21 Containers library [containers]

21.3 Sequence containers [sequences]

21.3.7 Class template array [array]

21.3.7.1 Overview [array.overview]

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The header <array> defines a class template for storing fixed-size sequences of objects.
An array is a contiguous container.
An instance of array<T, N> stores N elements of type T, so that size() == N is an invariant.
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An array is an aggregate that can be list-initialized with up to N elements whose types are convertible to T.
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An array satisfies 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 satisfies 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.
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The types iterator and const_­iterator satisfy 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

    void fill(const T& u);
    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)>;
}

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

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The conditions for an aggregate shall be met.
Class array relies on the implicitly-declared special member functions ([class.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)>;
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Requires: (is_­same_­v<T, U> && ...) is true.
Otherwise the program is ill-formed.

21.3.7.3 Member functions [array.members]

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constexpr size_type size() const noexcept;
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Returns: N.
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constexpr T* data() noexcept; constexpr const T* data() const noexcept;
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Returns: A pointer such that [data(), data() + size()) is a valid range.
For a non-empty array, data() == addressof(front()).
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void fill(const T& u);
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Effects: As if by fill_­n(begin(), N, u).
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void swap(array& y) noexcept(is_nothrow_swappable_v<T>);
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Effects: Equivalent to swap_­ranges(begin(), end(), y.begin()).
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[ 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
 ]

21.3.7.4 Specialized algorithms [array.special]

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template<class T, size_t N> void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y)));
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Remarks: This function shall not participate in overload resolution unless N == 0 or is_­swappable_­v<T> is true.
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Effects: As if by x.swap(y).
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Complexity: Linear in N.

21.3.7.5 Zero-sized arrays [array.zero]

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array shall provide support for the special case N == 0.
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In the case that N == 0, begin() == end() == unique value.
The return value of data() is unspecified.
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The effect of calling front() or back() for a zero-sized array is undefined.
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Member function swap() shall have a non-throwing exception specification.

21.3.7.6 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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tuple_element<I, array<T, N>>::type
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Requires: I < N.
The program is ill-formed if I is out of bounds.
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Value: The type T.
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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;
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Requires: I < N.
The program is ill-formed if I is out of bounds.
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Returns: A reference to the I element of a, where indexing is zero-based.