# Standard library header <span> (C++20)

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This header is part of the containers library.

### Classes

span (C++20) | a non-owning view over a contiguous sequence of objects (class template)

### Constants

dynamic_extent (C++20) | a constant of type std::size_t signifying that the `span` has dynamic extent (constant)

### Functions

as_bytesas_writable_bytes (C++20) | converts a `span` into a view of its underlying bytes (function template)

##### Range access

begincbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)
endcend (C++11)(C++14) | returns an iterator to the end of a container or array (function template)
rbegincrbegin (C++14) | returns a reverse iterator to the beginning of a container or array (function template)
rendcrend (C++14) | returns a reverse end iterator for a container or array (function template)
sizessize (C++17)(C++20) | returns the size of a container or array (function template)
empty (C++17) | checks whether the container is empty (function template)
data (C++17) | obtains the pointer to the underlying array (function template)

### Synopsis

    #include <initializer_list>

    // mostly freestanding
    namespace std {
      // constants
      inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();

      template<class T>
      concept /*integral-constant-like*/ = // exposition only
        is_integral_v<decltype(T::value)> &&
        !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
        convertible_to<T, decltype(T::value)> &&
        equality_comparable_with<T, decltype(T::value)> &&
        bool_constant<T() == T::value>::value &&
        bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;

      template<class T>
      constexpr size_t /*maybe-static-ext*/ = dynamic_extent; // exposition only
      template</*integral-constant-like*/ T>
      constexpr size_t /*maybe-static-ext*/<T> = { T::value };

      // class template span
      template<class ElementType, size_t Extent = dynamic_extent>
      class span; // partially freestanding

      template<class ElementType, size_t Extent>
      constexpr bool ranges::enable_view<span<ElementType, Extent>> = true;
      template<class ElementType, size_t Extent>
      constexpr bool ranges::enable_borrowed_range<span<ElementType, Extent>> = true;

      // views of object representation
      template<class ElementType, size_t Extent>
      span<const byte,
           Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
      as_bytes(span<ElementType, Extent> s) noexcept;

      template<class ElementType, size_t Extent>
      span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
      as_writable_bytes(span<ElementType, Extent> s) noexcept;
    }

#### Class template std::span

    namespace std {
      template<class ElementType, size_t Extent = dynamic_extent>
      class span
      {
      public:
        // constants and types
        using element_type                = ElementType;
        using value_type                  = remove_cv_t<ElementType>;
        using size_type                   = size_t;
        using difference_type             = ptrdiff_t;
        using pointer                     = element_type*;
        using const_pointer               = const element_type*;
        using reference                   = element_type&;
        using const_reference             = const element_type&;
        using iterator                    = /* implementation-defined */;
        using const_iterator              = std::const_iterator<iterator>;
        using reverse_iterator            = std::reverse_iterator<iterator>;
        using const_reverse_iterator      = std::const_iterator<reverse_iterator>;
        static constexpr size_type extent = Extent;

        // constructors, copy, and assignment
        constexpr span() noexcept;
        template<class It>
        constexpr explicit(extent != dynamic_extent) span(It first, size_type count);
        template<class It, class End>
        constexpr explicit(extent != dynamic_extent) span(It first, End last);
        template<size_t N>
        constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
        template<class T, size_t N>
        constexpr span(array<T, N>& arr) noexcept;
        template<class T, size_t N>
        constexpr span(const array<T, N>& arr) noexcept;
        template<class R>
        constexpr explicit(extent != dynamic_extent) span(R&& r);
        constexpr explicit(extent != dynamic_extent)
          span(std::initializer_list<value_type> il);
        constexpr span(const span& other) noexcept = default;
        template<class OtherElementType, size_t OtherExtent>
        constexpr explicit(/* see description */)
          span(const span<OtherElementType, OtherExtent>& s) noexcept;

        constexpr span& operator=(const span& other) noexcept = default;

        // subviews
        template<size_t Count>
        constexpr span<element_type, Count> first() const;
        template<size_t Count>
        constexpr span<element_type, Count> last() const;
        template<size_t Offset, size_t Count = dynamic_extent>
        constexpr span<element_type, /* see description */> subspan() const;

        constexpr span<element_type, dynamic_extent> first(size_type count) const;
        constexpr span<element_type, dynamic_extent> last(size_type count) const;
        constexpr span<element_type, dynamic_extent> subspan(
          size_type offset,
          size_type count = dynamic_extent) const;

        // observers
        constexpr size_type size() const noexcept;
        constexpr size_type size_bytes() const noexcept;
        constexpr bool empty() const noexcept;

        // element access
        constexpr reference operator[](size_type idx) const;
        constexpr reference at(size_type idx) const; // freestanding-deleted
        constexpr reference front() const;
        constexpr reference back() const;
        constexpr pointer data() const noexcept;

        // iterator support
        constexpr iterator begin() const noexcept;
        constexpr iterator end() const noexcept;
        constexpr const_iterator cbegin() const noexcept { return begin(); }
        constexpr const_iterator cend() const noexcept { return end(); }
        constexpr reverse_iterator rbegin() const noexcept;
        constexpr reverse_iterator rend() const noexcept;
        constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); }
        constexpr const_reverse_iterator crend() const noexcept { return rend(); }

      private:
        pointer /*data_*/;   // exposition only
        size_type /*size_*/; // exposition only
      };

      template<class It, class EndOrSize>
      span(It, EndOrSize)
        -> span<remove_reference_t<iter_reference_t<It>>, /*maybe-static-ext*/<EndOrSize>>;
      template<class T, size_t N>
      span(T (&)[N]) -> span<T, N>;
      template<class T, size_t N>
      span(array<T, N>&) -> span<T, N>;
      template<class T, size_t N>
      span(const array<T, N>&) -> span<const T, N>;
      template<class R>
      span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;
    }

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