# Standard library header <span>

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< cpp‎ | header

This header is part of the container library.

### Classes

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

### Functions

as_bytes as_writable_bytes | converts a `span` into a view of its underlying bytes (function template)

### Constants

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

## Synopsis

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

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

      // 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;

      // tuple interface
      template<class T> class tuple_size;
      template<size_t I, class T> class tuple_element;
      template<class ElementType, size_t Extent>
        struct tuple_size<span<ElementType, Extent>>;
      template<class ElementType>
        struct tuple_size<span<ElementType, dynamic_extent>>;       // not defined
      template<size_t I, class ElementType, size_t Extent>
        struct tuple_element<I, span<ElementType, Extent>>;
      template<size_t I, class ElementType, size_t Extent>
        constexpr ElementType& get(span<ElementType, Extent>) noexcept;
    }

#### Class 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 index_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 = /* implementation-defined */;
        using reverse_iterator = std::reverse_iterator<iterator>;
        using const_reverse_iterator = std::reverse_iterator<const_iterator>;
        static constexpr index_type extent = Extent;

        // constructors, copy, and assignment
        constexpr span() noexcept;
        constexpr span(pointer ptr, index_type count);
        constexpr span(pointer first, pointer last);
        template<size_t N>
          constexpr span(element_type (&arr)[N]) noexcept;
        template<size_t N>
          constexpr span(array<value_type, N>& arr) noexcept;
        template<size_t N>
          constexpr span(const array<value_type, N>& arr) noexcept;
        template<class Container>
          constexpr span(Container& cont);
        template<class Container>
          constexpr span(const Container& cont);
        constexpr span(const span& other) noexcept = default;
        template<class OtherElementType, size_t OtherExtent>
          constexpr span(const span<OtherElementType, OtherExtent>& s) noexcept;

        ~span() noexcept = default;

        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 below> subspan() const;

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

        // observers
        constexpr index_type size() const noexcept;
        constexpr index_type size_bytes() const noexcept;
        [[nodiscard]] constexpr bool empty() const noexcept;

        // element access
        constexpr reference operator[](index_type idx) const;
        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;
        constexpr const_iterator cend() const noexcept;
        constexpr reverse_iterator rbegin() const noexcept;
        constexpr reverse_iterator rend() const noexcept;
        constexpr const_reverse_iterator crbegin() const noexcept;
        constexpr const_reverse_iterator crend() const noexcept;

        friend constexpr iterator begin(span s) noexcept { return s.begin(); }
        friend constexpr iterator end(span s) noexcept { return s.end(); }

      private:
        pointer data_;    // exposition only
        index_type size_; // exposition only
      };

      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 Container>
        span(Container&) -> span<typename Container::value_type>;
      template<class Container>
        span(const Container&) -> span<const typename Container::value_type>;
    }

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