22 Containers library [containers]

22.3 Sequence containers [sequences]

22.3.7 Class template array [array]

22.3.7.1 Overview [array.overview]

The header <array> defines a class template for storing fixed-size sequences of objects.

An instance of array<T, N> stores N elements of type T, so that size() == N is an invariant.

An array is an aggregate that can be list-initialized with up to N elements whose types are convertible to T.

An array meets all of the requirements of a container and of a reversible container ([container.requirements]), except that a default constructed array object is not empty and that swap does not have constant complexity.

An array meets some of the requirements of a sequence container .

Descriptions are provided here only for operations on array that are not described in one of these tables and for operations where there is additional semantic information.

array<T, N> is a structural type ([temp.param]) if T is a structural type.

Two values a1 and a2 of type array<T, N> are template-argument-equivalent ([temp.type]) if and only if each pair of corresponding elements in a1 and a2 are template-argument-equivalent.

The types iterator and const_iterator meet the constexpr iterator requirements ([iterator.requirements.general]).

namespace std {
  template<class T, size_t N>
  struct array {
    // types
    using value_type             = T;
    using pointer                = T*;
    using const_pointer          = const T*;
    using reference              = T&;
    using const_reference        = const T&;
    using size_type              = size_t;
    using difference_type        = ptrdiff_t;
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // no explicit construct/copy/destroy for aggregate type

    constexpr void fill(const T& u);
    constexpr void swap(array&) noexcept(is_nothrow_swappable_v<T>);

    // iterators
    constexpr iterator               begin() noexcept;
    constexpr const_iterator         begin() const noexcept;
    constexpr iterator               end() noexcept;
    constexpr const_iterator         end() const noexcept;

    constexpr reverse_iterator       rbegin() noexcept;
    constexpr const_reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator       rend() noexcept;
    constexpr const_reverse_iterator rend() const noexcept;

    constexpr const_iterator         cbegin() const noexcept;
    constexpr const_iterator         cend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

    // capacity
    [[nodiscard]] constexpr bool empty() const noexcept;
    constexpr size_type size() const noexcept;
    constexpr size_type max_size() const noexcept;

    // element access
    constexpr reference       operator[](size_type n);
    constexpr const_reference operator[](size_type n) const;
    constexpr reference       at(size_type n);
    constexpr const_reference at(size_type n) const;
    constexpr reference       front();
    constexpr const_reference front() const;
    constexpr reference       back();
    constexpr const_reference back() const;

    constexpr T *       data() noexcept;
    constexpr const T * data() const noexcept;
  };

  template<class T, class... U>
    array(T, U...) -> array<T, 1 + sizeof...(U)>;
}

22.3.7.2 Constructors, copy, and assignment [array.cons]

The conditions for an aggregate shall be met.

Class array relies on the implicitly-declared special member functions ([class.default.ctor], [class.dtor], and [class.copy.ctor]) to conform to the container requirements table in [container.requirements].

In addition to the requirements specified in the container requirements table, the implicit move constructor and move assignment operator for array require that T be Cpp17MoveConstructible or Cpp17MoveAssignable, respectively.

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template<class T, class... U>
  array(T, U...) -> array<T, 1 + sizeof...(U)>;

Mandates: (is_same_v<T, U> && ...) is true.

22.3.7.3 Member functions [array.members]

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constexpr size_type size() const noexcept;

Returns: N.

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constexpr T* data() noexcept;
constexpr const T* data() const noexcept;

Returns: A pointer such that [data(), data() + size()) is a valid range.

For a non-empty array, data() == addressof(front()).

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constexpr void fill(const T& u);

Effects: As if by fill_n(begin(), N, u).

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constexpr void swap(array& y) noexcept(is_nothrow_swappable_v<T>);

Effects: Equivalent to swap_ranges(begin(), end(), y.begin()).

[Note

Unlike the swap function for other containers, array::swap takes linear time, may exit via an exception, and does not cause iterators to become associated with the other container.

— end note

]

22.3.7.4 Specialized algorithms [array.special]

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template<class T, size_t N>
  constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));

Constraints: N == 0 or is_swappable_v<T> is true.

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

Complexity: Linear in N.

22.3.7.5 Zero-sized arrays [array.zero]

array shall provide support for the special case N == 0.

In the case that N == 0, begin() == end() == unique value.

The return value of data() is unspecified.

The effect of calling front() or back() for a zero-sized array is undefined.

Member function swap() shall have a non-throwing exception specification.

22.3.7.6 Array creation functions [array.creation]

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template<class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]);

Mandates: is_array_v<T> is false and is_constructible_v<T, T&> is true.

Preconditions: T meets the Cpp17CopyConstructible requirements.

Returns: {{ a[0], …, a[N - 1] }}.

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template<class T, size_t N>
  constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);

Mandates: is_array_v<T> is false and is_move_constructible_v<T> is true.

Preconditions: T meets the Cpp17MoveConstructible requirements.

Returns: {{ std::move(a[0]), …, std::move(a[N - 1]) }}.

22.3.7.7 Tuple interface [array.tuple]

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template<class T, size_t N>
  struct tuple_size<array<T, N>> : integral_constant<size_t, N> { };

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template<size_t I, class T, size_t N>
  struct tuple_element<I, array<T, N>> {
    using type = T;
  };

Mandates: I < N is true.

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template<size_t I, class T, size_t N>
  constexpr T& get(array<T, N>& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr T&& get(array<T, N>&& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr const T& get(const array<T, N>& a) noexcept;
template<size_t I, class T, size_t N>
  constexpr const T&& get(const array<T, N>&& a) noexcept;

Mandates: I < N is true.

Returns: A reference to the

I^{th}

element of a, where indexing is zero-based.