

An element of the constructed object has value zero if the corresponding character in str, beginning at position pos, is zero.

Otherwise, the element has the value one.

Character position pos + M - 1 corresponds to bit position zero.

Subsequent decreasing character positions correspond to increasing bit positions.

If M < N, remaining bit positions are initialized to zero.

The function uses traits::eq to compare the character values.

Throws: out_of_range if pos > str.size() or invalid_argument if any of the rlen characters in str beginning at position pos is other than zero or one.

template<class charT>
  explicit bitset(
    const charT* str,
    typename basic_string<charT>::size_type n = basic_string<charT>::npos,
    charT zero = charT('0'),
    charT one = charT('1'));

Effects: As if by:

bitset(n == basic_string<charT>::npos
          ? basic_string<charT>(str)
          : basic_string<charT>(str, n),
       0, n, zero, one)

20.9.2.2 Members [bitset.members]

bitset<N>& operator&=(const bitset<N>& rhs) noexcept;

Effects: Clears each bit in *this for which the corresponding bit in rhs is clear, and leaves all other bits unchanged.

Returns: *this.

bitset<N>& operator|=(const bitset<N>& rhs) noexcept;

Effects: Sets each bit in *this for which the corresponding bit in rhs is set, and leaves all other bits unchanged.

Returns: *this.

bitset<N>& operator^=(const bitset<N>& rhs) noexcept;

Effects: Toggles each bit in *this for which the corresponding bit in rhs is set, and leaves all other bits unchanged.

Returns: *this.

bitset<N>& operator<<=(size_t pos) noexcept;

Effects: Replaces each bit at position I in *this with a value determined as follows:

(7.1)
If I < pos, the new value is zero;
(7.2)
If I >= pos, the new value is the previous value of the bit at position I - pos.

Returns: *this.

bitset<N>& operator>>=(size_t pos) noexcept;

Effects: Replaces each bit at position I in *this with a value determined as follows:

(9.1)
If pos >= N - I, the new value is zero;
(9.2)
If pos < N - I, the new value is the previous value of the bit at position I + pos.

Returns: *this.

bitset<N>& set() noexcept;

Effects: Sets all bits in *this.

Returns: *this.

bitset<N>& set(size_t pos, bool val = true);

Effects: Stores a new value in the bit at position pos in *this.

If val is true, the stored value is one, otherwise it is zero.

Returns: *this.

Throws: out_of_range if pos does not correspond to a valid bit position.

bitset<N>& reset() noexcept;

Effects: Resets all bits in *this.

Returns: *this.

bitset<N>& reset(size_t pos);

Effects: Resets the bit at position pos in *this.

Returns: *this.

Throws: out_of_range if pos does not correspond to a valid bit position.

bitset<N> operator~() const noexcept;

Effects: Constructs an object x of class bitset<N> and initializes it with *this.

Returns: x.flip().

bitset<N>& flip() noexcept;

Effects: Toggles all bits in *this.

Returns: *this.

bitset<N>& flip(size_t pos);

Effects: Toggles the bit at position pos in *this.

Returns: *this.

Throws: out_of_range if pos does not correspond to a valid bit position.

unsigned long to_ulong() const;

Returns: x.

Throws: overflow_error if the integral value x corresponding to the bits in *this cannot be represented as type unsigned long.

unsigned long long to_ullong() const;

Returns: x.

Throws: overflow_error if the integral value x corresponding to the bits in *this cannot be represented as type unsigned long long.

template<class charT = char,
         class traits = char_traits<charT>,
         class Allocator = allocator<charT>>
  basic_string<charT, traits, Allocator>
    to_string(charT zero = charT('0'), charT one = charT('1')) const;

Effects: Constructs a string object of the appropriate type and initializes it to a string of length N characters.

Each character is determined by the value of its corresponding bit position in *this.

Character position N - 1 corresponds to bit position zero.

Subsequent decreasing character positions correspond to increasing bit positions.

Bit value zero becomes the character zero, bit value one becomes the character one.

Returns: The created object.

size_t count() const noexcept;

Returns: A count of the number of bits set in *this.

constexpr size_t size() const noexcept;

Returns: N.

bool operator==(const bitset<N>& rhs) const noexcept;

Returns: true if the value of each bit in *this equals the value of the corresponding bit in rhs.

bool test(size_t pos) const;

Returns: true if the bit at position pos in *this has the value one.

Throws: out_of_range if pos does not correspond to a valid bit position.

bool all() const noexcept;

Returns: count() == size().

bool any() const noexcept;

Returns: count() != 0.

bool none() const noexcept;

Returns: count() == 0.

bitset<N> operator<<(size_t pos) const noexcept;

Returns: bitset<N>(*this) <<= pos.

bitset<N> operator>>(size_t pos) const noexcept;

Returns: bitset<N>(*this) >>= pos.

constexpr bool operator[](size_t pos) const;

Preconditions: pos is valid.

Returns: true if the bit at position pos in *this has the value one, otherwise false.

Throws: Nothing.

bitset<N>::reference operator[](size_t pos);

Preconditions: pos is valid.

Returns: An object of type bitset<N>::reference such that (*this)[pos] == this->test(pos), and such that (*this)[pos] = val is equivalent to this->set(pos, val).

Throws: Nothing.

Remarks: For the purpose of determining the presence of a data race, any access or update through the resulting reference potentially accesses or modifies, respectively, the entire underlying bitset.

20.9.3 bitset hash support [bitset.hash]

template<size_t N> struct hash<bitset<N>>;

The specialization is enabled ([unord.hash]).

20.9.4 bitset operators [bitset.operators]

bitset<N> operator&(const bitset<N>& lhs, const bitset<N>& rhs) noexcept;

Returns: bitset<N>(lhs) &= rhs.

bitset<N> operator|(const bitset<N>& lhs, const bitset<N>& rhs) noexcept;

Returns: bitset<N>(lhs) |= rhs.

bitset<N> operator^(const bitset<N>& lhs, const bitset<N>& rhs) noexcept;

Returns: bitset<N>(lhs) ^= rhs.

template<class charT, class traits, size_t N>
  basic_istream<charT, traits>&
    operator>>(basic_istream<charT, traits>& is, bitset<N>& x);

A formatted input function .

Effects: Extracts up to N characters from is.

Stores these characters in a temporary object str of type basic_string<charT, traits>, then evaluates the expression x = bitset<N>(str).

Characters are extracted and stored until any of the following occurs:

(5.1)
N characters have been extracted and stored;
(5.2)
end-of-file occurs on the input sequence;
(5.3)
the next input character is neither is.widen('0') nor is.widen('1') (in which case the input character is not extracted).

If N > 0 and no characters are stored in str, calls is.setstate(ios_base::failbit) (which may throw ios_base::failure ([iostate.flags])).

Returns: is.

template<class charT, class traits, size_t N>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, const bitset<N>& x);

Returns:

os << x.template to_string<charT, traits, allocator<charT>>(
  use_facet<ctype<charT>>(os.getloc()).widen('0'),
  use_facet<ctype<charT>>(os.getloc()).widen('1'))

(see [ostream.formatted]).

20.10 Memory [memory]

20.10.1 In general [memory.general]

Subclause [memory] describes the contents of the header <memory> ([memory.syn]) and some of the contents of the header <cstdlib> ([cstdlib.syn]).

20.10.2 Header <memory> synopsis [memory.syn]

The header <memory> defines several types and function templates that describe properties of pointers and pointer-like types, manage memory for containers and other template types, destroy objects, and construct objects in uninitialized memory buffers ([pointer.traits]–[specialized.addressof] and [specialized.algorithms]).

The header also defines the templates unique_ptr, shared_ptr, weak_ptr, and various function templates that operate on objects of these types ([smartptr]).

#include <compare>              // see [compare.syn]

namespace std {
  // [pointer.traits], pointer traits
  template<class Ptr> struct pointer_traits;
  template<class T> struct pointer_traits<T*>;

  // [pointer.conversion], pointer conversion
  template<class T>
    constexpr T* to_address(T* p) noexcept;
  template<class Ptr>
    constexpr auto to_address(const Ptr& p) noexcept;

  // [util.dynamic.safety], pointer safety
  enum class pointer_safety { relaxed, preferred, strict };
  void declare_reachable(void* p);
  template<class T>
    T* undeclare_reachable(T* p);
  void declare_no_pointers(char* p, size_t n);
  void undeclare_no_pointers(char* p, size_t n);
  pointer_safety get_pointer_safety() noexcept;

  // [ptr.align], pointer alignment
  void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
  template<size_t N, class T>
    [[nodiscard]] constexpr T* assume_aligned(T* ptr);

  // [allocator.tag], allocator argument tag
  struct allocator_arg_t { explicit allocator_arg_t() = default; };
  inline constexpr allocator_arg_t allocator_arg{};

  // [allocator.uses], uses_allocator
  template<class T, class Alloc> struct uses_allocator;

  // [allocator.uses.trait], uses_allocator
  template<class T, class Alloc>
    inline constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;

  // [allocator.uses.construction], uses-allocator construction
  template<class T, class Alloc, class... Args>
    constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                    Args&&... args) noexcept -> see below;
  template<class T, class Alloc, class Tuple1, class Tuple2>
    constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
                                                    Tuple1&& x, Tuple2&& y)
                                                    noexcept ->  see below;
  template<class T, class Alloc>
    constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;
  template<class T, class Alloc, class U, class V>
    constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                    U&& u, V&& v) noexcept -> see below;
  template<class T, class Alloc, class U, class V>
    constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                    const pair<U,V>& pr) noexcept -> see below;
  template<class T, class Alloc, class U, class V>
    constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                    pair<U,V>&& pr) noexcept -> see below;
  template<class T, class Alloc, class... Args>
    constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args);
  template<class T, class Alloc, class... Args>
    constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc,
                                                         Args&&... args);

  // [allocator.traits], allocator traits
  template<class Alloc> struct allocator_traits;

  // [default.allocator], the default allocator
  template<class T> class allocator;
  template<class T, class U>
    constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept;

  // [specialized.addressof], addressof
  template<class T>
    constexpr T* addressof(T& r) noexcept;
  template<class T>
    const T* addressof(const T&&) = delete;

  // [specialized.algorithms], specialized algorithms
  // [special.mem.concepts], special memory concepts
  template<class I>
    concept no-throw-input-iterator = see below;    // exposition only
  template<class I>
    concept no-throw-forward-iterator = see below;  // exposition only
  template<class S, class I>
    concept no-throw-sentinel = see below;          // exposition only
  template<class R>
    concept no-throw-input-range = see below;       // exposition only
  template<class R>
    concept no-throw-forward-range = see below;     // exposition only

  template<class NoThrowForwardIterator>
    void uninitialized_default_construct(NoThrowForwardIterator first,
                                         NoThrowForwardIterator last);
  template<class ExecutionPolicy, class NoThrowForwardIterator>
    void uninitialized_default_construct(ExecutionPolicy&& exec,        // see [algorithms.parallel.overloads]
                                         NoThrowForwardIterator first,
                                         NoThrowForwardIterator last);
  template<class NoThrowForwardIterator, class Size>
    NoThrowForwardIterator
      uninitialized_default_construct_n(NoThrowForwardIterator first, Size n);
  template<class ExecutionPolicy, class NoThrowForwardIterator, class Size>
    NoThrowForwardIterator
      uninitialized_default_construct_n(ExecutionPolicy&& exec,         // see [algorithms.parallel.overloads]
                                        NoThrowForwardIterator first, Size n);

  namespace ranges {
    template<no-throw-forward-iterator I, no-throw-sentinel<I> S>
      requires default_initializable<iter_value_t<I>>
        I uninitialized_default_construct(I first, S last);
    template<no-throw-forward-range R>
      requires default_initializable<range_value_t<R>>
        borrowed_iterator_t<R> uninitialized_default_construct(R&& r);

    template<no-throw-forward-iterator I>
      requires default_initializable<iter_value_t<I>>
        I uninitialized_default_construct_n(I first, iter_difference_t<I> n);
  }

  template<class NoThrowForwardIterator>
    void uninitialized_value_construct(NoThrowForwardIterator first,
                                       NoThrowForwardIterator last);
  template<class ExecutionPolicy, class NoThrowForwardIterator>
    void uninitialized_value_construct(ExecutionPolicy&& exec,  // see [algorithms.parallel.overloads]
                                       NoThrowForwardIterator first,
                                       NoThrowForwardIterator last);
  template<class NoThrowForwardIterator, class Size>
    NoThrowForwardIterator
      uninitialized_value_construct_n(NoThrowForwardIterator first, Size n);
  template<class ExecutionPolicy, class NoThrowForwardIterator, class Size>
    NoThrowForwardIterator
      uninitialized_value_construct_n(ExecutionPolicy&& exec,   // see [algorithms.parallel.overloads]
                                      NoThrowForwardIterator first, Size n);

  namespace ranges {
    template<no-throw-forward-iterator I, no-throw-sentinel<I> S>
      requires default_initializable<iter_value_t<I>>
        I uninitialized_value_construct(I first, S last);
    template<no-throw-forward-range R>
      requires default_initializable<range_value_t<R>>
        borrowed_iterator_t<R> uninitialized_value_construct(R&& r);

    template<no-throw-forward-iterator I>
      requires default_initializable<iter_value_t<I>>
        I uninitialized_value_construct_n(I first, iter_difference_t<I> n);
  }

  template<class InputIterator, class NoThrowForwardIterator>
    NoThrowForwardIterator uninitialized_copy(InputIterator first, InputIterator last,
                                              NoThrowForwardIterator result);
  template<class ExecutionPolicy, class InputIterator, class NoThrowForwardIterator>
    NoThrowForwardIterator uninitialized_copy(ExecutionPolicy&& exec,   // see [algorithms.parallel.overloads]
                                              InputIterator first, InputIterator last,
                                              NoThrowForwardIterator result);
  template<class InputIterator, class Size, class NoThrowForwardIterator>
    NoThrowForwardIterator uninitialized_copy_n(InputIterator first, Size n,
                                                NoThrowForwardIterator result);
  template<class ExecutionPolicy, class InputIterator, class Size, class NoThrowForwardIterator>
    NoThrowForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads]
                                                InputIterator first, Size n,
                                                NoThrowForwardIterator result);

  namespace ranges {
    template<class I, class O>
      using uninitialized_copy_result = in_out_result<I, O>;
    template<input_iterator I, sentinel_for<I> S1,
             no-throw-forward-iterator O, no-throw-sentinel<O> S2>
      requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
        uninitialized_copy_result<I, O>
          uninitialized_copy(I ifirst, S1 ilast, O ofirst, S2 olast);
    template<input_range IR, no-throw-forward-range OR>
      requires constructible_from<range_value_t<OR>, range_reference_t<IR>>
        uninitialized_copy_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
          uninitialized_copy(IR&& in_range, OR&& out_range);

    template<class I, class O>
      using uninitialized_copy_n_result = in_out_result<I, O>;
    template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel<O> S>
      requires constructible_from<iter_value_t<O>, iter_reference_t<I>>
        uninitialized_copy_n_result<I, O>
          uninitialized_copy_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
  }

  template<class InputIterator, class NoThrowForwardIterator>
    NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last,
                                              NoThrowForwardIterator result);
  template<class ExecutionPolicy, class InputIterator, class NoThrowForwardIterator>
    NoThrowForwardIterator uninitialized_move(ExecutionPolicy&& exec,   // see [algorithms.parallel.overloads]
                                              InputIterator first, InputIterator last,
                                              NoThrowForwardIterator result);
  template<class InputIterator, class Size, class NoThrowForwardIterator>
    pair<InputIterator, NoThrowForwardIterator>
      uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);
  template<class ExecutionPolicy, class InputIterator, class Size, class NoThrowForwardIterator>
    pair<InputIterator, NoThrowForwardIterator>
      uninitialized_move_n(ExecutionPolicy&& exec,              // see [algorithms.parallel.overloads]
                           InputIterator first, Size n, NoThrowForwardIterator result);

  namespace ranges {
    template<class I, class O>
      using uninitialized_move_result = in_out_result<I, O>;
    template<input_iterator I, sentinel_for<I> S1,
             no-throw-forward-iterator O, no-throw-sentinel<O> S2>
      requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
        uninitialized_move_result<I, O>
          uninitialized_move(I ifirst, S1 ilast, O ofirst, S2 olast);
    template<input_range IR, no-throw-forward-range OR>
      requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>>
        uninitialized_move_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>>
          uninitialized_move(IR&& in_range, OR&& out_range);

    template<class I, class O>
      using uninitialized_move_n_result = in_out_result<I, O>;
    template<input_iterator I,
             no-throw-forward-iterator O, no-throw-sentinel<O> S>
      requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>>
        uninitialized_move_n_result<I, O>
          uninitialized_move_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast);
  }

  template<class NoThrowForwardIterator, class T>
    void uninitialized_fill(NoThrowForwardIterator first, NoThrowForwardIterator last,
                            const T& x);
  template<class ExecutionPolicy, class NoThrowForwardIterator, class T>
    void uninitialized_fill(ExecutionPolicy&& exec,             // see [algorithms.parallel.overloads]
                            NoThrowForwardIterator first, NoThrowForwardIterator last,
                            const T& x);
  template<class NoThrowForwardIterator, class Size, class T>
    NoThrowForwardIterator
      uninitialized_fill_n(NoThrowForwardIterator first, Size n, const T& x);
  template<class ExecutionPolicy, class NoThrowForwardIterator, class Size, class T>
    NoThrowForwardIterator
      uninitialized_fill_n(ExecutionPolicy&& exec,              // see [algorithms.parallel.overloads]
                           NoThrowForwardIterator first, Size n, const T& x);

  namespace ranges {
    template<no-throw-forward-iterator I, no-throw-sentinel<I> S, class T>
      requires constructible_from<iter_value_t<I>, const T&>
        I uninitialized_fill(I first, S last, const T& x);
    template<no-throw-forward-range R, class T>
      requires constructible_from<range_value_t<R>, const T&>
        borrowed_iterator_t<R> uninitialized_fill(R&& r, const T& x);

    template<no-throw-forward-iterator I, class T>
      requires constructible_from<iter_value_t<I>, const T&>
        I uninitialized_fill_n(I first, iter_difference_t<I> n, const T& x);
  }

  // [specialized.construct], construct_at
  template<class T, class... Args>
    constexpr T* construct_at(T* location, Args&&... args);

  namespace ranges {
    template<class T, class... Args>
      constexpr T* construct_at(T* location, Args&&... args);
  }

  // [specialized.destroy], destroy
  template<class T>
    constexpr void destroy_at(T* location);
  template<class NoThrowForwardIterator>
    constexpr void destroy(NoThrowForwardIterator first, NoThrowForwardIterator last);
  template<class ExecutionPolicy, class NoThrowForwardIterator>
    void destroy(ExecutionPolicy&& exec,                        // see [algorithms.parallel.overloads]
                 NoThrowForwardIterator first, NoThrowForwardIterator last);
  template<class NoThrowForwardIterator, class Size>
    constexpr NoThrowForwardIterator destroy_n(NoThrowForwardIterator first, Size n);
  template<class ExecutionPolicy, class NoThrowForwardIterator, class Size>
    NoThrowForwardIterator destroy_n(ExecutionPolicy&& exec,    // see [algorithms.parallel.overloads]
                                     NoThrowForwardIterator first, Size n);

  namespace ranges {
    template<destructible T>
      constexpr void destroy_at(T* location) noexcept;

    template<no-throw-input-iterator I, no-throw-sentinel<I> S>
      requires destructible<iter_value_t<I>>
        constexpr I destroy(I first, S last) noexcept;
    template<no-throw-input-range R>
      requires destructible<range_value_t<R>>
        constexpr borrowed_iterator_t<R> destroy(R&& r) noexcept;

    template<no-throw-input-iterator I>
      requires destructible<iter_value_t<I>>
        constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept;
  }

  // [unique.ptr], class template unique_ptr
  template<class T> struct default_delete;
  template<class T> struct default_delete<T[]>;
  template<class T, class D = default_delete<T>> class unique_ptr;
  template<class T, class D> class unique_ptr<T[], D>;

  template<class T, class... Args>
    unique_ptr<T> make_unique(Args&&... args);                                  // T is not array
  template<class T>
    unique_ptr<T> make_unique(size_t n);                                        // T is U[]
  template<class T, class... Args>
    unspecified make_unique(Args&&...) = delete;                                // T is U[N]

  template<class T>
    unique_ptr<T> make_unique_for_overwrite();                                  // T is not array
  template<class T>
    unique_ptr<T> make_unique_for_overwrite(size_t n);                          // T is U[]
  template<class T, class... Args>
    unspecified make_unique_for_overwrite(Args&&...) = delete;                  // T is U[N]

  template<class T, class D>
    void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;

  template<class T1, class D1, class T2, class D2>
    bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
  template<class T1, class D1, class T2, class D2>
    requires three_way_comparable_with<typename unique_ptr<T1, D1>::pointer,
                                       typename unique_ptr<T2, D2>::pointer>
    compare_three_way_result_t<typename unique_ptr<T1, D1>::pointer,
                               typename unique_ptr<T2, D2>::pointer>
      operator<=>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

  template<class T, class D>
    bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
  template<class T, class D>
    bool operator<(const unique_ptr<T, D>& x, nullptr_t);
  template<class T, class D>
    bool operator<(nullptr_t, const unique_ptr<T, D>& y);
  template<class T, class D>
    bool operator>(const unique_ptr<T, D>& x, nullptr_t);
  template<class T, class D>
    bool operator>(nullptr_t, const unique_ptr<T, D>& y);
  template<class T, class D>
    bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
  template<class T, class D>
    bool operator<=(nullptr_t, const unique_ptr<T, D>& y);
  template<class T, class D>
    bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
  template<class T, class D>
    bool operator>=(nullptr_t, const unique_ptr<T, D>& y);
  template<class T, class D>
    requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t>
    compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t>
      operator<=>(const unique_ptr<T, D>& x, nullptr_t);

  template<class E, class T, class Y, class D>
    basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);

  // [util.smartptr.weak.bad], class bad_weak_ptr
  class bad_weak_ptr;

  // [util.smartptr.shared], class template shared_ptr
  template<class T> class shared_ptr;

  // [util.smartptr.shared.create], shared_ptr creation
  template<class T, class... Args>
    shared_ptr<T> make_shared(Args&&... args);                                  // T is not array
  template<class T, class A, class... Args>
    shared_ptr<T> allocate_shared(const A& a, Args&&... args);                  // T is not array

  template<class T>
    shared_ptr<T> make_shared(size_t N);                                        // T is U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, size_t N);                        // T is U[]

  template<class T>
    shared_ptr<T> make_shared();                                                // T is U[N]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a);                                  // T is U[N]

  template<class T>
    shared_ptr<T> make_shared(size_t N, const remove_extent_t<T>& u);           // T is U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, size_t N,
                                  const remove_extent_t<T>& u);                 // T is U[]

  template<class T>
    shared_ptr<T> make_shared(const remove_extent_t<T>& u);                     // T is U[N]
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, const remove_extent_t<T>& u);     // T is U[N]

  template<class T>
    shared_ptr<T> make_shared_for_overwrite();                                  // T is not U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared_for_overwrite(const A& a);                    // T is not U[]

  template<class T>
    shared_ptr<T> make_shared_for_overwrite(size_t N);                          // T is U[]
  template<class T, class A>
    shared_ptr<T> allocate_shared_for_overwrite(const A& a, size_t N);          // T is U[]

  // [util.smartptr.shared.cmp], shared_ptr comparisons
  template<class T, class U>
    bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
  template<class T, class U>
    strong_ordering operator<=>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

  template<class T>
    bool operator==(const shared_ptr<T>& x, nullptr_t) noexcept;
  template<class T>
    strong_ordering operator<=>(const shared_ptr<T>& x, nullptr_t) noexcept;

  // [util.smartptr.shared.spec], shared_ptr specialized algorithms
  template<class T>
    void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

  // [util.smartptr.shared.cast], shared_ptr casts
  template<class T, class U>
    shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;
  template<class T, class U>
    shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;
  template<class T, class U>
    shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;
  template<class T, class U>
    shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;
  template<class T, class U>
    shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;

  // [util.smartptr.getdeleter], shared_ptr get_deleter
  template<class D, class T>
    D* get_deleter(const shared_ptr<T>& p) noexcept;

  // [util.smartptr.shared.io], shared_ptr I/O
  template<class E, class T, class Y>
    basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const shared_ptr<Y>& p);

  // [util.smartptr.weak], class template weak_ptr
  template<class T> class weak_ptr;

  // [util.smartptr.weak.spec], weak_ptr specialized algorithms
  template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;

  // [util.smartptr.ownerless], class template owner_less
  template<class T = void> struct owner_less;

  // [util.smartptr.enab], class template enable_shared_from_this
  template<class T> class enable_shared_from_this;

  // [util.smartptr.hash], hash support
  template<class T> struct hash;
  template<class T, class D> struct hash<unique_ptr<T, D>>;
  template<class T> struct hash<shared_ptr<T>>;

  // [util.smartptr.atomic], atomic smart pointers
  template<class T> struct atomic;
  template<class T> struct atomic<shared_ptr<T>>;
  template<class T> struct atomic<weak_ptr<T>>;
}

20.10.3 Pointer traits [pointer.traits]

The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.

namespace std {
  template<class Ptr> struct pointer_traits {
    using pointer         = Ptr;
    using element_type    = see below;
    using difference_type = see below;

    template<class U> using rebind = see below;

    static pointer pointer_to(see below r);
  };

  template<class T> struct pointer_traits<T*> {
    using pointer         = T*;
    using element_type    = T;
    using difference_type = ptrdiff_t;

    template<class U> using rebind = U*;

    static constexpr pointer pointer_to(see below r) noexcept;
  };
}

20.10.3.1 Member types [pointer.traits.types]

using element_type = see below;

Type: Ptr::element_type if the qualified-id Ptr::element_type is valid and denotes a type ([temp.deduct]); otherwise, T if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the specialization is ill-formed.

using difference_type = see below;

Type: Ptr::difference_type if the qualified-id Ptr::difference_type is valid and denotes a type ([temp.deduct]); otherwise, ptrdiff_t.

template<class U> using rebind = see below;

Alias template: Ptr::rebind if the qualified-id Ptr::rebind is valid and denotes a type ([temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.

20.10.3.2 Member functions [pointer.traits.functions]

static pointer pointer_traits::pointer_to(see below r);
static constexpr pointer pointer_traits<T*>::pointer_to(see below r) noexcept;

Mandates: For the first member function, Ptr::pointer_to(r) is well-formed.

Preconditions: For the first member function, Ptr::pointer_to(r) returns a pointer to r through which indirection is valid.

Returns: The first member function returns Ptr::pointer_to(r).

The second member function returns addressof(r).

Remarks: If element_type is cv void, the type of r is unspecified; otherwise, it is element_type&.

20.10.3.3 Optional members [pointer.traits.optmem]

Specializations of pointer_traits may define the member declared in this subclause to customize the behavior of the standard library.

static element_type* to_address(pointer p) noexcept;

Returns: A pointer of type element_type* that references the same location as the argument p.

This function should be the inverse of pointer_to.

If defined, it customizes the behavior of the non-member function to_address ([pointer.conversion]).

— end note

]

20.10.4 Pointer conversion [pointer.conversion]

template<class T> constexpr T* to_address(T* p) noexcept;

Mandates: T is not a function type.

Returns: p.

template<class Ptr> constexpr auto to_address(const Ptr& p) noexcept;

Returns: pointer_traits<Ptr>::to_address(p) if that expression is well-formed (see [pointer.traits.optmem]), otherwise to_address(p.operator->()).

20.10.5 Pointer safety [util.dynamic.safety]

A complete object is declared reachable while the number of calls to declare_reachable with an argument referencing the object exceeds the number of calls to undeclare_reachable with an argument referencing the object.

void declare_reachable(void* p);

Preconditions: p is a safely-derived pointer ([basic.stc.dynamic.safety]) or a null pointer value.

Effects: If p is not null, the complete object referenced by p is subsequently declared reachable ([basic.stc.dynamic.safety]).

Throws: May throw bad_alloc if the system cannot allocate additional memory that may be required to track objects declared reachable.

template<class T> T* undeclare_reachable(T* p);

Preconditions: If p is not null, the complete object referenced by p has been previously declared reachable, and is live ([basic.life]) from the time of the call until the last undeclare_reachable(p) call on the object.

Returns: A safely derived copy of p which compares equal to p.

Throws: Nothing.

It is expected that calls to declare_reachable(p) will consume a small amount of memory in addition to that occupied by the referenced object until the matching call to undeclare_reachable(p) is encountered.

Long running programs should arrange that calls are matched.

— end note

]

void declare_no_pointers(char* p, size_t n);

Preconditions: No bytes in the specified range are currently registered with declare_no_pointers().

If the specified range is in an allocated object, then it is entirely within a single allocated object.

The object is live until the corresponding undeclare_no_pointers() call.

In a garbage-collecting implementation, the fact that a region in an object is registered with declare_no_pointers() should not prevent the object from being collected.

— end note

]

Effects: The n bytes starting at p no longer contain traceable pointer locations, independent of their type.

Hence indirection through a pointer located there is undefined if the object it points to was created by global operator new and not previously declared reachable.

This may be used to inform a garbage collector or leak detector that this region of memory need not be traced.

— end note

]

Throws: Nothing.

Under some conditions implementations may need to allocate memory.

However, the request can be ignored if memory allocation fails.

— end note

]

void undeclare_no_pointers(char* p, size_t n);

Preconditions: The same range has previously been passed to declare_no_pointers().

Effects: Unregisters a range registered with declare_no_pointers() for destruction.

It shall be called before the lifetime of the object ends.

Throws: Nothing.

pointer_safety get_pointer_safety() noexcept;

Returns: pointer_safety::strict if the implementation has strict pointer safety .

It is implementation-defined whether get_pointer_safety returns pointer_safety::relaxed or pointer_safety::preferred if the implementation has relaxed pointer safety.219

219)

pointer_safety::preferred might be returned to indicate that a leak detector is running so that the program can avoid spurious leak reports.

⮥

20.10.6 Pointer alignment [ptr.align]

void* align(size_t alignment, size_t size, void*& ptr, size_t& space);

Preconditions:

(1.1)
alignment is a power of two
(1.2)
ptr represents the address of contiguous storage of at least space bytes

Effects: If it is possible to fit size bytes of storage aligned by alignment into the buffer pointed to by ptr with length space, the function updates ptr to represent the first possible address of such storage and decreases space by the number of bytes used for alignment.

Otherwise, the function does nothing.

Returns: A null pointer if the requested aligned buffer would not fit into the available space, otherwise the adjusted value of ptr.

The function updates its ptr and space arguments so that it can be called repeatedly with possibly different alignment and size arguments for the same buffer.

— end note

]

template<size_t N, class T>
  [[nodiscard]] constexpr T* assume_aligned(T* ptr);

Mandates: N is a power of two.

Preconditions: ptr points to an object X of a type similar ([conv.qual]) to T, where X has alignment N ([basic.align]).

Returns: ptr.

Throws: Nothing.

The alignment assumption on an object X expressed by a call to assume_aligned may result in generation of more efficient code.

It is up to the program to ensure that the assumption actually holds.

The call does not cause the compiler to verify or enforce this.

An implementation might only make the assumption for those operations on X that access X through the pointer returned by assume_aligned.

— end note

]

20.10.7 Allocator argument tag [allocator.tag]

namespace std {
  struct allocator_arg_t { explicit allocator_arg_t() = default; };
  inline constexpr allocator_arg_t allocator_arg{};
}

The allocator_arg_t struct is an empty class type used as a unique type to disambiguate constructor and function overloading.

Specifically, several types (see tuple [tuple]) have constructors with allocator_arg_t as the first argument, immediately followed by an argument of a type that meets the Cpp17Allocator requirements (Table 36).

20.10.8 uses_allocator [allocator.uses]

20.10.8.1 uses_allocator trait [allocator.uses.trait]

template<class T, class Alloc> struct uses_allocator;

Remarks: Automatically detects whether T has a nested allocator_type that is convertible from Alloc.

Meets the Cpp17BinaryTypeTrait requirements ([meta.rqmts]).

The implementation shall provide a definition that is derived from true_type if the qualified-id T::allocator_type is valid and denotes a type ([temp.deduct]) and is_convertible_v<Alloc, T::allocator_type> != false, otherwise it shall be derived from false_type.

A program may specialize this template to derive from true_type for a program-defined type T that does not have a nested allocator_type but nonetheless can be constructed with an allocator where either:

(1.1)
the first argument of a constructor has type allocator_arg_t and the second argument has type Alloc or
(1.2)
the last argument of a constructor has type Alloc.

20.10.8.2 Uses-allocator construction [allocator.uses.construction]

Uses-allocator construction with allocator alloc and constructor arguments args... refers to the construction of an object of type T such that alloc is passed to the constructor of T if T uses an allocator type compatible with alloc.

When applied to the construction of an object of type T, it is equivalent to initializing it with the value of the expression make_obj_using_allocator<T>(alloc, args...), described below.

The following utility functions support three conventions for passing alloc to a constructor:

(2.1)
If T does not use an allocator compatible with alloc, then alloc is ignored.
(2.2)
Otherwise, if T has a constructor invocable as T(allocator_arg, alloc, args...) (leading-allocator convention), then uses-allocator construction chooses this constructor form.
(2.3)
Otherwise, if T has a constructor invocable as T(args..., alloc) (trailing-allocator convention), then uses-allocator construction chooses this constructor form.

The uses_allocator_construction_args function template takes an allocator and argument list and produces (as a tuple) a new argument list matching one of the above conventions.

Additionally, overloads are provided that treat specializations of pair such that uses-allocator construction is applied individually to the first and second data members.

The make_obj_using_allocator and uninitialized_construct_using_allocator function templates apply the modified constructor arguments to construct an object of type T as a return value or in-place, respectively.

For uses_allocator_construction_args and make_obj_using_allocator, type T is not deduced and must therefore be specified explicitly by the caller.

— end note

]

template<class T, class Alloc, class... Args>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                  Args&&... args) noexcept -> see below;

Constraints: T is not a specialization of pair.

Returns: A tuple value determined as follows:

(5.1)
If uses_allocator_v<T, Alloc> is false and is_constructible_v<T, Args...> is true, return forward_as_tuple(std::forward<Args>(args)...).
(5.2)
Otherwise, if uses_allocator_v<T, Alloc> is true and is_constructible_v<T, allocator_arg_t, const Alloc&, Args...> is true, return
```
tuple<allocator_arg_t, const Alloc&, Args&&...>(
 allocator_arg, alloc, std::forward<Args>(args)...)
```
(5.3)
Otherwise, if uses_allocator_v<T, Alloc> is true and is_constructible_v<T, Args..., const Alloc&> is true, return forward_as_tuple(std::forward<Args>(args)..., alloc).
(5.4)
Otherwise, the program is ill-formed.

This definition prevents a silent failure to pass the allocator to a constructor of a type for which uses_allocator_v<T, Alloc> is true.

— end note

]

template<class T, class Alloc, class Tuple1, class Tuple2>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t,
                                                  Tuple1&& x, Tuple2&& y)
                                                  noexcept -> see below;

Constraints: T is a specialization of pair.

Effects: For T specified as pair<T1, T2>, equivalent to:

return make_tuple(
  piecewise_construct,
  apply([&alloc](auto&&... args1) {
          return uses_allocator_construction_args<T1>(
            alloc, std::forward<decltype(args1)>(args1)...);
        }, std::forward<Tuple1>(x)),
  apply([&alloc](auto&&... args2) {
          return uses_allocator_construction_args<T2>(
            alloc, std::forward<decltype(args2)>(args2)...);
        }, std::forward<Tuple2>(y)));

template<class T, class Alloc>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;

Constraints: T is a specialization of pair.

Effects: Equivalent to:

return uses_allocator_construction_args<T>(alloc, piecewise_construct,
                                           tuple<>{}, tuple<>{});

template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                  U&& u, V&& v) noexcept -> see below;

Constraints: T is a specialization of pair.

Effects: Equivalent to:

return uses_allocator_construction_args<T>(alloc, piecewise_construct,
                                           forward_as_tuple(std::forward<U>(u)),
                                           forward_as_tuple(std::forward<V>(v)));

template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                  const pair<U,V>& pr) noexcept -> see below;

Constraints: T is a specialization of pair.

Effects: Equivalent to:

return uses_allocator_construction_args<T>(alloc, piecewise_construct,
                                           forward_as_tuple(pr.first),
                                           forward_as_tuple(pr.second));

template<class T, class Alloc, class U, class V>
  constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                  pair<U,V>&& pr) noexcept -> see below;

Constraints: T is a specialization of pair.

Effects: Equivalent to:

return uses_allocator_construction_args<T>(alloc, piecewise_construct,
                                           forward_as_tuple(std::move(pr).first),
                                           forward_as_tuple(std::move(pr).second));

template<class T, class Alloc, class... Args>
  constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args);

Effects: Equivalent to:

return make_from_tuple<T>(uses_allocator_construction_args<T>(
                            alloc, std::forward<Args>(args)...));

template<class T, class Alloc, class... Args>
  constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);

Effects: Equivalent to:

return apply([&]<class... U>(U&&... xs) {
       return construct_at(p, std::forward<U>(xs)...);
     }, uses_allocator_construction_args<T>(alloc, std::forward<Args>(args)...));

20.10.9 Allocator traits [allocator.traits]

The class template allocator_traits supplies a uniform interface to all allocator types.

An allocator cannot be a non-class type, however, even if allocator_traits supplies the entire required interface.

Thus, it is always possible to create a derived class from an allocator.

— end note

]

namespace std {
  template<class Alloc> struct allocator_traits {
    using allocator_type     = Alloc;

    using value_type         = typename Alloc::value_type;

    using pointer            = see below;
    using const_pointer      = see below;
    using void_pointer       = see below;
    using const_void_pointer = see below;

    using difference_type    = see below;
    using size_type          = see below;

    using propagate_on_container_copy_assignment = see below;
    using propagate_on_container_move_assignment = see below;
    using propagate_on_container_swap            = see below;
    using is_always_equal                        = see below;

    template<class T> using rebind_alloc = see below;
    template<class T> using rebind_traits = allocator_traits<rebind_alloc<T>>;

    [[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n);
    [[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n,
                                                    const_void_pointer hint);

    static constexpr void deallocate(Alloc& a, pointer p, size_type n);

    template<class T, class... Args>
      static constexpr void construct(Alloc& a, T* p, Args&&... args);

    template<class T>
      static constexpr void destroy(Alloc& a, T* p);

    static constexpr size_type max_size(const Alloc& a) noexcept;

    static constexpr Alloc select_on_container_copy_construction(const Alloc& rhs);
  };
}

20.10.9.1 Member types [allocator.traits.types]

using pointer = see below;

Type: Alloc::pointer if the qualified-id Alloc::pointer is valid and denotes a type ([temp.deduct]); otherwise, value_type*.

using const_pointer = see below;

Type: Alloc::const_pointer if the qualified-id Alloc::const_pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>::rebind<const value_type>.

using void_pointer = see below;

Type: Alloc::void_pointer if the qualified-id Alloc::void_pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>::rebind<void>.

using const_void_pointer = see below;

Type: Alloc::const_void_pointer if the qualified-id Alloc::const_void_pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>::rebind<const void>.

using difference_type = see below;

Type: Alloc::difference_type if the qualified-id Alloc::difference_type is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>::difference_type.

using size_type = see below;

Type: Alloc::size_type if the qualified-id Alloc::size_type is valid and denotes a type ([temp.deduct]); otherwise, make_unsigned_t<difference_type>.

using propagate_on_container_copy_assignment = see below;

Type: Alloc::propagate_on_container_copy_assignment if the qualified-id Alloc::propagate_on_container_copy_assignment is valid and denotes a type ([temp.deduct]); otherwise false_type.

using propagate_on_container_move_assignment = see below;

Type: Alloc::propagate_on_container_move_assignment if the qualified-id Alloc::propagate_on_container_move_assignment is valid and denotes a type ([temp.deduct]); otherwise false_type.

using propagate_on_container_swap = see below;

Type: Alloc::propagate_on_container_swap if the qualified-id Alloc::propagate_on_container_swap is valid and denotes a type ([temp.deduct]); otherwise false_type.

using is_always_equal = see below;

Type: Alloc::is_always_equal if the qualified-id Alloc::is_always_equal is valid and denotes a type ([temp.deduct]); otherwise is_empty<Alloc>::type.

template<class T> using rebind_alloc = see below;

Alias template: Alloc::rebind<T>::other if the qualified-id Alloc::rebind<T>::other is valid and denotes a type ([temp.deduct]); otherwise, Alloc<T, Args> if Alloc is a class template instantiation of the form Alloc<U, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind_alloc is ill-formed.

20.10.9.2 Static member functions [allocator.traits.members]

[[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n);

Returns: a.allocate(n).

[[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n, const_void_pointer hint);

Returns: a.allocate(n, hint) if that expression is well-formed; otherwise, a.allocate(n).

static constexpr void deallocate(Alloc& a, pointer p, size_type n);

Effects: Calls a.deallocate(p, n).

Throws: Nothing.

template<class T, class... Args>
  static constexpr void construct(Alloc& a, T* p, Args&&... args);

Effects: Calls a.construct(p, std::forward<Args>(args)...) if that call is well-formed; otherwise, invokes construct_at(p, std::forward<Args>(args)...).

template<class T>
  static constexpr void destroy(Alloc& a, T* p);

Effects: Calls a.destroy(p) if that call is well-formed; otherwise, invokes destroy_at(p).

static constexpr size_type max_size(const Alloc& a) noexcept;

Returns: a.max_size() if that expression is well-formed; otherwise, numeric_limits<size_type>::max()/sizeof(value_type).

static constexpr Alloc select_on_container_copy_construction(const Alloc& rhs);

Returns: rhs.select_on_container_copy_construction() if that expression is well-formed; otherwise, rhs.

20.10.10 The default allocator [default.allocator]

All specializations of the default allocator meet the allocator completeness requirements ([allocator.requirements.completeness]).

namespace std {
  template<class T> class allocator {
   public:
    using value_type                             = T;
    using size_type                              = size_t;
    using difference_type                        = ptrdiff_t;
    using propagate_on_container_move_assignment = true_type;
    using is_always_equal                        = true_type;

    constexpr allocator() noexcept;
    constexpr allocator(const allocator&) noexcept;
    template<class U> constexpr allocator(const allocator<U>&) noexcept;
    constexpr ~allocator();
    constexpr allocator& operator=(const allocator&) = default;

    [[nodiscard]] constexpr T* allocate(size_t n);
    constexpr void deallocate(T* p, size_t n);
  };
}

20.10.10.1 Members [allocator.members]

Except for the destructor, member functions of the default allocator shall not introduce data races as a result of concurrent calls to those member functions from different threads.

Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.

[[nodiscard]] constexpr T* allocate(size_t n);

Mandates: T is not an incomplete type ([basic.types]).

Returns: A pointer to the initial element of an array of n T.

Remarks: The storage for the array is obtained by calling ::operator new, but it is unspecified when or how often this function is called.

This function starts the lifetime of the array object, but not that of any of the array elements.

Throws: bad_array_new_length if numeric_limits<size_t>::max() / sizeof(T) < n, or bad_alloc if the storage cannot be obtained.

constexpr void deallocate(T* p, size_t n);

Preconditions: p is a pointer value obtained from allocate().

n equals the value passed as the first argument to the invocation of allocate which returned p.

Effects: Deallocates the storage referenced by p.

Remarks: Uses ::operator delete, but it is unspecified when this function is called.

20.10.10.2 Operators [allocator.globals]

template<class T, class U>
  constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept;

Returns: true.

20.10.11 addressof [specialized.addressof]

template<class T> constexpr T* addressof(T& r) noexcept;

Returns: The actual address of the object or function referenced by r, even in the presence of an overloaded operator&.

Remarks: An expression addressof(E) is a constant subexpression ([defns.const.subexpr]) if E is an lvalue constant subexpression.

20.10.12 C library memory allocation [c.malloc]

The header <cstdlib> ([cstdlib.syn]) declares the functions described in this subclause.

— end note

]

void* aligned_alloc(size_t alignment, size_t size);
void* calloc(size_t nmemb, size_t size);
void* malloc(size_t size);
void* realloc(void* ptr, size_t size);

Effects: These functions have the semantics specified in the C standard library.

Remarks: These functions do not attempt to allocate storage by calling ::operator new() ([new.delete]).

Storage allocated directly with these functions is implicitly declared reachable (see [basic.stc.dynamic.safety]) on allocation, ceases to be declared reachable on deallocation, and need not cease to be declared reachable as the result of an undeclare_reachable() call.

This allows existing C libraries to remain unaffected by restrictions on pointers that are not safely derived, at the expense of providing far fewer garbage collection and leak detection options for malloc()-allocated objects.

It also allows malloc() to be implemented with a separate allocation arena, bypassing the normal declare_reachable() implementation.

The above functions should never intentionally be used as a replacement for declare_reachable(), and newly written code is strongly encouraged to treat memory allocated with these functions as though it were allocated with operator new.

— end note

]

These functions implicitly create objects ([intro.object]) in the returned region of storage and return a pointer to a suitable created object.

In the case of calloc and realloc, the objects are created before the storage is zeroed or copied, respectively.

void free(void* ptr);

Effects: This function has the semantics specified in the C standard library.

Remarks: This function does not attempt to deallocate storage by calling ::operator delete().

20.11 Smart pointers [smartptr]

20.11.1 Class template unique_ptr [unique.ptr]

A unique pointer is an object that owns another object and manages that other object through a pointer.

More precisely, a unique pointer is an object u that stores a pointer to a second object p and will dispose of p when u is itself destroyed (e.g., when leaving block scope ([stmt.dcl])).

In this context, u is said to own p.

The mechanism by which u disposes of p is known as p's associated deleter, a function object whose correct invocation results in p's appropriate disposition (typically its deletion).

Let the notation u.p denote the pointer stored by u, and let u.d denote the associated deleter.

Upon request, u can reset (replace) u.p and u.d with another pointer and deleter, but properly disposes of its owned object via the associated deleter before such replacement is considered completed.

Each object of a type U instantiated from the unique_ptr template specified in this subclause has the strict ownership semantics, specified above, of a unique pointer.

In partial satisfaction of these semantics, each such U is Cpp17MoveConstructible and Cpp17MoveAssignable, but is not Cpp17CopyConstructible nor Cpp17CopyAssignable.

The template parameter T of unique_ptr may be an incomplete type.

The uses of unique_ptr include providing exception safety for dynamically allocated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function.

— end note

]

20.11.1.1 Default deleters [unique.ptr.dltr]

20.11.1.1.1 In general [unique.ptr.dltr.general]

The class template default_delete serves as the default deleter (destruction policy) for the class template unique_ptr.

The template parameter T of default_delete may be an incomplete type.

20.11.1.1.2 default_delete [unique.ptr.dltr.dflt]

namespace std {
  template<class T> struct default_delete {
    constexpr default_delete() noexcept = default;
    template<class U> default_delete(const default_delete<U>&) noexcept;
    void operator()(T*) const;
  };
}

template<class U> default_delete(const default_delete<U>& other) noexcept;

Constraints: U* is implicitly convertible to T*.

Effects: Constructs a default_delete object from another default_delete object.

void operator()(T* ptr) const;

Mandates: T is a complete type.

Effects: Calls delete on ptr.

20.11.1.1.3 default_delete<T[]> [unique.ptr.dltr.dflt1]

namespace std {
  template<class T> struct default_delete<T[]> {
    constexpr default_delete() noexcept = default;
    template<class U> default_delete(const default_delete<U[]>&) noexcept;
    template<class U> void operator()(U* ptr) const;
  };
}

template<class U> default_delete(const default_delete<U[]>& other) noexcept;

Constraints: U(*)[] is convertible to T(*)[].

Effects: Constructs a default_delete object from another default_delete<U[]> object.

template<class U> void operator()(U* ptr) const;

Mandates: U is a complete type.

Constraints: U(*)[] is convertible to T(*)[].

Effects: Calls delete[] on ptr.

20.11.1.2 unique_ptr for single objects [unique.ptr.single]

namespace std {
  template<class T, class D = default_delete<T>> class unique_ptr {
  public:
    using pointer      = see below;
    using element_type = T;
    using deleter_type = D;

    // [unique.ptr.single.ctor], constructors
    constexpr unique_ptr() noexcept;
    explicit unique_ptr(pointer p) noexcept;
    unique_ptr(pointer p, see below d1) noexcept;
    unique_ptr(pointer p, see below d2) noexcept;
    unique_ptr(unique_ptr&& u) noexcept;
    constexpr unique_ptr(nullptr_t) noexcept;
    template<class U, class E>
      unique_ptr(unique_ptr<U, E>&& u) noexcept;

    // [unique.ptr.single.dtor], destructor
    ~unique_ptr();

    // [unique.ptr.single.asgn], assignment
    unique_ptr& operator=(unique_ptr&& u) noexcept;
    template<class U, class E>
      unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
    unique_ptr& operator=(nullptr_t) noexcept;

    // [unique.ptr.single.observers], observers
    add_lvalue_reference_t<T> operator*() const;
    pointer operator->() const noexcept;
    pointer get() const noexcept;
    deleter_type& get_deleter() noexcept;
    const deleter_type& get_deleter() const noexcept;
    explicit operator bool() const noexcept;

    // [unique.ptr.single.modifiers], modifiers
    pointer release() noexcept;
    void reset(pointer p = pointer()) noexcept;
    void swap(unique_ptr& u) noexcept;

    // disable copy from lvalue
    unique_ptr(const unique_ptr&) = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
  };
}

The default type for the template parameter D is default_delete.

A client-supplied template argument D shall be a function object type, lvalue reference to function, or lvalue reference to function object type for which, given a value d of type D and a value ptr of type unique_ptr<T, D>::pointer, the expression d(ptr) is valid and has the effect of disposing of the pointer as appropriate for that deleter.

If the deleter's type D is not a reference type, D shall meet the Cpp17Destructible requirements (Table 32).

If the qualified-id remove_reference_t<D>::pointer is valid and denotes a type ([temp.deduct]), then unique_ptr<T, D>::pointer shall be a synonym for remove_reference_t<D>::pointer.

Otherwise unique_ptr<T, D>::pointer shall be a synonym for element_type*.

The type unique_ptr<T, D>::pointer shall meet the Cpp17NullablePointer requirements (Table 33).

Given an allocator type X (Table 36) and letting A be a synonym for allocator_traits<X>, the types A::pointer, A::const_pointer, A::void_pointer, and A::const_void_pointer may be used as unique_ptr<T, D>::pointer.

— end example

]

20.11.1.2.1 Constructors [unique.ptr.single.ctor]

constexpr unique_ptr() noexcept;
constexpr unique_ptr(nullptr_t) noexcept;

Preconditions: D meets the Cpp17DefaultConstructible requirements (Table 27), and that construction does not throw an exception.

Constraints: is_pointer_v<deleter_type> is false and is_default_constructible_v<deleter_type> is true.

Effects: Constructs a unique_ptr object that owns nothing, value-initializing the stored pointer and the stored deleter.

Postconditions: get() == nullptr.

get_deleter() returns a reference to the stored deleter.

explicit unique_ptr(pointer p) noexcept;

Constraints: is_pointer_v<deleter_type> is false and is_default_constructible_v<deleter_type> is true.

Mandates: This constructor is not selected by class template argument deduction ([over.match.class.deduct]).

Preconditions: D meets the Cpp17DefaultConstructible requirements (Table 27), and that construction does not throw an exception.

Effects: Constructs a unique_ptr which owns p, initializing the stored pointer with p and value-initializing the stored deleter.

Postconditions: get() == p.

get_deleter() returns a reference to the stored deleter.

unique_ptr(pointer p, const D& d) noexcept;
unique_ptr(pointer p, remove_reference_t<D>&& d) noexcept;

Constraints: is_constructible_v<D, decltype(d)> is true.

Mandates: These constructors are not selected by class template argument deduction ([over.match.class.deduct]).

Preconditions: For the first constructor, if D is not a reference type, D meets the Cpp17CopyConstructible requirements and such construction does not exit via an exception.

For the second constructor, if D is not a reference type, D meets the Cpp17MoveConstructible requirements and such construction does not exit via an exception.

Effects: Constructs a unique_ptr object which owns p, initializing the stored pointer with p and initializing the deleter from std::forward<decltype(d)>(d).

Postconditions: get() == p.

get_deleter() returns a reference to the stored deleter.

If D is a reference type then get_deleter() returns a reference to the lvalue d.

Remarks: If D is a reference type, the second constructor is defined as deleted.

D d;
unique_ptr<int, D> p1(new int, D());        // D must be Cpp17MoveConstructible
unique_ptr<int, D> p2(new int, d);          // D must be Cpp17CopyConstructible
unique_ptr<int, D&> p3(new int, d);         // p3 holds a reference to d
unique_ptr<int, const D&> p4(new int, D()); // error: rvalue deleter object combined
                                            // with reference deleter type

— end example

]

unique_ptr(unique_ptr&& u) noexcept;

Constraints: is_move_constructible_v<D> is true.

Preconditions: If D is not a reference type, D meets the Cpp17MoveConstructible requirements (Table 28).

Construction of the deleter from an rvalue of type D does not throw an exception.

Effects: Constructs a unique_ptr from u.

If D is a reference type, this deleter is copy constructed from u's deleter; otherwise, this deleter is move constructed from u's deleter.

The construction of the deleter can be implemented with std::forward<D>.

— end note

]

Postconditions: get() yields the value u.get() yielded before the construction.

u.get() == nullptr.

get_deleter() returns a reference to the stored deleter that was constructed from u.get_deleter().

If D is a reference type then get_deleter() and u.get_deleter() both reference the same lvalue deleter.

template<class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;

Constraints:

(21.1)
unique_ptr<U, E>::pointer is implicitly convertible to pointer,
(21.2)
U is not an array type, and
(21.3)
either D is a reference type and E is the same type as D, or D is not a reference type and E is implicitly convertible to D.

Preconditions: If E is not a reference type, construction of the deleter from an rvalue of type E is well-formed and does not throw an exception.

Otherwise, E is a reference type and construction of the deleter from an lvalue of type E is well-formed and does not throw an exception.

Effects: Constructs a unique_ptr from u.

If E is a reference type, this deleter is copy constructed from u's deleter; otherwise, this deleter is move constructed from u's deleter.

The deleter constructor can be implemented with std::forward<E>.

— end note

]

Postconditions: get() yields the value u.get() yielded before the construction.

u.get() == nullptr.

get_deleter() returns a reference to the stored deleter that was constructed from u.get_deleter().

20.11.1.2.2 Destructor [unique.ptr.single.dtor]

~unique_ptr();

Preconditions: The expression get_deleter()(get()) is well-formed, has well-defined behavior, and does not throw exceptions.

The use of default_delete requires T to be a complete type.

— end note

]

Effects: If get() == nullptr there are no effects.

Otherwise get_deleter()(get()).

20.11.1.2.3 Assignment [unique.ptr.single.asgn]

unique_ptr& operator=(unique_ptr&& u) noexcept;

Constraints: is_move_assignable_v<D> is true.

Preconditions: If D is not a reference type, D meets the Cpp17MoveAssignable requirements (Table 30) and assignment of the deleter from an rvalue of type D does not throw an exception.

Otherwise, D is a reference type; remove_reference_t<D> meets the Cpp17CopyAssignable requirements and assignment of the deleter from an lvalue of type D does not throw an exception.

Effects: Calls reset(u.release()) followed by get_deleter() = std::forward<D>(u.get_deleter()).

Returns: *this.

Postconditions: u.get() == nullptr.

template<class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

Constraints:

(6.1)
unique_ptr<U, E>::pointer is implicitly convertible to pointer, and
(6.2)
U is not an array type, and
(6.3)
is_assignable_v<D&, E&&> is true.

Preconditions: If E is not a reference type, assignment of the deleter from an rvalue of type E is well-formed and does not throw an exception.

Otherwise, E is a reference type and assignment of the deleter from an lvalue of type E is well-formed and does not throw an exception.

Effects: Calls reset(u.release()) followed by get_deleter() = std::forward<E>(u.get_deleter()).

Returns: *this.

Postconditions: u.get() == nullptr.

unique_ptr& operator=(nullptr_t) noexcept;

Effects: As if by reset().

Postconditions: get() == nullptr.

Returns: *this.

20.11.1.2.4 Observers [unique.ptr.single.observers]

add_lvalue_reference_t<T> operator*() const;

Preconditions: get() != nullptr.

Returns: *get().

pointer operator->() const noexcept;

Preconditions: get() != nullptr.

Returns: get().

The use of this function typically requires that T be a complete type.

— end note

]

pointer get() const noexcept;

Returns: The stored pointer.

deleter_type& get_deleter() noexcept;
const deleter_type& get_deleter() const noexcept;

Returns: A reference to the stored deleter.

explicit operator bool() const noexcept;

Returns: get() != nullptr.

20.11.1.2.5 Modifiers [unique.ptr.single.modifiers]

pointer release() noexcept;

Postconditions: get() == nullptr.

Returns: The value get() had at the start of the call to release.

void reset(pointer p = pointer()) noexcept;

Preconditions: The expression get_deleter()(get()) is well-formed, has well-defined behavior, and does not throw exceptions.

Effects: Assigns p to the stored pointer, and then if and only if the old value of the stored pointer, old_p, was not equal to nullptr, calls get_deleter()(old_p).

The order of these operations is significant because the call to get_deleter() may destroy *this.

— end note

]

Postconditions: get() == p.

The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression.

— end note

]

void swap(unique_ptr& u) noexcept;

Preconditions: get_deleter() is swappable ([swappable.requirements]) and does not throw an exception under swap.

Effects: Invokes swap on the stored pointers and on the stored deleters of *this and u.

20.11.1.3 unique_ptr for array objects with a runtime length [unique.ptr.runtime]

namespace std {
  template<class T, class D> class unique_ptr<T[], D> {
  public:
    using pointer      = see below;
    using element_type = T;
    using deleter_type = D;

    // [unique.ptr.runtime.ctor], constructors
    constexpr unique_ptr() noexcept;
    template<class U> explicit unique_ptr(U p) noexcept;
    template<class U> unique_ptr(U p, see below d) noexcept;
    template<class U> unique_ptr(U p, see below d) noexcept;
    unique_ptr(unique_ptr&& u) noexcept;
    template<class U, class E>
      unique_ptr(unique_ptr<U, E>&& u) noexcept;
    constexpr unique_ptr(nullptr_t) noexcept;

    // destructor
    ~unique_ptr();

    // assignment
    unique_ptr& operator=(unique_ptr&& u) noexcept;
    template<class U, class E>
      unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
    unique_ptr& operator=(nullptr_t) noexcept;

    // [unique.ptr.runtime.observers], observers
    T& operator[](size_t i) const;
    pointer get() const noexcept;
    deleter_type& get_deleter() noexcept;
    const deleter_type& get_deleter() const noexcept;
    explicit operator bool() const noexcept;

    // [unique.ptr.runtime.modifiers], modifiers
    pointer release() noexcept;
    template<class U> void reset(U p) noexcept;
    void reset(nullptr_t = nullptr) noexcept;
    void swap(unique_ptr& u) noexcept;

    // disable copy from lvalue
    unique_ptr(const unique_ptr&) = delete;
    unique_ptr& operator=(const unique_ptr&) = delete;
  };
}

A specialization for array types is provided with a slightly altered interface.

(1.1)
Conversions between different types of unique_ptr<T[], D> that would be disallowed for the corresponding pointer-to-array types, and conversions to or from the non-array forms of unique_ptr, produce an ill-formed program.
(1.2)
Pointers to types derived from T are rejected by the constructors, and by reset.
(1.3)
The observers operator* and operator-> are not provided.
(1.4)
The indexing observer operator[] is provided.
(1.5)
The default deleter will call delete[].

Descriptions are provided below only for members that differ from the primary template.

The template argument T shall be a complete type.

20.11.1.3.1 Constructors [unique.ptr.runtime.ctor]

template<class U> explicit unique_ptr(U p) noexcept;

This constructor behaves the same as the constructor in the primary template that takes a single parameter of type pointer.

Constraints:

(2.1)
U is the same type as pointer, or
(2.2)
pointer is the same type as element_type*, U is a pointer type V*, and V(*)[] is convertible to element_type(*)[].

template<class U> unique_ptr(U p, see below d) noexcept;
template<class U> unique_ptr(U p, see below d) noexcept;

These constructors behave the same as the constructors in the primary template that take a parameter of type pointer and a second parameter.

Constraints:

(4.1)
U is the same type as pointer,
(4.2)
U is nullptr_t, or
(4.3)
pointer is the same type as element_type*, U is a pointer type V*, and V(*)[] is convertible to element_type(*)[].

template<class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;

This constructor behaves the same as in the primary template.

Constraints: Where UP is unique_ptr<U, E>:

(6.1)
U is an array type, and
(6.2)
pointer is the same type as element_type*, and
(6.3)
UP::pointer is the same type as UP::element_type*, and
(6.4)
UP::element_type(*)[] is convertible to element_type(*)[], and
(6.5)
either D is a reference type and E is the same type as D, or D is not a reference type and E is implicitly convertible to D.

This replaces the Constraints: specification of the primary template.

— end note

]

20.11.1.3.2 Assignment [unique.ptr.runtime.asgn]

template<class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

This operator behaves the same as in the primary template.

Constraints: Where UP is unique_ptr<U, E>:

(2.1)
U is an array type, and
(2.2)
pointer is the same type as element_type*, and
(2.3)
UP::pointer is the same type as UP::element_type*, and
(2.4)
UP::element_type(*)[] is convertible to element_type(*)[], and
(2.5)
is_assignable_v<D&, E&&> is true.

This replaces the Constraints: specification of the primary template.

— end note

]

20.11.1.3.3 Observers [unique.ptr.runtime.observers]

T& operator[](size_t i) const;

Preconditions:

i <

the number of elements in the array to which the stored pointer points.

Returns: get()[i].

20.11.1.3.4 Modifiers [unique.ptr.runtime.modifiers]

void reset(nullptr_t p = nullptr) noexcept;

Effects: Equivalent to reset(pointer()).

template<class U> void reset(U p) noexcept;

This function behaves the same as the reset member of the primary template.

Constraints:

(3.1)
U is the same type as pointer, or
(3.2)
pointer is the same type as element_type*, U is a pointer type V*, and V(*)[] is convertible to element_type(*)[].

20.11.1.4 Creation [unique.ptr.create]

template<class T, class... Args> unique_ptr<T> make_unique(Args&&... args);

Constraints: T is not an array type.

Returns: unique_ptr<T>(new T(std::forward<Args>(args)...)).

template<class T> unique_ptr<T> make_unique(size_t n);

Constraints: T is an array of unknown bound.

Returns: unique_ptr<T>(new remove_extent_t<T>[n]()).

template<class T, class... Args> unspecified make_unique(Args&&...) = delete;

Constraints: T is an array of known bound.

template<class T> unique_ptr<T> make_unique_for_overwrite();

Constraints: T is not an array type.

Returns: unique_ptr<T>(new T).

template<class T> unique_ptr<T> make_unique_for_overwrite(size_t n);

Constraints: T is an array of unknown bound.

Returns: unique_ptr<T>(new remove_extent_t<T>[n]).

template<class T, class... Args> unspecified make_unique_for_overwrite(Args&&...) = delete;

Constraints: T is an array of known bound.

20.11.1.5 Specialized algorithms [unique.ptr.special]

template<class T, class D> void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept;

Constraints: is_swappable_v<D> is true.

Effects: Calls x.swap(y).

template<class T1, class D1, class T2, class D2>
  bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

Returns: x.get() == y.get().

template<class T1, class D1, class T2, class D2>
  bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

Let CT denote

common_type_t<typename unique_ptr<T1, D1>::pointer,
              typename unique_ptr<T2, D2>::pointer>

Mandates:

(5.1)
unique_ptr<T1, D1>::pointer is implicitly convertible to CT and
(5.2)
unique_ptr<T2, D2>::pointer is implicitly convertible to CT.

Preconditions: The specialization less<CT> is a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

Returns: less<CT>()(x.get(), y.get()).

template<class T1, class D1, class T2, class D2>
  bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

Returns: y < x.

template<class T1, class D1, class T2, class D2>
  bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

Returns: !(y < x).

template<class T1, class D1, class T2, class D2>
  bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

Returns: !(x < y).

template<class T1, class D1, class T2, class D2>
  requires three_way_comparable_with<typename unique_ptr<T1, D1>::pointer,
                                     typename unique_ptr<T2, D2>::pointer>
  compare_three_way_result_t<typename unique_ptr<T1, D1>::pointer,
                             typename unique_ptr<T2, D2>::pointer>
    operator<=>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

Returns: compare_three_way()(x.get(), y.get()).

template<class T, class D>
  bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;

Returns: !x.

template<class T, class D>
  bool operator<(const unique_ptr<T, D>& x, nullptr_t);
template<class T, class D>
  bool operator<(nullptr_t, const unique_ptr<T, D>& x);

Preconditions: The specialization less<unique_ptr<T, D>::pointer> is a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

Returns: The first function template returns

less<unique_ptr<T, D>::pointer>()(x.get(), nullptr)

The second function template returns

less<unique_ptr<T, D>::pointer>()(nullptr, x.get())

template<class T, class D>
  bool operator>(const unique_ptr<T, D>& x, nullptr_t);
template<class T, class D>
  bool operator>(nullptr_t, const unique_ptr<T, D>& x);

Returns: The first function template returns nullptr < x.

The second function template returns x < nullptr.

template<class T, class D>
  bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
template<class T, class D>
  bool operator<=(nullptr_t, const unique_ptr<T, D>& x);

Returns: The first function template returns !(nullptr < x).

The second function template returns !(x < nullptr).

template<class T, class D>
  bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
template<class T, class D>
  bool operator>=(nullptr_t, const unique_ptr<T, D>& x);

Returns: The first function template returns !(x < nullptr).

The second function template returns !(nullptr < x).

template<class T, class D>
  requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t>
  compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t>
    operator<=>(const unique_ptr<T, D>& x, nullptr_t);

Returns: compare_three_way()(x.get(), nullptr).

20.11.1.6 I/O [unique.ptr.io]

template<class E, class T, class Y, class D>
  basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);

Constraints: os << p.get() is a valid expression.

Effects: Equivalent to: os << p.get();

Returns: os.

20.11.2 Class bad_weak_ptr [util.smartptr.weak.bad]

namespace std {
  class bad_weak_ptr : public exception {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}

An exception of type bad_weak_ptr is thrown by the shared_ptr constructor taking a weak_ptr.

const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

20.11.3 Class template shared_ptr [util.smartptr.shared]

The shared_ptr class template stores a pointer, usually obtained via new.

shared_ptr implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer.

A shared_ptr is said to be empty if it does not own a pointer.

namespace std {
  template<class T> class shared_ptr {
  public:
    using element_type = remove_extent_t<T>;
    using weak_type    = weak_ptr<T>;

    // [util.smartptr.shared.const], constructors
    constexpr shared_ptr() noexcept;
    constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }
    template<class Y>
      explicit shared_ptr(Y* p);
    template<class Y, class D>
      shared_ptr(Y* p, D d);
    template<class Y, class D, class A>
      shared_ptr(Y* p, D d, A a);
    template<class D>
      shared_ptr(nullptr_t p, D d);
    template<class D, class A>
      shared_ptr(nullptr_t p, D d, A a);
    template<class Y>
      shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
    template<class Y>
      shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;
    shared_ptr(const shared_ptr& r) noexcept;
    template<class Y>
      shared_ptr(const shared_ptr<Y>& r) noexcept;
    shared_ptr(shared_ptr&& r) noexcept;
    template<class Y>
      shared_ptr(shared_ptr<Y>&& r) noexcept;
    template<class Y>
      explicit shared_ptr(const weak_ptr<Y>& r);
    template<class Y, class D>
      shared_ptr(unique_ptr<Y, D>&& r);

    // [util.smartptr.shared.dest], destructor
    ~shared_ptr();

    // [util.smartptr.shared.assign], assignment
    shared_ptr& operator=(const shared_ptr& r) noexcept;
    template<class Y>
      shared_ptr& operator=(const shared_ptr<Y>& r) noexcept;
    shared_ptr& operator=(shared_ptr&& r) noexcept;
    template<class Y>
      shared_ptr& operator=(shared_ptr<Y>&& r) noexcept;
    template<class Y, class D>
      shared_ptr& operator=(unique_ptr<Y, D>&& r);

    // [util.smartptr.shared.mod], modifiers
    void swap(shared_ptr& r) noexcept;
    void reset() noexcept;
    template<class Y>
      void reset(Y* p);
    template<class Y, class D>
      void reset(Y* p, D d);
    template<class Y, class D, class A>
      void reset(Y* p, D d, A a);

    // [util.smartptr.shared.obs], observers
    element_type* get() const noexcept;
    T& operator*() const noexcept;
    T* operator->() const noexcept;
    element_type& operator[](ptrdiff_t i) const;
    long use_count() const noexcept;
    explicit operator bool() const noexcept;
    template<class U>
      bool owner_before(const shared_ptr<U>& b) const noexcept;
    template<class U>
      bool owner_before(const weak_ptr<U>& b) const noexcept;
  };

  template<class T>
    shared_ptr(weak_ptr<T>) -> shared_ptr<T>;
  template<class T, class D>
    shared_ptr(unique_ptr<T, D>) -> shared_ptr<T>;
}

Specializations of shared_ptr shall be Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17LessThanComparable, allowing their use in standard containers.

Specializations of shared_ptr shall be contextually convertible to bool, allowing their use in boolean expressions and declarations in conditions.

The template parameter T of shared_ptr may be an incomplete type.

T may be a function type.

— end note

]

if (shared_ptr<X> px = dynamic_pointer_cast<X>(py)) {
  // do something with px
}

— end example

]

For purposes of determining the presence of a data race, member functions shall access and modify only the shared_ptr and weak_ptr objects themselves and not objects they refer to.

Changes in use_count() do not reflect modifications that can introduce data races.

For the purposes of subclause [smartptr], a pointer type Y* is said to be compatible with a pointer type T* when either Y* is convertible to T* or Y is U[N] and T is cv U[].

20.11.3.1 Constructors [util.smartptr.shared.const]

In the constructor definitions below, enables shared_from_this with p, for a pointer p of type Y*, means that if Y has an unambiguous and accessible base class that is a specialization of enable_shared_from_this, then remove_cv_t<Y>* shall be implicitly convertible to T* and the constructor evaluates the statement:

if (p != nullptr && p->weak_this.expired())
  p->weak_this = shared_ptr<remove_cv_t<Y>>(*this, const_cast<remove_cv_t<Y>*>(p));

The assignment to the weak_this member is not atomic and conflicts with any potentially concurrent access to the same object ([intro.multithread]).

constexpr shared_ptr() noexcept;

Postconditions: use_count() == 0 && get() == nullptr.

template<class Y> explicit shared_ptr(Y* p);

Mandates: Y is a complete type.

Constraints: When T is an array type, the expression delete[] p is well-formed and either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*.

When T is not an array type, the expression delete p is well-formed and Y* is convertible to T*.

Preconditions: The expression delete[] p, when T is an array type, or delete p, when T is not an array type, has well-defined behavior, and does not throw exceptions.

Effects: When T is not an array type, constructs a shared_ptr object that owns the pointer p.

Otherwise, constructs a shared_ptr that owns p and a deleter of an unspecified type that calls delete[] p.

When T is not an array type, enables shared_from_this with p.

If an exception is thrown, delete p is called when T is not an array type, delete[] p otherwise.

Postconditions: use_count() == 1 && get() == p.

Throws: bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

template<class Y, class D> shared_ptr(Y* p, D d);
template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
template<class D> shared_ptr(nullptr_t p, D d);
template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

Constraints: is_move_constructible_v<D> is true, and d(p) is a well-formed expression.

For the first two overloads:

(9.1)
If T is an array type, then either T is U[N] and Y(*)[N] is convertible to T*, or T is U[] and Y(*)[] is convertible to T*.
(9.2)
If T is not an array type, then Y* is convertible to T*.

Preconditions: Construction of d and a deleter of type D initialized with std::move(d) do not throw exceptions.

The expression d(p) has well-defined behavior and does not throw exceptions.

A meets the Cpp17Allocator requirements (Table 36).

Effects: Constructs a shared_ptr object that owns the object p and the deleter d.

When T is not an array type, the first and second constructors enable shared_from_this with p.

The second and fourth constructors shall use a copy of a to allocate memory for internal use.

If an exception is thrown, d(p) is called.

Postconditions: use_count() == 1 && get() == p.

Throws: bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept;
template<class Y> shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;

Effects: Constructs a shared_ptr instance that stores p and shares ownership with the initial value of r.

Postconditions: get() == p.

For the second overload, r is empty and r.get() == nullptr.

To avoid the possibility of a dangling pointer, the user of this constructor should ensure that p remains valid at least until the ownership group of r is destroyed.

— end note

]

This constructor allows creation of an empty shared_ptr instance with a non-null stored pointer.

— end note

]

shared_ptr(const shared_ptr& r) noexcept;
template<class Y> shared_ptr(const shared_ptr<Y>& r) noexcept;

Constraints: For the second constructor, Y* is compatible with T*.

Effects: If r is empty, constructs an empty shared_ptr object; otherwise, constructs a shared_ptr object that shares ownership with r.

Postconditions: get() == r.get() && use_count() == r.use_count().

shared_ptr(shared_ptr&& r) noexcept;
template<class Y> shared_ptr(shared_ptr<Y>&& r) noexcept;

Constraints: For the second constructor, Y* is compatible with T*.

Effects: Move constructs a shared_ptr instance from r.

Postconditions: *this shall contain the old value of r.

r shall be empty.

r.get() == nullptr.

template<class Y> explicit shared_ptr(const weak_ptr<Y>& r);

Constraints: Y* is compatible with T*.

Effects: Constructs a shared_ptr object that shares ownership with r and stores a copy of the pointer stored in r.

If an exception is thrown, the constructor has no effect.

Postconditions: use_count() == r.use_count().

Throws: bad_weak_ptr when r.expired().

template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

Constraints: Y* is compatible with T* and unique_ptr<Y, D>::pointer is convertible to element_type*.

Effects: If r.get() == nullptr, equivalent to shared_ptr().

Otherwise, if D is not a reference type, equivalent to shared_ptr(r.release(), r.get_deleter()).

Otherwise, equivalent to shared_ptr(r.release(), ref(r.get_deleter())).

If an exception is thrown, the constructor has no effect.

20.11.3.2 Destructor [util.smartptr.shared.dest]

~shared_ptr();

Effects:

(1.1)
If *this is empty or shares ownership with another shared_ptr instance (use_count() > 1), there are no side effects.
(1.2)
Otherwise, if *this owns an object p and a deleter d, d(p) is called.
(1.3)
Otherwise, *this owns a pointer p, and delete p is called.

Since the destruction of *this decreases the number of instances that share ownership with *this by one, after *this has been destroyed all shared_ptr instances that shared ownership with *this will report a use_count() that is one less than its previous value.

— end note

]

20.11.3.3 Assignment [util.smartptr.shared.assign]

shared_ptr& operator=(const shared_ptr& r) noexcept;
template<class Y> shared_ptr& operator=(const shared_ptr<Y>& r) noexcept;

Effects: Equivalent to shared_ptr(r).swap(*this).

Returns: *this.

The use count updates caused by the temporary object construction and destruction are not observable side effects, so the implementation may meet the effects (and the implied guarantees) via different means, without creating a temporary.

In particular, in the example:

shared_ptr<int> p(new int);
shared_ptr<void> q(p);
p = p;
q = p;

both assignments may be no-ops.

— end note

]

shared_ptr& operator=(shared_ptr&& r) noexcept;
template<class Y> shared_ptr& operator=(shared_ptr<Y>&& r) noexcept;

Effects: Equivalent to shared_ptr(std::move(r)).swap(*this).

Returns: *this.

template<class Y, class D> shared_ptr& operator=(unique_ptr<Y, D>&& r);

Effects: Equivalent to shared_ptr(std::move(r)).swap(*this).

Returns: *this.

20.11.3.4 Modifiers [util.smartptr.shared.mod]

void swap(shared_ptr& r) noexcept;

Effects: Exchanges the contents of *this and r.

void reset() noexcept;

Effects: Equivalent to shared_ptr().swap(*this).

template<class Y> void reset(Y* p);

Effects: Equivalent to shared_ptr(p).swap(*this).

template<class Y, class D> void reset(Y* p, D d);

Effects: Equivalent to shared_ptr(p, d).swap(*this).

template<class Y, class D, class A> void reset(Y* p, D d, A a);

Effects: Equivalent to shared_ptr(p, d, a).swap(*this).

20.11.3.5 Observers [util.smartptr.shared.obs]

element_type* get() const noexcept;

Returns: The stored pointer.

T& operator*() const noexcept;

Preconditions: get() != 0.

Returns: *get().

Remarks: When T is an array type or cv void, it is unspecified whether this member function is declared.

If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

T* operator->() const noexcept;

Preconditions: get() != 0.

Returns: get().

Remarks: When T is an array type, it is unspecified whether this member function is declared.

If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

element_type& operator[](ptrdiff_t i) const;

Preconditions: get() != 0 && i >= 0.

If T is U[N], i < N.

Returns: get()[i].

Remarks: When T is not an array type, it is unspecified whether this member function is declared.

If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

Throws: Nothing.

long use_count() const noexcept;

Returns: The number of shared_ptr objects, *this included, that share ownership with *this, or 0 when *this is empty.

Synchronization: None.

get() == nullptr does not imply a specific return value of use_count().

— end note

]

weak_ptr<T>::lock() can affect the return value of use_count().

— end note

]

When multiple threads can affect the return value of use_count(), the result should be treated as approximate.

In particular, use_count() == 1 does not imply that accesses through a previously destroyed shared_ptr have in any sense completed.

— end note

]

explicit operator bool() const noexcept;

Returns: get() != 0.

template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept;
template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;

Returns: An unspecified value such that

(18.1)
x.owner_before(y) defines a strict weak ordering as defined in [alg.sorting];
(18.2)
under the equivalence relation defined by owner_before, !a.owner_before(b) && !b.owner_before(a), two shared_ptr or weak_ptr instances are equivalent if and only if they share ownership or are both empty.

20.11.3.6 Creation [util.smartptr.shared.create]

The common requirements that apply to all make_shared, allocate_shared, make_shared_for_overwrite, and allocate_shared_for_overwrite overloads, unless specified otherwise, are described below.

template<class T, ...>
  shared_ptr<T> make_shared(args);
template<class T, class A, ...>
  shared_ptr<T> allocate_shared(const A& a, args);
template<class T, ...>
  shared_ptr<T> make_shared_for_overwrite(args);
template<class T, class A, ...>
  shared_ptr<T> allocate_shared_for_overwrite(const A& a, args);

Preconditions: A meets the Cpp17Allocator requirements (Table 36).

Effects: Allocates memory for an object of type T (or U[N] when T is U[], where N is determined from args as specified by the concrete overload).

The object is initialized from args as specified by the concrete overload.

The allocate_shared and allocate_shared_for_overwrite templates use a copy of a (rebound for an unspecified value_type) to allocate memory.

If an exception is thrown, the functions have no effect.

Returns: A shared_ptr instance that stores and owns the address of the newly constructed object.

Postconditions: r.get() != 0 && r.use_count() == 1, where r is the return value.

Throws: bad_alloc, or an exception thrown from allocate or from the initialization of the object.

Remarks:

(7.1)
Implementations should perform no more than one memory allocation.
[Note
: This provides efficiency equivalent to an intrusive smart pointer. — end note
]
(7.2)
When an object of an array type U is specified to have an initial value of u (of the same type), this shall be interpreted to mean that each array element of the object has as its initial value the corresponding element from u.
(7.3)
When an object of an array type is specified to have a default initial value, this shall be interpreted to mean that each array element of the object has a default initial value.
(7.4)
When a (sub)object of a non-array type U is specified to have an initial value of v, or U(l...), where l... is a list of constructor arguments, make_shared shall initialize this (sub)object via the expression ::new(pv) U(v) or ::new(pv) U(l...) respectively, where pv has type void* and points to storage suitable to hold an object of type U.
(7.5)
When a (sub)object of a non-array type U is specified to have an initial value of v, or U(l...), where l... is a list of constructor arguments, allocate_shared shall initialize this (sub)object via the expression
- (7.5.1)
 allocator_traits<A2>::construct(a2, pv, v) or
- (7.5.2)
 allocator_traits<A2>::construct(a2, pv, l...)
respectively, where pv points to storage suitable to hold an object of type U and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t.
(7.6)
When a (sub)object of non-array type U is specified to have a default initial value, make_shared shall initialize this (sub)object via the expression ::new(pv) U(), where pv has type void* and points to storage suitable to hold an object of type U.
(7.7)
When a (sub)object of non-array type U is specified to have a default initial value, allocate_shared shall initialize this (sub)object via the expression allocator_traits<A2>::construct(a2, pv), where pv points to storage suitable to hold an object of type U and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t.
(7.8)
When a (sub)object of non-array type U is initialized by make_shared_for_overwrite or
allocate_shared_for_overwrite, it is initialized via the expression ::new(pv) U, where pv has type void* and points to storage suitable to hold an object of type U.
(7.9)
Array elements are initialized in ascending order of their addresses.
(7.10)
When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements are destroyed in the reverse order of their original construction.
(7.11)
When a (sub)object of non-array type U that was initialized by make_shared is to be destroyed, it is destroyed via the expression pv->~U() where pv points to that object of type U.
(7.12)
When a (sub)object of non-array type U that was initialized by allocate_shared is to be destroyed, it is destroyed via the expression allocator_traits<A2>::destroy(a2, pv) where pv points to that object of type remove_cv_t and a2 of type A2 is a rebound copy of the allocator a passed to allocate_shared such that its value_type is remove_cv_t.

These functions will typically allocate more memory than sizeof(T) to allow for internal bookkeeping structures such as reference counts.

— end note

]

template<class T, class... Args>
  shared_ptr<T> make_shared(Args&&... args);                    // T is not array
template<class T, class A, class... Args>
  shared_ptr<T> allocate_shared(const A& a, Args&&... args);    // T is not array

Constraints: T is not an array type.

Returns: A shared_ptr to an object of type T with an initial value T(forward<Args>(args)...).

Remarks: The shared_ptr constructors called by these functions enable shared_from_this with the address of the newly constructed object of type T.

shared_ptr<int> p = make_shared<int>(); // shared_ptr to int()
shared_ptr<vector<int>> q = make_shared<vector<int>>(16, 1);
  // shared_ptr to vector of 16 elements with value 1

— end example

]

template<class T> shared_ptr<T>
  make_shared(size_t N);                                        // T is U[]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a, size_t N);          // T is U[]

Constraints: T is of the form U[].

Returns: A shared_ptr to an object of type U[N] with a default initial value, where U is remove_extent_t<T>.

shared_ptr<double[]> p = make_shared<double[]>(1024);
  // shared_ptr to a value-initialized double[1024]
shared_ptr<double[][2][2]> q = make_shared<double[][2][2]>(6);
  // shared_ptr to a value-initialized double[6][2][2]

— end example

]

template<class T>
  shared_ptr<T> make_shared();                                  // T is U[N]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a);                    // T is U[N]

Constraints: T is of the form U[N].

Returns: A shared_ptr to an object of type T with a default initial value.

shared_ptr<double[1024]> p = make_shared<double[1024]>();
  // shared_ptr to a value-initialized double[1024]
shared_ptr<double[6][2][2]> q = make_shared<double[6][2][2]>();
  // shared_ptr to a value-initialized double[6][2][2]

— end example

]

template<class T>
  shared_ptr<T> make_shared(size_t N,
                            const remove_extent_t<T>& u);       // T is U[]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a, size_t N,
                                const remove_extent_t<T>& u);   // T is U[]

Constraints: T is of the form U[].

Returns: A shared_ptr to an object of type U[N], where U is remove_extent_t<T> and each array element has an initial value of u.

shared_ptr<double[]> p = make_shared<double[]>(1024, 1.0);
  // shared_ptr to a double[1024], where each element is 1.0
shared_ptr<double[][2]> q = make_shared<double[][2]>(6, {1.0, 0.0});
  // shared_ptr to a double[6][2], where each double[2] element is {1.0, 0.0}
shared_ptr<vector<int>[]> r = make_shared<vector<int>[]>(4, {1, 2});
  // shared_ptr to a vector<int>[4], where each vector has contents {1, 2}

— end example

]

template<class T>
  shared_ptr<T> make_shared(const remove_extent_t<T>& u);       // T is U[N]
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a,
                                const remove_extent_t<T>& u);   // T is U[N]

Constraints: T is of the form U[N].

Returns: A shared_ptr to an object of type T, where each array element of type remove_extent_t<T> has an initial value of u.

shared_ptr<double[1024]> p = make_shared<double[1024]>(1.0);
  // shared_ptr to a double[1024], where each element is 1.0
shared_ptr<double[6][2]> q = make_shared<double[6][2]>({1.0, 0.0});
  // shared_ptr to a double[6][2], where each double[2] element is {1.0, 0.0}
shared_ptr<vector<int>[4]> r = make_shared<vector<int>[4]>({1, 2});
  // shared_ptr to a vector<int>[4], where each vector has contents {1, 2}

— end example

]

template<class T>
  shared_ptr<T> make_shared_for_overwrite();
template<class T, class A>
  shared_ptr<T> allocate_shared_for_overwrite(const A& a);

Constraints: T is not an array of unknown bound.

Returns: A shared_ptr to an object of type T.

struct X { double data[1024]; };
shared_ptr<X> p = make_shared_for_overwrite<X>();
  // shared_ptr to a default-initialized X, where each element in X::data has an indeterminate value

shared_ptr<double[1024]> q = make_shared_for_overwrite<double[1024]>();
  // shared_ptr to a default-initialized double[1024], where each element has an indeterminate value

— end example

]

template<class T>
  shared_ptr<T> make_shared_for_overwrite(size_t N);
template<class T, class A>
  shared_ptr<T> allocate_shared_for_overwrite(const A& a, size_t N);

Constraints: T is an array of unknown bound.

Returns: A shared_ptr to an object of type U[N], where U is remove_extent_t<T>.

shared_ptr<double[]> p = make_shared_for_overwrite<double[]>(1024);
  // shared_ptr to a default-initialized double[1024], where each element has an indeterminate value

— end example

]

20.11.3.7 Comparison [util.smartptr.shared.cmp]

template<class T, class U>
  bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

Returns: a.get() == b.get().

template<class T>
  bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;

Returns: !a.

template<class T, class U>
  strong_ordering operator<=>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

Returns: compare_three_way()(a.get(), b.get()).

Defining a comparison function allows shared_ptr objects to be used as keys in associative containers.

— end note

]

template<class T>
  strong_ordering operator<=>(const shared_ptr<T>& a, nullptr_t) noexcept;

Returns: compare_three_way()(a.get(), nullptr).

20.11.3.8 Specialized algorithms [util.smartptr.shared.spec]

template<class T>
  void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

Effects: Equivalent to a.swap(b).

20.11.3.9 Casts [util.smartptr.shared.cast]

template<class T, class U>
  shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression static_cast<T*>((U*)nullptr) is well-formed.

Returns:

shared_ptr<T>(R, static_cast<typename shared_ptr<T>::element_type*>(r.get()))

where R is r for the first overload, and std::move(r) for the second.

The seemingly equivalent expression shared_ptr<T>(static_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note

]

template<class T, class U>
  shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression dynamic_cast<T*>((U*)nullptr) is well-formed.

The expression dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) is well formed.

Preconditions: The expression dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) has well-defined behavior.

Returns:

(6.1)
When dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) returns a non-null value p, shared_ptr<T>(R, p), where R is r for the first overload, and std::move(r) for the second.
(6.2)
Otherwise, shared_ptr<T>().

The seemingly equivalent expression shared_ptr<T>(dynamic_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note

]

template<class T, class U>
  shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression const_cast<T*>((U*)nullptr) is well-formed.

Returns:

shared_ptr<T>(R, const_cast<typename shared_ptr<T>::element_type*>(r.get()))

where R is r for the first overload, and std::move(r) for the second.

The seemingly equivalent expression shared_ptr<T>(const_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note

]

template<class T, class U>
  shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept;
template<class T, class U>
  shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;

Mandates: The expression reinterpret_cast<T*>((U*)nullptr) is well-formed.

Returns:

shared_ptr<T>(R, reinterpret_cast<typename shared_ptr<T>::element_type*>(r.get()))

where R is r for the first overload, and std::move(r) for the second.

The seemingly equivalent expression shared_ptr<T>(reinterpret_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

— end note

]

20.11.3.10 get_deleter [util.smartptr.getdeleter]

template<class D, class T>
  D* get_deleter(const shared_ptr<T>& p) noexcept;

Returns: If p owns a deleter d of type cv-unqualified D, returns addressof(d); otherwise returns nullptr.

The returned pointer remains valid as long as there exists a shared_ptr instance that owns d.

It is unspecified whether the pointer remains valid longer than that.

This can happen if the implementation doesn't destroy the deleter until all weak_ptr instances that share ownership with p have been destroyed.

— end note

]

20.11.3.11 I/O [util.smartptr.shared.io]

template<class E, class T, class Y>
  basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const shared_ptr<Y>& p);

Effects: As if by: os << p.get();

Returns: os.

20.11.4 Class template weak_ptr [util.smartptr.weak]

The weak_ptr class template stores a weak reference to an object that is already managed by a shared_ptr.

To access the object, a weak_ptr can be converted to a shared_ptr using the member function lock.

namespace std {
  template<class T> class weak_ptr {
  public:
    using element_type = remove_extent_t<T>;

    // [util.smartptr.weak.const], constructors
    constexpr weak_ptr() noexcept;
    template<class Y>
      weak_ptr(const shared_ptr<Y>& r) noexcept;
    weak_ptr(const weak_ptr& r) noexcept;
    template<class Y>
      weak_ptr(const weak_ptr<Y>& r) noexcept;
    weak_ptr(weak_ptr&& r) noexcept;
    template<class Y>
      weak_ptr(weak_ptr<Y>&& r) noexcept;

    // [util.smartptr.weak.dest], destructor
    ~weak_ptr();

    // [util.smartptr.weak.assign], assignment
    weak_ptr& operator=(const weak_ptr& r) noexcept;
    template<class Y>
      weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
    template<class Y>
      weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;
    weak_ptr& operator=(weak_ptr&& r) noexcept;
    template<class Y>
      weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

    // [util.smartptr.weak.mod], modifiers
    void swap(weak_ptr& r) noexcept;
    void reset() noexcept;

    // [util.smartptr.weak.obs], observers
    long use_count() const noexcept;
    bool expired() const noexcept;
    shared_ptr<T> lock() const noexcept;
    template<class U>
      bool owner_before(const shared_ptr<U>& b) const noexcept;
    template<class U>
      bool owner_before(const weak_ptr<U>& b) const noexcept;
  };

  template<class T>
    weak_ptr(shared_ptr<T>) -> weak_ptr<T>;

  // [util.smartptr.weak.spec], specialized algorithms
  template<class T>
    void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
}

Specializations of weak_ptr shall be Cpp17CopyConstructible and Cpp17CopyAssignable, allowing their use in standard containers.

The template parameter T of weak_ptr may be an incomplete type.

20.11.4.1 Constructors [util.smartptr.weak.const]

constexpr weak_ptr() noexcept;

Effects: Constructs an empty weak_ptr object.

Postconditions: use_count() == 0.

weak_ptr(const weak_ptr& r) noexcept;
template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;

Constraints: For the second and third constructors, Y* is compatible with T*.

Effects: If r is empty, constructs an empty weak_ptr object; otherwise, constructs a weak_ptr object that shares ownership with r and stores a copy of the pointer stored in r.

Postconditions: use_count() == r.use_count().

weak_ptr(weak_ptr&& r) noexcept;
template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

Constraints: For the second constructor, Y* is compatible with T*.

Effects: Move constructs a weak_ptr instance from r.

Postconditions: *this shall contain the old value of r.

r shall be empty.

r.use_count() == 0.

20.11.4.2 Destructor [util.smartptr.weak.dest]

~weak_ptr();

Effects: Destroys this weak_ptr object but has no effect on the object its stored pointer points to.

20.11.4.3 Assignment [util.smartptr.weak.assign]

weak_ptr& operator=(const weak_ptr& r) noexcept;
template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;

Effects: Equivalent to weak_ptr(r).swap(*this).

Remarks: The implementation may meet the effects (and the implied guarantees) via different means, without creating a temporary object.

Returns: *this.

weak_ptr& operator=(weak_ptr&& r) noexcept;
template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

Effects: Equivalent to weak_ptr(std::move(r)).swap(*this).

Returns: *this.

20.11.4.4 Modifiers [util.smartptr.weak.mod]

void swap(weak_ptr& r) noexcept;

Effects: Exchanges the contents of *this and r.

void reset() noexcept;

Effects: Equivalent to weak_ptr().swap(*this).

20.11.4.5 Observers [util.smartptr.weak.obs]

long use_count() const noexcept;

Returns: 0 if *this is empty; otherwise, the number of shared_ptr instances that share ownership with *this.

bool expired() const noexcept;

Returns: use_count() == 0.

shared_ptr<T> lock() const noexcept;

Returns: expired() ? shared_ptr<T>() : shared_ptr<T>(*this), executed atomically.

template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept;
template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;

Returns: An unspecified value such that

(4.1)
x.owner_before(y) defines a strict weak ordering as defined in [alg.sorting];
(4.2)
under the equivalence relation defined by owner_before, !a.owner_before(b) && !b.owner_before(a), two shared_ptr or weak_ptr instances are equivalent if and only if they share ownership or are both empty.

20.11.4.6 Specialized algorithms [util.smartptr.weak.spec]

template<class T>
  void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;

Effects: Equivalent to a.swap(b).

20.11.5 Class template owner_less [util.smartptr.ownerless]

The class template owner_less allows ownership-based mixed comparisons of shared and weak pointers.

namespace std {
  template<class T = void> struct owner_less;

  template<class T> struct owner_less<shared_ptr<T>> {
    bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const noexcept;
    bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
    bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
  };

  template<class T> struct owner_less<weak_ptr<T>> {
    bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const noexcept;
    bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept;
    bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept;
  };

  template<> struct owner_less<void> {
    template<class T, class U>
      bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const noexcept;
    template<class T, class U>
      bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const noexcept;
    template<class T, class U>
      bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const noexcept;
    template<class T, class U>
      bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const noexcept;

    using is_transparent = unspecified;
  };
}

operator()(x, y) returns x.owner_before(y).

Note that

(2.1)
operator() defines a strict weak ordering as defined in [alg.sorting];
(2.2)
under the equivalence relation defined by operator(), !operator()(a, b) && !operator()(b, a), two shared_ptr or weak_ptr instances are equivalent if and only if they share ownership or are both empty.

— end note

]

20.11.6 Class template enable_shared_from_this [util.smartptr.enab]

A class T can inherit from enable_shared_from_this<T> to inherit the shared_from_this member functions that obtain a shared_ptr instance pointing to *this.

struct X: public enable_shared_from_this<X> { };

int main() {
  shared_ptr<X> p(new X);
  shared_ptr<X> q = p->shared_from_this();
  assert(p == q);
  assert(!p.owner_before(q) && !q.owner_before(p)); // p and q share ownership
}

— end example

]

namespace std {
  template<class T> class enable_shared_from_this {
  protected:
    constexpr enable_shared_from_this() noexcept;
    enable_shared_from_this(const enable_shared_from_this&) noexcept;
    enable_shared_from_this& operator=(const enable_shared_from_this&) noexcept;
    ~enable_shared_from_this();

  public:
    shared_ptr<T> shared_from_this();
    shared_ptr<T const> shared_from_this() const;
    weak_ptr<T> weak_from_this() noexcept;
    weak_ptr<T const> weak_from_this() const noexcept;

  private:
    mutable weak_ptr<T> weak_this;  // exposition only
  };
}

The template parameter T of enable_shared_from_this may be an incomplete type.

constexpr enable_shared_from_this() noexcept;
enable_shared_from_this(const enable_shared_from_this<T>&) noexcept;

Effects: Value-initializes weak_this.

enable_shared_from_this<T>& operator=(const enable_shared_from_this<T>&) noexcept;

Returns: *this.

weak_this is not changed.

— end note

]

shared_ptr<T>       shared_from_this();
shared_ptr<T const> shared_from_this() const;

Returns: shared_ptr<T>(weak_this).

weak_ptr<T>       weak_from_this() noexcept;
weak_ptr<T const> weak_from_this() const noexcept;

Returns: weak_this.

20.11.7 Smart pointer hash support [util.smartptr.hash]

template<class T, class D> struct hash<unique_ptr<T, D>>;

Letting UP be unique_ptr<T,D>, the specialization hash<UP> is enabled ([unord.hash]) if and only if hash<typename UP::pointer> is enabled.

When enabled, for an object p of type UP, hash<UP>()(p) evaluates to the same value as hash<typename UP::pointer>()(p.get()).

The member functions are not guaranteed to be noexcept.

template<class T> struct hash<shared_ptr<T>>;

For an object p of type shared_ptr<T>, hash<shared_ptr<T>>()(p) evaluates to the same value as hash<typename shared_ptr<T>::element_type*>()(p.get()).

20.12 Memory resources [mem.res]

20.12.1 Header <memory_resource> synopsis [mem.res.syn]

namespace std::pmr {
  // [mem.res.class], class memory_resource
  class memory_resource;

  bool operator==(const memory_resource& a, const memory_resource& b) noexcept;

  // [mem.poly.allocator.class], class template polymorphic_allocator
  template<class Tp> class polymorphic_allocator;

  template<class T1, class T2>
    bool operator==(const polymorphic_allocator<T1>& a,
                    const polymorphic_allocator<T2>& b) noexcept;

  // [mem.res.global], global memory resources
  memory_resource* new_delete_resource() noexcept;
  memory_resource* null_memory_resource() noexcept;
  memory_resource* set_default_resource(memory_resource* r) noexcept;
  memory_resource* get_default_resource() noexcept;

  // [mem.res.pool], pool resource classes
  struct pool_options;
  class synchronized_pool_resource;
  class unsynchronized_pool_resource;
  class monotonic_buffer_resource;
}

20.12.2 Class memory_resource [mem.res.class]

The memory_resource class is an abstract interface to an unbounded set of classes encapsulating memory resources.

namespace std::pmr {
  class memory_resource {
    static constexpr size_t max_align = alignof(max_align_t);   // exposition only

  public:
    memory_resource() = default;
    memory_resource(const memory_resource&) = default;
    virtual ~memory_resource();

    memory_resource& operator=(const memory_resource&) = default;

    [[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
    void deallocate(void* p, size_t bytes, size_t alignment = max_align);

    bool is_equal(const memory_resource& other) const noexcept;

  private:
    virtual void* do_allocate(size_t bytes, size_t alignment) = 0;
    virtual void do_deallocate(void* p, size_t bytes, size_t alignment) = 0;

    virtual bool do_is_equal(const memory_resource& other) const noexcept = 0;
  };
}

20.12.2.1 Public member functions [mem.res.public]

~memory_resource();

Effects: Destroys this memory_resource.

[[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);

Effects: Equivalent to: return do_allocate(bytes, alignment);

void deallocate(void* p, size_t bytes, size_t alignment = max_align);

Effects: Equivalent to do_deallocate(p, bytes, alignment).

bool is_equal(const memory_resource& other) const noexcept;

Effects: Equivalent to: return do_is_equal(other);

20.12.2.2 Private virtual member functions [mem.res.private]

virtual void* do_allocate(size_t bytes, size_t alignment) = 0;

Preconditions: alignment is a power of two.

Returns: A derived class shall implement this function to return a pointer to allocated storage ([basic.stc.dynamic.allocation]) with a size of at least bytes, aligned to the specified alignment.

Throws: A derived class implementation shall throw an appropriate exception if it is unable to allocate memory with the requested size and alignment.

virtual void do_deallocate(void* p, size_t bytes, size_t alignment) = 0;

Preconditions: p was returned from a prior call to allocate(bytes, alignment) on a memory resource equal to *this, and the storage at p has not yet been deallocated.

Effects: A derived class shall implement this function to dispose of allocated storage.

Throws: Nothing.

virtual bool do_is_equal(const memory_resource& other) const noexcept = 0;

Returns: A derived class shall implement this function to return true if memory allocated from this can be deallocated from other and vice-versa, otherwise false.

The most-derived type of other might not match the type of this.

For a derived class D, an implementation of this function could immediately return false if dynamic_cast<const D*>(&other) == nullptr.

— end note

]

20.12.2.3 Equality [mem.res.eq]

bool operator==(const memory_resource& a, const memory_resource& b) noexcept;

Returns: &a == &b || a.is_equal(b).

20.12.3 Class template polymorphic_allocator [mem.poly.allocator.class]

A specialization of class template pmr::polymorphic_allocator meets the Cpp17Allocator requirements (Table 36).

Constructed with different memory resources, different instances of the same specialization of pmr::polymorphic_allocator can exhibit entirely different allocation behavior.

This runtime polymorphism allows objects that use polymorphic_allocator to behave as if they used different allocator types at run time even though they use the same static allocator type.

All specializations of class template pmr::polymorphic_allocator meet the allocator completeness requirements ([allocator.requirements.completeness]).

namespace std::pmr {
  template<class Tp = byte> class polymorphic_allocator {
    memory_resource* memory_rsrc;       // exposition only

  public:
    using value_type = Tp;

    // [mem.poly.allocator.ctor], constructors
    polymorphic_allocator() noexcept;
    polymorphic_allocator(memory_resource* r);

    polymorphic_allocator(const polymorphic_allocator& other) = default;

    template<class U>
      polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept;

    polymorphic_allocator& operator=(const polymorphic_allocator&) = delete;

    // [mem.poly.allocator.mem], member functions
    [[nodiscard]] Tp* allocate(size_t n);
    void deallocate(Tp* p, size_t n);

    [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));
    void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t));
    template<class T> [[nodiscard]] T* allocate_object(size_t n = 1);
    template<class T> void deallocate_object(T* p, size_t n = 1);
    template<class T, class... CtorArgs> [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);
    template<class T> void delete_object(T* p);

    template<class T, class... Args>
      void construct(T* p, Args&&... args);

    template<class T>
      void destroy(T* p);

    polymorphic_allocator select_on_container_copy_construction() const;

    memory_resource* resource() const;
  };
}

20.12.3.1 Constructors [mem.poly.allocator.ctor]

polymorphic_allocator() noexcept;

Effects: Sets memory_rsrc to get_default_resource().

polymorphic_allocator(memory_resource* r);

Preconditions: r is non-null.

Effects: Sets memory_rsrc to r.

Throws: Nothing.

This constructor provides an implicit conversion from memory_resource*.

— end note

]

template<class U> polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept;

Effects: Sets memory_rsrc to other.resource().

20.12.3.2 Member functions [mem.poly.allocator.mem]

[[nodiscard]] Tp* allocate(size_t n);

Effects: If numeric_limits<size_t>::max() / sizeof(Tp) < n, throws bad_array_new_length.

Otherwise equivalent to:

return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));

void deallocate(Tp* p, size_t n);

Preconditions: p was allocated from a memory resource x, equal to *memory_rsrc, using x.allocate(n * sizeof(Tp), alignof(Tp)).

Effects: Equivalent to memory_rsrc->deallocate(p, n * sizeof(Tp), alignof(Tp)).

Throws: Nothing.

[[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));

Effects: Equivalent to: return memory_rsrc->allocate(nbytes, alignment);

The return type is void* (rather than, e.g., byte*) to support conversion to an arbitrary pointer type U* by static_cast<U*>, thus facilitating construction of a U object in the allocated memory.

— end note

]

void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t));

Effects: Equivalent to memory_rsrc->deallocate(p, nbytes, alignment).

template<class T>
  [[nodiscard]] T* allocate_object(size_t n = 1);

Effects: Allocates memory suitable for holding an array of n objects of type T, as follows:

(8.1)
if numeric_limits<size_t>::max() / sizeof(T) < n, throws bad_array_new_length,

(8.2)

otherwise equivalent to:

return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));

T is not deduced and must therefore be provided as a template argument.

— end note

]

template<class T>
  void deallocate_object(T* p, size_t n = 1);

Effects: Equivalent to deallocate_bytes(p, n*sizeof(T), alignof(T)).

template<class T, class CtorArgs...>
  [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);

Effects: Allocates and constructs an object of type T, as follows.

Equivalent to:

T* p = allocate_object<T>();
try {
  construct(p, std::forward<CtorArgs>(ctor_args)...);
} catch (...) {
  deallocate_object(p);
  throw;
}
return p;

T is not deduced and must therefore be provided as a template argument.

— end note

]

template<class T>
  void delete_object(T* p);

Effects: Equivalent to:

destroy(p);
deallocate_object(p);

template<class T, class... Args>
  void construct(T* p, Args&&... args);

Mandates: Uses-allocator construction of T with allocator *this (see [allocator.uses.construction]) and constructor arguments std::forward<Args>(args)... is well-formed.

Effects: Construct a T object in the storage whose address is represented by p by uses-allocator construction with allocator *this and constructor arguments std::forward<Args>(args)....

Throws: Nothing unless the constructor for T throws.

template<class T>
  void destroy(T* p);

Effects: As if by p->~T().

polymorphic_allocator select_on_container_copy_construction() const;

Returns: polymorphic_allocator().

The memory resource is not propagated.

— end note

]

memory_resource* resource() const;

Returns: memory_rsrc.

20.12.3.3 Equality [mem.poly.allocator.eq]

template<class T1, class T2>
  bool operator==(const polymorphic_allocator<T1>& a,
                  const polymorphic_allocator<T2>& b) noexcept;

Returns: *a.resource() == *b.resource().

20.12.4 Access to program-wide memory_resource objects [mem.res.global]

memory_resource* new_delete_resource() noexcept;

Returns: A pointer to a static-duration object of a type derived from memory_resource that can serve as a resource for allocating memory using ::operator new and ::operator delete.

The same value is returned every time this function is called.

For a return value p and a memory resource r, p->is_equal(r) returns &r == p.

memory_resource* null_memory_resource() noexcept;

Returns: A pointer to a static-duration object of a type derived from memory_resource for which allocate() always throws bad_alloc and for which deallocate() has no effect.

The same value is returned every time this function is called.

For a return value p and a memory resource r, p->is_equal(r) returns &r == p.

The default memory resource pointer is a pointer to a memory resource that is used by certain facilities when an explicit memory resource is not supplied through the interface.

Its initial value is the return value of new_delete_resource().

memory_resource* set_default_resource(memory_resource* r) noexcept;

Effects: If r is non-null, sets the value of the default memory resource pointer to r, otherwise sets the default memory resource pointer to new_delete_resource().

Returns: The previous value of the default memory resource pointer.

Remarks: Calling the set_default_resource and get_default_resource functions shall not incur a data race.

A call to the set_default_resource function shall synchronize with subsequent calls to the set_default_resource and get_default_resource functions.

memory_resource* get_default_resource() noexcept;

Returns: The current value of the default memory resource pointer.

20.12.5 Pool resource classes [mem.res.pool]

20.12.5.1 Classes synchronized_pool_resource and unsynchronized_pool_resource [mem.res.pool.overview]

The synchronized_pool_resource and unsynchronized_pool_resource classes (collectively called pool resource classes) are general-purpose memory resources having the following qualities:

(1.1)
Each resource frees its allocated memory on destruction, even if deallocate has not been called for some of the allocated blocks.
(1.2)
A pool resource consists of a collection of pools, serving requests for different block sizes. Each individual pool manages a collection of chunks that are in turn divided into blocks of uniform size, returned via calls to do_allocate. Each call to do_allocate(size, alignment) is dispatched to the pool serving the smallest blocks accommodating at least size bytes.
(1.3)
When a particular pool is exhausted, allocating a block from that pool results in the allocation of an additional chunk of memory from the upstream allocator (supplied at construction), thus replenishing the pool. With each successive replenishment, the chunk size obtained increases geometrically.
[Note
: By allocating memory in chunks, the pooling strategy increases the chance that consecutive allocations will be close together in memory. — end note
]
(1.4)
Allocation requests that exceed the largest block size of any pool are fulfilled directly from the upstream allocator.
(1.5)
A pool_options struct may be passed to the pool resource constructors to tune the largest block size and the maximum chunk size.

A synchronized_pool_resource may be accessed from multiple threads without external synchronization and may have thread-specific pools to reduce synchronization costs.

An unsynchronized_pool_resource class may not be accessed from multiple threads simultaneously and thus avoids the cost of synchronization entirely in single-threaded applications.

namespace std::pmr {
  struct pool_options {
    size_t max_blocks_per_chunk = 0;
    size_t largest_required_pool_block = 0;
  };

  class synchronized_pool_resource : public memory_resource {
  public:
    synchronized_pool_resource(const pool_options& opts, memory_resource* upstream);

    synchronized_pool_resource()
        : synchronized_pool_resource(pool_options(), get_default_resource()) {}
    explicit synchronized_pool_resource(memory_resource* upstream)
        : synchronized_pool_resource(pool_options(), upstream) {}
    explicit synchronized_pool_resource(const pool_options& opts)
        : synchronized_pool_resource(opts, get_default_resource()) {}

    synchronized_pool_resource(const synchronized_pool_resource&) = delete;
    virtual ~synchronized_pool_resource();

    synchronized_pool_resource& operator=(const synchronized_pool_resource&) = delete;

    void release();
    memory_resource* upstream_resource() const;
    pool_options options() const;

  protected:
    void* do_allocate(size_t bytes, size_t alignment) override;
    void do_deallocate(void* p, size_t bytes, size_t alignment) override;

    bool do_is_equal(const memory_resource& other) const noexcept override;
  };

  class unsynchronized_pool_resource : public memory_resource {
  public:
    unsynchronized_pool_resource(const pool_options& opts, memory_resource* upstream);

    unsynchronized_pool_resource()
        : unsynchronized_pool_resource(pool_options(), get_default_resource()) {}
    explicit unsynchronized_pool_resource(memory_resource* upstream)
        : unsynchronized_pool_resource(pool_options(), upstream) {}
    explicit unsynchronized_pool_resource(const pool_options& opts)
        : unsynchronized_pool_resource(opts, get_default_resource()) {}

    unsynchronized_pool_resource(const unsynchronized_pool_resource&) = delete;
    virtual ~unsynchronized_pool_resource();

    unsynchronized_pool_resource& operator=(const unsynchronized_pool_resource&) = delete;

    void release();
    memory_resource* upstream_resource() const;
    pool_options options() const;

  protected:
    void* do_allocate(size_t bytes, size_t alignment) override;
    void do_deallocate(void* p, size_t bytes, size_t alignment) override;

    bool do_is_equal(const memory_resource& other) const noexcept override;
  };
}

20.12.5.2 pool_options data members [mem.res.pool.options]

The members of pool_options comprise a set of constructor options for pool resources.

The effect of each option on the pool resource behavior is described below:

size_t max_blocks_per_chunk;

The maximum number of blocks that will be allocated at once from the upstream memory resource ([mem.res.monotonic.buffer]) to replenish a pool.

If the value of max_blocks_per_chunk is zero or is greater than an implementation-defined limit, that limit is used instead.

The implementation may choose to use a smaller value than is specified in this field and may use different values for different pools.

size_t largest_required_pool_block;

The largest allocation size that is required to be fulfilled using the pooling mechanism.

Attempts to allocate a single block larger than this threshold will be allocated directly from the upstream memory resource.

If largest_required_pool_block is zero or is greater than an implementation-defined limit, that limit is used instead.

The implementation may choose a pass-through threshold larger than specified in this field.

20.12.5.3 Constructors and destructors [mem.res.pool.ctor]

synchronized_pool_resource(const pool_options& opts, memory_resource* upstream);
unsynchronized_pool_resource(const pool_options& opts, memory_resource* upstream);

Preconditions: upstream is the address of a valid memory resource.

Effects: Constructs a pool resource object that will obtain memory from upstream whenever the pool resource is unable to satisfy a memory request from its own internal data structures.

The resulting object will hold a copy of upstream, but will not own the resource to which upstream points.

The intention is that calls to upstream->allocate() will be substantially fewer than calls to this->allocate() in most cases.

— end note

]

The behavior of the pooling mechanism is tuned according to the value of the opts argument.

Throws: Nothing unless upstream->allocate() throws.

It is unspecified if, or under what conditions, this constructor calls upstream->allocate().

virtual ~synchronized_pool_resource();
virtual ~unsynchronized_pool_resource();

Effects: Calls release().

20.12.5.4 Members [mem.res.pool.mem]

void release();

Effects: Calls upstream_resource()->deallocate() as necessary to release all allocated memory.

The memory is released back to upstream_resource() even if deallocate has not been called for some of the allocated blocks.

— end note

]

memory_resource* upstream_resource() const;

Returns: The value of the upstream argument provided to the constructor of this object.

pool_options options() const;

Returns: The options that control the pooling behavior of this resource.

The values in the returned struct may differ from those supplied to the pool resource constructor in that values of zero will be replaced with implementation-defined defaults, and sizes may be rounded to unspecified granularity.

void* do_allocate(size_t bytes, size_t alignment) override;

Returns: A pointer to allocated storage ([basic.stc.dynamic.allocation]) with a size of at least bytes.

The size and alignment of the allocated memory shall meet the requirements for a class derived from memory_resource ([mem.res.class]).

Effects: If the pool selected for a block of size bytes is unable to satisfy the memory request from its own internal data structures, it will call upstream_resource()->allocate() to obtain more memory.

If bytes is larger than that which the largest pool can handle, then memory will be allocated using upstream_resource()->allocate().

Throws: Nothing unless upstream_resource()->allocate() throws.

void do_deallocate(void* p, size_t bytes, size_t alignment) override;

Effects: Returns the memory at p to the pool.

It is unspecified if, or under what circumstances, this operation will result in a call to upstream_resource()->deallocate().

Throws: Nothing.

bool do_is_equal(const memory_resource& other) const noexcept override;

Returns: this == &other.

20.12.6 Class monotonic_buffer_resource [mem.res.monotonic.buffer]

A monotonic_buffer_resource is a special-purpose memory resource intended for very fast memory allocations in situations where memory is used to build up a few objects and then is released all at once when the memory resource object is destroyed.

It has the following qualities:

(1.1)
A call to deallocate has no effect, thus the amount of memory consumed increases monotonically until the resource is destroyed.
(1.2)
The program can supply an initial buffer, which the allocator uses to satisfy memory requests.
(1.3)
When the initial buffer (if any) is exhausted, it obtains additional buffers from an upstream memory resource supplied at construction. Each additional buffer is larger than the previous one, following a geometric progression.
(1.4)
It is intended for access from one thread of control at a time. Specifically, calls to allocate and deallocate do not synchronize with one another.
(1.5)
It frees the allocated memory on destruction, even if deallocate has not been called for some of the allocated blocks.

namespace std::pmr {
  class monotonic_buffer_resource : public memory_resource {
    memory_resource* upstream_rsrc;     // exposition only
    void* current_buffer;               // exposition only
    size_t next_buffer_size;            // exposition only

  public:
    explicit monotonic_buffer_resource(memory_resource* upstream);
    monotonic_buffer_resource(size_t initial_size, memory_resource* upstream);
    monotonic_buffer_resource(void* buffer, size_t buffer_size, memory_resource* upstream);

    monotonic_buffer_resource()
      : monotonic_buffer_resource(get_default_resource()) {}
    explicit monotonic_buffer_resource(size_t initial_size)
      : monotonic_buffer_resource(initial_size, get_default_resource()) {}
    monotonic_buffer_resource(void* buffer, size_t buffer_size)
      : monotonic_buffer_resource(buffer, buffer_size, get_default_resource()) {}

    monotonic_buffer_resource(const monotonic_buffer_resource&) = delete;

    virtual ~monotonic_buffer_resource();

    monotonic_buffer_resource& operator=(const monotonic_buffer_resource&) = delete;

    void release();
    memory_resource* upstream_resource() const;

  protected:
    void* do_allocate(size_t bytes, size_t alignment) override;
    void do_deallocate(void* p, size_t bytes, size_t alignment) override;

    bool do_is_equal(const memory_resource& other) const noexcept override;
  };
}

20.12.6.1 Constructors and destructor [mem.res.monotonic.buffer.ctor]

explicit monotonic_buffer_resource(memory_resource* upstream);
monotonic_buffer_resource(size_t initial_size, memory_resource* upstream);

Preconditions: upstream is the address of a valid memory resource.

initial_size, if specified, is greater than zero.

Effects: Sets upstream_rsrc to upstream and current_buffer to nullptr.

If initial_size is specified, sets next_buffer_size to at least initial_size; otherwise sets next_buffer_size to an implementation-defined size.

monotonic_buffer_resource(void* buffer, size_t buffer_size, memory_resource* upstream);

Preconditions: upstream is the address of a valid memory resource.

buffer_size is no larger than the number of bytes in buffer.

Effects: Sets upstream_rsrc to upstream, current_buffer to buffer, and next_buffer_size to buffer_size (but not less than 1), then increases next_buffer_size by an implementation-defined growth factor (which need not be integral).

~monotonic_buffer_resource();

Effects: Calls release().

20.12.6.2 Members [mem.res.monotonic.buffer.mem]

void release();

Effects: Calls upstream_rsrc->deallocate() as necessary to release all allocated memory.

The memory is released back to upstream_rsrc even if some blocks that were allocated from this have not been deallocated from this.

— end note

]

memory_resource* upstream_resource() const;

Returns: The value of upstream_rsrc.

void* do_allocate(size_t bytes, size_t alignment) override;

Returns: A pointer to allocated storage ([basic.stc.dynamic.allocation]) with a size of at least bytes.

The size and alignment of the allocated memory shall meet the requirements for a class derived from memory_resource ([mem.res.class]).

Effects: If the unused space in current_buffer can fit a block with the specified bytes and alignment, then allocate the return block from current_buffer; otherwise set current_buffer to upstream_rsrc->allocate(n, m), where n is not less than max(bytes, next_buffer_size) and m is not less than alignment, and increase next_buffer_size by an implementation-defined growth factor (which need not be integral), then allocate the return block from the newly-allocated current_buffer.

Throws: Nothing unless upstream_rsrc->allocate() throws.

void do_deallocate(void* p, size_t bytes, size_t alignment) override;

Effects: None.

Throws: Nothing.

Remarks: Memory used by this resource increases monotonically until its destruction.

bool do_is_equal(const memory_resource& other) const noexcept override;

Returns: this == &other.

20.13 Class template scoped_allocator_adaptor [allocator.adaptor]

20.13.1 Header <scoped_allocator> synopsis [allocator.adaptor.syn]

namespace std {
  // class template scoped allocator adaptor
  template<class OuterAlloc, class... InnerAlloc>
    class scoped_allocator_adaptor;

  // [scoped.adaptor.operators], scoped allocator operators
  template<class OuterA1, class OuterA2, class... InnerAllocs>
    bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
                    const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b) noexcept;
}

The class template scoped_allocator_adaptor is an allocator template that specifies an allocator resource (the outer allocator) to be used by a container (as any other allocator does) and also specifies an inner allocator resource to be passed to the constructor of every element within the container.

This adaptor is instantiated with one outer and zero or more inner allocator types.

If instantiated with only one allocator type, the inner allocator becomes the scoped_allocator_adaptor itself, thus using the same allocator resource for the container and every element within the container and, if the elements themselves are containers, each of their elements recursively.

If instantiated with more than one allocator, the first allocator is the outer allocator for use by the container, the second allocator is passed to the constructors of the container's elements, and, if the elements themselves are containers, the third allocator is passed to the elements' elements, and so on.

If containers are nested to a depth greater than the number of allocators, the last allocator is used repeatedly, as in the single-allocator case, for any remaining recursions.

The scoped_allocator_adaptor is derived from the outer allocator type so it can be substituted for the outer allocator type in most expressions.

— end note

]

namespace std {
  template<class OuterAlloc, class... InnerAllocs>
  class scoped_allocator_adaptor : public OuterAlloc {
  private:
    using OuterTraits = allocator_traits<OuterAlloc>;   // exposition only
    scoped_allocator_adaptor<InnerAllocs...> inner;     // exposition only

  public:
    using outer_allocator_type = OuterAlloc;
    using inner_allocator_type = see below;

    using value_type           = typename OuterTraits::value_type;
    using size_type            = typename OuterTraits::size_type;
    using difference_type      = typename OuterTraits::difference_type;
    using pointer              = typename OuterTraits::pointer;
    using const_pointer        = typename OuterTraits::const_pointer;
    using void_pointer         = typename OuterTraits::void_pointer;
    using const_void_pointer   = typename OuterTraits::const_void_pointer;

    using propagate_on_container_copy_assignment = see below;
    using propagate_on_container_move_assignment = see below;
    using propagate_on_container_swap            = see below;
    using is_always_equal                        = see below;

    template<class Tp> struct rebind {
      using other = scoped_allocator_adaptor<
        OuterTraits::template rebind_alloc<Tp>, InnerAllocs...>;
    };

    scoped_allocator_adaptor();
    template<class OuterA2>
      scoped_allocator_adaptor(OuterA2&& outerAlloc,
                               const InnerAllocs&... innerAllocs) noexcept;

    scoped_allocator_adaptor(const scoped_allocator_adaptor& other) noexcept;
    scoped_allocator_adaptor(scoped_allocator_adaptor&& other) noexcept;

    template<class OuterA2>
      scoped_allocator_adaptor(
        const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& other) noexcept;
    template<class OuterA2>
      scoped_allocator_adaptor(
        scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;

    scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&) = default;
    scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&) = default;

    ~scoped_allocator_adaptor();

    inner_allocator_type& inner_allocator() noexcept;
    const inner_allocator_type& inner_allocator() const noexcept;
    outer_allocator_type& outer_allocator() noexcept;
    const outer_allocator_type& outer_allocator() const noexcept;

    [[nodiscard]] pointer allocate(size_type n);
    [[nodiscard]] pointer allocate(size_type n, const_void_pointer hint);
    void deallocate(pointer p, size_type n);
    size_type max_size() const;

    template<class T, class... Args>
      void construct(T* p, Args&&... args);

    template<class T>
      void destroy(T* p);

    scoped_allocator_adaptor select_on_container_copy_construction() const;
  };

  template<class OuterAlloc, class... InnerAllocs>
    scoped_allocator_adaptor(OuterAlloc, InnerAllocs...)
      -> scoped_allocator_adaptor<OuterAlloc, InnerAllocs...>;
}

20.13.2 Member types [allocator.adaptor.types]

using inner_allocator_type = see below;

Type: scoped_allocator_adaptor<OuterAlloc> if sizeof...(InnerAllocs) is zero; otherwise,

scoped_allocator_adaptor<InnerAllocs...>.

using propagate_on_container_copy_assignment = see below;

Type: true_type if allocator_traits<A>::propagate_on_container_copy_assignment::value is true for any A in the set of OuterAlloc and InnerAllocs...; otherwise, false_type.

using propagate_on_container_move_assignment = see below;

Type: true_type if allocator_traits<A>::propagate_on_container_move_assignment::value is true for any A in the set of OuterAlloc and InnerAllocs...; otherwise, false_type.

using propagate_on_container_swap = see below;

Type: true_type if allocator_traits<A>::propagate_on_container_swap::value is true for any A in the set of OuterAlloc and InnerAllocs...; otherwise, false_type.

using is_always_equal = see below;

Type: true_type if allocator_traits<A>::is_always_equal::value is true for every A in the set of OuterAlloc and InnerAllocs...; otherwise, false_type.

20.13.3 Constructors [allocator.adaptor.cnstr]

scoped_allocator_adaptor();

Effects: Value-initializes the OuterAlloc base class and the inner allocator object.

template<class OuterA2>
  scoped_allocator_adaptor(OuterA2&& outerAlloc, const InnerAllocs&... innerAllocs) noexcept;

Constraints: is_constructible_v<OuterAlloc, OuterA2> is true.

Effects: Initializes the OuterAlloc base class with std::forward<OuterA2>(outerAlloc) and inner with innerAllocs... (hence recursively initializing each allocator within the adaptor with the corresponding allocator from the argument list).

scoped_allocator_adaptor(const scoped_allocator_adaptor& other) noexcept;

Effects: Initializes each allocator within the adaptor with the corresponding allocator from other.

scoped_allocator_adaptor(scoped_allocator_adaptor&& other) noexcept;

Effects: Move constructs each allocator within the adaptor with the corresponding allocator from other.

template<class OuterA2>
  scoped_allocator_adaptor(
    const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& other) noexcept;

Constraints: is_constructible_v<OuterAlloc, const OuterA2&> is true.

Effects: Initializes each allocator within the adaptor with the corresponding allocator from other.

template<class OuterA2>
  scoped_allocator_adaptor(scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;

Constraints: is_constructible_v<OuterAlloc, OuterA2> is true.

Effects: Initializes each allocator within the adaptor with the corresponding allocator rvalue from other.

20.13.4 Members [allocator.adaptor.members]

In the construct member functions, OUTERMOST(x) is OUTERMOST(x.outer_allocator()) if the expression x.outer_allocator() is valid ([temp.deduct]) and x otherwise; OUTERMOST_ALLOC_TRAITS(x) is allocator_traits<remove_reference_t<decltype(OUTERMOST(x))>>.

OUTERMOST(x) and OUTERMOST_ALLOC_TRAITS(x) are recursive operations.

It is incumbent upon the definition of outer_allocator() to ensure that the recursion terminates.

It will terminate for all instantiations of scoped_allocator_adaptor.

— end note

]

inner_allocator_type& inner_allocator() noexcept;
const inner_allocator_type& inner_allocator() const noexcept;

Returns: *this if sizeof...(InnerAllocs) is zero; otherwise, inner.

outer_allocator_type& outer_allocator() noexcept;

Returns: static_cast<OuterAlloc&>(*this).

const outer_allocator_type& outer_allocator() const noexcept;

Returns: static_cast<const OuterAlloc&>(*this).

[[nodiscard]] pointer allocate(size_type n);

Returns: allocator_traits<OuterAlloc>::allocate(outer_allocator(), n).

[[nodiscard]] pointer allocate(size_type n, const_void_pointer hint);

Returns: allocator_traits<OuterAlloc>::allocate(outer_allocator(), n, hint).

void deallocate(pointer p, size_type n) noexcept;

Effects: As if by: allocator_traits<OuterAlloc>::deallocate(outer_allocator(), p, n);

size_type max_size() const;

Returns: allocator_traits<OuterAlloc>::max_size(outer_allocator()).

template<class T, class... Args>
  void construct(T* p, Args&&... args);

Effects: Equivalent to:

apply([p, this](auto&&... newargs) {
        OUTERMOST_ALLOC_TRAITS(*this)::construct(
          OUTERMOST(*this), p,
          std::forward<decltype(newargs)>(newargs)...);
      },
      uses_allocator_construction_args<T>(inner_allocator(),
                                          std::forward<Args>(args)...));

template<class T>
  void destroy(T* p);

Effects: Calls OUTERMOST_ALLOC_TRAITS(*this)::destroy(OUTERMOST(*this), p).

scoped_allocator_adaptor select_on_container_copy_construction() const;

Returns: A new scoped_allocator_adaptor object where each allocator A in the adaptor is initialized from the result of calling allocator_traits<A>::select_on_container_copy_construction() on the corresponding allocator in *this.

20.13.5 Operators [scoped.adaptor.operators]

template<class OuterA1, class OuterA2, class... InnerAllocs>
  bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
                  const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b) noexcept;

Returns: If sizeof...(InnerAllocs) is zero,

a.outer_allocator() == b.outer_allocator()

otherwise

a.outer_allocator() == b.outer_allocator() && a.inner_allocator() == b.inner_allocator()

20.14 Function objects [function.objects]

A function object type is an object type that can be the type of the postfix-expression in a function call ([expr.call], [over.match.call]).220

A function object is an object of a function object type.

In the places where one would expect to pass a pointer to a function to an algorithmic template, the interface is specified to accept a function object.

This not only makes algorithmic templates work with pointers to functions, but also enables them to work with arbitrary function objects.

220)

Such a type is a function pointer or a class type which has a member operator() or a class type which has a conversion to a pointer to function.

⮥

20.14.1 Header <functional> synopsis [functional.syn]

namespace std {
  // [func.invoke], invoke
  template<class F, class... Args>
    constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
      noexcept(is_nothrow_invocable_v<F, Args...>);

  // [refwrap], reference_wrapper
  template<class T> class reference_wrapper;

  template<class T> constexpr reference_wrapper<T> ref(T&) noexcept;
  template<class T> constexpr reference_wrapper<const T> cref(const T&) noexcept;
  template<class T> void ref(const T&&) = delete;
  template<class T> void cref(const T&&) = delete;

  template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T>) noexcept;
  template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T>) noexcept;

  // [arithmetic.operations], arithmetic operations
  template<class T = void> struct plus;
  template<class T = void> struct minus;
  template<class T = void> struct multiplies;
  template<class T = void> struct divides;
  template<class T = void> struct modulus;
  template<class T = void> struct negate;
  template<> struct plus<void>;
  template<> struct minus<void>;
  template<> struct multiplies<void>;
  template<> struct divides<void>;
  template<> struct modulus<void>;
  template<> struct negate<void>;

  // [comparisons], comparisons
  template<class T = void> struct equal_to;
  template<class T = void> struct not_equal_to;
  template<class T = void> struct greater;
  template<class T = void> struct less;
  template<class T = void> struct greater_equal;
  template<class T = void> struct less_equal;
  template<> struct equal_to<void>;
  template<> struct not_equal_to<void>;
  template<> struct greater<void>;
  template<> struct less<void>;
  template<> struct greater_equal<void>;
  template<> struct less_equal<void>;

  // [comparisons.three.way], class compare_three_way
  struct compare_three_way;

  // [logical.operations], logical operations
  template<class T = void> struct logical_and;
  template<class T = void> struct logical_or;
  template<class T = void> struct logical_not;
  template<> struct logical_and<void>;
  template<> struct logical_or<void>;
  template<> struct logical_not<void>;

  // [bitwise.operations], bitwise operations
  template<class T = void> struct bit_and;
  template<class T = void> struct bit_or;
  template<class T = void> struct bit_xor;
  template<class T = void> struct bit_not;
  template<> struct bit_and<void>;
  template<> struct bit_or<void>;
  template<> struct bit_xor<void>;
  template<> struct bit_not<void>;

  // [func.identity], identity
  struct identity;

  // [func.not.fn], function template not_fn
  template<class F> constexpr unspecified not_fn(F&& f);

  // [func.bind.front], function template bind_front
  template<class F, class... Args> constexpr unspecified bind_front(F&&, Args&&...);

  // [func.bind], bind
  template<class T> struct is_bind_expression;
  template<class T>
    inline constexpr bool is_bind_expression_v = is_bind_expression<T>::value;
  template<class T> struct is_placeholder;
  template<class T>
    inline constexpr int is_placeholder_v = is_placeholder<T>::value;

  template<class F, class... BoundArgs>
    constexpr unspecified bind(F&&, BoundArgs&&...);
  template<class R, class F, class... BoundArgs>
    constexpr unspecified bind(F&&, BoundArgs&&...);

  namespace placeholders {
    // M is the implementation-defined number of placeholders
    see below _1;
    see below _2;
               .
               .
               .
    see below _M;
  }

  // [func.memfn], member function adaptors
  template<class R, class T>
    constexpr unspecified mem_fn(R T::*) noexcept;

  // [func.wrap], polymorphic function wrappers
  class bad_function_call;

  template<class> class function;       // not defined
  template<class R, class... ArgTypes> class function<R(ArgTypes...)>;

  template<class R, class... ArgTypes>
    void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;

  template<class R, class... ArgTypes>
    bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;

  // [func.search], searchers
  template<class ForwardIterator, class BinaryPredicate = equal_to<>>
    class default_searcher;

  template<class RandomAccessIterator,
           class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>,
           class BinaryPredicate = equal_to<>>
    class boyer_moore_searcher;

  template<class RandomAccessIterator,
           class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>,
           class BinaryPredicate = equal_to<>>
    class boyer_moore_horspool_searcher;

  // [unord.hash], class template hash
  template<class T>
    struct hash;

  namespace ranges {
    // [range.cmp], concept-constrained comparisons
    struct equal_to;
    struct not_equal_to;
    struct greater;
    struct less;
    struct greater_equal;
    struct less_equal;
  }
}

If a C++ program wants to have a by-element addition of two vectors a and b containing double and put the result into a, it can do:

transform(a.begin(), a.end(), b.begin(), a.begin(), plus<double>());

— end example

]

To negate every element of a:

transform(a.begin(), a.end(), a.begin(), negate<double>());

— end example

]

20.14.2 Definitions [func.def]

The following definitions apply to this Clause:

A call signature is the name of a return type followed by a parenthesized comma-separated list of zero or more argument types.

A callable type is a function object type or a pointer to member.

A callable object is an object of a callable type.

A call wrapper type is a type that holds a callable object and supports a call operation that forwards to that object.

A call wrapper is an object of a call wrapper type.

A target object is the callable object held by a call wrapper.

A call wrapper type may additionally hold a sequence of objects and references that may be passed as arguments to the target object.

These entities are collectively referred to as bound argument entities.

The target object and bound argument entities of the call wrapper are collectively referred to as state entities.

20.14.3 Requirements [func.require]

Define INVOKE(f, t

_{1}

, t

_{2}

, …, t

_{N}

) as follows:

(1.1)
(t $_{1}$ .*f)(t $_{2}$ , …, t $_{N}$ ) when f is a pointer to a member function of a class T and is_base_of_v<T, remove_reference_t<decltype(t $_{1}$ )>> is true;
(1.2)
(t $_{1}$ .get().*f)(t $_{2}$ , …, t $_{N}$ ) when f is a pointer to a member function of a class T and remove_cvref_t<decltype(t $_{1}$ )> is a specialization of reference_wrapper;
(1.3)
((*t $_{1}$ ).*f)(t $_{2}$ , …, t $_{N}$ ) when f is a pointer to a member function of a class T and t $_{1}$ does not satisfy the previous two items;
(1.4)
t $_{1}$ .*f when N == 1 and f is a pointer to data member of a class T and is_base_of_v<T, remove_reference_t<decltype(t $_{1}$ )>> is true;
(1.5)
t $_{1}$ .get().*f when N == 1 and f is a pointer to data member of a class T and remove_cvref_t<decltype(t $_{1}$ )> is a specialization of reference_wrapper;
(1.6)
(*t $_{1}$ ).*f when N == 1 and f is a pointer to data member of a class T and t $_{1}$ does not satisfy the previous two items;
(1.7)
f(t $_{1}$ , t $_{2}$ , …, t $_{N}$ ) in all other cases.

Define INVOKE<R>(f, t

_{1}

, t

_{2}

, …, t

_{N}

) as static_cast<void>(INVOKE(f, t

_{1}

, t

_{2}

, …, t

_{N}

)) if R is cv void, otherwise INVOKE(f, t

_{1}

, t

_{2}

, …, t

_{N}

) implicitly converted to R.

Every call wrapper ([func.def]) meets the Cpp17MoveConstructible and Cpp17Destructible requirements.

An argument forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments to the wrapped callable object as references.

This forwarding step delivers rvalue arguments as rvalue references and lvalue arguments as lvalue references.

In a typical implementation, argument forwarding call wrappers have an overloaded function call operator of the form

template<class... UnBoundArgs>
  constexpr R operator()(UnBoundArgs&&... unbound_args) cv-qual;

— end note

]

A perfect forwarding call wrapper is an argument forwarding call wrapper that forwards its state entities to the underlying call expression.

This forwarding step delivers a state entity of type T as cv T& when the call is performed on an lvalue of the call wrapper type and as cv T&& otherwise, where cv represents the cv-qualifiers of the call wrapper and where cv shall be neither volatile nor const volatile.

A call pattern defines the semantics of invoking a perfect forwarding call wrapper.

A postfix call performed on a perfect forwarding call wrapper is expression-equivalent ([defns.expression-equivalent]) to an expression e determined from its call pattern cp by replacing all occurrences of the arguments of the call wrapper and its state entities with references as described in the corresponding forwarding steps.

A simple call wrapper is a perfect forwarding call wrapper that meets the Cpp17CopyConstructible and Cpp17CopyAssignable requirements and whose copy constructor, move constructor, and assignment operators are constexpr functions that do not throw exceptions.

The copy/move constructor of an argument forwarding call wrapper has the same apparent semantics as if memberwise copy/move of its state entities were performed ([class.copy.ctor]).

This implies that each of the copy/move constructors has the same exception-specification as the corresponding implicit definition and is declared as constexpr if the corresponding implicit definition would be considered to be constexpr.

— end note

]

Argument forwarding call wrappers returned by a given standard library function template have the same type if the types of their corresponding state entities are the same.

20.14.4 Function template invoke [func.invoke]

template<class F, class... Args>
  constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
    noexcept(is_nothrow_invocable_v<F, Args...>);

Returns: INVOKE(std::forward<F>(f), std::forward<Args>(args)...).

20.14.5 Class template reference_wrapper [refwrap]

namespace std {
  template<class T> class reference_wrapper {
  public:
    // types
    using type = T;

    // construct/copy/destroy
    template<class U>
      constexpr reference_wrapper(U&&) noexcept(see below);
    constexpr reference_wrapper(const reference_wrapper& x) noexcept;

    // assignment
    constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;

    // access
    constexpr operator T& () const noexcept;
    constexpr T& get() const noexcept;

    // invocation
    template<class... ArgTypes>
      constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const;
  };

  template<class T>
    reference_wrapper(T&) -> reference_wrapper<T>;
}

reference_wrapper<T> is a Cpp17CopyConstructible and Cpp17CopyAssignable wrapper around a reference to an object or function of type T.

reference_wrapper<T> is a trivially copyable type .

The template parameter T of reference_wrapper may be an incomplete type.

20.14.5.1 Constructors and destructor [refwrap.const]

template<class U>
  constexpr reference_wrapper(U&& u) noexcept(see below);

Let FUN denote the exposition-only functions

void FUN(T&) noexcept;
void FUN(T&&) = delete;

Constraints: The expression FUN(declval()) is well-formed and is_same_v<remove_cvref_t, reference_wrapper> is false.

Effects: Creates a variable r as if by T& r = std::forward(u), then constructs a reference_wrapper object that stores a reference to r.

Remarks: The expression inside noexcept is equivalent to noexcept(FUN(declval())).

constexpr reference_wrapper(const reference_wrapper& x) noexcept;

Effects: Constructs a reference_wrapper object that stores a reference to x.get().

20.14.5.2 Assignment [refwrap.assign]

constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;

Postconditions: *this stores a reference to x.get().

20.14.5.3 Access [refwrap.access]

constexpr operator T& () const noexcept;

Returns: The stored reference.

constexpr T& get() const noexcept;

Returns: The stored reference.

20.14.5.4 Invocation [refwrap.invoke]

template<class... ArgTypes>
  constexpr invoke_result_t<T&, ArgTypes...>
    operator()(ArgTypes&&... args) const;

Mandates: T is a complete type.

Returns: INVOKE(get(), std::forward<ArgTypes>(args)...).

([func.require])

20.14.5.5 Helper functions [refwrap.helpers]

The template parameter T of the following ref and cref function templates may be an incomplete type.

template<class T> constexpr reference_wrapper<T> ref(T& t) noexcept;

Returns: reference_wrapper<T>(t).

template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;

Returns: ref(t.get()).

template<class T> constexpr reference_wrapper<const T> cref(const T& t) noexcept;

Returns: reference_wrapper <const T>(t).

template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;

Returns: cref(t.get()).

20.14.6 Arithmetic operations [arithmetic.operations]

The library provides basic function object classes for all of the arithmetic operators in the language ([expr.mul], [expr.add]).

20.14.6.1 Class template plus [arithmetic.operations.plus]

template<class T = void> struct plus {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x + y.

template<> struct plus<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) + std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) + std::forward<U>(u));

Returns: std::forward<T>(t) + std::forward(u).

20.14.6.2 Class template minus [arithmetic.operations.minus]

template<class T = void> struct minus {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x - y.

template<> struct minus<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) - std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) - std::forward<U>(u));

Returns: std::forward<T>(t) - std::forward(u).

20.14.6.3 Class template multiplies [arithmetic.operations.multiplies]

template<class T = void> struct multiplies {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x * y.

template<> struct multiplies<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) * std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) * std::forward<U>(u));

Returns: std::forward<T>(t) * std::forward(u).

20.14.6.4 Class template divides [arithmetic.operations.divides]

template<class T = void> struct divides {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x / y.

template<> struct divides<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) / std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) / std::forward<U>(u));

Returns: std::forward<T>(t) / std::forward(u).

20.14.6.5 Class template modulus [arithmetic.operations.modulus]

template<class T = void> struct modulus {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x % y.

template<> struct modulus<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) % std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) % std::forward<U>(u));

Returns: std::forward<T>(t) % std::forward(u).

20.14.6.6 Class template negate [arithmetic.operations.negate]

template<class T = void> struct negate {
  constexpr T operator()(const T& x) const;
};

constexpr T operator()(const T& x) const;

Returns: -x.

template<> struct negate<void> {
  template<class T> constexpr auto operator()(T&& t) const
    -> decltype(-std::forward<T>(t));

  using is_transparent = unspecified;
};

template<class T> constexpr auto operator()(T&& t) const
    -> decltype(-std::forward<T>(t));

Returns: -std::forward<T>(t).

20.14.7 Comparisons [comparisons]

The library provides basic function object classes for all of the comparison operators in the language ([expr.rel], [expr.eq]).

For templates less, greater, less_equal, and greater_equal, the specializations for any pointer type yield a result consistent with the implementation-defined strict total order over pointers ([defns.order.ptr]).

If a ()(a, b), (a > b) == greater()(a, b), and so forth.

— end note

]

For template specializations less<void>, greater<void>, less_equal<void>, and greater_equal<void>, if the call operator calls a built-in operator comparing pointers, the call operator yields a result consistent with the implementation-defined strict total order over pointers.

20.14.7.1 Class template equal_to [comparisons.equal.to]

template<class T = void> struct equal_to {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x == y.

template<> struct equal_to<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) == std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) == std::forward<U>(u));

Returns: std::forward<T>(t) == std::forward(u).

20.14.7.2 Class template not_equal_to [comparisons.not.equal.to]

template<class T = void> struct not_equal_to {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x != y.

template<> struct not_equal_to<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) != std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) != std::forward<U>(u));

Returns: std::forward<T>(t) != std::forward(u).

20.14.7.3 Class template greater [comparisons.greater]

template<class T = void> struct greater {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x > y.

template<> struct greater<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) > std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) > std::forward<U>(u));

Returns: std::forward<T>(t) > std::forward(u).

20.14.7.4 Class template less [comparisons.less]

template<class T = void> struct less {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x < y.

template<> struct less<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));

Returns: std::forward<T>(t) < std::forward(u).

20.14.7.5 Class template greater_equal [comparisons.greater.equal]

template<class T = void> struct greater_equal {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x >= y.

template<> struct greater_equal<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) >= std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) >= std::forward<U>(u));

Returns: std::forward<T>(t) >= std::forward(u).

20.14.7.6 Class template less_equal [comparisons.less.equal]

template<class T = void> struct less_equal {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x <= y.

template<> struct less_equal<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) <= std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) <= std::forward<U>(u));

Returns: std::forward<T>(t) <= std::forward(u).

20.14.7.7 Class compare_three_way [comparisons.three.way]

In this subclause, BUILTIN-PTR-THREE-WAY(T, U) for types T and U is a boolean constant expression.

BUILTIN-PTR-THREE-WAY(T, U) is true if and only if <=> in the expression

declval<T>() <=> declval<U>()

resolves to a built-in operator comparing pointers.

struct compare_three_way {
  template<class T, class U>
    requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U)
  constexpr auto operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

template<class T, class U>
  requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U)
constexpr auto operator()(T&& t, U&& u) const;

Preconditions: If the expression std::forward<T>(t) <=> std::forward(u) results in a call to a built-in operator <=> comparing pointers of type P, the conversion sequences from both T and U to P are equality-preserving ([concepts.equality]).

Effects:

(3.1)
If the expression std::forward<T>(t) <=> std::forward(u) results in a call to a built-in operator <=> comparing pointers of type P, returns strong_ordering::less if (the converted value of) t precedes u in the implementation-defined strict total order over pointers ([defns.order.ptr]), strong_ordering::greater if u precedes t, and otherwise strong_ordering::equal.
(3.2)
Otherwise, equivalent to: return std::forward<T>(t) <=> std::forward(u);

20.14.8 Concept-constrained comparisons [range.cmp]

In this subclause, BUILTIN-PTR-CMP(T, op, U) for types T and U and where op is an equality ([expr.eq]) or relational operator ([expr.rel]) is a boolean constant expression.

BUILTIN-PTR-CMP(T, op, U) is true if and only if op in the expression declval<T>() op declval() resolves to a built-in operator comparing pointers.

struct ranges::equal_to {
  template<class T, class U>
    requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

Preconditions: If the expression std::forward<T>(t) == std::forward(u) results in a call to a built-in operator == comparing pointers of type P, the conversion sequences from both T and U to P are equality-preserving ([concepts.equality]).

Effects:

(3.1)
If the expression std::forward<T>(t) == std::forward(u) results in a call to a built-in operator == comparing pointers: returns false if either (the converted value of) t precedes u or u precedes t in the implementation-defined strict total order over pointers ([defns.order.ptr]) and otherwise true.
(3.2)
Otherwise, equivalent to: return std::forward<T>(t) == std::forward(u);

struct ranges::not_equal_to {
  template<class T, class U>
    requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to:

return !ranges::equal_to{}(std::forward<T>(t), std::forward<U>(u));

struct ranges::greater {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(U, <, T)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to:

return ranges::less{}(std::forward<U>(u), std::forward<T>(t));

struct ranges::less {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

Preconditions: If the expression std::forward<T>(t) < std::forward(u) results in a call to a built-in operator < comparing pointers of type P, the conversion sequences from both T and U to P are equality-preserving ([concepts.equality]).

For any expressions ET and EU such that decltype((ET)) is T and decltype((EU)) is U, exactly one of ranges::less{}(ET, EU), ranges::less{}(EU, ET), or ranges::equal_to{}(ET, EU) is true.

Effects:

(7.1)
If the expression std::forward<T>(t) < std::forward(u) results in a call to a built-in operator < comparing pointers: returns true if (the converted value of) t precedes u in the implementation-defined strict total order over pointers ([defns.order.ptr]) and otherwise false.
(7.2)
Otherwise, equivalent to: return std::forward<T>(t) < std::forward(u);

struct ranges::greater_equal {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to:

return !ranges::less{}(std::forward<T>(t), std::forward<U>(u));

struct ranges::less_equal {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(U, <, T)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to:

return !ranges::less{}(std::forward<U>(u), std::forward<T>(t));

20.14.9 Logical operations [logical.operations]

The library provides basic function object classes for all of the logical operators in the language ([expr.log.and], [expr.log.or], [expr.unary.op]).

20.14.9.1 Class template logical_and [logical.operations.and]

template<class T = void> struct logical_and {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x && y.

template<> struct logical_and<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) && std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) && std::forward<U>(u));

Returns: std::forward<T>(t) && std::forward(u).

20.14.9.2 Class template logical_or [logical.operations.or]

template<class T = void> struct logical_or {
  constexpr bool operator()(const T& x, const T& y) const;
};

constexpr bool operator()(const T& x, const T& y) const;

Returns: x || y.

template<> struct logical_or<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) || std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) || std::forward<U>(u));

Returns: std::forward<T>(t) || std::forward(u).

20.14.9.3 Class template logical_not [logical.operations.not]

template<class T = void> struct logical_not {
  constexpr bool operator()(const T& x) const;
};

constexpr bool operator()(const T& x) const;

Returns: !x.

template<> struct logical_not<void> {
  template<class T> constexpr auto operator()(T&& t) const
    -> decltype(!std::forward<T>(t));

  using is_transparent = unspecified;
};

template<class T> constexpr auto operator()(T&& t) const
    -> decltype(!std::forward<T>(t));

Returns: !std::forward<T>(t).

20.14.10 Bitwise operations [bitwise.operations]

The library provides basic function object classes for all of the bitwise operators in the language ([expr.bit.and], [expr.or], [expr.xor], [expr.unary.op]).

20.14.10.1 Class template bit_and [bitwise.operations.and]

template<class T = void> struct bit_and {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x & y.

template<> struct bit_and<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) & std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) & std::forward<U>(u));

Returns: std::forward<T>(t) & std::forward(u).

20.14.10.2 Class template bit_or [bitwise.operations.or]

template<class T = void> struct bit_or {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x | y.

template<> struct bit_or<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) | std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) | std::forward<U>(u));

Returns: std::forward<T>(t) | std::forward(u).

20.14.10.3 Class template bit_xor [bitwise.operations.xor]

template<class T = void> struct bit_xor {
  constexpr T operator()(const T& x, const T& y) const;
};

constexpr T operator()(const T& x, const T& y) const;

Returns: x ^ y.

template<> struct bit_xor<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) ^ std::forward<U>(u));

  using is_transparent = unspecified;
};

template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) ^ std::forward<U>(u));

Returns: std::forward<T>(t) ^ std::forward(u).

20.14.10.4 Class template bit_not [bitwise.operations.not]

template<class T = void> struct bit_not {
  constexpr T operator()(const T& x) const;
};

constexpr T operator()(const T& x) const;

Returns: ~x.

template<> struct bit_not<void> {
  template<class T> constexpr auto operator()(T&& t) const
    -> decltype(~std::forward<T>(t));

  using is_transparent = unspecified;
};

template<class T> constexpr auto operator()(T&&) const
    -> decltype(~std::forward<T>(t));

Returns: ~std::forward<T>(t).

20.14.11 Class identity [func.identity]

struct identity {
  template<class T>
    constexpr T&& operator()(T&& t) const noexcept;

  using is_transparent = unspecified;
};

template<class T>
  constexpr T&& operator()(T&& t) const noexcept;

Effects: Equivalent to: return std::forward<T>(t);

20.14.12 Function template not_fn [func.not.fn]

template<class F> constexpr unspecified not_fn(F&& f);

In the text that follows:

(1.1)
g is a value of the result of a not_fn invocation,
(1.2)
FD is the type decay_t<F>,
(1.3)
fd is the target object of g ([func.def]) of type FD, direct-non-list-initialized with std::forward<F>(f),
(1.4)
call_args is an argument pack used in a function call expression ([expr.call]) of g.

Mandates: is_constructible_v<FD, F> && is_move_constructible_v<FD> is true.

Preconditions: FD meets the Cpp17MoveConstructible requirements.

Returns: A perfect forwarding call wrapper g with call pattern !invoke(fd, call_args...).

Throws: Any exception thrown by the initialization of fd.

20.14.13 Function template bind_front [func.bind.front]

template<class F, class... Args>
  constexpr unspecified bind_front(F&& f, Args&&... args);

In the text that follows:

(1.1)
g is a value of the result of a bind_front invocation,
(1.2)
FD is the type decay_t<F>,
(1.3)
fd is the target object of g ([func.def]) of type FD, direct-non-list-initialized with std::forward<F>(f),
(1.4)
BoundArgs is a pack that denotes decay_t<Args>...,
(1.5)
bound_args is a pack of bound argument entities of g ([func.def]) of types BoundArgs..., direct-non-list-initialized with std::forward<Args>(args)..., respectively, and
(1.6)
call_args is an argument pack used in a function call expression ([expr.call]) of g.

Mandates:

is_constructible_v<FD, F> &&
is_move_constructible_v<FD> &&
(is_constructible_v<BoundArgs, Args> && ...) &&
(is_move_constructible_v<BoundArgs> && ...)

is true.

Preconditions: FD meets the Cpp17MoveConstructible requirements.

For each

T_{i}

in BoundArgs, if

T_{i}

is an object type,

T_{i}

meets the Cpp17MoveConstructible requirements.

Returns: A perfect forwarding call wrapper g with call pattern invoke(fd, bound_args..., call_args...).

Throws: Any exception thrown by the initialization of the state entities of g ([func.def]).

20.14.14 Function object binders [func.bind]

This subclause describes a uniform mechanism for binding arguments of callable objects.

20.14.14.1 Class template is_bind_expression [func.bind.isbind]

namespace std {
  template<class T> struct is_bind_expression;  // see below
}

The class template is_bind_expression can be used to detect function objects generated by bind.

The function template bind uses is_bind_expression to detect subexpressions.

Specializations of the is_bind_expression template shall meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]).

The implementation provides a definition that has a base characteristic of true_type if T is a type returned from bind, otherwise it has a base characteristic of false_type.

A program may specialize this template for a program-defined type T to have a base characteristic of true_type to indicate that T should be treated as a subexpression in a bind call.

20.14.14.2 Class template is_placeholder [func.bind.isplace]

namespace std {
  template<class T> struct is_placeholder;      // see below
}

The class template is_placeholder can be used to detect the standard placeholders _1, _2, and so on.

The function template bind uses is_placeholder to detect placeholders.

Specializations of the is_placeholder template shall meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]).

The implementation provides a definition that has the base characteristic of integral_constant<int, J> if T is the type of std::placeholders::_J, otherwise it has a base characteristic of integral_constant<int, 0>.

A program may specialize this template for a program-defined type T to have a base characteristic of integral_constant<int, N> with N > 0 to indicate that T should be treated as a placeholder type.

20.14.14.3 Function template bind [func.bind.bind]

In the text that follows:

(1.1)
g is a value of the result of a bind invocation,
(1.2)
FD is the type decay_t<F>,
(1.3)
fd is an lvalue that is a target object of g ([func.def]) of type FD direct-non-list-initialized with std::forward<F>(f),
(1.4)
$T_{i}$ is the $i^{th}$ type in the template parameter pack BoundArgs,
(1.5)
${TD}_{i}$ is the type decay_t< $T_{i}$ >,
(1.6)
$t_{i}$ is the $i^{th}$ argument in the function parameter pack bound_args,
(1.7)
${td}_{i}$ is a bound argument entity of g ([func.def]) of type ${TD}_{i}$ direct-non-list-initialized with std::forward< $T_{i}$ >( $t_{i}$ ),
(1.8)
$U_{j}$ is the $j^{th}$ deduced type of the UnBoundArgs&&... parameter of the argument forwarding call wrapper, and
(1.9)
$u_{j}$ is the $j^{th}$ argument associated with $U_{j}$ .

template<class F, class... BoundArgs>
  constexpr unspecified bind(F&& f, BoundArgs&&... bound_args);
template<class R, class F, class... BoundArgs>
  constexpr unspecified bind(F&& f, BoundArgs&&... bound_args);

Mandates: is_constructible_v<FD, F> is true.

For each

T_{i}

in BoundArgs, is_constructible_v<

{TD}_{i}

T_{i}

> is true.

Preconditions: FD and each

{TD}_{i}

meet the Cpp17MoveConstructible and Cpp17Destructible requirements.

INVOKE(fd,

w_{1}

w_{2}

, …,

w_{N}

) ([func.require]) is a valid expression for some values

w_{1}

w_{2}

, …,

w_{N}

, where N has the value sizeof...(bound_args).

Returns: An argument forwarding call wrapper g ([func.require]).

A program that attempts to invoke a volatile-qualified g is ill-formed.

When g is not volatile-qualified, invocation of g(

u_{1}

u_{2}

, …,

u_{M}

) is expression-equivalent ([defns.expression-equivalent]) to

INVOKE(static_cast< $V_{fd}$ >( $v_{fd}$ ),
       static_cast< $V_{1}$ >( $v_{1}$ ), static_cast< $V_{2}$ >( $v_{2}$ ), …, static_cast< $V_{N}$ >( $v_{N}$ ))

for the first overload, and

INVOKE<R>(static_cast< $V_{fd}$ >( $v_{fd}$ ),
          static_cast< $V_{1}$ >( $v_{1}$ ), static_cast< $V_{2}$ >( $v_{2}$ ), …, static_cast< $V_{N}$ >( $v_{N}$ ))

for the second overload, where the values and types of the target argument

v_{fd}

and of the bound arguments

v_{1}

v_{2}

, …,

v_{N}

are determined as specified below.

Throws: Any exception thrown by the initialization of the state entities of g.

If all of FD and

{TD}_{i}

meet the requirements of Cpp17CopyConstructible, then the return type meets the requirements of Cpp17CopyConstructible.

— end note

]

The values of the bound arguments

v_{1}

v_{2}

, …,

v_{N}

and their corresponding types

V_{1}

V_{2}

, …,

V_{N}

depend on the types

{TD}_{i}

derived from the call to bind and the cv-qualifiers cv of the call wrapper g as follows:

(7.1)
if ${TD}_{i}$ is reference_wrapper<T>, the argument is ${td}_{i}$ .get() and its type $V_{i}$ is T&;
(7.2)
if the value of is_bind_expression_v< ${TD}_{i}$ > is true, the argument is
```
static_cast<cv ${TD}_{i}$ &>( ${td}_{i}$ )(std::forward< $U_{j}$ >( $u_{j}$ )...)
```
and its type $V_{i}$ is invoke_result_t<cv ${TD}_{i}$ &, $U_{j}$ ...>&&;
(7.3)
if the value j of is_placeholder_v< ${TD}_{i}$ > is not zero, the argument is std::forward< $U_{j}$ >( $u_{j}$ ) and its type $V_{i}$ is $U_{j}$ &&;
(7.4)
otherwise, the value is ${td}_{i}$ and its type $V_{i}$ is cv ${TD}_{i}$ &.

The value of the target argument

v_{fd}

is fd and its corresponding type

V_{fd}

is cv FD&.

20.14.14.4 Placeholders [func.bind.place]

namespace std::placeholders {
  // M is the implementation-defined number of placeholders
  see below _1;
  see below _2;
              .
              .
              .
  see below _M;
}

All placeholder types meet the Cpp17DefaultConstructible and Cpp17CopyConstructible requirements, and their default constructors and copy/move constructors are constexpr functions that do not throw exceptions.

It is implementation-defined whether placeholder types meet the Cpp17CopyAssignable requirements, but if so, their copy assignment operators are constexpr functions that do not throw exceptions.

Placeholders should be defined as:

inline constexpr unspecified _1{};

If they are not, they are declared as:

extern unspecified _1;

20.14.15 Function template mem_fn [func.memfn]

template<class R, class T> constexpr unspecified mem_fn(R T::* pm) noexcept;

Returns: A simple call wrapper ([func.def]) fn with call pattern invoke(pmd, call_args...), where pmd is the target object of fn of type R T::* direct-non-list-initialized with pm, and call_args is an argument pack used in a function call expression ([expr.call]) of pm.

20.14.16 Polymorphic function wrappers [func.wrap]

This subclause describes a polymorphic wrapper class that encapsulates arbitrary callable objects.

20.14.16.1 Class bad_function_call [func.wrap.badcall]

An exception of type bad_function_call is thrown by function::operator() ([func.wrap.func.inv]) when the function wrapper object has no target.

namespace std {
  class bad_function_call : public exception {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}

const char* what() const noexcept override;

Returns: An implementation-defined ntbs.

20.14.16.2 Class template function [func.wrap.func]

namespace std {
  template<class> class function;       // not defined

  template<class R, class... ArgTypes>
  class function<R(ArgTypes...)> {
  public:
    using result_type = R;

    // [func.wrap.func.con], construct/copy/destroy
    function() noexcept;
    function(nullptr_t) noexcept;
    function(const function&);
    function(function&&) noexcept;
    template<class F> function(F);

    function& operator=(const function&);
    function& operator=(function&&);
    function& operator=(nullptr_t) noexcept;
    template<class F> function& operator=(F&&);
    template<class F> function& operator=(reference_wrapper<F>) noexcept;

    ~function();

    // [func.wrap.func.mod], function modifiers
    void swap(function&) noexcept;

    // [func.wrap.func.cap], function capacity
    explicit operator bool() const noexcept;

    // [func.wrap.func.inv], function invocation
    R operator()(ArgTypes...) const;

    // [func.wrap.func.targ], function target access
    const type_info& target_type() const noexcept;
    template<class T>       T* target() noexcept;
    template<class T> const T* target() const noexcept;
  };

  template<class R, class... ArgTypes>
    function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>;

  template<class F> function(F) -> function<see below>;

  // [func.wrap.func.nullptr], null pointer comparison functions
  template<class R, class... ArgTypes>
    bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;

  // [func.wrap.func.alg], specialized algorithms
  template<class R, class... ArgTypes>
    void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;
}

The function class template provides polymorphic wrappers that generalize the notion of a function pointer.

Wrappers can store, copy, and call arbitrary callable objects, given a call signature, allowing functions to be first-class objects.

A callable type F is Lvalue-Callable for argument types ArgTypes and return type R if the expression INVOKE<R>(declval<F&>(), declval<ArgTypes>()...), considered as an unevaluated operand, is well-formed ([func.require]).

The function class template is a call wrapper whose call signature is R(ArgTypes...).

The types deduced by the deduction guides for function may change in future versions of this International Standard.

— end note

]

20.14.16.2.1 Constructors and destructor [func.wrap.func.con]

function() noexcept;

Postconditions: !*this.

function(nullptr_t) noexcept;

Postconditions: !*this.

function(const function& f);

Postconditions: !*this if !f; otherwise, *this targets a copy of f.target().

Throws: Nothing if f's target is a specialization of reference_wrapper or a function pointer.

Otherwise, may throw bad_alloc or any exception thrown by the copy constructor of the stored callable object.

Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where f's target is an object holding only a pointer or reference to an object and a member function pointer.

— end note

]

function(function&& f) noexcept;

Postconditions: If !f, *this has no target; otherwise, the target of *this is equivalent to the target of f before the construction, and f is in a valid state with an unspecified value.

— end note

]

template<class F> function(F f);

Constraints: F is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.

Preconditions: F meets the Cpp17CopyConstructible requirements.

Postconditions: !*this if any of the following hold:

(9.1)
f is a null function pointer value.
(9.2)
f is a null member pointer value.
(9.3)
F is an instance of the function class template, and !f.

Otherwise, *this targets a copy of f initialized with std::move(f).

Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where f is an object holding only a pointer or reference to an object and a member function pointer.

— end note

]

Throws: Nothing if f is a specialization of reference_wrapper or a function pointer.

Otherwise, may throw bad_alloc or any exception thrown by F's copy or move constructor.

template<class F> function(F) -> function<see below>;

Constraints: &F::operator() is well-formed when treated as an unevaluated operand and decltype(&F::operator()) is of the form R(G::*)(A...) cv &

_{o p t}

noexcept

_{o p t}

for a class type G.

Remarks: The deduced type is function<R(A...)>.

void f() {
  int i{5};
  function g = [&](double) { return i; };       // deduces function<int(double)>
}

— end example

]

function& operator=(const function& f);

Effects: As if by function(f).swap(*this);

Returns: *this.

function& operator=(function&& f);

Effects: Replaces the target of *this with the target of f.

Returns: *this.

function& operator=(nullptr_t) noexcept;

Effects: If *this != nullptr, destroys the target of this.

Postconditions: !(*this).

Returns: *this.

template<class F> function& operator=(F&& f);

Constraints: decay_t<F> is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.

Effects: As if by: function(std::forward<F>(f)).swap(*this);

Returns: *this.

template<class F> function& operator=(reference_wrapper<F> f) noexcept;

Effects: As if by: function(f).swap(*this);

Returns: *this.

~function();

Effects: If *this != nullptr, destroys the target of this.

20.14.16.2.2 Modifiers [func.wrap.func.mod]

void swap(function& other) noexcept;

Effects: Interchanges the targets of *this and other.

20.14.16.2.3 Capacity [func.wrap.func.cap]

explicit operator bool() const noexcept;

Returns: true if *this has a target, otherwise false.

20.14.16.2.4 Invocation [func.wrap.func.inv]

R operator()(ArgTypes... args) const;

Returns: INVOKE<R>(f, std::forward<ArgTypes>(args)...) ([func.require]), where f is the target object of *this.

Throws: bad_function_call if !*this; otherwise, any exception thrown by the wrapped callable object.

20.14.16.2.5 Target access [func.wrap.func.targ]

const type_info& target_type() const noexcept;

Returns: If *this has a target of type T, typeid(T); otherwise, typeid(void).

template<class T>       T* target() noexcept;
template<class T> const T* target() const noexcept;

Returns: If target_type() == typeid(T) a pointer to the stored function target; otherwise a null pointer.

20.14.16.2.6 Null pointer comparison functions [func.wrap.func.nullptr]

template<class R, class... ArgTypes>
  bool operator==(const function<R(ArgTypes...)>& f, nullptr_t) noexcept;

Returns: !f.

20.14.16.2.7 Specialized algorithms [func.wrap.func.alg]

template<class R, class... ArgTypes>
  void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;

Effects: As if by: f1.swap(f2);

20.14.17 Searchers [func.search]

This subclause provides function object types for operations that search for a sequence [pat_first, pat_last) in another sequence [first, last) that is provided to the object's function call operator.

The first sequence (the pattern to be searched for) is provided to the object's constructor, and the second (the sequence to be searched) is provided to the function call operator.

Each specialization of a class template specified in this subclause [func.search] shall meet the Cpp17CopyConstructible and Cpp17CopyAssignable requirements.

Template parameters named

(2.1)
ForwardIterator,
(2.2)
ForwardIterator1,
(2.3)
ForwardIterator2,
(2.4)
RandomAccessIterator,
(2.5)
RandomAccessIterator1,
(2.6)
RandomAccessIterator2, and
(2.7)
BinaryPredicate

of templates specified in this subclause [func.search] shall meet the same requirements and semantics as specified in [algorithms.general].

Template parameters named Hash shall meet the Cpp17Hash requirements (Table 34).

The Boyer-Moore searcher implements the Boyer-Moore search algorithm.

The Boyer-Moore-Horspool searcher implements the Boyer-Moore-Horspool search algorithm.

In general, the Boyer-Moore searcher will use more memory and give better runtime performance than Boyer-Moore-Horspool.

20.14.17.1 Class template default_searcher [func.search.default]

template<class ForwardIterator1, class BinaryPredicate = equal_to<>>
  class default_searcher {
  public:
    constexpr default_searcher(ForwardIterator1 pat_first, ForwardIterator1 pat_last,
                               BinaryPredicate pred = BinaryPredicate());

    template<class ForwardIterator2>
      constexpr pair<ForwardIterator2, ForwardIterator2>
        operator()(ForwardIterator2 first, ForwardIterator2 last) const;

  private:
    ForwardIterator1 pat_first_;        // exposition only
    ForwardIterator1 pat_last_;         // exposition only
    BinaryPredicate pred_;              // exposition only
  };

constexpr default_searcher(ForwardIterator pat_first, ForwardIterator pat_last,
                           BinaryPredicate pred = BinaryPredicate());

Effects: Constructs a default_searcher object, initializing pat_first_ with pat_first, pat_last_ with pat_last, and pred_ with pred.

Throws: Any exception thrown by the copy constructor of BinaryPredicate or ForwardIterator1.

template<class ForwardIterator2>
  constexpr pair<ForwardIterator2, ForwardIterator2>
    operator()(ForwardIterator2 first, ForwardIterator2 last) const;

Effects: Returns a pair of iterators i and j such that

(3.1)
i == search(first, last, pat_first_, pat_last_, pred_), and
(3.2)
if i == last, then j == last, otherwise j == next(i, distance(pat_first_, pat_last_)).

20.14.17.2 Class template boyer_moore_searcher [func.search.bm]

template<class RandomAccessIterator1,
         class Hash = hash<typename iterator_traits<RandomAccessIterator1>::value_type>,
         class BinaryPredicate = equal_to<>>
  class boyer_moore_searcher {
  public:
    boyer_moore_searcher(RandomAccessIterator1 pat_first,
                         RandomAccessIterator1 pat_last,
                         Hash hf = Hash(),
                         BinaryPredicate pred = BinaryPredicate());

    template<class RandomAccessIterator2>
      pair<RandomAccessIterator2, RandomAccessIterator2>
        operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;

  private:
    RandomAccessIterator1 pat_first_;   // exposition only
    RandomAccessIterator1 pat_last_;    // exposition only
    Hash hash_;                         // exposition only
    BinaryPredicate pred_;              // exposition only
  };

boyer_moore_searcher(RandomAccessIterator1 pat_first,
                     RandomAccessIterator1 pat_last,
                     Hash hf = Hash(),
                     BinaryPredicate pred = BinaryPredicate());

Preconditions: The value type of RandomAccessIterator1 meets the Cpp17DefaultConstructible requirements, the Cpp17CopyConstructible requirements, and the Cpp17CopyAssignable requirements.

Preconditions: Let V be iterator_traits<RandomAccessIterator1>::value_type.

For any two values A and B of type V, if pred(A, B) == true, then hf(A) == hf(B) is true.

Effects: Initializes pat_first_ with pat_first, pat_last_ with pat_last, hash_ with hf, and pred_ with pred.

Throws: Any exception thrown by the copy constructor of RandomAccessIterator1, or by the default constructor, copy constructor, or the copy assignment operator of the value type of RandomAccessIterator1, or the copy constructor or operator() of BinaryPredicate or Hash.

May throw bad_alloc if additional memory needed for internal data structures cannot be allocated.

template<class RandomAccessIterator2>
  pair<RandomAccessIterator2, RandomAccessIterator2>
    operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;

Mandates: RandomAccessIterator1 and RandomAccessIterator2 have the same value type.

Effects: Finds a subsequence of equal values in a sequence.

Returns: A pair of iterators i and j such that

(7.1)
i is the first iterator in the range [first, last - (pat_last_ - pat_first_)) such that for every non-negative integer n less than pat_last_ - pat_first_ the following condition holds: pred(*(i + n), *(pat_first_ + n)) != false, and
(7.2)
j == next(i, distance(pat_first_, pat_last_)).

Returns make_pair(first, first) if [pat_first_, pat_last_) is empty, otherwise returns make_pair(last, last) if no such iterator is found.

Complexity: At most (last - first) * (pat_last_ - pat_first_) applications of the predicate.

20.14.17.3 Class template boyer_moore_horspool_searcher [func.search.bmh]

template<class RandomAccessIterator1,
         class Hash = hash<typename iterator_traits<RandomAccessIterator1>::value_type>,
         class BinaryPredicate = equal_to<>>
  class boyer_moore_horspool_searcher {
  public:
    boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
                                  RandomAccessIterator1 pat_last,
                                  Hash hf = Hash(),
                                  BinaryPredicate pred = BinaryPredicate());

    template<class RandomAccessIterator2>
      pair<RandomAccessIterator2, RandomAccessIterator2>
        operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;

  private:
    RandomAccessIterator1 pat_first_;   // exposition only
    RandomAccessIterator1 pat_last_;    // exposition only
    Hash hash_;                         // exposition only
    BinaryPredicate pred_;              // exposition only
  };

boyer_moore_horspool_searcher(RandomAccessIterator1 pat_first,
                              RandomAccessIterator1 pat_last,
                              Hash hf = Hash(),
                              BinaryPredicate pred = BinaryPredicate());

Preconditions: The value type of RandomAccessIterator1 meets the Cpp17DefaultConstructible, Cpp17CopyConstructible, and Cpp17CopyAssignable requirements.

Preconditions: Let V be iterator_traits<RandomAccessIterator1>::value_type.

For any two values A and B of type V, if pred(A, B) == true, then hf(A) == hf(B) is true.

Effects: Initializes pat_first_ with pat_first, pat_last_ with pat_last, hash_ with hf, and pred_ with pred.

Throws: Any exception thrown by the copy constructor of RandomAccessIterator1, or by the default constructor, copy constructor, or the copy assignment operator of the value type of RandomAccessIterator1 or the copy constructor or operator() of BinaryPredicate or Hash.

May throw bad_alloc if additional memory needed for internal data structures cannot be allocated.

template<class RandomAccessIterator2>
  pair<RandomAccessIterator2, RandomAccessIterator2>
    operator()(RandomAccessIterator2 first, RandomAccessIterator2 last) const;

Mandates: RandomAccessIterator1 and RandomAccessIterator2 have the same value type.

Effects: Finds a subsequence of equal values in a sequence.

Returns: A pair of iterators i and j such that

(7.1)
i is the first iterator i in the range [first, last - (pat_last_ - pat_first_)) such that for every non-negative integer n less than pat_last_ - pat_first_ the following condition holds: pred(*(i + n), *(pat_first_ + n)) != false, and
(7.2)
j == next(i, distance(pat_first_, pat_last_)).

Returns make_pair(first, first) if [pat_first_, pat_last_) is empty, otherwise returns make_pair(last, last) if no such iterator is found.

Complexity: At most (last - first) * (pat_last_ - pat_first_) applications of the predicate.

20.14.18 Class template hash [unord.hash]

The unordered associative containers defined in [unord] use specializations of the class template hash ([functional.syn]) as the default hash function.

Each specialization of hash is either enabled or disabled, as described below.

Enabled specializations meet the Cpp17Hash requirements, and disabled specializations do not.

— end note

]

Each header that declares the template hash provides enabled specializations of hash for nullptr_t and all cv-unqualified arithmetic, enumeration, and pointer types.

For any type Key for which neither the library nor the user provides an explicit or partial specialization of the class template hash, hash<Key> is disabled.

If the library provides an explicit or partial specialization of hash<Key>, that specialization is enabled except as noted otherwise, and its member functions are noexcept except as noted otherwise.

If H is a disabled specialization of hash, these values are false: is_default_constructible_v<H>, is_copy_constructible_v<H>, is_move_constructible_v<H>, is_copy_assignable_v<H>, and is_move_assignable_v<H>.

Disabled specializations of hash are not function object types .

This means that the specialization of hash exists, but any attempts to use it as a Cpp17Hash will be ill-formed.

— end note

]

An enabled specialization hash<Key> will:

(5.1)
meet the Cpp17Hash requirements (Table 34), with Key as the function call argument type, the Cpp17DefaultConstructible requirements (Table 27), the Cpp17CopyAssignable requirements (Table 31),
(5.2)
be swappable ([swappable.requirements]) for lvalues,
(5.3)
meet the requirement that if k1 == k2 is true, h(k1) == h(k2) is also true, where h is an object of type hash<Key> and k1 and k2 are objects of type Key;
(5.4)
meet the requirement that the expression h(k), where h is an object of type hash<Key> and k is an object of type Key, shall not throw an exception unless hash<Key> is a program-defined specialization that depends on at least one program-defined type.

20.15 Metaprogramming and type traits [meta]

This subclause describes components used by C++ programs, particularly in templates, to support the widest possible range of types, optimise template code usage, detect type related user errors, and perform type inference and transformation at compile time.

It includes type classification traits, type property inspection traits, and type transformations.

The type classification traits describe a complete taxonomy of all possible C++ types, and state where in that taxonomy a given type belongs.

The type property inspection traits allow important characteristics of types or of combinations of types to be inspected.

The type transformations allow certain properties of types to be manipulated.

All functions specified in this subclause are signal-safe .

20.15.1 Requirements [meta.rqmts]

A Cpp17UnaryTypeTrait describes a property of a type.

It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the property being described.

It shall be Cpp17DefaultConstructible, Cpp17CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from its base characteristic, which is a specialization of the template integral_constant, with the arguments to the template integral_constant determined by the requirements for the particular property being described.

The member names of the base characteristic shall not be hidden and shall be unambiguously available in the Cpp17UnaryTypeTrait.

A Cpp17BinaryTypeTrait describes a relationship between two types.

It shall be a class template that takes two template type arguments and, optionally, additional arguments that help define the relationship being described.

The member names of the base characteristic shall not be hidden and shall be unambiguously available in the Cpp17BinaryTypeTrait.

A Cpp17TransformationTrait modifies a property of a type.

It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the modification.

It shall define a publicly accessible nested type named type, which shall be a synonym for the modified type.

Unless otherwise specified, the behavior of a program that adds specializations for any of the templates specified in this subclause [meta] is undefined.

Unless otherwise specified, an incomplete type may be used to instantiate a template specified in this subclause.

The behavior of a program is undefined if:

(5.1)
an instantiation of a template specified in subclause [meta] directly or indirectly depends on an incompletely-defined object type T, and
(5.2)
that instantiation could yield a different result were T hypothetically completed.

20.15.2 Header <type_traits> synopsis [meta.type.synop]

namespace std {
  // [meta.help], helper class
  template<class T, T v> struct integral_constant;

  template<bool B>
    using bool_constant = integral_constant<bool, B>;
  using true_type  = bool_constant<true>;
  using false_type = bool_constant<false>;

  // [meta.unary.cat], primary type categories
  template<class T> struct is_void;
  template<class T> struct is_null_pointer;
  template<class T> struct is_integral;
  template<class T> struct is_floating_point;
  template<class T> struct is_array;
  template<class T> struct is_pointer;
  template<class T> struct is_lvalue_reference;
  template<class T> struct is_rvalue_reference;
  template<class T> struct is_member_object_pointer;
  template<class T> struct is_member_function_pointer;
  template<class T> struct is_enum;
  template<class T> struct is_union;
  template<class T> struct is_class;
  template<class T> struct is_function;

  // [meta.unary.comp], composite type categories
  template<class T> struct is_reference;
  template<class T> struct is_arithmetic;
  template<class T> struct is_fundamental;
  template<class T> struct is_object;
  template<class T> struct is_scalar;
  template<class T> struct is_compound;
  template<class T> struct is_member_pointer;

  // [meta.unary.prop], type properties
  template<class T> struct is_const;
  template<class T> struct is_volatile;
  template<class T> struct is_trivial;
  template<class T> struct is_trivially_copyable;
  template<class T> struct is_standard_layout;
  template<class T> struct is_empty;
  template<class T> struct is_polymorphic;
  template<class T> struct is_abstract;
  template<class T> struct is_final;
  template<class T> struct is_aggregate;

  template<class T> struct is_signed;
  template<class T> struct is_unsigned;
  template<class T> struct is_bounded_array;
  template<class T> struct is_unbounded_array;

  template<class T, class... Args> struct is_constructible;
  template<class T> struct is_default_constructible;
  template<class T> struct is_copy_constructible;
  template<class T> struct is_move_constructible;

  template<class T, class U> struct is_assignable;
  template<class T> struct is_copy_assignable;
  template<class T> struct is_move_assignable;

  template<class T, class U> struct is_swappable_with;
  template<class T> struct is_swappable;

  template<class T> struct is_destructible;

  template<class T, class... Args> struct is_trivially_constructible;
  template<class T> struct is_trivially_default_constructible;
  template<class T> struct is_trivially_copy_constructible;
  template<class T> struct is_trivially_move_constructible;

  template<class T, class U> struct is_trivially_assignable;
  template<class T> struct is_trivially_copy_assignable;
  template<class T> struct is_trivially_move_assignable;
  template<class T> struct is_trivially_destructible;

  template<class T, class... Args> struct is_nothrow_constructible;
  template<class T> struct is_nothrow_default_constructible;
  template<class T> struct is_nothrow_copy_constructible;
  template<class T> struct is_nothrow_move_constructible;

  template<class T, class U> struct is_nothrow_assignable;
  template<class T> struct is_nothrow_copy_assignable;
  template<class T> struct is_nothrow_move_assignable;

  template<class T, class U> struct is_nothrow_swappable_with;
  template<class T> struct is_nothrow_swappable;

  template<class T> struct is_nothrow_destructible;

  template<class T> struct has_virtual_destructor;

  template<class T> struct has_unique_object_representations;

  // [meta.unary.prop.query], type property queries
  template<class T> struct alignment_of;
  template<class T> struct rank;
  template<class T, unsigned I = 0> struct extent;

  // [meta.rel], type relations
  template<class T, class U> struct is_same;
  template<class Base, class Derived> struct is_base_of;
  template<class From, class To> struct is_convertible;
  template<class From, class To> struct is_nothrow_convertible;
  template<class T, class U> struct is_layout_compatible;
  template<class Base, class Derived> struct is_pointer_interconvertible_base_of;

  template<class Fn, class... ArgTypes> struct is_invocable;
  template<class R, class Fn, class... ArgTypes> struct is_invocable_r;

  template<class Fn, class... ArgTypes> struct is_nothrow_invocable;
  template<class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r;

  // [meta.trans.cv], const-volatile modifications
  template<class T> struct remove_const;
  template<class T> struct remove_volatile;
  template<class T> struct remove_cv;
  template<class T> struct add_const;
  template<class T> struct add_volatile;
  template<class T> struct add_cv;

  template<class T>
    using remove_const_t    = typename remove_const<T>::type;
  template<class T>
    using remove_volatile_t = typename remove_volatile<T>::type;
  template<class T>
    using remove_cv_t       = typename remove_cv<T>::type;
  template<class T>
    using add_const_t       = typename add_const<T>::type;
  template<class T>
    using add_volatile_t    = typename add_volatile<T>::type;
  template<class T>
    using add_cv_t          = typename add_cv<T>::type;

  // [meta.trans.ref], reference modifications
  template<class T> struct remove_reference;
  template<class T> struct add_lvalue_reference;
  template<class T> struct add_rvalue_reference;

  template<class T>
    using remove_reference_t     = typename remove_reference<T>::type;
  template<class T>
    using add_lvalue_reference_t = typename add_lvalue_reference<T>::type;
  template<class T>
    using add_rvalue_reference_t = typename add_rvalue_reference<T>::type;

  // [meta.trans.sign], sign modifications
  template<class T> struct make_signed;
  template<class T> struct make_unsigned;

  template<class T>
    using make_signed_t   = typename make_signed<T>::type;
  template<class T>
    using make_unsigned_t = typename make_unsigned<T>::type;

  // [meta.trans.arr], array modifications
  template<class T> struct remove_extent;
  template<class T> struct remove_all_extents;

  template<class T>
    using remove_extent_t      = typename remove_extent<T>::type;
  template<class T>
    using remove_all_extents_t = typename remove_all_extents<T>::type;

  // [meta.trans.ptr], pointer modifications
  template<class T> struct remove_pointer;
  template<class T> struct add_pointer;

  template<class T>
    using remove_pointer_t = typename remove_pointer<T>::type;
  template<class T>
    using add_pointer_t    = typename add_pointer<T>::type;

  // [meta.trans.other], other transformations
  template<class T> struct type_identity;
  template<size_t Len, size_t Align = default-alignment> // see [meta.trans.other]
    struct aligned_storage;
  template<size_t Len, class... Types> struct aligned_union;
  template<class T> struct remove_cvref;
  template<class T> struct decay;
  template<bool, class T = void> struct enable_if;
  template<bool, class T, class F> struct conditional;
  template<class... T> struct common_type;
  template<class T, class U, template<class> class TQual, template<class> class UQual>
    struct basic_common_reference { };
  template<class... T> struct common_reference;
  template<class T> struct underlying_type;
  template<class Fn, class... ArgTypes> struct invoke_result;
  template<class T> struct unwrap_reference;
  template<class T> struct unwrap_ref_decay;

  template<class T>
    using type_identity_t    = typename type_identity<T>::type;
  template<size_t Len, size_t Align = default-alignment> // see [meta.trans.other]
    using aligned_storage_t  = typename aligned_storage<Len, Align>::type;
  template<size_t Len, class... Types>
    using aligned_union_t    = typename aligned_union<Len, Types...>::type;
  template<class T>
    using remove_cvref_t     = typename remove_cvref<T>::type;
  template<class T>
    using decay_t            = typename decay<T>::type;
  template<bool b, class T = void>
    using enable_if_t        = typename enable_if<b, T>::type;
  template<bool b, class T, class F>
    using conditional_t      = typename conditional<b, T, F>::type;
  template<class... T>
    using common_type_t      = typename common_type<T...>::type;
  template<class... T>
    using common_reference_t = typename common_reference<T...>::type;
  template<class T>
    using underlying_type_t  = typename underlying_type<T>::type;
  template<class Fn, class... ArgTypes>
    using invoke_result_t    = typename invoke_result<Fn, ArgTypes...>::type;
  template<class T>
    using unwrap_reference_t = typename unwrap_reference<T>::type;
  template<class T>
    using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;
  template<class...>
    using void_t             = void;

  // [meta.logical], logical operator traits
  template<class... B> struct conjunction;
  template<class... B> struct disjunction;
  template<class B> struct negation;

  // [meta.unary.cat], primary type categories
  template<class T>
    inline constexpr bool is_void_v = is_void<T>::value;
  template<class T>
    inline constexpr bool is_null_pointer_v = is_null_pointer<T>::value;
  template<class T>
    inline constexpr bool is_integral_v = is_integral<T>::value;
  template<class T>
    inline constexpr bool is_floating_point_v = is_floating_point<T>::value;
  template<class T>
    inline constexpr bool is_array_v = is_array<T>::value;
  template<class T>
    inline constexpr bool is_pointer_v = is_pointer<T>::value;
  template<class T>
    inline constexpr bool is_lvalue_reference_v = is_lvalue_reference<T>::value;
  template<class T>
    inline constexpr bool is_rvalue_reference_v = is_rvalue_reference<T>::value;
  template<class T>
    inline constexpr bool is_member_object_pointer_v = is_member_object_pointer<T>::value;
  template<class T>
    inline constexpr bool is_member_function_pointer_v = is_member_function_pointer<T>::value;
  template<class T>
    inline constexpr bool is_enum_v = is_enum<T>::value;
  template<class T>
    inline constexpr bool is_union_v = is_union<T>::value;
  template<class T>
    inline constexpr bool is_class_v = is_class<T>::value;
  template<class T>
    inline constexpr bool is_function_v = is_function<T>::value;

  // [meta.unary.comp], composite type categories
  template<class T>
    inline constexpr bool is_reference_v = is_reference<T>::value;
  template<class T>
    inline constexpr bool is_arithmetic_v = is_arithmetic<T>::value;
  template<class T>
    inline constexpr bool is_fundamental_v = is_fundamental<T>::value;
  template<class T>
    inline constexpr bool is_object_v = is_object<T>::value;
  template<class T>
    inline constexpr bool is_scalar_v = is_scalar<T>::value;
  template<class T>
    inline constexpr bool is_compound_v = is_compound<T>::value;
  template<class T>
    inline constexpr bool is_member_pointer_v = is_member_pointer<T>::value;

  // [meta.unary.prop], type properties
  template<class T>
    inline constexpr bool is_const_v = is_const<T>::value;
  template<class T>
    inline constexpr bool is_volatile_v = is_volatile<T>::value;
  template<class T>
    inline constexpr bool is_trivial_v = is_trivial<T>::value;
  template<class T>
    inline constexpr bool is_trivially_copyable_v = is_trivially_copyable<T>::value;
  template<class T>
    inline constexpr bool is_standard_layout_v = is_standard_layout<T>::value;
  template<class T>
    inline constexpr bool is_empty_v = is_empty<T>::value;
  template<class T>
    inline constexpr bool is_polymorphic_v = is_polymorphic<T>::value;
  template<class T>
    inline constexpr bool is_abstract_v = is_abstract<T>::value;
  template<class T>
    inline constexpr bool is_final_v = is_final<T>::value;
  template<class T>
    inline constexpr bool is_aggregate_v = is_aggregate<T>::value;
  template<class T>
    inline constexpr bool is_signed_v = is_signed<T>::value;
  template<class T>
    inline constexpr bool is_unsigned_v = is_unsigned<T>::value;
  template<class T>
    inline constexpr bool is_bounded_array_v = is_bounded_array<T>::value;
  template<class T>
    inline constexpr bool is_unbounded_array_v = is_unbounded_array<T>::value;
  template<class T, class... Args>
    inline constexpr bool is_constructible_v = is_constructible<T, Args...>::value;
  template<class T>
    inline constexpr bool is_default_constructible_v = is_default_constructible<T>::value;
  template<class T>
    inline constexpr bool is_copy_constructible_v = is_copy_constructible<T>::value;
  template<class T>
    inline constexpr bool is_move_constructible_v = is_move_constructible<T>::value;
  template<class T, class U>
    inline constexpr bool is_assignable_v = is_assignable<T, U>::value;
  template<class T>
    inline constexpr bool is_copy_assignable_v = is_copy_assignable<T>::value;
  template<class T>
    inline constexpr bool is_move_assignable_v = is_move_assignable<T>::value;
  template<class T, class U>
    inline constexpr bool is_swappable_with_v = is_swappable_with<T, U>::value;
  template<class T>
    inline constexpr bool is_swappable_v = is_swappable<T>::value;
  template<class T>
    inline constexpr bool is_destructible_v = is_destructible<T>::value;
  template<class T, class... Args>
    inline constexpr bool is_trivially_constructible_v
      = is_trivially_constructible<T, Args...>::value;
  template<class T>
    inline constexpr bool is_trivially_default_constructible_v
      = is_trivially_default_constructible<T>::value;
  template<class T>
    inline constexpr bool is_trivially_copy_constructible_v
      = is_trivially_copy_constructible<T>::value;
  template<class T>
    inline constexpr bool is_trivially_move_constructible_v
      = is_trivially_move_constructible<T>::value;
  template<class T, class U>
    inline constexpr bool is_trivially_assignable_v = is_trivially_assignable<T, U>::value;
  template<class T>
    inline constexpr bool is_trivially_copy_assignable_v
      = is_trivially_copy_assignable<T>::value;
  template<class T>
    inline constexpr bool is_trivially_move_assignable_v
      = is_trivially_move_assignable<T>::value;
  template<class T>
    inline constexpr bool is_trivially_destructible_v = is_trivially_destructible<T>::value;
  template<class T, class... Args>
    inline constexpr bool is_nothrow_constructible_v
      = is_nothrow_constructible<T, Args...>::value;
  template<class T>
    inline constexpr bool is_nothrow_default_constructible_v
      = is_nothrow_default_constructible<T>::value;
  template<class T>
    inline constexpr bool is_nothrow_copy_constructible_v
    = is_nothrow_copy_constructible<T>::value;
  template<class T>
    inline constexpr bool is_nothrow_move_constructible_v
      = is_nothrow_move_constructible<T>::value;
  template<class T, class U>
    inline constexpr bool is_nothrow_assignable_v = is_nothrow_assignable<T, U>::value;
  template<class T>
    inline constexpr bool is_nothrow_copy_assignable_v = is_nothrow_copy_assignable<T>::value;
  template<class T>
    inline constexpr bool is_nothrow_move_assignable_v = is_nothrow_move_assignable<T>::value;
  template<class T, class U>
    inline constexpr bool is_nothrow_swappable_with_v = is_nothrow_swappable_with<T, U>::value;
  template<class T>
    inline constexpr bool is_nothrow_swappable_v = is_nothrow_swappable<T>::value;
  template<class T>
    inline constexpr bool is_nothrow_destructible_v = is_nothrow_destructible<T>::value;
  template<class T>
    inline constexpr bool has_virtual_destructor_v = has_virtual_destructor<T>::value;
  template<class T>
    inline constexpr bool has_unique_object_representations_v
      = has_unique_object_representations<T>::value;

  // [meta.unary.prop.query], type property queries
  template<class T>
    inline constexpr size_t alignment_of_v = alignment_of<T>::value;
  template<class T>
    inline constexpr size_t rank_v = rank<T>::value;
  template<class T, unsigned I = 0>
    inline constexpr size_t extent_v = extent<T, I>::value;

  // [meta.rel], type relations
  template<class T, class U>
    inline constexpr bool is_same_v = is_same<T, U>::value;
  template<class Base, class Derived>
    inline constexpr bool is_base_of_v = is_base_of<Base, Derived>::value;
  template<class From, class To>
    inline constexpr bool is_convertible_v = is_convertible<From, To>::value;
  template<class From, class To>
    inline constexpr bool is_nothrow_convertible_v = is_nothrow_convertible<From, To>::value;
  template<class T, class U>
    inline constexpr bool is_layout_compatible_v = is_layout_compatible<T, U>::value;
  template<class Base, class Derived>
    inline constexpr bool is_pointer_interconvertible_base_of_v
      = is_pointer_interconvertible_base_of<Base, Derived>::value;
  template<class Fn, class... ArgTypes>
    inline constexpr bool is_invocable_v = is_invocable<Fn, ArgTypes...>::value;
  template<class R, class Fn, class... ArgTypes>
    inline constexpr bool is_invocable_r_v = is_invocable_r<R, Fn, ArgTypes...>::value;
  template<class Fn, class... ArgTypes>
    inline constexpr bool is_nothrow_invocable_v = is_nothrow_invocable<Fn, ArgTypes...>::value;
  template<class R, class Fn, class... ArgTypes>
    inline constexpr bool is_nothrow_invocable_r_v
      = is_nothrow_invocable_r<R, Fn, ArgTypes...>::value;

  // [meta.logical], logical operator traits
  template<class... B>
    inline constexpr bool conjunction_v = conjunction<B...>::value;
  template<class... B>
    inline constexpr bool disjunction_v = disjunction<B...>::value;
  template<class B>
    inline constexpr bool negation_v = negation<B>::value;

  // [meta.member], member relationships
  template<class S, class M>
    constexpr bool is_pointer_interconvertible_with_class(M S::*m) noexcept;
  template<class S1, class S2, class M1, class M2>
    constexpr bool is_corresponding_member(M1 S1::*m1, M2 S2::*m2) noexcept;

  // [meta.const.eval], constant evaluation context
  constexpr bool is_constant_evaluated() noexcept;
}

20.15.3 Helper classes [meta.help]

namespace std {
  template<class T, T v> struct integral_constant {
    static constexpr T value = v;

    using value_type = T;
    using type = integral_constant<T, v>;

    constexpr operator value_type() const noexcept { return value; }
    constexpr value_type operator()() const noexcept { return value; }
  };
}

The class template integral_constant, alias template bool_constant, and its associated typedef-names true_type and false_type are used as base classes to define the interface for various type traits.

20.15.4 Unary type traits [meta.unary]

This subclause contains templates that may be used to query the properties of a type at compile time.

Each of these templates shall be a Cpp17UnaryTypeTrait ([meta.rqmts]) with a base characteristic of true_type if the corresponding condition is true, otherwise false_type.

20.15.4.1 Primary type categories [meta.unary.cat]

The primary type categories correspond to the descriptions given in subclause [basic.types] of the C++ standard.

For any given type T, the result of applying one of these templates to T and to cv T shall yield the same result.

For any given type T, exactly one of the primary type categories has a value member that evaluates to true.

— end note

]

Table 47: Primary type category predicates [tab:meta.unary.cat]

Template	Condition	Comments
template<class T> struct is_void;	T is void
template<class T> struct is_null_pointer;	T is nullptr_t ([basic.fundamental])
template<class T> struct is_integral;	T is an integral type ([basic.fundamental])
template<class T> struct is_floating_point;	T is a floating-point type ([basic.fundamental])
template<class T> struct is_array;	T is an array type ([basic.compound]) of known or unknown extent	Class template array is not an array type.
template<class T> struct is_pointer;	T is a pointer type ([basic.compound])	Includes pointers to functions but not pointers to non-static members.
template<class T> struct is_lvalue_reference;	T is an lvalue reference type ([dcl.ref])
template<class T> struct is_rvalue_reference;	T is an rvalue reference type ([dcl.ref])
template<class T> struct is_member_object_pointer;	T is a pointer to data member
template<class T> struct is_member_function_pointer;	T is a pointer to member function
template<class T> struct is_enum;	T is an enumeration type ([basic.compound])
template<class T> struct is_union;	T is a union type ([basic.compound])
template<class T> struct is_class;	T is a non-union class type ([basic.compound])
template<class T> struct is_function;	T is a function type ([basic.compound])

20.15.4.2 Composite type traits [meta.unary.comp]

These templates provide convenient compositions of the primary type categories, corresponding to the descriptions given in subclause [basic.types].

For any given type T, the result of applying one of these templates to T and to cv T shall yield the same result.

Table 48: Composite type category predicates [tab:meta.unary.comp]

Template	Condition	Comments
template<class T> struct is_reference;	T is an lvalue reference or an rvalue reference
template<class T> struct is_arithmetic;	T is an arithmetic type ([basic.fundamental])
template<class T> struct is_fundamental;	T is a fundamental type ([basic.fundamental])
template<class T> struct is_object;	T is an object type ([basic.types])
template<class T> struct is_scalar;	T is a scalar type ([basic.types])
template<class T> struct is_compound;	T is a compound type ([basic.compound])
template<class T> struct is_member_pointer;	T is a pointer-to-member type

20.15.4.3 Type properties [meta.unary.prop]

These templates provide access to some of the more important properties of types.

It is unspecified whether the library defines any full or partial specializations of any of these templates.

For all of the class templates X declared in this subclause, instantiating that template with a template-argument that is a class template specialization may result in the implicit instantiation of the template argument if and only if the semantics of X require that the argument is a complete type.

For the purpose of defining the templates in this subclause, a function call expression declval<T>() for any type T is considered to be a trivial ([basic.types], [special]) function call that is not an odr-use of declval in the context of the corresponding definition notwithstanding the restrictions of [declval].

Table 49: Type property predicates [tab:meta.unary.prop]

Template	Condition	Preconditions
template<class T> struct is_const;	T is const-qualified ([basic.type.qualifier])
template<class T> struct is_volatile;	T is volatile-qualified ([basic.type.qualifier])
template<class T> struct is_trivial;	T is a trivial type ([basic.types])	remove_all_extents_t<T> shall be a complete type or cv void.
template<class T> struct is_trivially_copyable;	T is a trivially copyable type ([basic.types])	remove_all_extents_t<T> shall be a complete type or cv void.
template<class T> struct is_standard_layout;	T is a standard-layout type ([basic.types])	remove_all_extents_t<T> shall be a complete type or cv void.
template<class T> struct is_empty;	T is a class type, but not a union type, with no non-static data members other than subobjects of zero size, no virtual member functions, no virtual base classes, and no base class B for which is_empty_v<B> is false.	If T is a non-union class type, T shall be a complete type.
template<class T> struct is_polymorphic;	T is a polymorphic class ([class.virtual])	If T is a non-union class type, T shall be a complete type.
template<class T> struct is_abstract;	T is an abstract class ([class.abstract])	If T is a non-union class type, T shall be a complete type.
template<class T> struct is_final;	T is a class type marked with the class-virt-specifier final ([class.pre]). [Note : A union is a class type that can be marked with final. — end note ]	If T is a class type, T shall be a complete type.
template<class T> struct is_aggregate;	T is an aggregate type ([dcl.init.aggr])	remove_all_extents_t<T> shall be a complete type or cv void.
template<class T> struct is_signed;	If is_arithmetic_v<T> is true, the same result as T(-1) < T(0); otherwise, false
template<class T> struct is_unsigned;	If is_arithmetic_v<T> is true, the same result as T(0) < T(-1); otherwise, false
template<class T> struct is_bounded_array;	T is an array type of known bound ([dcl.array])
template<class T> struct is_unbounded_array;	T is an array type of unknown bound ([dcl.array])
template<class T, class... Args> struct is_constructible;	For a function type T or for a cv void type T, is_constructible_v<T, Args...> is false, otherwise see below	T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_default_constructible;	is_constructible_v<T> is true.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_copy_constructible;	For a referenceable type T ([defns.referenceable]), the same result as is_constructible_v<T, const T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_move_constructible;	For a referenceable type T, the same result as is_constructible_v<T, T&&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U> struct is_assignable;	The expression declval<T>() = declval<U>() is well-formed when treated as an unevaluated operand . Access checking is performed as if in a context unrelated to T and U. Only the validity of the immediate context of the assignment expression is considered. [Note : The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the “immediate context” and can result in the program being ill-formed. — end note ]	T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_copy_assignable;	For a referenceable type T, the same result as is_assignable_v<T&, const T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_move_assignable;	For a referenceable type T, the same result as is_assignable_v<T&, T&&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U> struct is_swappable_with;	The expressions swap(declval<T>(), declval<U>()) and swap(declval<U>(), declval<T>()) are each well-formed when treated as an unevaluated operand in an overload-resolution context for swappable values ([swappable.requirements]). Access checking is performed as if in a context unrelated to T and U. Only the validity of the immediate context of the swap expressions is considered. [Note : The compilation of the expressions can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the “immediate context” and can result in the program being ill-formed. — end note ]	T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_swappable;	For a referenceable type T, the same result as is_swappable_with_v<T&, T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_destructible;	Either T is a reference type, or T is a complete object type for which the expression declval<U&>().~U() is well-formed when treated as an unevaluated operand, where U is remove_all_extents_t<T>.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class... Args> struct is_trivially_constructible;	is_constructible_v<T, Args...> is true and the variable definition for is_constructible, as defined below, is known to call no operation that is not trivial ([basic.types], [special]).	T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_trivially_default_constructible;	is_trivially_constructible_v<T> is true.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_trivially_copy_constructible;	For a referenceable type T, the same result as is_trivially_constructible_v<T, const T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_trivially_move_constructible;	For a referenceable type T, the same result as is_trivially_constructible_v<T, T&&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U> struct is_trivially_assignable;	is_assignable_v<T, U> is true and the assignment, as defined by is_assignable, is known to call no operation that is not trivial ([basic.types], [special]).	T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_trivially_copy_assignable;	For a referenceable type T, the same result as is_trivially_assignable_v<T&, const T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_trivially_move_assignable;	For a referenceable type T, the same result as is_trivially_assignable_v<T&, T&&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_trivially_destructible;	is_destructible_v<T> is true and remove_all_extents_t<T> is either a non-class type or a class type with a trivial destructor.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class... Args> struct is_nothrow_constructible;	is_constructible_v<T, Args...> is true and the variable definition for is_constructible, as defined below, is known not to throw any exceptions ([expr.unary.noexcept]).	T and all types in the template parameter pack Args shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_nothrow_default_constructible;	is_nothrow_constructible_v<T> is true.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_nothrow_copy_constructible;	For a referenceable type T, the same result as is_nothrow_constructible_v<T, const T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_nothrow_move_constructible;	For a referenceable type T, the same result as is_nothrow_constructible_v<T, T&&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U> struct is_nothrow_assignable;	is_assignable_v<T, U> is true and the assignment is known not to throw any exceptions ([expr.unary.noexcept]).	T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_nothrow_copy_assignable;	For a referenceable type T, the same result as is_nothrow_assignable_v<T&, const T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_nothrow_move_assignable;	For a referenceable type T, the same result as is_nothrow_assignable_v<T&, T&&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T, class U> struct is_nothrow_swappable_with;	is_swappable_with_v<T, U> is true and each swap expression of the definition of is_swappable_with<T, U> is known not to throw any exceptions ([expr.unary.noexcept]).	T and U shall be complete types, cv void, or arrays of unknown bound.
template<class T> struct is_nothrow_swappable;	For a referenceable type T, the same result as is_nothrow_swappable_with_v<T&, T&>, otherwise false.	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct is_nothrow_destructible;	is_destructible_v<T> is true and the indicated destructor is known not to throw any exceptions ([expr.unary.noexcept]).	T shall be a complete type, cv void, or an array of unknown bound.
template<class T> struct has_virtual_destructor;	T has a virtual destructor ([class.dtor])	If T is a non-union class type, T shall be a complete type.
template<class T> struct has_unique_object_representations;	For an array type T, the same result as has_unique_object_representations_v<remove_all_extents_t<T>>, otherwise see below.	T shall be a complete type, cv void, or an array of unknown bound.

is_const_v<const volatile int>      // true
is_const_v<const int*>              // false
is_const_v<const int&>              // false
is_const_v<int[3]>                  // false
is_const_v<const int[3]>            // true

— end example

]

remove_const_t<const volatile int>  // volatile int
remove_const_t<const int* const>    // const int*
remove_const_t<const int&>          // const int&
remove_const_t<const int[3]>        // int[3]

— end example

]

// Given:
struct P final { };
union U1 { };
union U2 final { };

// the following assertions hold:
static_assert(!is_final_v<int>);
static_assert(is_final_v<P>);
static_assert(!is_final_v<U1>);
static_assert(is_final_v<U2>);

— end example

]

The predicate condition for a template specialization is_constructible<T, Args...> shall be satisfied if and only if the following variable definition would be well-formed for some invented variable t:

T t(declval<Args>()...);

These tokens are never interpreted as a function declaration.

— end note

]

Access checking is performed as if in a context unrelated to T and any of the Args.

Only the validity of the immediate context of the variable initialization is considered.

The evaluation of the initialization can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.

Such side effects are not in the “immediate context” and can result in the program being ill-formed.

— end note

]

The predicate condition for a template specialization has_unique_object_representations<T> shall be satisfied if and only if:

(9.1)
T is trivially copyable, and
(9.2)
any two objects of type T with the same value have the same object representation, where two objects of array or non-union class type are considered to have the same value if their respective sequences of direct subobjects have the same values, and two objects of union type are considered to have the same value if they have the same active member and the corresponding members have the same value.

The set of scalar types for which this condition holds is implementation-defined.

If a type has padding bits, the condition does not hold; otherwise, the condition holds true for integral types.

— end note

]

20.15.5 Type property queries [meta.unary.prop.query]

This subclause contains templates that may be used to query properties of types at compile time.

Table 50: Type property queries [tab:meta.unary.prop.query]

Template	Value
template<class T> struct alignment_of;	alignof(T). Mandates: alignof(T) is a valid expression ([expr.alignof])
template<class T> struct rank;	If T names an array type, an integer value representing the number of dimensions of T; otherwise, 0.
template<class T, unsigned I = 0> struct extent;	If T is not an array type, or if it has rank less than or equal to I, or if I is 0 and T has type “array of unknown bound of U”, then 0; otherwise, the bound ([dcl.array]) of the $I^{th}$ dimension of T, where indexing of I is zero-based

Each of these templates shall be a Cpp17UnaryTypeTrait ([meta.rqmts]) with a base characteristic of integral_constant<size_t, Value>.

// the following assertions hold:
assert(rank_v<int> == 0);
assert(rank_v<int[2]> == 1);
assert(rank_v<int[][4]> == 2);

— end example

]

// the following assertions hold:
assert(extent_v<int> == 0);
assert(extent_v<int[2]> == 2);
assert(extent_v<int[2][4]> == 2);
assert(extent_v<int[][4]> == 0);
assert((extent_v<int, 1>) == 0);
assert((extent_v<int[2], 1>) == 0);
assert((extent_v<int[2][4], 1>) == 4);
assert((extent_v<int[][4], 1>) == 4);

— end example

]

20.15.6 Relationships between types [meta.rel]

This subclause contains templates that may be used to query relationships between types at compile time.

Each of these templates shall be a Cpp17BinaryTypeTrait ([meta.rqmts]) with a base characteristic of true_type if the corresponding condition is true, otherwise false_type.

Table 51: Type relationship predicates [tab:meta.rel]

Template	Condition	Comments
template<class T, class U> struct is_same;	T and U name the same type with the same cv-qualifications
template<class Base, class Derived> struct is_base_of;	Base is a base class of Derived ([class.derived]) without regard to cv-qualifiers or Base and Derived are not unions and name the same class type without regard to cv-qualifiers	If Base and Derived are non-union class types and are not possibly cv-qualified versions of the same type, Derived shall be a complete type. [Note : Base classes that are private, protected, or ambiguous are, nonetheless, base classes. — end note ]
template<class From, class To> struct is_convertible;	see below	From and To shall be complete types, cv void, or arrays of unknown bound.
template<class From, class To> struct is_nothrow_convertible;	is_convertible_v<From, To> is true and the conversion, as defined by is_convertible, is known not to throw any exceptions ([expr.unary.noexcept])	From and To shall be complete types, cv void, or arrays of unknown bound.
template<class T, class U> struct is_layout_compatible;	T and U are layout-compatible ([basic.types])	T and U shall be complete types, cv void, or arrays of unknown bound.
template<class Base, class Derived> struct is_pointer_interconvertible_base_of;	Derived is unambiguously derived from Base without regard to cv-qualifiers, and each object of type Derived is pointer-interconvertible ([basic.compound]) with its Base subobject, or Base and Derived are not unions and name the same class type without regard to cv-qualifiers.	If Base and Derived are non-union class types and are not (possibly cv-qualified versions of) the same type, Derived shall be a complete type.
template<class Fn, class... ArgTypes> struct is_invocable;	The expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well-formed when treated as an unevaluated operand	Fn and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
template<class R, class Fn, class... ArgTypes> struct is_invocable_r;	The expression INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is well-formed when treated as an unevaluated operand	Fn, R, and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
template<class Fn, class... ArgTypes> struct is_nothrow_invocable;	is_invocable_v< Fn, ArgTypes...> is true and the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is known not to throw any exceptions ([expr.unary.noexcept])	Fn and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.
template<class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r;	is_invocable_r_v< R, Fn, ArgTypes...> is true and the expression INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is known not to throw any exceptions ([expr.unary.noexcept])	Fn, R, and all types in the template parameter pack ArgTypes shall be complete types, cv void, or arrays of unknown bound.

struct B {};
struct B1 : B {};
struct B2 : B {};
struct D : private B1, private B2 {};

is_base_of_v<B, D>              // true
is_base_of_v<const B, D>        // true
is_base_of_v<B, const D>        // true
is_base_of_v<B, const B>        // true
is_base_of_v<D, B>              // false
is_base_of_v<B&, D&>            // false
is_base_of_v<B[3], D[3]>        // false
is_base_of_v<int, int>          // false

— end example

]

The predicate condition for a template specialization is_convertible<From, To> shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

To test() {
  return declval<From>();
}

This requirement gives well-defined results for reference types, void types, array types, and function types.

— end note

]

Access checking is performed in a context unrelated to To and From.

Only the validity of the immediate context of the expression of the return statement ([stmt.return]) (including initialization of the returned object or reference) is considered.

The initialization can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.

Such side effects are not in the “immediate context” and can result in the program being ill-formed.

— end note

]

20.15.7 Transformations between types [meta.trans]

This subclause contains templates that may be used to transform one type to another following some predefined rule.

Each of the templates in this subclause shall be a Cpp17TransformationTrait ([meta.rqmts]).

20.15.7.1 Const-volatile modifications [meta.trans.cv]

Table 52: Const-volatile modifications [tab:meta.trans.cv]

Template	Comments
template<class T> struct remove_const;	The member typedef type names the same type as T except that any top-level const-qualifier has been removed. [Example : remove_const_t<const volatile int> evaluates to volatile int, whereas remove_const_t<const int> evaluates to const int. — end example ]
template<class T> struct remove_volatile;	The member typedef type names the same type as T except that any top-level volatile-qualifier has been removed. [Example : remove_volatile_t<const volatile int> evaluates to const int, whereas remove_volatile_t<volatile int> evaluates to volatile int. — end example ]
template<class T> struct remove_cv;	The member typedef type shall be the same as T except that any top-level cv-qualifier has been removed. [Example : remove_cv_t<const volatile int> evaluates to int, whereas remove_cv_t<const volatile int> evaluates to const volatile int. — end example ]
template<class T> struct add_const;	If T is a reference, function, or top-level const-qualified type, then type names the same type as T, otherwise T const.
template<class T> struct add_volatile;	If T is a reference, function, or top-level volatile-qualified type, then type names the same type as T, otherwise T volatile.
template<class T> struct add_cv;	The member typedef type names the same type as add_const_t<add_volatile_t<T>>.

20.15.7.2 Reference modifications [meta.trans.ref]

Table 53: Reference modifications [tab:meta.trans.ref]

Template	Comments
template<class T> struct remove_reference;	If T has type “reference to T1” then the member typedef type names T1; otherwise, type names T.
template<class T> struct add_lvalue_reference;	If T names a referenceable type then the member typedef type names T&; otherwise, type names T. [Note : This rule reflects the semantics of reference collapsing ([dcl.ref]). — end note ]
template<class T> struct add_rvalue_reference;	If T names a referenceable type then the member typedef type names T&&; otherwise, type names T. [Note : This rule reflects the semantics of reference collapsing ([dcl.ref]). For example, when a type T names a type T1&, the type add_rvalue_reference_t<T> is not an rvalue reference. — end note ]

20.15.7.3 Sign modifications [meta.trans.sign]

Table 54: Sign modifications [tab:meta.trans.sign]

Template	Comments
template<class T> struct make_signed;	If T names a (possibly cv-qualified) signed integer type then the member typedef type names the type T; otherwise, if T names a (possibly cv-qualified) unsigned integer type then type names the corresponding signed integer type, with the same cv-qualifiers as T; otherwise, type names the signed integer type with smallest rank for which sizeof(T) == sizeof(type), with the same cv-qualifiers as T. Mandates: T is an integral or enumeration type other than cv bool.
template<class T> struct make_unsigned;	If T names a (possibly cv-qualified) unsigned integer type then the member typedef type names the type T; otherwise, if T names a (possibly cv-qualified) signed integer type then type names the corresponding unsigned integer type, with the same cv-qualifiers as T; otherwise, type names the unsigned integer type with smallest rank for which sizeof(T) == sizeof(type), with the same cv-qualifiers as T. Mandates: T is an integral or enumeration type other than cv bool.

20.15.7.4 Array modifications [meta.trans.arr]

Table 55: Array modifications [tab:meta.trans.arr]

Template	Comments
template<class T> struct remove_extent;	If T names a type “array of U”, the member typedef type shall be U, otherwise T. [Note : For multidimensional arrays, only the first array dimension is removed. For a type “array of const U”, the resulting type is const U. — end note ]
template<class T> struct remove_all_extents;	If T is “multi-dimensional array of U”, the resulting member typedef type is U, otherwise T.

// the following assertions hold:
assert((is_same_v<remove_extent_t<int>, int>));
assert((is_same_v<remove_extent_t<int[2]>, int>));
assert((is_same_v<remove_extent_t<int[2][3]>, int[3]>));
assert((is_same_v<remove_extent_t<int[][3]>, int[3]>));

— end example

]

// the following assertions hold:
assert((is_same_v<remove_all_extents_t<int>, int>));
assert((is_same_v<remove_all_extents_t<int[2]>, int>));
assert((is_same_v<remove_all_extents_t<int[2][3]>, int>));
assert((is_same_v<remove_all_extents_t<int[][3]>, int>));

— end example

]

20.15.7.5 Pointer modifications [meta.trans.ptr]

Table 56: Pointer modifications [tab:meta.trans.ptr]

Template	Comments
template<class T> struct remove_pointer;	If T has type “(possibly cv-qualified) pointer to T1” then the member typedef type names T1; otherwise, it names T.
template<class T> struct add_pointer;	If T names a referenceable type or a cv void type then the member typedef type names the same type as remove_reference_t<T>*; otherwise, type names T.

20.15.7.6 Other transformations [meta.trans.other]

Table 57: Other transformations [tab:meta.trans.other]

Template	Comments
template<class T> struct type_identity;	The member typedef type names the type T.
template<size_t Len, size_t Align = default-alignment> struct aligned_storage;	The value of default-alignment shall be the most stringent alignment requirement for any object type whose size is no greater than Len ([basic.types]). The member typedef type shall be a trivial standard-layout type suitable for use as uninitialized storage for any object whose size is at most Len and whose alignment is a divisor of Align. Mandates: Len is not zero. Align is equal to alignof(T) for some type T or to default-alignment.
template<size_t Len, class... Types> struct aligned_union;	The member typedef type shall be a trivial standard-layout type suitable for use as uninitialized storage for any object whose type is listed in Types; its size shall be at least Len. The static member alignment_value shall be an integral constant of type size_t whose value is the strictest alignment of all types listed in Types. Mandates: At least one type is provided. Each type in the template parameter pack Types is a complete object type.
template<class T> struct remove_cvref;	The member typedef type names the same type as remove_cv_t<remove_reference_t<T>>.
template<class T> struct decay;	Let U be remove_reference_t<T>. If is_array_v<U> is true, the member typedef type equals remove_extent_t<U>. If is_function_v<U> is true, the member typedef type equals add_pointer_t<U>. Otherwise the member typedef type equals remove_cv_t<U>. [Note : This behavior is similar to the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) conversions applied when an lvalue is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. — end note* ]
template<bool B, class T = void> struct enable_if;	If B is true, the member typedef type shall equal T; otherwise, there shall be no member type.
template<bool B, class T, class F> struct conditional;	If B is true, the member typedef type shall equal T. If B is false, the member typedef type shall equal F.
template<class... T> struct common_type;	Unless this trait is specialized (as specified in Note B, below), the member type is defined or omitted as specified in Note A, below. If it is omitted, there shall be no member type. Each type in the template parameter pack T shall be complete, cv void, or an array of unknown bound.
template<class, class, template<class> class, template<class> class> struct basic_common_reference;	Unless this trait is specialized (as specified in Note D, below), there shall be no member type.
template<class... T> struct common_reference;	The member typedef-name type is defined or omitted as specified in Note C, below. Each type in the parameter pack T shall be complete or cv void.
template<class T> struct underlying_type;	If T is an enumeration type, the member typedef type names the underlying type of T ([dcl.enum]); otherwise, there is no member type. Mandates: T is not an incomplete enumeration type.
template<class Fn, class... ArgTypes> struct invoke_result;	If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well-formed when treated as an unevaluated operand, the member typedef type names the type decltype(INVOKE(declval<Fn>(), declval<ArgTypes>()...)); otherwise, there shall be no member type. Access checking is performed as if in a context unrelated to Fn and ArgTypes. Only the validity of the immediate context of the expression is considered. [Note : The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the “immediate context” and can result in the program being ill-formed. — end note ] Preconditions: Fn and all types in the template parameter pack ArgTypes are complete types, cv void, or arrays of unknown bound.
template<class T> struct unwrap_reference;	If T is a specialization reference_wrapper<X> for some type X, the member typedef type of unwrap_reference<T> is X&, otherwise it is T.
template<class T> unwrap_ref_decay;	The member typedef type of unwrap_ref_decay<T> denotes the type unwrap_reference_t<decay_t<T>>.

A typical implementation would define aligned_storage as:

template<size_t Len, size_t Alignment>
struct aligned_storage {
  typedef struct {
    alignas(Alignment) unsigned char __data[Len];
  } type;
};

— end note

]

In addition to being available via inclusion of the <type_traits> header, the templates unwrap_reference, unwrap_ref_decay, unwrap_reference_t, and unwrap_ref_decay_t are available when the header <functional> ([functional.syn]) is included.

Let:

(3.1)
CREF(A) be add_lvalue_reference_t<const remove_reference_t<A>>,
(3.2)
XREF(A) denote a unary alias template T such that T denotes the same type as U with the addition of A's cv and reference qualifiers, for a non-reference cv-unqualified type U,
(3.3)
COPYCV(FROM, TO) be an alias for type TO with the addition of FROM's top-level cv-qualifiers,
[Example
: COPYCV(const int, volatile short) is an alias for const volatile short. — end example
]
(3.4)
COND-RES(X, Y) be decltype(false ? declval<X(&)()>()() : declval<Y(&)()>()()).

Given types A and B, let X be remove_reference_t<A>, let Y be remove_reference_t, and let COMMON-REF(A, B) be:

(3.5)
If A and B are both lvalue reference types, COMMON-REF(A, B) is COND-RES(COPYCV(X, Y) &, COPYCV(Y, X) &) if that type exists and is a reference type.
(3.6)
Otherwise, let C be remove_reference_t<COMMON-REF(X&, Y&)>&&. If A and B are both rvalue reference types, C is well-formed, and is_convertible_v<A, C> && is_convertible_v<B, C> is true, then COMMON-REF(A, B) is C.
(3.7)
Otherwise, let D be COMMON-REF(const X&, Y&). If A is an rvalue reference and B is an lvalue reference and D is well-formed and is_convertible_v<A, D> is true, then COMMON-REF(A, B) is D.
(3.8)
Otherwise, if A is an lvalue reference and B is an rvalue reference, then COMMON-REF(A, B) is COMMON-REF(B, A).
(3.9)
Otherwise, COMMON-REF(A, B) is ill-formed.

If any of the types computed above is ill-formed, then COMMON-REF(A, B) is ill-formed.

Note A: For the common_type trait applied to a template parameter pack T of types, the member type shall be either defined or not present as follows:

(4.1)
If sizeof...(T) is zero, there shall be no member type.
(4.2)
If sizeof...(T) is one, let T0 denote the sole type constituting the pack T. The member typedef-name type shall denote the same type, if any, as common_type_t<T0, T0>; otherwise there shall be no member type.
(4.3)
If sizeof...(T) is two, let the first and second types constituting T be denoted by T1 and T2, respectively, and let D1 and D2 denote the same types as decay_t<T1> and decay_t<T2>, respectively.
- (4.3.1)
 If is_same_v<T1, D1> is false or is_same_v<T2, D2> is false, let C denote the same type, if any, as common_type_t<D1, D2>.
- (4.3.2)
 [Note
 : None of the following will apply if there is a specialization common_type<D1, D2>. — end note
 ]
- (4.3.3)
 Otherwise, if
```
decay_t<decltype(false ? declval<D1>() : declval<D2>())>
```
 denotes a valid type, let C denote that type.
- (4.3.4)
 Otherwise, if COND-RES(CREF(D1), CREF(D2)) denotes a type, let C denote the type decay_t<COND-RES(CREF(D1), CREF(D2))>.
In either case, the member typedef-name type shall denote the same type, if any, as C. Otherwise, there shall be no member type.
(4.4)
If sizeof...(T) is greater than two, let T1, T2, and R, respectively, denote the first, second, and (pack of) remaining types constituting T. Let C denote the same type, if any, as common_type_t<T1, T2>. If there is such a type C, the member typedef-name type shall denote the same type, if any, as common_type_t<C, R...>. Otherwise, there shall be no member type.

Note B: Notwithstanding the provisions of [meta.type.synop], and pursuant to [namespace.std], a program may specialize common_type<T1, T2> for types T1 and T2 such that is_same_v<T1, decay_t<T1>> and is_same_v<T2, decay_t<T2>> are each true.

Such specializations are needed when only explicit conversions are desired between the template arguments.

— end note

]

Such a specialization need not have a member named type, but if it does, that member shall be a typedef-name for an accessible and unambiguous cv-unqualified non-reference type C to which each of the types T1 and T2 is explicitly convertible.

Moreover, common_type_t<T1, T2> shall denote the same type, if any, as does common_type_t<T2, T1>.

No diagnostic is required for a violation of this Note's rules.

Note C: For the common_reference trait applied to a parameter pack T of types, the member type shall be either defined or not present as follows:

(6.1)
If sizeof...(T) is zero, there shall be no member type.
(6.2)
Otherwise, if sizeof...(T) is one, let T0 denote the sole type in the pack T. The member typedef type shall denote the same type as T0.
(6.3)
Otherwise, if sizeof...(T) is two, let T1 and T2 denote the two types in the pack T. Then
- (6.3.1)
 If T1 and T2 are reference types and COMMON-REF(T1, T2) is well-formed, then the member typedef type denotes that type.
- (6.3.2)
 Otherwise, if basic_common_reference<remove_cvref_t<T1>, remove_cvref_t<T2>, XREF(T1), XREF(T2)>::type is well-formed, then the member typedef type denotes that type.
- (6.3.3)
 Otherwise, if COND-RES(T1, T2) is well-formed, then the member typedef type denotes that type.
- (6.3.4)
 Otherwise, if common_type_t<T1, T2> is well-formed, then the member typedef type denotes that type.
- (6.3.5)
 Otherwise, there shall be no member type.
(6.4)
Otherwise, if sizeof...(T) is greater than two, let T1, T2, and Rest, respectively, denote the first, second, and (pack of) remaining types comprising T. Let C be the type common_reference_t<T1, T2>. Then:
- (6.4.1)
 If there is such a type C, the member typedef type shall denote the same type, if any, as common_reference_t<C, Rest...>.
- (6.4.2)
 Otherwise, there shall be no member type.

Note D: Notwithstanding the provisions of [meta.type.synop], and pursuant to [namespace.std], a program may partially specialize basic_common_reference<T, U, TQual, UQual> for types T and U such that is_same_v<T, decay_t<T>> and is_same_v<U, decay_t> are each true.

Such specializations can be used to influence the result of common_reference, and are needed when only explicit conversions are desired between the template arguments.

— end note

]

Such a specialization need not have a member named type, but if it does, that member shall be a typedef-name for an accessible and unambiguous type C to which each of the types TQual<T> and UQual is convertible.

Moreover, basic_common_reference<T, U, TQual, UQual>::type shall denote the same type, if any, as does basic_common_reference<U, T, UQual, TQual>::type.

No diagnostic is required for a violation of these rules.

Given these definitions:

using PF1 = bool  (&)();
using PF2 = short (*)(long);

struct S {
  operator PF2() const;
  double operator()(char, int&);
  void fn(long) const;
  char data;
};

using PMF = void (S::*)(long) const;
using PMD = char  S::*;

the following assertions will hold:

static_assert(is_same_v<invoke_result_t<S, int>, short>);
static_assert(is_same_v<invoke_result_t<S&, unsigned char, int&>, double>);
static_assert(is_same_v<invoke_result_t<PF1>, bool>);
static_assert(is_same_v<invoke_result_t<PMF, unique_ptr<S>, int>, void>);
static_assert(is_same_v<invoke_result_t<PMD, S>, char&&>);
static_assert(is_same_v<invoke_result_t<PMD, const S*>, const char&>);

— end example

]

20.15.8 Logical operator traits [meta.logical]

This subclause describes type traits for applying logical operators to other type traits.

template<class... B> struct conjunction : see below { };

The class template conjunction forms the logical conjunction of its template type arguments.

For a specialization conjunction<

B_{1}

, …,

B_{N}

>, if there is a template type argument

B_{i}

for which bool(

B_{i}

::value) is false, then instantiating conjunction<

B_{1}

, …,

B_{N}

>::value does not require the instantiation of

B_{j}

::value for

j > i

This is analogous to the short-circuiting behavior of the built-in operator &&.

— end note

]

Every template type argument for which

B_{i}

::value is instantiated shall be usable as a base class and shall have a member value which is convertible to bool, is not hidden, and is unambiguously available in the type.

The specialization conjunction<

B_{1}

, …,

B_{N}

> has a public and unambiguous base that is either

(5.1)
the first type $B_{i}$ in the list true_type, $B_{1}$ , …, $B_{N}$ for which bool( $B_{i}$ ::value) is false, or
(5.2)
if there is no such $B_{i}$ , the last type in the list.

This means a specialization of conjunction does not necessarily inherit from either true_type or false_type.

— end note

]

The member names of the base class, other than conjunction and operator=, shall not be hidden and shall be unambiguously available in conjunction.

template<class... B> struct disjunction : see below { };

The class template disjunction forms the logical disjunction of its template type arguments.

For a specialization disjunction<

B_{1}

, …,

B_{N}

>, if there is a template type argument

B_{i}

for which bool(

B_{i}

::value) is true, then instantiating disjunction<

B_{1}

, …,

B_{N}

>::value does not require the instantiation of

B_{j}

::value for

j > i

This is analogous to the short-circuiting behavior of the built-in operator ||.

— end note

]

Every template type argument for which

B_{i}

::value is instantiated shall be usable as a base class and shall have a member value which is convertible to bool, is not hidden, and is unambiguously available in the type.

The specialization disjunction<

B_{1}

, …,

B_{N}

> has a public and unambiguous base that is either

(10.1)
the first type $B_{i}$ in the list false_type, $B_{1}$ , …, $B_{N}$ for which bool( $B_{i}$ ::value) is true, or
(10.2)
if there is no such $B_{i}$ , the last type in the list.

This means a specialization of disjunction does not necessarily inherit from either true_type or false_type.

— end note

]

The member names of the base class, other than disjunction and operator=, shall not be hidden and shall be unambiguously available in disjunction.

template<class B> struct negation : see below { };

The class template negation forms the logical negation of its template type argument.

The type negation is a Cpp17UnaryTypeTrait with a base characteristic of bool_constant<!bool(B::value)>.

20.15.9 Member relationships [meta.member]

template<class S, class M>
  constexpr bool is_pointer_interconvertible_with_class(M S::*m) noexcept;

Mandates: S is a complete type.

Returns: true if and only if S is a standard-layout type, M is an object type, m is not null, and each object s of type S is pointer-interconvertible ([basic.compound]) with its subobject s.*m.

template<class S1, class S2, class M1, class M2>
  constexpr bool is_corresponding_member(M1 S1::*m1, M2 S2::*m2) noexcept;

Mandates: S1 and S2 are complete types.

Returns: true if and only if S1 and S2 are standard-layout types, M1 and M2 are object types, m1 and m2 are not null, and m1 and m2 point to corresponding members of the common initial sequence ([class.mem]) of S1 and S2.

The type of a pointer-to-member expression &C::b is not always a pointer to member of C, leading to potentially surprising results when using these functions in conjunction with inheritance.

struct A { int a; };                    // a standard-layout class
struct B { int b; };                    // a standard-layout class
struct C: public A, public B { };       // not a standard-layout class

static_assert( is_pointer_interconvertible_with_class( &C::b ) );
  // Succeeds because, despite its appearance, &C::b has type
  // “pointer to member of B of type int”.
static_assert( is_pointer_interconvertible_with_class<C>( &C::b ) );
  // Forces the use of class C, and fails.

static_assert( is_corresponding_member( &C::a, &C::b ) );
  // Succeeds because, despite its appearance, &C::a and &C::b have types
  // “pointer to member of A of type int” and
  // “pointer to member of B of type int”, respectively.
static_assert( is_corresponding_member<C, C>( &C::a, &C::b ) );
  // Forces the use of class C, and fails.

— end example

]

— end note

]

20.15.10 Constant evaluation context [meta.const.eval]

constexpr bool is_constant_evaluated() noexcept;

Returns: true if and only if evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated ([expr.const]).

constexpr void f(unsigned char *p, int n) {
  if (std::is_constant_evaluated()) {           // should not be a constexpr if statement
    for (int k = 0; k<n; ++k) p[k] = 0;
  } else {
    memset(p, 0, n);                            // not a core constant expression
  }
}

— end example

]

20.16 Compile-time rational arithmetic [ratio]

20.16.1 In general [ratio.general]

Subclause [ratio] describes the ratio library.

It provides a class template ratio which exactly represents any finite rational number with a numerator and denominator representable by compile-time constants of type intmax_t.

Throughout subclause [ratio], the names of template parameters are used to express type requirements.

If a template parameter is named R1 or R2, and the template argument is not a specialization of the ratio template, the program is ill-formed.

20.16.2 Header <ratio> synopsis [ratio.syn]

namespace std {
  // [ratio.ratio], class template ratio
  template<intmax_t N, intmax_t D = 1> class ratio;

  // [ratio.arithmetic], ratio arithmetic
  template<class R1, class R2> using ratio_add = see below;
  template<class R1, class R2> using ratio_subtract = see below;
  template<class R1, class R2> using ratio_multiply = see below;
  template<class R1, class R2> using ratio_divide = see below;

  // [ratio.comparison], ratio comparison
  template<class R1, class R2> struct ratio_equal;
  template<class R1, class R2> struct ratio_not_equal;
  template<class R1, class R2> struct ratio_less;
  template<class R1, class R2> struct ratio_less_equal;
  template<class R1, class R2> struct ratio_greater;
  template<class R1, class R2> struct ratio_greater_equal;

  template<class R1, class R2>
    inline constexpr bool ratio_equal_v = ratio_equal<R1, R2>::value;
  template<class R1, class R2>
    inline constexpr bool ratio_not_equal_v = ratio_not_equal<R1, R2>::value;
  template<class R1, class R2>
    inline constexpr bool ratio_less_v = ratio_less<R1, R2>::value;
  template<class R1, class R2>
    inline constexpr bool ratio_less_equal_v = ratio_less_equal<R1, R2>::value;
  template<class R1, class R2>
    inline constexpr bool ratio_greater_v = ratio_greater<R1, R2>::value;
  template<class R1, class R2>
    inline constexpr bool ratio_greater_equal_v = ratio_greater_equal<R1, R2>::value;

  // [ratio.si], convenience SI typedefs
  using yocto = ratio<1, 1'000'000'000'000'000'000'000'000>;  // see below
  using zepto = ratio<1,     1'000'000'000'000'000'000'000>;  // see below
  using atto  = ratio<1,         1'000'000'000'000'000'000>;
  using femto = ratio<1,             1'000'000'000'000'000>;
  using pico  = ratio<1,                 1'000'000'000'000>;
  using nano  = ratio<1,                     1'000'000'000>;
  using micro = ratio<1,                         1'000'000>;
  using milli = ratio<1,                             1'000>;
  using centi = ratio<1,                               100>;
  using deci  = ratio<1,                                10>;
  using deca  = ratio<                               10, 1>;
  using hecto = ratio<                              100, 1>;
  using kilo  = ratio<                            1'000, 1>;
  using mega  = ratio<                        1'000'000, 1>;
  using giga  = ratio<                    1'000'000'000, 1>;
  using tera  = ratio<                1'000'000'000'000, 1>;
  using peta  = ratio<            1'000'000'000'000'000, 1>;
  using exa   = ratio<        1'000'000'000'000'000'000, 1>;
  using zetta = ratio<    1'000'000'000'000'000'000'000, 1>;  // see below
  using yotta = ratio<1'000'000'000'000'000'000'000'000, 1>;  // see below
}

20.16.3 Class template ratio [ratio.ratio]

namespace std {
  template<intmax_t N, intmax_t D = 1> class ratio {
  public:
    static constexpr intmax_t num;
    static constexpr intmax_t den;
    using type = ratio<num, den>;
  };
}

If the template argument D is zero or the absolute values of either of the template arguments N and D is not representable by type intmax_t, the program is ill-formed.

These rules ensure that infinite ratios are avoided and that for any negative input, there exists a representable value of its absolute value which is positive.

This excludes the most negative value.

— end note

]

The static data members num and den shall have the following values, where gcd represents the greatest common divisor of the absolute values of N and D:

(2.1)
num shall have the value sign(N) * sign(D) * abs(N) / gcd.
(2.2)
den shall have the value abs(D) / gcd.

20.16.4 Arithmetic on ratios [ratio.arithmetic]

Each of the alias templates ratio_add, ratio_subtract, ratio_multiply, and ratio_divide denotes the result of an arithmetic computation on two ratios R1 and R2.

With X and Y computed (in the absence of arithmetic overflow) as specified by Table 58, each alias denotes a ratio<U, V> such that U is the same as ratio<X, Y>::num and V is the same as ratio<X, Y>::den.

If it is not possible to represent U or V with intmax_t, the program is ill-formed.

Otherwise, an implementation should yield correct values of U and V.

If it is not possible to represent X or Y with intmax_t, the program is ill-formed unless the implementation yields correct values of U and V.

Table 58: Expressions used to perform ratio arithmetic [tab:ratio.arithmetic]

Type	Value of X	Value of Y
ratio_add<R1, R2>	R1::num * R2::den +	R1::den * R2::den
	R2::num * R1::den
ratio_subtract<R1, R2>	R1::num * R2::den -	R1::den * R2::den
	R2::num * R1::den
ratio_multiply<R1, R2>	R1::num * R2::num	R1::den * R2::den
ratio_divide<R1, R2>	R1::num * R2::den	R1::den * R2::num

static_assert(ratio_add<ratio<1, 3>, ratio<1, 6>>::num == 1, "1/3+1/6 == 1/2");
static_assert(ratio_add<ratio<1, 3>, ratio<1, 6>>::den == 2, "1/3+1/6 == 1/2");
static_assert(ratio_multiply<ratio<1, 3>, ratio<3, 2>>::num == 1, "1/3*3/2 == 1/2");
static_assert(ratio_multiply<ratio<1, 3>, ratio<3, 2>>::den == 2, "1/3*3/2 == 1/2");

// The following cases may cause the program to be ill-formed under some implementations
static_assert(ratio_add<ratio<1, INT_MAX>, ratio<1, INT_MAX>>::num == 2,
  "1/MAX+1/MAX == 2/MAX");
static_assert(ratio_add<ratio<1, INT_MAX>, ratio<1, INT_MAX>>::den == INT_MAX,
  "1/MAX+1/MAX == 2/MAX");
static_assert(ratio_multiply<ratio<1, INT_MAX>, ratio<INT_MAX, 2>>::num == 1,
  "1/MAX * MAX/2 == 1/2");
static_assert(ratio_multiply<ratio<1, INT_MAX>, ratio<INT_MAX, 2>>::den == 2,
  "1/MAX * MAX/2 == 1/2");

— end example

]

20.16.5 Comparison of ratios [ratio.comparison]

template<class R1, class R2>
  struct ratio_equal : bool_constant<R1::num == R2::num && R1::den == R2::den> { };

template<class R1, class R2>
  struct ratio_not_equal : bool_constant<!ratio_equal_v<R1, R2>> { };

template<class R1, class R2>
  struct ratio_less : bool_constant<see below> { };

If R1::num × R2::den is less than R2::num × R1::den, ratio_less<R1, R2> shall be derived from bool_constant<true>; otherwise it shall be derived from bool_constant<false>.

Implementations may use other algorithms to compute this relationship to avoid overflow.

If overflow occurs, the program is ill-formed.

template<class R1, class R2>
  struct ratio_less_equal : bool_constant<!ratio_less_v<R2, R1>> { };

template<class R1, class R2>
  struct ratio_greater : bool_constant<ratio_less_v<R2, R1>> { };

template<class R1, class R2>
  struct ratio_greater_equal : bool_constant<!ratio_less_v<R1, R2>> { };

20.16.6 SI types for ratio [ratio.si]

For each of the typedef-names yocto, zepto, zetta, and yotta, if both of the constants used in its specification are representable by intmax_t, the typedef is defined; if either of the constants is not representable by intmax_t, the typedef is not defined.

20.17 Class type_index [type.index]

20.17.1 Header <typeindex> synopsis [type.index.synopsis]

#include <compare>              // see [compare.syn]

namespace std {
  class type_index;
  template<class T> struct hash;
  template<> struct hash<type_index>;
}

20.17.2 type_index overview [type.index.overview]

namespace std {
  class type_index {
  public:
    type_index(const type_info& rhs) noexcept;
    bool operator==(const type_index& rhs) const noexcept;
    bool operator< (const type_index& rhs) const noexcept;
    bool operator> (const type_index& rhs) const noexcept;
    bool operator<=(const type_index& rhs) const noexcept;
    bool operator>=(const type_index& rhs) const noexcept;
    strong_ordering operator<=>(const type_index& rhs) const noexcept;
    size_t hash_code() const noexcept;
    const char* name() const noexcept;

  private:
    const type_info* target;    // exposition only
    // Note that the use of a pointer here, rather than a reference,
    // means that the default copy/move constructor and assignment
    // operators will be provided and work as expected.
  };
}

The class type_index provides a simple wrapper for type_info which can be used as an index type in associative containers and in unordered associative containers .

20.17.3 type_index members [type.index.members]

type_index(const type_info& rhs) noexcept;

Effects: Constructs a type_index object, the equivalent of target = &rhs.

bool operator==(const type_index& rhs) const noexcept;

Returns: *target == *rhs.target.

bool operator<(const type_index& rhs) const noexcept;

Returns: target->before(*rhs.target).

bool operator>(const type_index& rhs) const noexcept;

Returns: rhs.target->before(*target).

bool operator<=(const type_index& rhs) const noexcept;

Returns: !rhs.target->before(*target).

bool operator>=(const type_index& rhs) const noexcept;

Returns: !target->before(*rhs.target).

strong_ordering operator<=>(const type_index& rhs) const noexcept;

Effects: Equivalent to:

if (*target == *rhs.target) return strong_ordering::equal;
if (target->before(*rhs.target)) return strong_ordering::less;
return strong_ordering::greater;

size_t hash_code() const noexcept;

Returns: target->hash_code().

const char* name() const noexcept;

Returns: target->name().

20.17.4 Hash support [type.index.hash]

template<> struct hash<type_index>;

For an object index of type type_index, hash<type_index>()(index) shall evaluate to the same result as index.hash_code().

20.18 Execution policies [execpol]

20.18.1 In general [execpol.general]

Subclause [execpol] describes classes that are execution policy types.

An object of an execution policy type indicates the kinds of parallelism allowed in the execution of an algorithm and expresses the consequent requirements on the element access functions.

using namespace std;
vector<int> v = /* ... */;

// standard sequential sort
sort(v.begin(), v.end());

// explicitly sequential sort
sort(execution::seq, v.begin(), v.end());

// permitting parallel execution
sort(execution::par, v.begin(), v.end());

// permitting vectorization as well
sort(execution::par_unseq, v.begin(), v.end());

— end example

]

Because different parallel architectures may require idiosyncratic parameters for efficient execution, implementations may provide additional execution policies to those described in this standard as extensions.

— end note

]

20.18.2 Header <execution> synopsis [execution.syn]

namespace std {
  // [execpol.type], execution policy type trait
  template<class T> struct is_execution_policy;
  template<class T> inline constexpr bool is_execution_policy_v = is_execution_policy<T>::value;
}

namespace std::execution {
  // [execpol.seq], sequenced execution policy
  class sequenced_policy;

  // [execpol.par], parallel execution policy
  class parallel_policy;

  // [execpol.parunseq], parallel and unsequenced execution policy
  class parallel_unsequenced_policy;

  // [execpol.unseq], unsequenced execution policy
  class unsequenced_policy;

  // [execpol.objects], execution policy objects
  inline constexpr sequenced_policy            seq{ unspecified };
  inline constexpr parallel_policy             par{ unspecified };
  inline constexpr parallel_unsequenced_policy par_unseq{ unspecified };
  inline constexpr unsequenced_policy          unseq{ unspecified };
}

20.18.3 Execution policy type trait [execpol.type]

template<class T> struct is_execution_policy { see below };

is_execution_policy can be used to detect execution policies for the purpose of excluding function signatures from otherwise ambiguous overload resolution participation.

is_execution_policy<T> is a Cpp17UnaryTypeTrait with a base characteristic of true_type if T is the type of a standard or implementation-defined execution policy, otherwise false_type.

This provision reserves the privilege of creating non-standard execution policies to the library implementation.

— end note

]

The behavior of a program that adds specializations for is_execution_policy is undefined.

20.18.4 Sequenced execution policy [execpol.seq]

class execution::sequenced_policy { unspecified };

The class execution::sequenced_policy is an execution policy type used as a unique type to disambiguate parallel algorithm overloading and require that a parallel algorithm's execution may not be parallelized.

During the execution of a parallel algorithm with the execution::sequenced_policy policy, if the invocation of an element access function exits via an uncaught exception, terminate() is called.

20.18.5 Parallel execution policy [execpol.par]

class execution::parallel_policy { unspecified };

The class execution::parallel_policy is an execution policy type used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm's execution may be parallelized.

During the execution of a parallel algorithm with the execution::parallel_policy policy, if the invocation of an element access function exits via an uncaught exception, terminate() is called.

20.18.6 Parallel and unsequenced execution policy [execpol.parunseq]

class execution::parallel_unsequenced_policy { unspecified };

The class execution::parallel_unsequenced_policy is an execution policy type used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm's execution may be parallelized and vectorized.

During the execution of a parallel algorithm with the execution::parallel_unsequenced_policy policy, if the invocation of an element access function exits via an uncaught exception, terminate() is called.

20.18.7 Unsequenced execution policy [execpol.unseq]

class execution::unsequenced_policy { unspecified };

The class unsequenced_policy is an execution policy type used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm's execution may be vectorized, e.g., executed on a single thread using instructions that operate on multiple data items.

During the execution of a parallel algorithm with the execution::unsequenced_policy policy, if the invocation of an element access function exits via an uncaught exception, terminate() is called.

20.18.8 Execution policy objects [execpol.objects]

inline constexpr execution::sequenced_policy            execution::seq{ unspecified };
inline constexpr execution::parallel_policy             execution::par{ unspecified };
inline constexpr execution::parallel_unsequenced_policy execution::par_unseq{ unspecified };
inline constexpr execution::unsequenced_policy          execution::unseq{ unspecified };

The header <execution> declares global objects associated with each type of execution policy.

20.19 Primitive numeric conversions [charconv]

20.19.1 Header <charconv> synopsis [charconv.syn]

namespace std {
  // floating-point format for primitive numerical conversion
  enum class chars_format {
    scientific = unspecified,
    fixed = unspecified,
    hex = unspecified,
    general = fixed | scientific
  };


  // [charconv.to.chars], primitive numerical output conversion
  struct to_chars_result {
    char* ptr;
    errc ec;
    friend bool operator==(const to_chars_result&, const to_chars_result&) = default;
  };

  to_chars_result to_chars(char* first, char* last, see below value, int base = 10);
  to_chars_result to_chars(char* first, char* last, bool value, int base = 10) = delete;

  to_chars_result to_chars(char* first, char* last, float value);
  to_chars_result to_chars(char* first, char* last, double value);
  to_chars_result to_chars(char* first, char* last, long double value);

  to_chars_result to_chars(char* first, char* last, float value, chars_format fmt);
  to_chars_result to_chars(char* first, char* last, double value, chars_format fmt);
  to_chars_result to_chars(char* first, char* last, long double value, chars_format fmt);

  to_chars_result to_chars(char* first, char* last, float value,
                           chars_format fmt, int precision);
  to_chars_result to_chars(char* first, char* last, double value,
                           chars_format fmt, int precision);
  to_chars_result to_chars(char* first, char* last, long double value,
                           chars_format fmt, int precision);


  // [charconv.from.chars], primitive numerical input conversion
  struct from_chars_result {
    const char* ptr;
    errc ec;
    friend bool operator==(const from_chars_result&, const from_chars_result&) = default;
  };

  from_chars_result from_chars(const char* first, const char* last,
                               see below& value, int base = 10);

  from_chars_result from_chars(const char* first, const char* last, float& value,
                               chars_format fmt = chars_format::general);
  from_chars_result from_chars(const char* first, const char* last, double& value,
                               chars_format fmt = chars_format::general);
  from_chars_result from_chars(const char* first, const char* last, long double& value,
                               chars_format fmt = chars_format::general);
}

The type chars_format is a bitmask type ([bitmask.types]) with elements scientific, fixed, and hex.

The types to_chars_result and from_chars_result have the data members and special members specified above.

They have no base classes or members other than those specified.

20.19.2 Primitive numeric output conversion [charconv.to.chars]

All functions named to_chars convert value into a character string by successively filling the range [first, last), where [first, last) is required to be a valid range.

If the member ec of the return value is such that the value is equal to the value of a value-initialized errc, the conversion was successful and the member ptr is the one-past-the-end pointer of the characters written.

Otherwise, the member ec has the value errc::value_too_large, the member ptr has the value last, and the contents of the range [first, last) are unspecified.

The functions that take a floating-point value but not a precision parameter ensure that the string representation consists of the smallest number of characters such that there is at least one digit before the radix point (if present) and parsing the representation using the corresponding from_chars function recovers value exactly.

This guarantee applies only if to_chars and from_chars are executed on the same implementation.

— end note

]

If there are several such representations, the representation with the smallest difference from the floating-point argument value is chosen, resolving any remaining ties using rounding according to round_to_nearest .

The functions taking a chars_format parameter determine the conversion specifier for printf as follows: The conversion specifier is f if fmt is chars_format::fixed, e if fmt is chars_format::scientific, a (without leading "0x" in the result) if fmt is chars_format::hex, and g if fmt is chars_format::general.

to_chars_result to_chars(char* first, char* last, see below value, int base = 10);

Preconditions: base has a value between 2 and 36 (inclusive).

Effects: The value of value is converted to a string of digits in the given base (with no redundant leading zeroes).

Digits in the range 10.

35 (inclusive) are represented as lowercase characters a.

If value is less than zero, the representation starts with '-'.

Throws: Nothing.

Remarks: The implementation shall provide overloads for all signed and unsigned integer types and char as the type of the parameter value.

to_chars_result to_chars(char* first, char* last, float value);
to_chars_result to_chars(char* first, char* last, double value);
to_chars_result to_chars(char* first, char* last, long double value);

Effects: value is converted to a string in the style of printf in the "C" locale.

The conversion specifier is f or e, chosen according to the requirement for a shortest representation (see above); a tie is resolved in favor of f.

Throws: Nothing.

to_chars_result to_chars(char* first, char* last, float value, chars_format fmt);
to_chars_result to_chars(char* first, char* last, double value, chars_format fmt);
to_chars_result to_chars(char* first, char* last, long double value, chars_format fmt);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: value is converted to a string in the style of printf in the "C" locale.

Throws: Nothing.

to_chars_result to_chars(char* first, char* last, float value,
                         chars_format fmt, int precision);
to_chars_result to_chars(char* first, char* last, double value,
                         chars_format fmt, int precision);
to_chars_result to_chars(char* first, char* last, long double value,
                         chars_format fmt, int precision);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: value is converted to a string in the style of printf in the "C" locale with the given precision.

Throws: Nothing.

20.19.3 Primitive numeric input conversion [charconv.from.chars]

All functions named from_chars analyze the string [first, last) for a pattern, where [first, last) is required to be a valid range.

If no characters match the pattern, value is unmodified, the member ptr of the return value is first and the member ec is equal to errc::invalid_argument.

If the pattern allows for an optional sign, but the string has no digit characters following the sign, no characters match the pattern.

— end note

]

Otherwise, the characters matching the pattern are interpreted as a representation of a value of the type of value.

The member ptr of the return value points to the first character not matching the pattern, or has the value last if all characters match.

If the parsed value is not in the range representable by the type of value, value is unmodified and the member ec of the return value is equal to errc::result_out_of_range.

Otherwise, value is set to the parsed value, after rounding according to round_to_nearest, and the member ec is value-initialized.

from_chars_result from_chars(const char* first, const char* last,
                             see below& value, int base = 10);

Preconditions: base has a value between 2 and 36 (inclusive).

Effects: The pattern is the expected form of the subject sequence in the "C" locale for the given nonzero base, as described for strtol, except that no "0x" or "0X" prefix shall appear if the value of base is 16, and except that '-' is the only sign that may appear, and only if value has a signed type.

Throws: Nothing.

Remarks: The implementation shall provide overloads for all signed and unsigned integer types and char as the referenced type of the parameter value.

from_chars_result from_chars(const char* first, const char* last, float& value,
                             chars_format fmt = chars_format::general);
from_chars_result from_chars(const char* first, const char* last, double& value,
                             chars_format fmt = chars_format::general);
from_chars_result from_chars(const char* first, const char* last, long double& value,
                             chars_format fmt = chars_format::general);

Preconditions: fmt has the value of one of the enumerators of chars_format.

Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that

(7.1)
the sign '+' may only appear in the exponent part;
(7.2)
if fmt has chars_format::scientific set but not chars_format::fixed, the otherwise optional exponent part shall appear;
(7.3)
if fmt has chars_format::fixed set but not chars_format::scientific, the optional exponent part shall not appear; and
(7.4)
if fmt is chars_format::hex, the prefix "0x" or "0X" is assumed.
[Example
: The string 0x123 is parsed to have the value 0 with remaining characters x123. — end example
]

In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern.

Throws: Nothing.

20.20 Formatting [format]

20.20.1 Header <format> synopsis [format.syn]

namespace std {
  // [format.functions], formatting functions
  template<class... Args>
    string format(string_view fmt, const Args&... args);
  template<class... Args>
    wstring format(wstring_view fmt, const Args&... args);
  template<class... Args>
    string format(const locale& loc, string_view fmt, const Args&... args);
  template<class... Args>
    wstring format(const locale& loc, wstring_view fmt, const Args&... args);

  string vformat(string_view fmt, format_args args);
  wstring vformat(wstring_view fmt, wformat_args args);
  string vformat(const locale& loc, string_view fmt, format_args args);
  wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);

  template<class Out, class... Args>
    Out format_to(Out out, string_view fmt, const Args&... args);
  template<class Out, class... Args>
    Out format_to(Out out, wstring_view fmt, const Args&... args);
  template<class Out, class... Args>
    Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args);
  template<class Out, class... Args>
    Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);

  template<class Out>
    Out vformat_to(Out out, string_view fmt,
                   format_args_t<type_identity_t<Out>, char> args);
  template<class Out>
    Out vformat_to(Out out, wstring_view fmt,
                   format_args_t<type_identity_t<Out>, wchar_t> args);
  template<class Out>
    Out vformat_to(Out out, const locale& loc, string_view fmt,
                   format_args_t<type_identity_t<Out>, char> args);
  template<class Out>
    Out vformat_to(Out out, const locale& loc, wstring_view fmt,
                   format_args_t<type_identity_t<Out>, wchar_t> args);

  template<class Out> struct format_to_n_result {
    Out out;
    iter_difference_t<Out> size;
  };
  template<class Out, class... Args>
    format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                        string_view fmt, const Args&... args);
  template<class Out, class... Args>
    format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                        wstring_view fmt, const Args&... args);
  template<class Out, class... Args>
    format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                        const locale& loc, string_view fmt,
                                        const Args&... args);
  template<class Out, class... Args>
    format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                        const locale& loc, wstring_view fmt,
                                        const Args&... args);

  template<class... Args>
    size_t formatted_size(string_view fmt, const Args&... args);
  template<class... Args>
    size_t formatted_size(wstring_view fmt, const Args&... args);
  template<class... Args>
    size_t formatted_size(const locale& loc, string_view fmt, const Args&... args);
  template<class... Args>
    size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);

  // [format.formatter], formatter
  template<class T, class charT = char> struct formatter;

  // [format.parse.ctx], class template basic_format_parse_context
  template<class charT> class basic_format_parse_context;
  using format_parse_context = basic_format_parse_context<char>;
  using wformat_parse_context = basic_format_parse_context<wchar_t>;

  // [format.context], class template basic_format_context
  template<class Out, class charT> class basic_format_context;
  using format_context = basic_format_context<unspecified, char>;
  using wformat_context = basic_format_context<unspecified, wchar_t>;

  // [format.arguments], arguments
  // [format.arg], class template basic_format_arg
  template<class Context> class basic_format_arg;

  template<class Visitor, class Context>
    see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);

  // [format.arg.store], class template format-arg-store
  template<class Context, class... Args> struct format-arg-store;      // exposition only

  template<class Context = format_context, class... Args>
    format-arg-store<Context, Args...>
      make_format_args(const Args&... args);
  template<class... Args>
    format-arg-store<wformat_context, Args...>
      make_wformat_args(const Args&... args);

  // [format.args], class template basic_format_args
  template<class Context> class basic_format_args;
  using format_args = basic_format_args<format_context>;
  using wformat_args = basic_format_args<wformat_context>;

  template<class Out, class charT>
    using format_args_t = basic_format_args<basic_format_context<Out, charT>>;

  // [format.error], class format_error
  class format_error;
}

The class template format_to_n_result has the template parameters, data members, and special members specified above.

It has no base classes or members other than those specified.

20.20.2 Format string [format.string]

20.20.2.1 In general [format.string.general]

A format string for arguments args is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than { and }.

Let charT be the character type of the format string.

Each character that is not part of a replacement field or an escape sequence is copied unchanged to the output.

An escape sequence is one of {{ or }}.

It is replaced with { or }, respectively, in the output.

The syntax of replacement fields is as follows:

replacement-field:
{ arg-id

_{o p t}

format-specifier

_{o p t}

}

arg-id:
0
positive-integer

positive-integer:
nonzero-digit
positive-integer digit

nonnegative-integer:
digit
nonnegative-integer digit

nonzero-digit: one of
1 2 3 4 5 6 7 8 9

digit: one of
0 1 2 3 4 5 6 7 8 9

format-specifier:
: format-spec

format-spec:
as specified by the formatter specialization for the argument type

The arg-id field specifies the index of the argument in args whose value is to be formatted and inserted into the output instead of the replacement field.

If there is no argument with the index arg-id in args, the string is not a format string for args.

The optional format-specifier field explicitly specifies a format for the replacement value.

string s = format("{0}-{{", 8);         // value of s is "8-{"

— end example

]

If all arg-ids in a format string are omitted (including those in the format-spec, as interpreted by the corresponding formatter specialization), argument indices 0, 1, 2, … will automatically be used in that order.

If some arg-ids are omitted and some are present, the string is not a format string.

A format string cannot contain a mixture of automatic and manual indexing.

— end note

]

string s0 = format("{} to {}",   "a", "b"); // OK, automatic indexing
string s1 = format("{1} to {0}", "a", "b"); // OK, manual indexing
string s2 = format("{0} to {}",  "a", "b"); // not a format string (mixing automatic and manual indexing),
                                            // throws format_error
string s3 = format("{} to {1}",  "a", "b"); // not a format string (mixing automatic and manual indexing),
                                            // throws format_error

— end example

]

The format-spec field contains format specifications that define how the value should be presented.

Each type can define its own interpretation of the format-spec field.

If format-spec does not conform to the format specifications for the argument type referred to by arg-id, the string is not a format string for args.

For arithmetic, pointer, and string types the format-spec is interpreted as a std-format-spec as described in ([format.string.std]).
For chrono types the format-spec is interpreted as a chrono-format-spec as described in ([time.format]).
For user-defined formatter specializations, the behavior of the parse member function determines how the format-spec is interpreted.

— end example

]

20.20.2.2 Standard format specifiers [format.string.std]

Each formatter specializations described in [format.formatter.spec] for fundamental and string types interprets format-spec as a std-format-spec.

The format specification can be used to specify such details as field width, alignment, padding, and decimal precision.

Some of the formatting options are only supported for arithmetic types.

— end note

]

The syntax of format specifications is as follows:

std-format-spec:
fill-and-align

_{o p t}

sign

_{o p t}

_{o p t}

_{o p t}

width

_{o p t}

precision

_{o p t}

_{o p t}

type

_{o p t}

fill-and-align:
fill

_{o p t}

align

fill:
any character other than { or }

align: one of
< > ^

sign: one of
+ - space

width:
positive-integer
{ arg-id

_{o p t}

}

precision:
. nonnegative-integer
. { arg-id

_{o p t}

}

type: one of
a A b B c d e E f F g G o p s x X

The fill character can be any character other than { or }.

The presence of a fill character is signaled by the character following it, which must be one of the alignment options.

If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent.

— end note

]

The align specifier applies to all argument types.

The meaning of the various alignment options is as specified in Table 59 .

char c = 120;
string s0 = format("{:6}", 42);         // value of s0 is "    42"
string s1 = format("{:6}", 'x');        // value of s1 is "x     "
string s2 = format("{:*<6}", 'x');      // value of s2 is "x*****"
string s3 = format("{:*>6}", 'x');      // value of s3 is "*****x"
string s4 = format("{:*^6}", 'x');      // value of s4 is "**x***"
string s5 = format("{:6d}", c);         // value of s5 is "   120"
string s6 = format("{:6}", true);       // value of s6 is "true  "

— end example

]

Unless a minimum field width is defined, the field width is determined by the size of the content and the alignment option has no effect.

— end note

]

Table 59: Meaning of align options [tab:format.align]

Option	Meaning
<	Forces the field to be aligned to the start of the available space. This is the default for non-arithmetic types, charT, and bool, unless an integer presentation type is specified.
>	Forces the field to be aligned to the end of the available space. This is the default for arithmetic types other than charT and bool or when an integer presentation type is specified.
^	Forces the field to be centered within the available space by inserting $⌊ \frac{n}{2} ⌋$ characters before and $⌈ \frac{n}{2} ⌉$ characters after the value, where n is the total number of fill characters to insert.

The sign option is only valid for arithmetic types other than charT and bool or when an integer presentation type is specified.

The meaning of the various options is as specified in Table 60 .

Table 60: Meaning of sign options [tab:format.sign]

Option	Meaning
+	Indicates that a sign should be used for both non-negative and negative numbers. The + sign is inserted before the output of to_chars for non-negative numbers other than negative zero. [Note : For negative numbers and negative zero the output of to_chars will already contain the sign so no additional transformation is performed. — end note ]
-	Indicates that a sign should be used for negative numbers and negative zero only (this is the default behavior).
space	Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.

The sign option applies to floating-point infinity and NaN.

double inf = numeric_limits<double>::infinity();
double nan = numeric_limits<double>::quiet_NaN();
string s0 = format("{0:},{0:+},{0:-},{0: }", 1);        // value of s0 is "1,+1,1, 1"
string s1 = format("{0:},{0:+},{0:-},{0: }", -1);       // value of s1 is "-1,-1,-1,-1"
string s2 = format("{0:},{0:+},{0:-},{0: }", inf);      // value of s2 is "inf,+inf,inf, inf"
string s3 = format("{0:},{0:+},{0:-},{0: }", nan);      // value of s3 is "nan,+nan,nan, nan"

— end example

]

The # option causes the alternate form to be used for the conversion.

This option is valid for arithmetic types other than charT and bool or when an integer presentation type is specified, and not otherwise.

For integral types, the alternate form inserts the base prefix (if any) specified in Table 62 into the output after the sign character (possibly space) if there is one, or before the output of to_chars otherwise.

For floating-point types, the alternate form causes the result of the conversion of finite values to always contain a decimal-point character, even if no digits follow it.

Normally, a decimal-point character appears in the result of these conversions only if a digit follows it.

In addition, for g and G conversions, trailing zeros are not removed from the result.

If { arg-id

_{o p t}

} is used in a width or precision, the value of the corresponding formatting argument is used in its place.

If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of type format_error is thrown.

The positive-integer in width is a decimal integer defining the minimum field width.

If width is not specified, there is no minimum field width, and the field width is determined based on the content of the field.

The width of a string is defined as the estimated number of column positions appropriate for displaying it in a terminal.

This is similar to the semantics of the POSIX wcswidth function.

— end note

]

For the purposes of width computation, a string is assumed to be in a locale-independent, implementation-defined encoding.

Implementations should use a Unicode encoding on platforms capable of displaying Unicode text in a terminal.

This is the case for Windows-based and many POSIX-based operating systems.

— end note

]

For a string in a Unicode encoding, implementations should estimate the width of a string as the sum of estimated widths of the first code points in its extended grapheme clusters.

The extended grapheme clusters of a string are defined by UAX #29.

The estimated width of the following code points is 2:

(11.1)
U+1100-U+115F
(11.2)
U+2329-U+232A
(11.3)
U+2E80-U+303E
(11.4)
U+3040-U+A4CF
(11.5)
U+AC00-U+D7A3
(11.6)
U+F900-U+FAFF
(11.7)
U+FE10-U+FE19
(11.8)
U+FE30-U+FE6F
(11.9)
U+FF00-U+FF60
(11.10)
U+FFE0-U+FFE6
(11.11)
U+1F300-U+1F64F
(11.12)
U+1F900-U+1F9FF
(11.13)
U+20000-U+2FFFD
(11.14)
U+30000-U+3FFFD

The estimated width of other code points is 1.

For a string in a non-Unicode encoding, the width of a string is unspecified.

A zero (0) character preceding the width field pads the field with leading zeros (following any indication of sign or base) to the field width, except when applied to an infinity or NaN. This option is only valid for arithmetic types other than charT and bool or when an integer presentation type is specified.

If the 0 character and an align option both appear, the 0 character is ignored.

char c = 120;
string s1 = format("{:+06d}", c);       // value of s1 is "+00120"
string s2 = format("{:#06x}", 0xa);     // value of s2 is "0x000a"
string s3 = format("{:<06}", -42);      // value of s3 is "-42   " (0 is ignored because of < alignment)

— end example

]

The nonnegative-integer in precision is a decimal integer defining the precision or maximum field size.

It can only be used with floating-point and string types.

For floating-point types this field specifies the formatting precision.

For string types, this field provides an upper bound for the estimated width of the prefix of the input string that is copied into the output.

For a string in a Unicode encoding, the formatter copies to the output the longest prefix of whole extended grapheme clusters whose estimated width is no greater than the precision.

When the L option is used, the form used for the conversion is called the locale-specific form.

The L option is only valid for arithmetic types, and its effect depends upon the type.

(15.1)
For integral types, the locale-specific form causes the context's locale to be used to insert the appropriate digit group separator characters.
(15.2)
For floating-point types, the locale-specific form causes the context's locale to be used to insert the appropriate digit group and radix separator characters.
(15.3)
For the textual representation of bool, the locale-specific form causes the context's locale to be used to insert the appropriate string as if obtained with numpunct::truename or numpunct::falsename.

The type determines how the data should be presented.

The available string presentation types are specified in Table 61 .

Table 61: Meaning of type options for strings [tab:format.type.string]

Type	Meaning
none, s	Copies the string to the output.

The meaning of some non-string presentation types is defined in terms of a call to to_chars.

In such cases, let [first, last) be a range large enough to hold the to_chars output and value be the formatting argument value.

Formatting is done as if by calling to_chars as specified and copying the output through the output iterator of the format context.

Additional padding and adjustments are performed prior to copying the output through the output iterator as specified by the format specifiers.

— end note

]

The available integer presentation types for integral types other than bool and charT are specified in Table 62 .

string s0 = format("{}", 42);                           // value of s0 is "42"
string s1 = format("{0:b} {0:d} {0:o} {0:x}", 42);      // value of s1 is "101010 42 52 2a"
string s2 = format("{0:#x} {0:#X}", 42);                // value of s2 is "0x2a 0X2A"
string s3 = format("{:L}", 1234);                       // value of s3 might be "1,234"
                                                        // (depending on the locale)

— end example

]

Table 62: Meaning of type options for integer types [tab:format.type.int]

Type	Meaning
b	to_chars(first, last, value, 2); the base prefix is 0b.
B	The same as b, except that the base prefix is 0B.
c	Copies the character static_cast<charT>(value) to the output. Throws format_error if value is not in the range of representable values for charT.
d	to_chars(first, last, value).
o	to_chars(first, last, value, 8); the base prefix is 0 if value is nonzero and is empty otherwise.
x	to_chars(first, last, value, 16); the base prefix is 0x.
X	The same as x, except that it uses uppercase letters for digits above 9 and the base prefix is 0X.
none	The same as d. [Note : If the formatting argument type is charT or bool, the default is instead c or s, respectively. — end note ]

The available charT presentation types are specified in Table 63 .

Table 63: Meaning of type options for charT [tab:format.type.char]

Type	Meaning
none, c	Copies the character to the output.
b, B, d, o, x, X	As specified in Table 62 .

The available bool presentation types are specified in Table 64 .

Table 64: Meaning of type options for bool [tab:format.type.bool]

Type	Meaning
none, s	Copies textual representation, either true or false, to the output.
b, B, c, d, o, x, X	As specified in Table 62 for the value static_cast<unsigned char>(value).

The available floating-point presentation types and their meanings for values other than infinity and NaN are specified in Table 65 .

For lower-case presentation types, infinity and NaN are formatted as inf and nan, respectively.

For upper-case presentation types, infinity and NaN are formatted as INF and NAN, respectively.

In either case, a sign is included if indicated by the sign option.

— end note

]

Table 65: Meaning of type options for floating-point types [tab:format.type.float]

Type	Meaning
a	If precision is specified, equivalent to to_chars(first, last, value, chars_format::hex, precision) where precision is the specified formatting precision; equivalent to to_chars(first, last, value, chars_format::hex) otherwise.
A	The same as a, except that it uses uppercase letters for digits above 9 and P to indicate the exponent.
e	Equivalent to to_chars(first, last, value, chars_format::scientific, precision) where precision is the specified formatting precision, or 6 if precision is not specified.
E	The same as e, except that it uses E to indicate exponent.
f, F	Equivalent to to_chars(first, last, value, chars_format::fixed, precision) where precision is the specified formatting precision, or 6 if precision is not specified.
g	Equivalent to to_chars(first, last, value, chars_format::general, precision) where precision is the specified formatting precision, or 6 if precision is not specified.
G	The same as g, except that it uses E to indicate exponent.
none	If precision is specified, equivalent to to_chars(first, last, value, chars_format::general, precision) where precision is the specified formatting precision; equivalent to to_chars(first, last, value) otherwise.

The available pointer presentation types and their mapping to to_chars are specified in Table 66 .

Pointer presentation types also apply to nullptr_t.

— end note

]

Table 66: Meaning of type options for pointer types [tab:format.type.ptr]

Type	Meaning
none, p	If uintptr_t is defined, to_chars(first, last, reinterpret_cast<uintptr_t>(value), 16) with the prefix 0x added to the output; otherwise, implementation-defined.

20.20.3 Error reporting [format.err.report]

Formatting functions throw format_error if an argument fmt is passed that is not a format string for args.

They propagate exceptions thrown by operations of formatter specializations and iterators.

Failure to allocate storage is reported by throwing an exception as described in [res.on.exception.handling].

20.20.4 Formatting functions [format.functions]

In the description of the functions, operator + is used for some of the iterator categories for which it does not have to be defined.

In these cases the semantics of a + n are the same as in [algorithms.requirements].

template<class... Args>
  string format(string_view fmt, const Args&... args);

Effects: Equivalent to:

return vformat(fmt, make_format_args(args...));

template<class... Args>
  wstring format(wstring_view fmt, const Args&... args);

Effects: Equivalent to:

return vformat(fmt, make_wformat_args(args...));

template<class... Args>
  string format(const locale& loc, string_view fmt, const Args&... args);

Effects: Equivalent to:

return vformat(loc, fmt, make_format_args(args...));

template<class... Args>
  wstring format(const locale& loc, wstring_view fmt, const Args&... args);

Effects: Equivalent to:

return vformat(loc, fmt, make_wformat_args(args...));

string vformat(string_view fmt, format_args args);
wstring vformat(wstring_view fmt, wformat_args args);
string vformat(const locale& loc, string_view fmt, format_args args);
wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);

Returns: A string object holding the character representation of formatting arguments provided by args formatted according to specifications given in fmt.

If present, loc is used for locale-specific formatting.

Throws: As specified in [format.err.report].

template<class Out, class... Args>
  Out format_to(Out out, string_view fmt, const Args&... args);
template<class Out, class... Args>
  Out format_to(Out out, wstring_view fmt, const Args&... args);

Effects: Equivalent to:

using context = basic_format_context<Out, decltype(fmt)::value_type>;
return vformat_to(out, fmt, make_format_args<context>(args...));

template<class Out, class... Args>
  Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args);
template<class Out, class... Args>
  Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);

Effects: Equivalent to:

using context = basic_format_context<Out, decltype(fmt)::value_type>;
return vformat_to(out, loc, fmt, make_format_args<context>(args...));

template<class Out>
  Out vformat_to(Out out, string_view fmt,
                 format_args_t<type_identity_t<Out>, char> args);
template<class Out>
  Out vformat_to(Out out, wstring_view fmt,
                 format_args_t<type_identity_t<Out>, wchar_t> args);
template<class Out>
  Out vformat_to(Out out, const locale& loc, string_view fmt,
                 format_args_t<type_identity_t<Out>, char> args);
template<class Out>
  Out vformat_to(Out out, const locale& loc, wstring_view fmt,
                 format_args_t<type_identity_t<Out>, wchar_t> args);

Let charT be decltype(fmt)::value_type.

Constraints: Out satisfies output_iterator<const charT&>.

Preconditions: Out models output_iterator<const charT&>.

Effects: Places the character representation of formatting the arguments provided by args, formatted according to the specifications given in fmt, into the range [out, out + N), where N is formatted_size(fmt, args...) for the functions without a loc parameter and formatted_size(loc, fmt, args...) for the functions with a loc parameter.

If present, loc is used for locale-specific formatting.

Returns: out + N.

Throws: As specified in [format.err.report].

template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      string_view fmt, const Args&... args);
template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      wstring_view fmt, const Args&... args);
template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      const locale& loc, string_view fmt,
                                      const Args&... args);
template<class Out, class... Args>
  format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n,
                                      const locale& loc, wstring_view fmt,
                                      const Args&... args);

Let

(16.1)
charT be decltype(fmt)::value_type,
(16.2)
N be formatted_size(fmt, args...) for the functions without a loc parameter and formatted_size(loc, fmt, args...) for the functions with a loc parameter, and
(16.3)
M be clamp(n, 0, N).

Constraints: Out satisfies output_iterator<const charT&>.

Preconditions: Out models output_iterator<const charT&>, and formatter<

T_{i}

, charT> meets the Formatter requirements ([formatter.requirements]) for each

T_{i}

in Args.

Effects: Places the first M characters of the character representation of formatting the arguments provided by args, formatted according to the specifications given in fmt, into the range [out, out + M).

If present, loc is used for locale-specific formatting.

Returns: {out + M, N}.

Throws: As specified in [format.err.report].

template<class... Args>
  size_t formatted_size(string_view fmt, const Args&... args);
template<class... Args>
  size_t formatted_size(wstring_view fmt, const Args&... args);
template<class... Args>
  size_t formatted_size(const locale& loc, string_view fmt, const Args&... args);
template<class... Args>
  size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);

Let charT be decltype(fmt)::value_type.

Preconditions: formatter<

T_{i}

, charT> meets the Formatter requirements ([formatter.requirements]) for each

T_{i}

in Args.

Returns: The number of characters in the character representation of formatting arguments args formatted according to specifications given in fmt.

If present, loc is used for locale-specific formatting.

Throws: As specified in [format.err.report].

20.20.5 Formatter [format.formatter]

20.20.5.1 Formatter requirements [formatter.requirements]

A type F meets the Formatter requirements if:

(1.1)
it meets the
- (1.1.1)
  Cpp17DefaultConstructible (Table 27),
- (1.1.2)
  Cpp17CopyConstructible (Table 29),
- (1.1.3)
  Cpp17CopyAssignable (Table 31), and
- (1.1.4)
  Cpp17Destructible (Table 32)
requirements,
(1.2)
it is swappable ([swappable.requirements]) for lvalues, and
(1.3)
the expressions shown in Table 67 are valid and have the indicated semantics.

Given character type charT, output iterator type Out, and formatting argument type T, in Table 67:

(2.1)
f is a value of type F,
(2.2)
u is an lvalue of type T,
(2.3)
t is a value of a type convertible to (possibly const) T,
(2.4)
PC is basic_format_parse_context<charT>,
(2.5)
FC is basic_format_context<Out, charT>,
(2.6)
pc is an lvalue of type PC, and
(2.7)
fc is an lvalue of type FC.

pc.begin() points to the beginning of the format-spec ([format.string]) of the replacement field being formatted in the format string.

If format-spec is empty then either pc.begin() == pc.end() or *pc.begin() == '}'.

Table 67: Formatter requirements [tab:formatter]

Expression	Return type	Requirement
f.parse(pc)	PC::iterator	Parses format-spec ([format.string]) for type T in the range [pc.begin(), pc.end()) until the first unmatched character. Throws format_error unless the whole range is parsed or the unmatched character is }. [Note : This allows formatters to emit meaningful error messages. — end note ] Stores the parsed format specifiers in *this and returns an iterator past the end of the parsed range.
f.format(t, fc)	FC::iterator	Formats t according to the specifiers stored in *this, writes the output to fc.out() and returns an iterator past the end of the output range. The output shall only depend on t, fc.locale(), and the range [pc.begin(), pc.end()) from the last call to f.parse(pc).
f.format(u, fc)	FC::iterator	As above, but does not modify u.

20.20.5.2 Formatter specializations [format.formatter.spec]

The functions defined in [format.functions] use specializations of the class template formatter to format individual arguments.

Let charT be either char or wchar_t.

Each specialization of formatter is either enabled or disabled, as described below.

Enabled specializations meet the Formatter requirements, and disabled specializations do not.

— end note

]

Each header that declares the template formatter provides the following enabled specializations:

(2.1)

The specializations

template<> struct formatter<char, char>;
template<> struct formatter<char, wchar_t>;
template<> struct formatter<wchar_t, wchar_t>;

(2.2)

For each charT, the string type specializations

template<> struct formatter<charT*, charT>;
template<> struct formatter<const charT*, charT>;
template<size_t N> struct formatter<const charT[N], charT>;
template<class traits, class Allocator>
  struct formatter<basic_string<charT, traits, Allocator>, charT>;
template<class traits>
  struct formatter<basic_string_view<charT, traits>, charT>;

(2.3)
For each charT, for each cv-unqualified arithmetic type ArithmeticT other than char, wchar_t, char8_t, char16_t, or char32_t, a specialization
```
template<> struct formatter<ArithmeticT, charT>;
```

(2.4)

For each charT, the pointer type specializations

template<> struct formatter<nullptr_t, charT>;
template<> struct formatter<void*, charT>;
template<> struct formatter<const void*, charT>;

The parse member functions of these formatters interpret the format specification as a std-format-spec as described in [format.string.std].

Specializations such as formatter<wchar_t, char> and formatter<const char*, wchar_t> that would require implicit multibyte / wide string or character conversion are disabled.

— end note

]

For any types T and charT for which neither the library nor the user provides an explicit or partial specialization of the class template formatter, formatter<T, charT> is disabled.

If the library provides an explicit or partial specialization of formatter<T, charT>, that specialization is enabled except as noted otherwise.

If F is a disabled specialization of formatter, these values are false:

(5.1)
is_default_constructible_v<F>,
(5.2)
is_copy_constructible_v<F>,
(5.3)
is_move_constructible_v<F>,
(5.4)
is_copy_assignable_v<F>, and
(5.5)
is_move_assignable_v<F>.

An enabled specialization formatter<T, charT> meets the Formatter requirements ([formatter.requirements]).

#include <format>

enum color { red, green, blue };
const char* color_names[] = { "red", "green", "blue" };

template<> struct std::formatter<color> : std::formatter<const char*> {
  auto format(color c, format_context& ctx) {
    return formatter<const char*>::format(color_names[c], ctx);
  }
};

struct err {};

std::string s0 = std::format("{}", 42);         // OK, library-provided formatter
std::string s1 = std::format("{}", L"foo");     // error: disabled formatter
std::string s2 = std::format("{}", red);        // OK, user-provided formatter
std::string s3 = std::format("{}", err{});      // error: disabled formatter

— end example

]

20.20.5.3 Class template basic_format_parse_context [format.parse.ctx]

namespace std {
  template<class charT>
  class basic_format_parse_context {
  public:
    using char_type = charT;
    using const_iterator = typename basic_string_view<charT>::const_iterator;
    using iterator = const_iterator;

  private:
    iterator begin_;                                    // exposition only
    iterator end_;                                      // exposition only
    enum indexing { unknown, manual, automatic };       // exposition only
    indexing indexing_;                                 // exposition only
    size_t next_arg_id_;                                // exposition only
    size_t num_args_;                                   // exposition only

  public:
    constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt,
                                                  size_t num_args = 0) noexcept;
    basic_format_parse_context(const basic_format_parse_context&) = delete;
    basic_format_parse_context& operator=(const basic_format_parse_context&) = delete;

    constexpr const_iterator begin() const noexcept;
    constexpr const_iterator end() const noexcept;
    constexpr void advance_to(const_iterator it);

    constexpr size_t next_arg_id();
    constexpr void check_arg_id(size_t id);
  };
}

An instance of basic_format_parse_context holds the format string parsing state consisting of the format string range being parsed and the argument counter for automatic indexing.

constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt,
                                              size_t num_args = 0) noexcept;

Effects: Initializes begin_ with fmt.begin(), end_ with fmt.end(), indexing_ with unknown, next_arg_id_ with 0, and num_args_ with num_args.

constexpr const_iterator begin() const noexcept;

Returns: begin_.

constexpr const_iterator end() const noexcept;

Returns: end_.

constexpr void advance_to(const_iterator it);

Preconditions: end() is reachable from it.

Effects: Equivalent to: begin_ = it;

constexpr size_t next_arg_id();

Effects: If indexing_ != manual, equivalent to:

if (indexing_ == unknown)
  indexing_ = automatic;
return next_arg_id_++;

Throws: format_error if indexing_ == manual which indicates mixing of automatic and manual argument indexing.

constexpr void check_arg_id(size_t id);

Effects: If indexing_ != automatic, equivalent to:

if (indexing_ == unknown)
  indexing_ = manual;

Throws: format_error if indexing_ == automatic which indicates mixing of automatic and manual argument indexing.

Remarks: Call expressions where id >= num_args_ are not core constant expressions ([expr.const]).

20.20.5.4 Class template basic_format_context [format.context]

namespace std {
  template<class Out, class charT>
  class basic_format_context {
    basic_format_args<basic_format_context> args_;      // exposition only
    Out out_;                                           // exposition only

  public:
    using iterator = Out;
    using char_type = charT;
    template<class T> using formatter_type = formatter<T, charT>;

    basic_format_arg<basic_format_context> arg(size_t id) const;
    std::locale locale();

    iterator out();
    void advance_to(iterator it);
  };
}

An instance of basic_format_context holds formatting state consisting of the formatting arguments and the output iterator.

Out shall model output_iterator<const charT&>.

format_context is an alias for a specialization of basic_format_context with an output iterator that appends to string, such as back_insert_iterator<string>.

Similarly, wformat_context is an alias for a specialization of basic_format_context with an output iterator that appends to wstring.

For a given type charT, implementations are encouraged to provide a single instantiation of basic_format_context for appending to basic_string<charT>, vector<charT>, or any other container with contiguous storage by wrapping those in temporary objects with a uniform interface (such as a span<charT>) and polymorphic reallocation.

— end note

]

basic_format_arg<basic_format_context> arg(size_t id) const;

Returns: args_.get(id).

std::locale locale();

Returns: The locale passed to the formatting function if the latter takes one, and std::locale() otherwise.

iterator out();

Returns: out_.

void advance_to(iterator it);

Effects: Equivalent to: out_ = it;

struct S { int value; };

template<> struct std::formatter<S> {
  size_t width_arg_id = 0;

  // Parses a width argument id in the format { digit }.
  constexpr auto parse(format_parse_context& ctx) {
    auto iter = ctx.begin();
    auto get_char = [&]() { return iter != ctx.end() ? *iter : 0; };
    if (get_char() != '{')
      return iter;
    ++iter;
    char c = get_char();
    if (!isdigit(c) || (++iter, get_char()) != '}')
      throw format_error("invalid format");
    width_arg_id = c - '0';
    ctx.check_arg_id(width_arg_id);
    return ++iter;
  }

  // Formats an S with width given by the argument width_arg_id.
  auto format(S s, format_context& ctx) {
    int width = visit_format_arg([](auto value) -> int {
      if constexpr (!is_integral_v<decltype(value)>)
        throw format_error("width is not integral");
      else if (value < 0 || value > numeric_limits<int>::max())
        throw format_error("invalid width");
      else
        return value;
      }, ctx.arg(width_arg_id));
    return format_to(ctx.out(), "{0:x<{1}}", s.value, width);
  }
};

std::string s = std::format("{0:{1}}", S{42}, 10);  // value of s is "xxxxxxxx42"

— end example

]

20.20.6 Arguments [format.arguments]

20.20.6.1 Class template basic_format_arg [format.arg]

namespace std {
  template<class Context>
  class basic_format_arg {
  public:
    class handle;

  private:
    using char_type = typename Context::char_type;                              // exposition only

    variant<monostate, bool, char_type,
            int, unsigned int, long long int, unsigned long long int,
            float, double, long double,
            const char_type*, basic_string_view<char_type>,
            const void*, handle> value;                                         // exposition only

    template<class T> explicit basic_format_arg(const T& v) noexcept;           // exposition only
    explicit basic_format_arg(float n) noexcept;                                // exposition only
    explicit basic_format_arg(double n) noexcept;                               // exposition only
    explicit basic_format_arg(long double n) noexcept;                          // exposition only
    explicit basic_format_arg(const char_type* s);                              // exposition only

    template<class traits>
      explicit basic_format_arg(
        basic_string_view<char_type, traits> s) noexcept;                       // exposition only

    template<class traits, class Allocator>
      explicit basic_format_arg(
        const basic_string<char_type, traits, Allocator>& s) noexcept;          // exposition only

    explicit basic_format_arg(nullptr_t) noexcept;                              // exposition only

    template<class T>
      explicit basic_format_arg(const T* p) noexcept;                           // exposition only

  public:
    basic_format_arg() noexcept;

    explicit operator bool() const noexcept;
  };
}

An instance of basic_format_arg provides access to a formatting argument for user-defined formatters.

The behavior of a program that adds specializations of basic_format_arg is undefined.

basic_format_arg() noexcept;

Postconditions: !(*this).

template<class T> explicit basic_format_arg(const T& v) noexcept;

Constraints: The template specialization

typename Context::template formatter_type<T>

meets the Formatter requirements ([formatter.requirements]).

The extent to which an implementation determines that the specialization meets the Formatter requirements is unspecified, except that as a minimum the expression

typename Context::template formatter_type<T>()
  .format(declval<const T&>(), declval<Context&>())

shall be well-formed when treated as an unevaluated operand.

Effects:

(5.1)
if T is bool or char_type, initializes value with v;
(5.2)
otherwise, if T is char and char_type is wchar_t, initializes value with static_cast<wchar_t>(v);
(5.3)
otherwise, if T is a signed integer type ([basic.fundamental]) and sizeof(T) <= sizeof(int), initializes value with static_cast<int>(v);
(5.4)
otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned int), initializes value with static_cast<unsigned int>(v);
(5.5)
otherwise, if T is a signed integer type and sizeof(T) <= sizeof(long long int), initializes value with static_cast<long long int>(v);
(5.6)
otherwise, if T is an unsigned integer type and sizeof(T) <= sizeof(unsigned long long int), initializes value with static_cast<unsigned long long int>(v);
(5.7)
otherwise, initializes value with handle(v).

explicit basic_format_arg(float n) noexcept;
explicit basic_format_arg(double n) noexcept;
explicit basic_format_arg(long double n) noexcept;

Effects: Initializes value with n.

explicit basic_format_arg(const char_type* s);

Preconditions: s points to a NTCTS ([defns.ntcts]).

Effects: Initializes value with s.

template<class traits>
  explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;

Effects: Initializes value with s.

template<class traits, class Allocator>
  explicit basic_format_arg(
    const basic_string<char_type, traits, Allocator>& s) noexcept;

Effects: Initializes value with basic_string_view<char_type>(s.data(), s.size()).

explicit basic_format_arg(nullptr_t) noexcept;

Effects: Initializes value with static_cast<const void*>(nullptr).

template<class T> explicit basic_format_arg(const T* p) noexcept;

Constraints: is_void_v<T> is true.

Effects: Initializes value with p.

Constructing basic_format_arg from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type.

— end note

]

explicit operator bool() const noexcept;

Returns: !holds_alternative<monostate>(value).

The class handle allows formatting an object of a user-defined type.

namespace std {
  template<class Context>
  class basic_format_arg<Context>::handle {
    const void* ptr_;                                           // exposition only
    void (*format_)(basic_format_parse_context<char_type>&,
                    Context&, const void*);                     // exposition only

    template<class T> explicit handle(const T& val) noexcept;   // exposition only

    friend class basic_format_arg<Context>;                     // exposition only

  public:
    void format(basic_format_parse_context<char_type>&, Context& ctx) const;
  };
}

template<class T> explicit handle(const T& val) noexcept;

Effects: Initializes ptr_ with addressof(val) and format_ with

[](basic_format_parse_context<char_type>& parse_ctx,
   Context& format_ctx, const void* ptr) {
  typename Context::template formatter_type<T> f;
  parse_ctx.advance_to(f.parse(parse_ctx));
  format_ctx.advance_to(f.format(*static_cast<const T*>(ptr), format_ctx));
}

void format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const;

Effects: Equivalent to: format_(parse_ctx, format_ctx, ptr_);

template<class Visitor, class Context>
  see below visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);

Effects: Equivalent to: return visit(forward<Visitor>(vis), arg.value);

20.20.6.2 Class template format-arg-store [format.arg.store]

namespace std {
  template<class Context, class... Args>
  struct format-arg-store {      // exposition only
    array<basic_format_arg<Context>, sizeof...(Args)> args;
  };
}

An instance of format-arg-store stores formatting arguments.

template<class Context = format_context, class... Args>
  format-arg-store<Context, Args...> make_format_args(const Args&... args);

Preconditions: The type typename Context::template formatter_type<

T_{i}

> meets the Formatter requirements ([formatter.requirements]) for each

T_{i}

in Args.

Returns: {basic_format_arg<Context>(args)...}.

template<class... Args>
  format-arg-store<wformat_context, Args...> make_wformat_args(const Args&... args);

Effects: Equivalent to: return make_format_args<wformat_context>(args...);

20.20.6.3 Class template basic_format_args [format.args]

namespace std {
  template<class Context>
  class basic_format_args {
    size_t size_;                               // exposition only
    const basic_format_arg<Context>* data_;     // exposition only

  public:
    basic_format_args() noexcept;

    template<class... Args>
      basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;

    basic_format_arg<Context> get(size_t i) const noexcept;
  };
}

An instance of basic_format_args provides access to formatting arguments.

basic_format_args() noexcept;

Effects: Initializes size_ with 0.

template<class... Args>
  basic_format_args(const format-arg-store<Context, Args...>& store) noexcept;

Effects: Initializes size_ with sizeof...(Args) and data_ with store.args.data().

basic_format_arg<Context> get(size_t i) const noexcept;

Returns: i < size_ ? data_[i] : basic_format_arg<Context>().

Implementations are encouraged to optimize the representation of basic_format_args for small number of formatting arguments by storing indices of type alternatives separately from values and packing the former.

— end note

]

20.20.7 Class format_error [format.error]

namespace std {
  class format_error : public runtime_error {
  public:
    explicit format_error(const string& what_arg);
    explicit format_error(const char* what_arg);
  };
}

The class format_error defines the type of objects thrown as exceptions to report errors from the formatting library.

format_error(const string& what_arg);

Postconditions: strcmp(what(), what_arg.c_str()) == 0.