24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

1
#
This subclause defines range adaptors, which are utilities that transform a range into a view with custom behaviors.
These adaptors can be chained to create pipelines of range transformations that evaluate lazily as the resulting view is iterated.
2
#
Range adaptors are declared in namespace std​::​ranges​::​views.
3
#
The bitwise OR operator is overloaded for the purpose of creating adaptor chain pipelines.
The adaptors also support function call syntax with equivalent semantics.
4
#
[Example
: 
vector<int> ints{0,1,2,3,4,5};
auto even = [](int i){ return 0 == i % 2; };
auto square = [](int i) { return i * i; };
for (int i : ints | views::filter(even) | views::transform(square)) {
  cout << i << ' '; // prints: 0 4 16
}
assert(ranges::equal(ints | views::filter(even), views::filter(ints, even)));
— end example
]

24.7.1 Range adaptor objects [range.adaptor.object]

1
#
A range adaptor closure object is a unary function object that accepts a viewable_­range argument and returns a view.
For a range adaptor closure object C and an expression R such that decltype((R)) models viewable_­range, the following expressions are equivalent and yield a view: 
C(R)
R | C
Given an additional range adaptor closure object D, the expression C | D is well-formed and produces another range adaptor closure object such that the following two expressions are equivalent: 
R | C | D
R | (C | D)
2
#
A range adaptor object is a customization point object ([customization.point.object]) that accepts a viewable_­range as its first argument and returns a view.
3
#
If a range adaptor object accepts only one argument, then it is a range adaptor closure object.
4
#
If a range adaptor object accepts more than one argument, then the following expressions are equivalent: 
adaptor(range, args...)
adaptor(args...)(range)
range | adaptor(args...)
In this case, adaptor(args...) is a range adaptor closure object.

24.7.2 Semiregular wrapper [range.semi.wrap]

1
#
Many types in this subclause are specified in terms of an exposition-only class template semiregular-box.
semiregular-box<T> behaves exactly like optional<T> with the following differences:
  • (1.1)
    semiregular-box<T> constrains its type parameter T with copy_­constructible<T> && is_­object_­v<T>.
  • (1.2)
    If T models default_­initializable, the default constructor of semiregular-box<T> is equivalent to: 
    constexpr semiregular-box() noexcept(is_nothrow_default_constructible_v<T>)
  : semiregular-box{in_place}
{ }
  • (1.3)
    If assignable_­from<T&, const T&> is not modeled, the copy assignment operator is equivalent to: 
    semiregular-box& operator=(const semiregular-box& that)
  noexcept(is_nothrow_copy_constructible_v<T>)
{
  if (that) emplace(*that);
  else reset();
  return *this;
}
  • (1.4)
    If assignable_­from<T&, T> is not modeled, the move assignment operator is equivalent to: 
    semiregular-box& operator=(semiregular-box&& that)
  noexcept(is_nothrow_move_constructible_v<T>)
{
  if (that) emplace(std::move(*that));
  else reset();
  return *this;
}

24.7.3 All view [range.all]

1
#
views​::​all returns a view that includes all elements of its range argument.
2
#
The name views​::​all denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​all(E) is expression-equivalent to:
  • (2.1)
    decay-copy(E) if the decayed type of E models view.
  • (2.2)
    Otherwise, ref_­view{E} if that expression is well-formed.
  • (2.3)
    Otherwise, subrange{E}.

24.7.3.1 Class template ref_­view [range.ref.view]

1
#
ref_­view is a view of the elements of some other range.
namespace std::ranges {
  template<range R>
    requires is_object_v<R>
  class ref_view : public view_interface<ref_view<R>> {
  private:
    R* r_ = nullptr;            // exposition only
  public:
    constexpr ref_view() noexcept = default;

    template<not-same-as<ref_view> T>
      requires see below
    constexpr ref_view(T&& t);

    constexpr R& base() const { return *r_; }

    constexpr iterator_t<R> begin() const { return ranges::begin(*r_); }
    constexpr sentinel_t<R> end() const { return ranges::end(*r_); }

    constexpr bool empty() const
      requires requires { ranges::empty(*r_); }
    { return ranges::empty(*r_); }

    constexpr auto size() const requires sized_range<R>
    { return ranges::size(*r_); }

    constexpr auto data() const requires contiguous_range<R>
    { return ranges::data(*r_); }
  };
  template<class R>
    ref_view(R&) -> ref_view<R>;
}
🔗
template<not-same-as<ref_view> T> requires see below constexpr ref_view(T&& t);
2
#
Remarks: Let FUN denote the exposition-only functions 
void FUN(R&);
void FUN(R&&) = delete;
The expression in the requires-clause is equivalent to 
convertible_to<T, R&> && requires { FUN(declval<T>()); }
3
#
Effects: Initializes r_­ with addressof(static_­cast<R&>(std​::​forward<T>(t))).

24.7.4 Filter view [range.filter]

24.7.4.1 Overview [range.filter.overview]

1
#
filter_­view presents a view of the elements of an underlying sequence that satisfy a predicate.
2
#
The name views​::​filter denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and P, the expression views​::​filter(E, P) is expression-equivalent to filter_­view{E, P}.
3
#
[Example
: 
vector<int> is{ 0, 1, 2, 3, 4, 5, 6 };
filter_view evens{is, [](int i) { return 0 == i % 2; }};
for (int i : evens)
  cout << i << ' '; // prints: 0 2 4 6
— end example
]

24.7.4.2 Class template filter_­view [range.filter.view]

namespace std::ranges {
  template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class filter_view : public view_interface<filter_view<V, Pred>> {
  private:
    V base_ = V();                      // exposition only
    semiregular-box<Pred> pred_;  // exposition only

    // [range.filter.iterator], class filter_­view​::​iterator
    class iterator;                     // exposition only
    // [range.filter.sentinel], class filter_­view​::​sentinel
    class sentinel;                     // exposition only

  public:
    filter_view() = default;
    constexpr filter_view(V base, Pred pred);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr const Pred& pred() const;

    constexpr iterator begin();
    constexpr auto end() {
      if constexpr (common_range<V>)
        return iterator{*this, ranges::end(base_)};
      else
        return sentinel{*this};
    }
  };

  template<class R, class Pred>
    filter_view(R&&, Pred) -> filter_view<views::all_t<R>, Pred>;
}
🔗
constexpr filter_view(V base, Pred pred);
1
#
Effects: Initializes base_­ with std​::​move(base) and initializes pred_­ with std​::​move(pred).
🔗
constexpr const Pred& pred() const;
2
#
Effects: Equivalent to: return *pred_­;
🔗
constexpr iterator begin();
3
#
Preconditions: pred_­.has_­value().
4
#
Returns: {*this, ranges​::​find_­if(base_­, ref(*pred_­))}.
5
#
Remarks: In order to provide the amortized constant time complexity required by the range concept when filter_­view models forward_­range, this function caches the result within the filter_­view for use on subsequent calls.

24.7.4.3 Class filter_­view​::​iterator [range.filter.iterator]

namespace std::ranges {
  template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class filter_view<V, Pred>::iterator {
  private:
    iterator_t<V> current_ = iterator_t<V>();   // exposition only
    filter_view* parent_ = nullptr;             // exposition only
  public:
    using iterator_concept  = see below;
    using iterator_category = see below;
    using value_type        = range_value_t<V>;
    using difference_type   = range_difference_t<V>;

    iterator() = default;
    constexpr iterator(filter_view& parent, iterator_t<V> current);

    constexpr iterator_t<V> base() const &
      requires copyable<iterator_t<V>>;
    constexpr iterator_t<V> base() &&;
    constexpr range_reference_t<V> operator*() const;
    constexpr iterator_t<V> operator->() const
      requires has-arrow<iterator_t<V>> && copyable<iterator_t<V>>;

    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires forward_range<V>;

    constexpr iterator& operator--() requires bidirectional_range<V>;
    constexpr iterator operator--(int) requires bidirectional_range<V>;

    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<iterator_t<V>>;

    friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i)
      noexcept(noexcept(ranges::iter_move(i.current_)));
    friend constexpr void iter_swap(const iterator& x, const iterator& y)
      noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
      requires indirectly_swappable<iterator_t<V>>;
  };
}
1
#
Modification of the element a filter_­view​::​iterator denotes is permitted, but results in undefined behavior if the resulting value does not satisfy the filter predicate.
2
#
iterator​::​iterator_­concept is defined as follows:
  • (2.1)
    If V models bidirectional_­range, then iterator_­concept denotes bidirectional_­iterator_­tag.
  • (2.2)
    Otherwise, if V models forward_­range, then iterator_­concept denotes forward_­iterator_­tag.
  • (2.3)
    Otherwise, iterator_­concept denotes input_­iterator_­tag.
3
#
iterator​::​iterator_­category is defined as follows:
  • (3.1)
    Let C denote the type iterator_­traits<iterator_­t<V>>​::​iterator_­category.
  • (3.2)
    If C models derived_­from<bidirectional_­iterator_­tag>, then iterator_­category denotes bidirectional_­iterator_­tag.
  • (3.3)
    Otherwise, if C models derived_­from<forward_­iterator_­tag>, then iterator_­category denotes forward_­iterator_­tag.
  • (3.4)
    Otherwise, iterator_­category denotes C.
🔗
constexpr iterator(filter_view& parent, iterator_t<V> current);
4
#
Effects: Initializes current_­ with std​::​move(current) and parent_­ with addressof(parent).
🔗
constexpr iterator_t<V> base() const & requires copyable<iterator_t<V>>;
5
#
Effects: Equivalent to: return current_­;
🔗
constexpr iterator_t<V> base() &&;
6
#
Effects: Equivalent to: return std​::​move(current_­);
🔗
constexpr range_reference_t<V> operator*() const;
7
#
Effects: Equivalent to: return *current_­;
🔗
constexpr iterator_t<V> operator->() const requires has-arrow<iterator_t<V>> && copyable<iterator_t<V>>;
8
#
Effects: Equivalent to: return current_­;
🔗
constexpr iterator& operator++();
9
#
Effects: Equivalent to: 
current_ = ranges::find_if(std::move(++current_), ranges::end(parent_->base_),
                           ref(*parent_->pred_));
return *this;
🔗
constexpr void operator++(int);
10
#
Effects: Equivalent to ++*this.
🔗
constexpr iterator operator++(int) requires forward_­range<V>;
11
#
Effects: Equivalent to: 
auto tmp = *this;
++*this;
return tmp;
🔗
constexpr iterator& operator--() requires bidirectional_­range<V>;
12
#
Effects: Equivalent to: 
do
  --current_;
while (!invoke(*parent_->pred_, *current_));
return *this;
🔗
constexpr iterator operator--(int) requires bidirectional_­range<V>;
13
#
Effects: Equivalent to: 
auto tmp = *this;
--*this;
return tmp;
🔗
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<V>>;
14
#
Effects: Equivalent to: return x.current_­ == y.current_­;
🔗
friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(i.current_­)));
15
#
Effects: Equivalent to: return ranges​::​iter_­move(i.current_­);
🔗
friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_­, y.current_­))) requires indirectly_­swappable<iterator_t<V>>;
16
#
Effects: Equivalent to ranges​::​iter_­swap(x.current_­, y.current_­).

24.7.4.4 Class filter_­view​::​sentinel [range.filter.sentinel]

namespace std::ranges {
  template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class filter_view<V, Pred>::sentinel {
  private:
    sentinel_t<V> end_ = sentinel_t<V>();       // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(filter_view& parent);

    constexpr sentinel_t<V> base() const;

    friend constexpr bool operator==(const iterator& x, const sentinel& y);
  };
}
🔗
constexpr explicit sentinel(filter_view& parent);
1
#
Effects: Initializes end_­ with ranges​::​end(parent.base_­).
🔗
constexpr sentinel_t<V> base() const;
2
#
Effects: Equivalent to: return end_­;
🔗
friend constexpr bool operator==(const iterator& x, const sentinel& y);
3
#
Effects: Equivalent to: return x.current_­ == y.end_­;

24.7.5 Transform view [range.transform]

24.7.5.1 Overview [range.transform.overview]

1
#
transform_­view presents a view of an underlying sequence after applying a transformation function to each element.
2
#
The name views​::​transform denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and F, the expression views​::​transform(E, F) is expression-equivalent to transform_­view{E, F}.
3
#
[Example
: 
vector<int> is{ 0, 1, 2, 3, 4 };
transform_view squares{is, [](int i) { return i * i; }};
for (int i : squares)
  cout << i << ' '; // prints: 0 1 4 9 16
— end example
]

24.7.5.2 Class template transform_­view [range.transform.view]

namespace std::ranges {
  template<input_range V, copy_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             can-reference<invoke_result_t<F&, range_reference_t<V>>>
  class transform_view : public view_interface<transform_view<V, F>> {
  private:
    // [range.transform.iterator], class template transform_­view​::​iterator
    template<bool> struct iterator;             // exposition only
    // [range.transform.sentinel], class template transform_­view​::​sentinel
    template<bool> struct sentinel;             // exposition only

    V base_ = V();                              // exposition only
    semiregular-box<F> fun_;                    // exposition only

  public:
    transform_view() = default;
    constexpr transform_view(V base, F fun);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr iterator<false> begin();
    constexpr iterator<true> begin() const
      requires range<const V> &&
               regular_invocable<const F&, range_reference_t<const V>>;

    constexpr sentinel<false> end();
    constexpr iterator<false> end() requires common_range<V>;
    constexpr sentinel<true> end() const
      requires range<const V> &&
               regular_invocable<const F&, range_reference_t<const V>>;
    constexpr iterator<true> end() const
      requires common_range<const V> &&
               regular_invocable<const F&, range_reference_t<const V>>;

    constexpr auto size() requires sized_range<V> { return ranges::size(base_); }
    constexpr auto size() const requires sized_range<const V>
    { return ranges::size(base_); }
  };

  template<class R, class F>
    transform_view(R&&, F) -> transform_view<views::all_t<R>, F>;
}
🔗
constexpr transform_view(V base, F fun);
1
#
Effects: Initializes base_­ with std​::​move(base) and fun_­ with std​::​move(fun).
🔗
constexpr iterator<false> begin();
2
#
Effects: Equivalent to: 
return iterator<false>{*this, ranges::begin(base_)};
🔗
constexpr iterator<true> begin() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>;
3
#
Effects: Equivalent to: 
return iterator<true>{*this, ranges::begin(base_)};
🔗
constexpr sentinel<false> end();
4
#
Effects: Equivalent to: 
return sentinel<false>{ranges::end(base_)};
🔗
constexpr iterator<false> end() requires common_range<V>;
5
#
Effects: Equivalent to: 
return iterator<false>{*this, ranges::end(base_)};
🔗
constexpr sentinel<true> end() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>;
6
#
Effects: Equivalent to: 
return sentinel<true>{ranges::end(base_)};
🔗
constexpr iterator<true> end() const requires common_range<const V> && regular_invocable<const F&, range_reference_t<const V>>;
7
#
Effects: Equivalent to: 
return iterator<true>{*this, ranges::end(base_)};

24.7.5.3 Class template transform_­view​::​iterator [range.transform.iterator]

namespace std::ranges {
  template<input_range V, copy_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             can-reference<invoke_result_t<F&, range_reference_t<V>>>
  template<bool Const>
  class transform_view<V, F>::iterator {
  private:
    using Parent =                              // exposition only
      conditional_t<Const, const transform_view, transform_view>;
    using Base   =                              // exposition only
      conditional_t<Const, const V, V>;
    iterator_t<Base> current_ =                 // exposition only
      iterator_t<Base>();
    Parent* parent_ = nullptr;                  // exposition only
  public:
    using iterator_concept  = see below;
    using iterator_category = see below;
    using value_type        =
      remove_cvref_t<invoke_result_t<F&, range_reference_t<Base>>>;
    using difference_type   = range_difference_t<Base>;

    iterator() = default;
    constexpr iterator(Parent& parent, iterator_t<Base> current);
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;

    constexpr iterator_t<Base> base() const &
      requires copyable<iterator_t<Base>>;
    constexpr iterator_t<Base> base() &&;
    constexpr decltype(auto) operator*() const
    { return invoke(*parent_->fun_, *current_); }

    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires forward_range<Base>;

    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;

    constexpr iterator& operator+=(difference_type n)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type n)
      requires random_access_range<Base>;
    constexpr decltype(auto) operator[](difference_type n) const
      requires random_access_range<Base>
    { return invoke(*parent_->fun_, current_[n]); }

    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<iterator_t<Base>>;

    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;

    friend constexpr iterator operator+(iterator i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, iterator i)
      requires random_access_range<Base>;

    friend constexpr iterator operator-(iterator i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires random_access_range<Base>;

    friend constexpr decltype(auto) iter_move(const iterator& i)
      noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_)))
    {
      if constexpr (is_lvalue_reference_v<decltype(*i)>)
        return std::move(*i);
      else
        return *i;
    }

    friend constexpr void iter_swap(const iterator& x, const iterator& y)
      noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
      requires indirectly_swappable<iterator_t<Base>>;
  };
}
1
#
iterator​::​iterator_­concept is defined as follows:
  • (1.1)
    If V models random_­access_­range, then iterator_­concept denotes random_­access_­iterator_­tag.
  • (1.2)
    Otherwise, if V models bidirectional_­range, then iterator_­concept denotes bidirectional_­iterator_­tag.
  • (1.3)
    Otherwise, if V models forward_­range, then iterator_­concept denotes forward_­iterator_­tag.
  • (1.4)
    Otherwise, iterator_­concept denotes input_­iterator_­tag.
2
#
iterator​::​iterator_­category is defined as follows: Let C denote the type iterator_­traits<iterator_­t<Base>>​::​iterator_­category.
  • (2.1)
    If is_­lvalue_­reference_­v<invoke_­result_­t<F&, range_­reference_­t<Base>>> is true, then
    • (2.1.1)
      if C models derived_­from<contiguous_­iterator_­tag>, iterator_­category denotes random_­access_­iterator_­tag;
    • (2.1.2)
      otherwise, iterator_­category denotes C.
  • (2.2)
    Otherwise, iterator_­category denotes input_­iterator_­tag.
🔗
constexpr iterator(Parent& parent, iterator_t<Base> current);
3
#
Effects: Initializes current_­ with std​::​move(current) and parent_­ with addressof(parent).
🔗
constexpr iterator(iterator<!Const> i) requires Const && convertible_­to<iterator_t<V>, iterator_t<Base>>;
4
#
Effects: Initializes current_­ with std​::​move(i.current_­) and parent_­ with i.parent_­.
🔗
constexpr iterator_t<Base> base() const & requires copyable<iterator_t<Base>>;
5
#
Effects: Equivalent to: return current_­;
🔗
constexpr iterator_t<Base> base() &&;
6
#
Effects: Equivalent to: return std​::​move(current_­);
🔗
constexpr iterator& operator++();
7
#
Effects: Equivalent to: 
++current_;
return *this;
🔗
constexpr void operator++(int);
8
#
Effects: Equivalent to ++current_­.
🔗
constexpr iterator operator++(int) requires forward_­range<Base>;
9
#
Effects: Equivalent to: 
auto tmp = *this;
++*this;
return tmp;
🔗
constexpr iterator& operator--() requires bidirectional_­range<Base>;
10
#
Effects: Equivalent to: 
--current_;
return *this;
🔗
constexpr iterator operator--(int) requires bidirectional_­range<Base>;
11
#
Effects: Equivalent to: 
auto tmp = *this;
--*this;
return tmp;
🔗
constexpr iterator& operator+=(difference_type n) requires random_­access_­range<Base>;
12
#
Effects: Equivalent to: 
current_ += n;
return *this;
🔗
constexpr iterator& operator-=(difference_type n) requires random_­access_­range<Base>;
13
#
Effects: Equivalent to: 
current_ -= n;
return *this;
🔗
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<Base>>;
14
#
Effects: Equivalent to: return x.current_­ == y.current_­;
🔗
friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
15
#
Effects: Equivalent to: return x.current_­ < y.current_­;
🔗
friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
16
#
Effects: Equivalent to: return y < x;
🔗
friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
17
#
Effects: Equivalent to: return !(y < x);
🔗
friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
18
#
Effects: Equivalent to: return !(x < y);
🔗
friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_­access_­range<Base> && three_­way_­comparable<iterator_t<Base>>;
19
#
Effects: Equivalent to: return x.current_­ <=> y.current_­;
🔗
friend constexpr iterator operator+(iterator i, difference_type n) requires random_­access_­range<Base>; friend constexpr iterator operator+(difference_type n, iterator i) requires random_­access_­range<Base>;
20
#
Effects: Equivalent to: return iterator{*i.parent_­, i.current_­ + n};
🔗
friend constexpr iterator operator-(iterator i, difference_type n) requires random_­access_­range<Base>;
21
#
Effects: Equivalent to: return iterator{*i.parent_­, i.current_­ - n};
🔗
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
22
#
Effects: Equivalent to: return x.current_­ - y.current_­;
🔗
friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_­, y.current_­))) requires indirectly_­swappable<iterator_t<Base>>;
23
#
Effects: Equivalent to ranges​::​iter_­swap(x.current_­, y.current_­).

24.7.5.4 Class template transform_­view​::​sentinel [range.transform.sentinel]

namespace std::ranges {
  template<input_range V, copy_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             can-reference<invoke_result_t<F&, range_reference_t<V>>>
  template<bool Const>
  class transform_view<V, F>::sentinel {
  private:
    using Parent =                                      // exposition only
      conditional_t<Const, const transform_view, transform_view>;
    using Base = conditional_t<Const, const V, V>;      // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end);
    constexpr sentinel(sentinel<!Const> i)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

    constexpr sentinel_t<Base> base() const;

    friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);

    friend constexpr range_difference_t<Base>
      operator-(const iterator<Const>& x, const sentinel& y)
        requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
    friend constexpr range_difference_t<Base>
      operator-(const sentinel& y, const iterator<Const>& x)
        requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
  };
}
🔗
constexpr explicit sentinel(sentinel_t<Base> end);
1
#
Effects: Initializes end_­ with end.
🔗
constexpr sentinel(sentinel<!Const> i) requires Const && convertible_­to<sentinel_t<V>, sentinel_t<Base>>;
2
#
Effects: Initializes end_­ with std​::​move(i.end_­).
🔗
constexpr sentinel_t<Base> base() const;
3
#
Effects: Equivalent to: return end_­;
🔗
friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);
4
#
Effects: Equivalent to: return x.current_­ == y.end_­;
🔗
friend constexpr range_difference_t<Base> operator-(const iterator<Const>& x, const sentinel& y) requires sized_­sentinel_­for<sentinel_t<Base>, iterator_t<Base>>;
5
#
Effects: Equivalent to: return x.current_­ - y.end_­;
🔗
friend constexpr range_difference_t<Base> operator-(const sentinel& y, const iterator<Const>& x) requires sized_­sentinel_­for<sentinel_t<Base>, iterator_t<Base>>;
6
#
Effects: Equivalent to: return y.end_­ - x.current_­;

24.7.6 Take view [range.take]

24.7.6.1 Overview [range.take.overview]

1
#
take_­view produces a view of the first N elements from another view, or all the elements if the adapted view contains fewer than N.
2
#
The name views​::​take denotes a range adaptor object ([range.adaptor.object]).
Let E and F be expressions, let T be remove_­cvref_­t<decltype((E))>, and let D be range_­difference_­t<decltype((E))>.
If decltype((F)) does not model convertible_­to<D>, views​::​take(E, F) is ill-formed.
Otherwise, the expression views​::​take(E, F) is expression-equivalent to:
  • (2.1)
    If T is a specialization of ranges​::​empty_­view ([range.empty.view]), then ((void) F, decay-copy(E)).
  • (2.2)
    Otherwise, if T models random_­access_­range and sized_­range and is then T{ranges​::​begin(E), ranges​::​begin(E) + min<D>(ranges​::​size(E), F)}, except that E is evaluated only once.
  • (2.3)
    Otherwise, ranges​::​take_­view{E, F}.
3
#
[Example
: 
vector<int> is{0,1,2,3,4,5,6,7,8,9};
take_view few{is, 5};
for (int i : few)
  cout << i << ' '; // prints: 0 1 2 3 4
— end example
]

24.7.6.2 Class template take_­view [range.take.view]

namespace std::ranges {
  template<view V>
  class take_view : public view_interface<take_view<V>> {
  private:
    V base_ = V();                                      // exposition only
    range_difference_t<V> count_ = 0;                   // exposition only
    // [range.take.sentinel], class template take_­view​::​sentinel
    template<bool> struct sentinel;                     // exposition only
  public:
    take_view() = default;
    constexpr take_view(V base, range_difference_t<V> count);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr auto begin() requires (!simple-view<V>) {
      if constexpr (sized_range<V>) {
        if constexpr (random_access_range<V>)
          return ranges::begin(base_);
        else {
          auto sz = size();
          return counted_iterator{ranges::begin(base_), sz};
        }
      } else
        return counted_iterator{ranges::begin(base_), count_};
    }

    constexpr auto begin() const requires range<const V> {
      if constexpr (sized_range<const V>) {
        if constexpr (random_access_range<const V>)
          return ranges::begin(base_);
        else {
          auto sz = size();
          return counted_iterator{ranges::begin(base_), sz};
        }
      } else
        return counted_iterator{ranges::begin(base_), count_};
    }

    constexpr auto end() requires (!simple-view<V>) {
      if constexpr (sized_range<V>) {
        if constexpr (random_access_range<V>)
          return ranges::begin(base_) + size();
        else
          return default_sentinel;
      } else
        return sentinel<false>{ranges::end(base_)};
    }

    constexpr auto end() const requires range<const V> {
      if constexpr (sized_range<const V>) {
        if constexpr (random_access_range<const V>)
          return ranges::begin(base_) + size();
        else
          return default_sentinel;
      } else
        return sentinel<true>{ranges::end(base_)};
    }

    constexpr auto size() requires sized_range<V> {
      auto n = ranges::size(base_);
      return ranges::min(n, static_cast<decltype(n)>(count_));
    }

    constexpr auto size() const requires sized_range<const V> {
      auto n = ranges::size(base_);
      return ranges::min(n, static_cast<decltype(n)>(count_));
    }
  };

  template<range R>
    take_view(R&&, range_difference_t<R>)
      -> take_view<views::all_t<R>>;
}
🔗
constexpr take_view(V base, range_difference_t<V> count);
1
#
Effects: Initializes base_­ with std​::​move(base) and count_­ with count.

24.7.6.3 Class template take_­view​::​sentinel [range.take.sentinel]

namespace std::ranges {
  template<view V>
  template<bool Const>
  class take_view<V>::sentinel {
  private:
    using Base = conditional_t<Const, const V, V>;      // exposition only
    using CI = counted_iterator<iterator_t<Base>>;      // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end);
    constexpr sentinel(sentinel<!Const> s)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

    constexpr sentinel_t<Base> base() const;

    friend constexpr bool operator==(const CI& y, const sentinel& x);
  };
}
🔗
constexpr explicit sentinel(sentinel_t<Base> end);
1
#
Effects: Initializes end_­ with end.
🔗
constexpr sentinel(sentinel<!Const> s) requires Const && convertible_­to<sentinel_t<V>, sentinel_t<Base>>;
2
#
Effects: Initializes end_­ with std​::​move(s.end_­).
🔗
constexpr sentinel_t<Base> base() const;
3
#
Effects: Equivalent to: return end_­;
🔗
friend constexpr bool operator==(const CI& y, const sentinel& x);
4
#
Effects: Equivalent to: return y.count() == 0 || y.base() == x.end_­;

24.7.7 Take while view [range.take.while]

24.7.7.1 Overview [range.take.while.overview]

1
#
Given a unary predicate pred and a view r, take_­while_­view produces a view of the range [begin(r), ranges​::​find_­if_­not(r, pred)).
2
#
The name views​::​take_­while denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and F, the expression views​::​take_­while(E, F) is expression-equivalent to take_­while_­view{E, F}.
3
#
[Example
: 
auto input = istringstream{"0 1 2 3 4 5 6 7 8 9"};
auto small = [](const auto x) noexcept { return x < 5; };
auto small_ints = istream_view<int>(input) | views::take_while(small);
for (const auto i : small_ints) {
  cout << i << ' ';                             // prints 0 1 2 3 4
}
auto i = 0;
input >> i;
cout << i;                                      // prints 6
— end example
]

24.7.7.2 Class template take_­while_­view [range.take.while.view]

namespace std::ranges {
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
  class take_while_view : public view_interface<take_while_view<V, Pred>> {
    // [range.take.while.sentinel], class template take_­while_­view​::​sentinel
    template<bool> class sentinel;                      // exposition only

    V base_ = V();                                      // exposition only
    semiregular-box<Pred> pred_;                        // exposition only

  public:
    take_while_view() = default;
    constexpr take_while_view(V base, Pred pred);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr const Pred& pred() const;

    constexpr auto begin() requires (!simple-view<V>)
    { return ranges::begin(base_); }

    constexpr auto begin() const requires range<const V>
    { return ranges::begin(base_); }

    constexpr auto end() requires (!simple-view<V>)
    { return sentinel<false>(ranges::end(base_), addressof(*pred_)); }

    constexpr auto end() const requires range<const V>
    { return sentinel<true>(ranges::end(base_), addressof(*pred_)); }
  };

  template<class R, class Pred>
    take_while_view(R&&, Pred) -> take_while_view<views::all_t<R>, Pred>;
}
🔗
constexpr take_while_view(V base, Pred pred);
1
#
Effects: Initializes base_­ with std​::​move(base) and pred_­ with std​::​move(pred).
🔗
constexpr const Pred& pred() const;
2
#
Effects: Equivalent to: return *pred_­;

24.7.7.3 Class template take_­while_­view​::​sentinel [range.take.while.sentinel]

namespace std::ranges {
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
  template<bool Const>
  class take_while_view<V, Pred>::sentinel {            // exposition only
    using Base = conditional_t<Const, const V, V>;      // exposition only

    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
    const Pred* pred_ = nullptr;                        // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end, const Pred* pred);
    constexpr sentinel(sentinel<!Const> s)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

    constexpr sentinel_t<Base> base() const { return end_; }

    friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);
  };
}
🔗
constexpr explicit sentinel(sentinel_t<Base> end, const Pred* pred);
1
#
Effects: Initializes end_­ with end and pred_­ with pred.
🔗
constexpr sentinel(sentinel<!Const> s) requires Const && convertible_­to<sentinel_t<V>, sentinel_t<Base>>;
2
#
Effects: Initializes end_­ with s.end_­ and pred_­ with s.pred_­.
🔗
friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);
3
#
Effects: Equivalent to: return y.end_­ == x || !invoke(*y.pred_­, *x);

24.7.8 Drop view [range.drop]

24.7.8.1 Overview [range.drop.overview]

1
#
drop_­view produces a view excluding the first N elements from another view, or an empty range if the adapted view contains fewer than N elements.
2
#
The name views​::​drop denotes a range adaptor object ([range.adaptor.object]).
Let E and F be expressions, let T be remove_­cvref_­t<decltype((E))>, and let D be range_­difference_­t<decltype((E))>.
If decltype((F)) does not model convertible_­to<D>, views​::​drop(E, F) is ill-formed.
Otherwise, the expression views​::​drop(E, F) is expression-equivalent to:
  • (2.1)
    If T is a specialization of ranges​::​empty_­view ([range.empty.view]), then ((void) F, decay-copy(E)).
  • (2.2)
    Otherwise, if T models random_­access_­range and sized_­range and is then T{ranges​::​begin(E) + min<D>(ranges​::​size(E), F), ranges​::​end(E)}, except that E is evaluated only once.
  • (2.3)
    Otherwise, ranges​::​drop_­view{E, F}.
3
#
[Example
: 
auto ints = views::iota(0) | views::take(10);
auto latter_half = drop_view{ints, 5};
for (auto i : latter_half) {
  cout << i << ' ';                             // prints 5 6 7 8 9
}
— end example
]

24.7.8.2 Class template drop_­view [range.drop.view]

namespace std::ranges {
  template<view V>
  class drop_view : public view_interface<drop_view<V>> {
  public:
    drop_view() = default;
    constexpr drop_view(V base, range_difference_t<V> count);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr auto begin()
      requires (!(simple-view<V> && random_access_range<V>));
    constexpr auto begin() const
      requires random_access_range<const V>;

    constexpr auto end()
      requires (!simple-view<V>)
    { return ranges::end(base_); }

    constexpr auto end() const
      requires range<const V>
    { return ranges::end(base_); }

    constexpr auto size()
      requires sized_range<V>
    {
      const auto s = ranges::size(base_);
      const auto c = static_cast<decltype(s)>(count_);
      return s < c ? 0 : s - c;
    }

    constexpr auto size() const
      requires sized_range<const V>
    {
      const auto s = ranges::size(base_);
      const auto c = static_cast<decltype(s)>(count_);
      return s < c ? 0 : s - c;
    }
  private:
    V base_ = V();                              // exposition only
    range_difference_t<V> count_ = 0;           // exposition only
  };

  template<class R>
    drop_view(R&&, range_difference_t<R>) -> drop_view<views::all_t<R>>;
}
🔗
constexpr drop_view(V base, range_difference_t<V> count);
1
#
Preconditions: count >= 0 is true.
2
#
Effects: Initializes base_­ with std​::​move(base) and count_­ with count.
🔗
constexpr auto begin() requires (!(simple-view<V> && random_­access_­range<V>)); constexpr auto begin() const requires random_access_range<const V>;
3
#
Returns: ranges​::​next(ranges​::​begin(base_­), count_­, ranges​::​end(base_­)).
4
#
Remarks: In order to provide the amortized constant-time complexity required by the range concept when drop_­view models forward_­range, the first overload caches the result within the drop_­view for use on subsequent calls.
[Note
:
Without this, applying a reverse_­view over a drop_­view would have quadratic iteration complexity.
— end note
]

24.7.9 Drop while view [range.drop.while]

24.7.9.1 Overview [range.drop.while.overview]

1
#
Given a unary predicate pred and a view r, drop_­while_­view produces a view of the range [ranges​::​find_­if_­not(r, pred), ranges​::​end(r)).
2
#
The name views​::​drop_­while denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and F, the expression views​::​drop_­while(E, F) is expression-equivalent to drop_­while_­view{E, F}.
3
#
[Example
: 
constexpr auto source = "  \t   \t   \t   hello there";
auto is_invisible = [](const auto x) { return x == ' ' || x == '\t'; };
auto skip_ws = drop_while_view{source, is_invisible};
for (auto c : skip_ws) {
  cout << c;                                    // prints hello there with no leading space
}
— end example
]

24.7.9.2 Class template drop_­while_­view [range.drop.while.view]

namespace std::ranges {
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
  class drop_while_view : public view_interface<drop_while_view<V, Pred>> {
  public:
    drop_while_view() = default;
    constexpr drop_while_view(V base, Pred pred);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr const Pred& pred() const;

    constexpr auto begin();

    constexpr auto end()
    { return ranges::end(base_); }

  private:
    V base_ = V();                                      // exposition only
    semiregular-box<Pred> pred_;                        // exposition only
  };

  template<class R, class Pred>
    drop_while_view(R&&, Pred) -> drop_while_view<views::all_t<R>, Pred>;
}
🔗
constexpr drop_while_view(V base, Pred pred);
1
#
Effects: Initializes base_­ with std​::​move(base) and pred_­ with std​::​move(pred).
🔗
constexpr const Pred& pred() const;
2
#
Effects: Equivalent to: return *pred_­;
🔗
constexpr auto begin();
3
#
Returns: ranges​::​find_­if_­not(base_­, cref(*pred_­)).
4
#
Remarks: In order to provide the amortized constant-time complexity required by the range concept when drop_­while_­view models forward_­range, the first call caches the result within the drop_­while_­view for use on subsequent calls.
[Note
:
Without this, applying a reverse_­view over a drop_­while_­view would have quadratic iteration complexity.
— end note
]

24.7.10 Join view [range.join]

24.7.10.1 Overview [range.join.overview]

1
#
join_­view flattens a view of ranges into a view.
2
#
The name views​::​join denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​join(E) is expression-equivalent to join_­view{E}.
3
#
[Example
: 
vector<string> ss{"hello", " ", "world", "!"};
join_view greeting{ss};
for (char ch : greeting)
  cout << ch;                                   // prints: hello world!
— end example
]

24.7.10.2 Class template join_­view [range.join.view]

namespace std::ranges {
  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>> &&
             (is_reference_v<range_reference_t<V>> ||
              view<range_value_t<V>>)
  class join_view : public view_interface<join_view<V>> {
  private:
    using InnerRng =                    // exposition only
      range_reference_t<V>;
    // [range.join.iterator], class template join_­view​::​iterator
    template<bool Const>
      struct iterator;                  // exposition only
    // [range.join.sentinel], class template join_­view​::​sentinel
    template<bool Const>
      struct sentinel;                  // exposition only

    V base_ = V();                      // exposition only
    views::all_t<InnerRng> inner_ =     // exposition only, present only when !is_­reference_­v<InnerRng>
      views::all_t<InnerRng>();
  public:
    join_view() = default;
    constexpr explicit join_view(V base);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr auto begin() {
      constexpr bool use_const = simple-view<V> &&
                                 is_reference_v<range_reference_t<V>>;
      return iterator<use_const>{*this, ranges::begin(base_)};
    }

    constexpr auto begin() const
    requires input_range<const V> &&
             is_reference_v<range_reference_t<const V>> {
      return iterator<true>{*this, ranges::begin(base_)};
    }

    constexpr auto end() {
      if constexpr (forward_range<V> &&
                    is_reference_v<InnerRng> && forward_range<InnerRng> &&
                    common_range<V> && common_range<InnerRng>)
        return iterator<simple-view<V>>{*this, ranges::end(base_)};
      else
        return sentinel<simple-view<V>>{*this};
    }

    constexpr auto end() const
    requires input_range<const V> &&
             is_reference_v<range_reference_t<const V>> {
      if constexpr (forward_range<const V> &&
                    is_reference_v<range_reference_t<const V>> &&
                    forward_range<range_reference_t<const V>> &&
                    common_range<const V> &&
                    common_range<range_reference_t<const V>>)
        return iterator<true>{*this, ranges::end(base_)};
      else
        return sentinel<true>{*this};
    }
  };

  template<class R>
    explicit join_view(R&&) -> join_view<views::all_t<R>>;
}
🔗
constexpr explicit join_view(V base);
1
#
Effects: Initializes base_­ with std​::​move(base).

24.7.10.3 Class template join_­view​::​iterator [range.join.iterator]

namespace std::ranges {
  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>> &&
             (is_reference_v<range_reference_t<V>> ||
              view<range_value_t<V>>)
  template<bool Const>
  struct join_view<V>::iterator {
  private:
    using Parent =                                              // exposition only
      conditional_t<Const, const join_view, join_view>;
    using Base   = conditional_t<Const, const V, V>;            // exposition only

    static constexpr bool ref-is-glvalue =                      // exposition only
      is_reference_v<range_reference_t<Base>>;

    iterator_t<Base> outer_ = iterator_t<Base>();               // exposition only
    iterator_t<range_reference_t<Base>> inner_ =                // exposition only
      iterator_t<range_reference_t<Base>>();
    Parent* parent_ = nullptr;                                  // exposition only

    constexpr void satisfy();                                   // exposition only
  public:
    using iterator_concept  = see below;
    using iterator_category = see below;
    using value_type        = range_value_t<range_reference_t<Base>>;
    using difference_type   = see below;

    iterator() = default;
    constexpr iterator(Parent& parent, iterator_t<Base> outer);
    constexpr iterator(iterator<!Const> i)
      requires Const &&
               convertible_to<iterator_t<V>, iterator_t<Base>> &&
               convertible_to<iterator_t<InnerRng>,
                              iterator_t<range_reference_t<Base>>>;

    constexpr decltype(auto) operator*() const { return *inner_; }

    constexpr iterator_t<Base> operator->() const
      requires has-arrow<iterator_t<Base>> && copyable<iterator_t<Base>>;

    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int)
      requires ref-is-glvalue && forward_range<Base> &&
               forward_range<range_reference_t<Base>>;

    constexpr iterator& operator--()
      requires ref-is-glvalue && bidirectional_range<Base> &&
               bidirectional_range<range_reference_t<Base>> &&
               common_range<range_reference_t<Base>>;

    constexpr iterator operator--(int)
      requires ref-is-glvalue && bidirectional_range<Base> &&
               bidirectional_range<range_reference_t<Base>> &&
               common_range<range_reference_t<Base>>;

    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires ref-is-glvalue && equality_comparable<iterator_t<Base>> &&
               equality_comparable<iterator_t<range_reference_t<Base>>>;

    friend constexpr decltype(auto) iter_move(const iterator& i)
    noexcept(noexcept(ranges::iter_move(i.inner_))) {
      return ranges::iter_move(i.inner_);
    }

    friend constexpr void iter_swap(const iterator& x, const iterator& y)
      noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)));
  };
}
1
#
iterator​::​iterator_­concept is defined as follows:
  • (1.1)
    If ref-is-glvalue is true and Base and range_­reference_­t<Base> each model bidirectional_­range, then iterator_­concept denotes bidirectional_­iterator_­tag.
  • (1.2)
    Otherwise, if ref-is-glvalue is true and Base and range_­reference_­t<Base> each model forward_­range, then iterator_­concept denotes forward_­iterator_­tag.
  • (1.3)
    Otherwise, iterator_­concept denotes input_­iterator_­tag.
2
#
iterator​::​iterator_­category is defined as follows:
  • (2.1)
    Let OUTERC denote iterator_­traits<iterator_­t<Base>>​::​iterator_­category, and let INNERC denote iterator_­traits<iterator_­t<range_­reference_­t<Base>>>​::​iterator_­category.
  • (2.2)
    If ref-is-glvalue is true and OUTERC and INNERC each model derived_­from<bidirectional_­iterator_­tag>, iterator_­category denotes bidirectional_­iterator_­tag.
  • (2.3)
    Otherwise, if ref-is-glvalue is true and OUTERC and INNERC each model derived_­from<forward_­iterator_­tag>, iterator_­category denotes forward_­iterator_­tag.
  • (2.4)
    Otherwise, if OUTERC and INNERC each model derived_­from<input_­iterator_­tag>, iterator_­category denotes input_­iterator_­tag.
  • (2.5)
    Otherwise, iterator_­category denotes output_­iterator_­tag.
3
#
iterator​::​difference_­type denotes the type: 
common_type_t<
  range_difference_t<Base>,
  range_difference_t<range_reference_t<Base>>>
4
#
join_­view iterators use the satisfy function to skip over empty inner ranges.
🔗
constexpr void satisfy(); // exposition only
5
#
Effects: Equivalent to: 
auto update_inner = [this](range_reference_t<Base> x) -> auto& {
  if constexpr (ref-is-glvalue) // x is a reference
    return x;
  else
    return (parent_->inner_ = views::all(std::move(x)));
};

for (; outer_ != ranges::end(parent_->base_); ++outer_) {
  auto& inner = update_inner(*outer_);
  inner_ = ranges::begin(inner);
  if (inner_ != ranges::end(inner))
    return;
}
if constexpr (ref-is-glvalue)
  inner_ = iterator_t<range_reference_t<Base>>();
🔗
constexpr iterator(Parent& parent, iterator_t<Base> outer);
6
#
Effects: Initializes outer_­ with std​::​move(outer) and parent_­ with addressof(parent); then calls satisfy().
🔗
constexpr iterator(iterator<!Const> i) requires Const && convertible_­to<iterator_t<V>, iterator_t<Base>> && convertible_­to<iterator_t<InnerRng>, iterator_t<range_reference_t<Base>>>;
7
#
Effects: Initializes outer_­ with std​::​move(i.outer_­), inner_­ with std​::​move(i.inner_­), and parent_­ with i.parent_­.
🔗
constexpr iterator_t<Base> operator->() const requires has-arrow<iterator_t<Base>> && copyable<iterator_t<Base>>;
8
#
Effects: Equivalent to return inner_­;
🔗
constexpr iterator& operator++();
9
#
Let inner-range be:
  • (9.1)
    If ref-is-glvalue is true, *outer_­.
  • (9.2)
    Otherwise, parent_­->inner_­.
10
#
Effects: Equivalent to: 
auto&& inner_rng = inner-range;
if (++inner_ == ranges::end(inner_rng)) {
  ++outer_;
  satisfy();
}
return *this;
🔗
constexpr void operator++(int);
11
#
Effects: Equivalent to: ++*this.
🔗
constexpr iterator operator++(int) requires ref-is-glvalue && forward_­range<Base> && forward_­range<range_reference_t<Base>>;
12
#
Effects: Equivalent to: 
auto tmp = *this;
++*this;
return tmp;
🔗
constexpr iterator& operator--() requires ref-is-glvalue && bidirectional_­range<Base> && bidirectional_­range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;
13
#
Effects: Equivalent to: 
if (outer_ == ranges::end(parent_->base_))
  inner_ = ranges::end(*--outer_);
while (inner_ == ranges::begin(*outer_))
  inner_ = ranges::end(*--outer_);
--inner_;
return *this;
🔗
constexpr iterator operator--(int) requires ref-is-glvalue && bidirectional_­range<Base> && bidirectional_­range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;
14
#
Effects: Equivalent to: 
auto tmp = *this;
--*this;
return tmp;
🔗
friend constexpr bool operator==(const iterator& x, const iterator& y) requires ref-is-glvalue && equality_comparable<iterator_t<Base>> && equality_comparable<iterator_t<range_reference_t<Base>>>;
15
#
Effects: Equivalent to: return x.outer_­ == y.outer_­ && x.inner_­ == y.inner_­;
🔗
friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.inner_­, y.inner_­)));
16
#
Effects: Equivalent to: return ranges​::​iter_­swap(x.inner_­, y.inner_­);

24.7.10.4 Class template join_­view​::​sentinel [range.join.sentinel]

namespace std::ranges {
  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>> &&
             (is_reference_v<range_reference_t<V>> ||
              view<range_value_t<V>>)
  template<bool Const>
  struct join_view<V>::sentinel {
  private:
    using Parent =                                      // exposition only
      conditional_t<Const, const join_view, join_view>;
    using Base   = conditional_t<Const, const V, V>;    // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
  public:
    sentinel() = default;

    constexpr explicit sentinel(Parent& parent);
    constexpr sentinel(sentinel<!Const> s)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

    friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);
  };
}
🔗
constexpr explicit sentinel(Parent& parent);
1
#
Effects: Initializes end_­ with ranges​::​end(parent.base_­).
🔗
constexpr sentinel(sentinel<!Const> s) requires Const && convertible_­to<sentinel_t<V>, sentinel_t<Base>>;
2
#
Effects: Initializes end_­ with std​::​move(s.end_­).
🔗
friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);
3
#
Effects: Equivalent to: return x.outer_­ == y.end_­;

24.7.11 Split view [range.split]

24.7.11.1 Overview [range.split.overview]

1
#
split_­view takes a view and a delimiter, and splits the view into subranges on the delimiter.
The delimiter can be a single element or a view of elements.
2
#
The name views​::​split denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and F, the expression views​::​split(E, F) is expression-equivalent to split_­view{E, F}.
3
#
[Example
: 
string str{"the quick brown fox"};
split_view sentence{str, ' '};
for (auto word : sentence) {
  for (char ch : word)
    cout << ch;
  cout << '*';
}
// The above prints: the*quick*brown*fox*
— end example
]

24.7.11.2 Class template split_­view [range.split.view]

namespace std::ranges {
  template<auto> struct require-constant;       // exposition only

  template<class R>
  concept tiny-range =                          // exposition only
    sized_range<R> &&
    requires { typename require-constant<remove_reference_t<R>::size()>; } &&
    (remove_reference_t<R>::size() <= 1);

  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
             (forward_range<V> || tiny-range<Pattern>)
  class split_view : public view_interface<split_view<V, Pattern>> {
  private:
    V base_ = V();                              // exposition only
    Pattern pattern_ = Pattern();               // exposition only
    iterator_t<V> current_ = iterator_t<V>();   // exposition only, present only if !forward_­range<V>
    // [range.split.outer], class template split_­view​::​outer-iterator
    template<bool> struct outer-iterator;       // exposition only
    // [range.split.inner], class template split_­view​::​inner-iterator
    template<bool> struct inner-iterator;       // exposition only
  public:
    split_view() = default;
    constexpr split_view(V base, Pattern pattern);

    template<input_range R>
      requires constructible_from<V, views::all_t<R>> &&
               constructible_from<Pattern, single_view<range_value_t<R>>>
    constexpr split_view(R&& r, range_value_t<R> e);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr auto begin() {
      if constexpr (forward_range<V>)
        return outer-iterator<simple-view<V>>{*this, ranges::begin(base_)};
      else {
        current_ = ranges::begin(base_);
        return outer-iterator<false>{*this};
      }
    }

    constexpr auto begin() const requires forward_range<V> && forward_range<const V> {
      return outer-iterator<true>{*this, ranges::begin(base_)};
    }

    constexpr auto end() requires forward_range<V> && common_range<V> {
      return outer-iterator<simple-view<V>>{*this, ranges::end(base_)};
    }

    constexpr auto end() const {
      if constexpr (forward_range<V> && forward_range<const V> && common_range<const V>)
        return outer-iterator<true>{*this, ranges::end(base_)};
      else
        return default_sentinel;
    }
  };

  template<class R, class P>
    split_view(R&&, P&&) -> split_view<views::all_t<R>, views::all_t<P>>;

  template<input_range R>
    split_view(R&&, range_value_t<R>)
      -> split_view<views::all_t<R>, single_view<range_value_t<R>>>;
}
🔗
constexpr split_view(V base, Pattern pattern);
1
#
Effects: Initializes base_­ with std​::​move(base), and pattern_­ with std​::​move(pattern).
🔗
template<input_­range R> requires constructible_­from<V, views::all_t<R>> && constructible_­from<Pattern, single_view<range_value_t<R>>> constexpr split_view(R&& r, range_value_t<R> e);
2
#
Effects: Initializes base_­ with views​::​all(std​::​forward<R>(r)), and pattern_­ with single_­view{​std​::​move(e)}.

24.7.11.3 Class template split_­view​::​outer-iterator [range.split.outer]

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
             (forward_range<V> || tiny-range<Pattern>)
  template<bool Const>
  struct split_view<V, Pattern>::outer-iterator {
  private:
    using Parent =                          // exposition only
      conditional_t<Const, const split_view, split_view>;
    using Base   =                          // exposition only
      conditional_t<Const, const V, V>;
    Parent* parent_ = nullptr;              // exposition only
    iterator_t<Base> current_ =             // exposition only, present only if V models forward_­range
      iterator_t<Base>();

  public:
    using iterator_concept  =
      conditional_t<forward_range<Base>, forward_iterator_tag, input_iterator_tag>;
    using iterator_category = input_iterator_tag;
    // [range.split.outer.value], class split_­view​::​outer-iterator​::​value_­type
    struct value_type;
    using difference_type   = range_difference_t<Base>;

    outer-iterator() = default;
    constexpr explicit outer-iterator(Parent& parent)
      requires (!forward_range<Base>);
    constexpr outer-iterator(Parent& parent, iterator_t<Base> current)
      requires forward_range<Base>;
    constexpr outer-iterator(outer-iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;

    constexpr value_type operator*() const;

    constexpr outer-iterator& operator++();
    constexpr decltype(auto) operator++(int) {
      if constexpr (forward_range<Base>) {
        auto tmp = *this;
        ++*this;
        return tmp;
      } else
        ++*this;
    }

    friend constexpr bool operator==(const outer-iterator& x, const outer-iterator& y)
      requires forward_range<Base>;

    friend constexpr bool operator==(const outer-iterator& x, default_sentinel_t);
  };
}
1
#
Many of the following specifications refer to the notional member current of outer-iterator.
current is equivalent to current_­ if V models forward_­range, and parent_­->current_­ otherwise.
🔗
constexpr explicit outer-iterator(Parent& parent) requires (!forward_­range<Base>);
2
#
Effects: Initializes parent_­ with addressof(parent).
🔗
constexpr outer-iterator(Parent& parent, iterator_t<Base> current) requires forward_­range<Base>;
3
#
Effects: Initializes parent_­ with addressof(parent) and current_­ with std​::​move(current).
🔗
constexpr outer-iterator(outer-iterator<!Const> i) requires Const && convertible_­to<iterator_t<V>, iterator_t<Base>>;
4
#
Effects: Initializes parent_­ with i.parent_­ and current_­ with std​::​move(i.current_­).
🔗
constexpr value_type operator*() const;
5
#
Effects: Equivalent to: return value_­type{*this};
🔗
constexpr outer-iterator& operator++();
6
#
Effects: Equivalent to: 
const auto end = ranges::end(parent_->base_);
if (current == end) return *this;
const auto [pbegin, pend] = subrange{parent_->pattern_};
if (pbegin == pend) ++current;
else {
  do {
    auto [b, p] = ranges::mismatch(std::move(current), end, pbegin, pend);
    current = std::move(b);
    if (p == pend) {
      break;            // The pattern matched; skip it
    }
  } while (++current != end);
}
return *this;
🔗
friend constexpr bool operator==(const outer-iterator& x, const outer-iterator& y) requires forward_­range<Base>;
7
#
Effects: Equivalent to: return x.current_­ == y.current_­;
🔗
friend constexpr bool operator==(const outer-iterator& x, default_sentinel_t);
8
#
Effects: Equivalent to: return x.current == ranges​::​end(x.parent_­->base_­);

24.7.11.4 Class split_­view​::​outer-iterator​::​value_­type [range.split.outer.value]

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
             (forward_range<V> || tiny-range<Pattern>)
  template<bool Const>
  struct split_view<V, Pattern>::outer-iterator<Const>::value_type
    : view_interface<value_type> {
  private:
    outer-iterator i_ = outer-iterator();               // exposition only
  public:
    value_type() = default;
    constexpr explicit value_type(outer-iterator i);

    constexpr inner-iterator<Const> begin() const requires copyable<outer-iterator>;
    constexpr inner-iterator<Const> begin() requires (!copyable<outer-iterator>);
    constexpr default_sentinel_t end() const;
  };
}
🔗
constexpr explicit value_type(outer-iterator i);
1
#
Effects: Initializes i_­ with std​::​move(i).
🔗
constexpr inner-iterator<Const> begin() const requires copyable<outer-iterator>;
2
#
Effects: Equivalent to: return inner-iterator<Const>{i_­};
🔗
constexpr inner-iterator<Const> begin() requires (!copyable<outer-iterator>);
3
#
Effects: Equivalent to: return inner-iterator<Const>{std​::​move(i_­)};
🔗
constexpr default_sentinel_t end() const;
4
#
Effects: Equivalent to: return default_­sentinel;

24.7.11.5 Class template split_­view​::​inner-iterator [range.split.inner]

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
             (forward_range<V> || tiny-range<Pattern>)
  template<bool Const>
  struct split_view<V, Pattern>::inner-iterator {
  private:
    using Base = conditional_t<Const, const V, V>;      // exposition only
    outer-iterator<Const> i_ = outer-iterator<Const>(); // exposition only
    bool incremented_ = false;                          // exposition only
  public:
    using iterator_concept  = typename outer-iterator<Const>::iterator_concept;
    using iterator_category = see below;
    using value_type        = range_value_t<Base>;
    using difference_type   = range_difference_t<Base>;

    inner-iterator() = default;
    constexpr explicit inner-iterator(outer-iterator<Const> i);

    constexpr decltype(auto) operator*() const { return *i_.current; }

    constexpr inner-iterator& operator++();
    constexpr decltype(auto) operator++(int) {
      if constexpr (forward_range<V>) {
        auto tmp = *this;
        ++*this;
        return tmp;
      } else
        ++*this;
    }

    friend constexpr bool operator==(const inner-iterator& x, const inner-iterator& y)
      requires forward_range<Base>;

    friend constexpr bool operator==(const inner-iterator& x, default_sentinel_t);

    friend constexpr decltype(auto) iter_move(const inner-iterator& i)
    noexcept(noexcept(ranges::iter_move(i.i_.current))) {
      return ranges::iter_move(i.i_.current);
    }

    friend constexpr void iter_swap(const inner-iterator& x, const inner-iterator& y)
      noexcept(noexcept(ranges::iter_swap(x.i_.current, y.i_.current)))
      requires indirectly_swappable<iterator_t<Base>>;
  };
}
1
#
The typedef-name iterator_­category denotes:
  • (1.1)
    forward_­iterator_­tag if iterator_­traits<iterator_­t<Base>>​::​iterator_­category models
    derived_­from<forward_­iterator_­tag>;
  • (1.2)
    otherwise, iterator_­traits<iterator_­t<Base>>​::​iterator_­category.
🔗
constexpr explicit inner-iterator(outer-iterator<Const> i);
2
#
Effects: Initializes i_­ with std​::​move(i).
🔗
constexpr inner-iterator& operator++();
3
#
Effects: Equivalent to: 
incremented_ = true;
if constexpr (!forward_range<Base>) {
  if constexpr (Pattern::size() == 0) {
    return *this;
  }
}
++i_.current;
return *this;
🔗
friend constexpr bool operator==(const inner-iterator& x, const inner-iterator& y) requires forward_­range<Base>;
4
#
Effects: Equivalent to: return x.i_­.current == y.i_­.current;
🔗
friend constexpr bool operator==(const inner-iterator& x, default_sentinel_t);
5
#
Effects: Equivalent to: 
auto [pcur, pend] = subrange{x.i_.parent_->pattern_};
auto end = ranges::end(x.i_.parent_->base_);
if constexpr (tiny-range<Pattern>) {
  const auto & cur = x.i_.current;
  if (cur == end) return true;
  if (pcur == pend) return x.incremented_;
  return *cur == *pcur;
} else {
  auto cur = x.i_.current;
  if (cur == end) return true;
  if (pcur == pend) return x.incremented_;
  do {
    if (*cur != *pcur) return false;
    if (++pcur == pend) return true;
  } while (++cur != end);
  return false;
}
🔗
friend constexpr void iter_swap(const inner-iterator& x, const inner-iterator& y) noexcept(noexcept(ranges::iter_swap(x.i_­.current, y.i_­.current))) requires indirectly_­swappable<iterator_t<Base>>;
6
#
Effects: Equivalent to ranges​::​iter_­swap(x.i_­.current, y.i_­.current).

24.7.12 Counted view [range.counted]

1
#
A counted view presents a view of the elements of the counted range ([iterator.requirements.general]) for an iterator i and non-negative integer n.
2
#
The name views​::​counted denotes a customization point object ([customization.point.object]).
Let E and F be expressions, let T be decay_­t<decltype((E))>, and let D be iter_­difference_­t<T>.
If decltype((F)) does not model convertible_­to<D>, views​::​counted(E, F) is ill-formed.
[Note
:
This case can result in substitution failure when views​::​counted(E, F) appears in the immediate context of a template instantiation.
— end note
]
Otherwise, views​::​counted(E, F) is expression-equivalent to:
  • (2.1)
    If T models contiguous_­iterator, then span{to_­address(E), static_­cast<D>(F)}.
  • (2.2)
    Otherwise, if T models random_­access_­iterator, then subrange{E, E + static_­cast<D>(F)}, except that E is evaluated only once.
  • (2.3)
    Otherwise, subrange{counted_­iterator{E, F}, default_­sentinel}.

24.7.13 Common view [range.common]

24.7.13.1 Overview [range.common.overview]

1
#
common_­view takes a view which has different types for its iterator and sentinel and turns it into a view of the same elements with an iterator and sentinel of the same type.
2
#
[Note
:
common_­view is useful for calling legacy algorithms that expect a range's iterator and sentinel types to be the same.
— end note
]
3
#
The name views​::​common denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​common(E) is expression-equivalent to:
  • (3.1)
    views​::​all(E), if decltype((E)) models common_­range and views​::​all(E) is a well-formed expression.
  • (3.2)
    Otherwise, common_­view{E}.
4
#
[Example
: 
// Legacy algorithm:
template<class ForwardIterator>
size_t count(ForwardIterator first, ForwardIterator last);

template<forward_range R>
void my_algo(R&& r) {
  auto&& common = common_view{r};
  auto cnt = count(common.begin(), common.end());
  // ...
}
— end example
]

24.7.13.2 Class template common_­view [range.common.view]

namespace std::ranges {
  template<view V>
    requires (!common_range<V> && copyable<iterator_t<V>>)
  class common_view : public view_interface<common_view<V>> {
  private:
    V base_ = V();  // exposition only
  public:
    common_view() = default;

    constexpr explicit common_view(V r);

    template<viewable_range R>
      requires (!common_range<R> && constructible_from<V, views::all_t<R>>)
    constexpr explicit common_view(R&& r);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr auto begin() {
      if constexpr (random_access_range<V> && sized_range<V>)
        return ranges::begin(base_);
      else
        return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_));
    }

    constexpr auto begin() const requires range<const V> {
      if constexpr (random_access_range<const V> && sized_range<const V>)
        return ranges::begin(base_);
      else
        return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_));
    }

    constexpr auto end() {
      if constexpr (random_access_range<V> && sized_range<V>)
        return ranges::begin(base_) + ranges::size(base_);
      else
        return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_));
    }

    constexpr auto end() const requires range<const V> {
      if constexpr (random_access_range<const V> && sized_range<const V>)
        return ranges::begin(base_) + ranges::size(base_);
      else
        return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_));
    }

    constexpr auto size() requires sized_range<V> {
      return ranges::size(base_);
    }
    constexpr auto size() const requires sized_range<const V> {
      return ranges::size(base_);
    }
  };

  template<class R>
    common_view(R&&) -> common_view<views::all_t<R>>;
}
🔗
constexpr explicit common_view(V base);
1
#
Effects: Initializes base_­ with std​::​move(base).
🔗
template<viewable_range R> requires (!common_range<R> && constructible_­from<V, views::all_t<R>>) constexpr explicit common_view(R&& r);
2
#
Effects: Initializes base_­ with views​::​all(std​::​forward<R>(r)).

24.7.14 Reverse view [range.reverse]

24.7.14.1 Overview [range.reverse.overview]

1
#
reverse_­view takes a bidirectional view and produces another view that iterates the same elements in reverse order.
2
#
The name views​::​reverse denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​reverse(E) is expression-equivalent to:
  • (2.1)
    If the type of E is a (possibly cv-qualified) specialization of reverse_­view, equivalent to E.base().
  • (2.2)
    Otherwise, if the type of E is cv-qualified 
    subrange<reverse_iterator<I>, reverse_iterator<I>, K>
    for some iterator type I and value K of type subrange_­kind,
    • (2.2.1)
      if K is subrange_­kind​::​sized, equivalent to: 
      subrange<I, I, K>(E.end().base(), E.begin().base(), E.size())
    • (2.2.2)
      otherwise, equivalent to: 
      subrange<I, I, K>(E.end().base(), E.begin().base())
    However, in either case E is evaluated only once.
  • (2.3)
    Otherwise, equivalent to reverse_­view{E}.
3
#
[Example
: 
vector<int> is {0,1,2,3,4};
reverse_view rv {is};
for (int i : rv)
  cout << i << ' '; // prints: 4 3 2 1 0
— end example
]

24.7.14.2 Class template reverse_­view [range.reverse.view]

namespace std::ranges {
  template<view V>
    requires bidirectional_range<V>
  class reverse_view : public view_interface<reverse_view<V>> {
  private:
    V base_ = V();  // exposition only
  public:
    reverse_view() = default;

    constexpr explicit reverse_view(V r);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr reverse_iterator<iterator_t<V>> begin();
    constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
    constexpr auto begin() const requires common_range<const V>;

    constexpr reverse_iterator<iterator_t<V>> end();
    constexpr auto end() const requires common_range<const V>;

    constexpr auto size() requires sized_range<V> {
      return ranges::size(base_);
    }
    constexpr auto size() const requires sized_range<const V> {
      return ranges::size(base_);
    }
  };

  template<class R>
    reverse_view(R&&) -> reverse_view<views::all_t<R>>;
}
🔗
constexpr explicit reverse_view(V base);
1
#
Effects: Initializes base_­ with std​::​move(base).
🔗
constexpr reverse_iterator<iterator_t<V>> begin();
2
#
Returns: 
make_reverse_iterator(ranges::next(ranges::begin(base_), ranges::end(base_)))
3
#
Remarks: In order to provide the amortized constant time complexity required by the range concept, this function caches the result within the reverse_­view for use on subsequent calls.
🔗
constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>; constexpr auto begin() const requires common_range<const V>;
4
#
Effects: Equivalent to: return make_­reverse_­iterator(ranges​::​end(base_­));
🔗
constexpr reverse_iterator<iterator_t<V>> end(); constexpr auto end() const requires common_range<const V>;
5
#
Effects: Equivalent to: return make_­reverse_­iterator(ranges​::​begin(base_­));

24.7.15 Elements view [range.elements]

24.7.15.1 Overview [range.elements.overview]

1
#
elements_­view takes a view of tuple-like values and a size_­t, and produces a view with a value-type of the element of the adapted view's value-type.
2
#
The name views​::​elements<N> denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E and constant expression N, the expression views​::​elements<N>(E) is expression-equivalent to elements_­view<views​::​all_­t<decltype((E))>, N>{E}.
[Example
: 
auto historical_figures = map{
  {"Lovelace"sv, 1815},
  {"Turing"sv, 1912},
  {"Babbage"sv, 1791},
  {"Hamilton"sv, 1936}
};

auto names = historical_figures | views::elements<0>;
for (auto&& name : names) {
  cout << name << ' ';          // prints Babbage Hamilton Lovelace Turing 
}

auto birth_years = historical_figures | views::elements<1>;
for (auto&& born : birth_years) {
  cout << born << ' ';          // prints 1791 1936 1815 1912 
}
— end example
]
3
#
keys_­view is an alias for elements_­view<views​::​all_­t<R>, 0>, and is useful for extracting keys from associative containers.
[Example
: 
auto names = keys_view{historical_figures};
for (auto&& name : names) {
  cout << name << ' ';          // prints Babbage Hamilton Lovelace Turing 
}
— end example
]
4
#
values_­view is an alias for elements_­view<views​::​all_­t<R>, 1>, and is useful for extracting values from associative containers.
[Example
: 
auto is_even = [](const auto x) { return x % 2 == 0; };
cout << ranges::count_if(values_view{historical_figures}, is_even);     // prints 2
— end example
]

24.7.15.2 Class template elements_­view [range.elements.view]

namespace std::ranges {
  template<class T, size_t N>
  concept has-tuple-element =                   // exposition only
    requires(T t) {
      typename tuple_size<T>::type;
      requires N < tuple_size_v<T>;
      typename tuple_element_t<N, T>;
      { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
    };


  template<input_range V, size_t N>
    requires view<V> && has-tuple-element<range_value_t<V>, N> &&
             has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
  class elements_view : public view_interface<elements_view<V, N>> {
  public:
    elements_view() = default;
    constexpr explicit elements_view(V base);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr auto begin() requires (!simple-view<V>)
    { return iterator<false>(ranges::begin(base_)); }

    constexpr auto begin() const requires simple-view<V>
    { return iterator<true>(ranges::begin(base_)); }

    constexpr auto end()
    { return sentinel<false>{ranges::end(base_)}; }

    constexpr auto end() requires common_range<V>
    { return iterator<false>{ranges::end(base_)}; }

    constexpr auto end() const requires range<const V>
    { return sentinel<true>{ranges::end(base_)}; }

    constexpr auto end() const requires common_range<const V>
    { return iterator<true>{ranges::end(base_)}; }

    constexpr auto size() requires sized_range<V>
    { return ranges::size(base_); }

    constexpr auto size() const requires sized_range<const V>
    { return ranges::size(base_); }

  private:
    // [range.elements.iterator], class template elements_­view​::​iterator
    template<bool> struct iterator;                     // exposition only
    // [range.elements.sentinel], class template elements_­view​::​sentinel
    template<bool> struct sentinel;                     // exposition only
    V base_ = V();                                      // exposition only
  };
}
🔗
constexpr explicit elements_view(V base);
1
#
Effects: Initializes base_­ with std​::​move(base).

24.7.15.3 Class template elements_­view​::​iterator [range.elements.iterator]

namespace std::ranges {
  template<input_range V, size_t N>
    requires view<V> && has-tuple-element<range_value_t<V>, N> &&
             has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
  template<bool Const>
  class elements_view<V, N>::iterator {                 // exposition only
    using Base = conditional_t<Const, const V, V>;      // exposition only

    iterator_t<Base> current_ = iterator_t<Base>();
  public:
    using iterator_category = typename iterator_traits<iterator_t<Base>>::iterator_category;
    using value_type = remove_cvref_t<tuple_element_t<N, range_value_t<Base>>>;
    using difference_type = range_difference_t<Base>;

    iterator() = default;
    constexpr explicit iterator(iterator_t<Base> current);
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;

    constexpr iterator_t<Base> base() const&
      requires copyable<iterator_t<Base>>;
    constexpr iterator_t<Base> base() &&;

    constexpr decltype(auto) operator*() const
    { return get<N>(*current_); }

    constexpr iterator& operator++();
    constexpr void operator++(int) requires (!forward_range<Base>);
    constexpr iterator operator++(int) requires forward_range<Base>;

    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;

    constexpr iterator& operator+=(difference_type x)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x)
      requires random_access_range<Base>;

    constexpr decltype(auto) operator[](difference_type n) const
      requires random_access_range<Base>
    { return get<N>(*(current_ + n)); }

    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<iterator_t<Base>>;

    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;

    friend constexpr iterator operator+(const iterator& x, difference_type y)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& x, difference_type y)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
  };
}
🔗
constexpr explicit iterator(iterator_t<Base> current);
1
#
Effects: Initializes current_­ with std​::​move(current).
🔗
constexpr iterator(iterator<!Const> i) requires Const && convertible_­to<iterator_t<V>, iterator_t<Base>>;
2
#
Effects: Initializes current_­ with std​::​move(i.current_­).
🔗
constexpr iterator_t<Base> base() const& requires copyable<iterator_t<Base>>;
3
#
Effects: Equivalent to: return current_­;
🔗
constexpr iterator_t<Base> base() &&;
4
#
Effects: Equivalent to: return std​::​move(current_­);
🔗
constexpr iterator& operator++();
5
#
Effects: Equivalent to: 
++current_;
return *this;
🔗
constexpr void operator++(int) requires (!forward_­range<Base>);
6
#
Effects: Equivalent to: ++current_­.
🔗
constexpr iterator operator++(int) requires forward_­range<Base>;
7
#
Effects: Equivalent to: 
auto temp = *this;
++current_;
return temp;
🔗
constexpr iterator& operator--() requires bidirectional_­range<Base>;
8
#
Effects: Equivalent to: 
--current_;
return *this;
🔗
constexpr iterator operator--(int) requires bidirectional_­range<Base>;
9
#
Effects: Equivalent to: 
auto temp = *this;
--current_;
return temp;
🔗
constexpr iterator& operator+=(difference_type n); requires random_­access_­range<Base>;
10
#
Effects: Equivalent to: 
current_ += n;
return *this;
🔗
constexpr iterator& operator-=(difference_type n) requires random_­access_­range<Base>;
11
#
Effects: Equivalent to: 
current_ -= n;
return *this;
🔗
friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<Base>;
12
#
Effects: Equivalent to: return x.current_­ == y.current_­;
🔗
friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
13
#
Effects: Equivalent to: return x.current_­ < y.current_­;
🔗
friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
14
#
Effects: Equivalent to: return y < x;
🔗
friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
15
#
Effects: Equivalent to: return !(y < x);
🔗
friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
16
#
Effects: Equivalent to: return !(x < y);
🔗
friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_­access_­range<Base> && three_­way_­comparable<iterator_t<Base>>;
17
#
Effects: Equivalent to: return x.current_­ <=> y.current_­;
🔗
friend constexpr iterator operator+(const iterator& x, difference_type y) requires random_­access_­range<Base>;
18
#
Effects: Equivalent to: return iterator{x} += y;
🔗
friend constexpr iterator operator+(difference_type x, const iterator& y) requires random_­access_­range<Base>;
19
#
Effects: Equivalent to: return y + x;
🔗
constexpr iterator operator-(const iterator& x, difference_type y) requires random_­access_­range<Base>;
20
#
Effects: Equivalent to: return iterator{x} -= y;
🔗
constexpr difference_type operator-(const iterator& x, const iterator& y) requires random_­access_­range<Base>;
21
#
Effects: Equivalent to: return x.current_­ - y.current_­;

24.7.15.4 Class template elements_­view​::​sentinel [range.elements.sentinel]

namespace std::ranges {
  template<input_range V, size_t N>
    requires view<V> && has-tuple-element<range_value_t<V>, N> &&
             has-tuple-element<remove_reference_t<range_reference_t<V>>, N>
  template<bool Const>
  class elements_view<V, N>::sentinel {                 // exposition only
  private:
    using Base = conditional_t<Const, const V, V>;      // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end);
    constexpr sentinel(sentinel<!Const> other)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

    constexpr sentinel_t<Base> base() const;

    friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);

    friend constexpr range_difference_t<Base>
      operator-(const iterator<Const>& x, const sentinel& y)
        requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;

    friend constexpr range_difference_t<Base>
      operator-(const sentinel& x, const iterator<Const>& y)
        requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
  };
}
🔗
constexpr explicit sentinel(sentinel_t<Base> end);
1
#
Effects: Initializes end_­ with end.
🔗
constexpr sentinel(sentinel<!Const> other) requires Const && convertible_­to<sentinel_t<V>, sentinel_t<Base>>;
2
#
Effects: Initializes end_­ with std​::​move(other.end_­).
🔗
constexpr sentinel_t<Base> base() const;
3
#
Effects: Equivalent to: return end_­;
🔗
friend constexpr bool operator==(const iterator<Const>& x, const sentinel& y);
4
#
Effects: Equivalent to: return x.current_­ == y.end_­;
🔗
friend constexpr range_difference_t<Base> operator-(const iterator<Const>& x, const sentinel& y) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
5
#
Effects: Equivalent to: return x.current_­ - y.end_­;
🔗
friend constexpr range_difference_t<Base> operator-(const sentinel& x, const iterator<Const>& y) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
6
#
Effects: Equivalent to: return x.end_­ - y.current_­;