# 标准库头文件 <span>

< cpp‎ | header

此头文件是容器库的一部分。

### 类
span (C++20) | 对象的连续序列上的无所有权视图 (类模板)
### 常量
dynamic_extent (C++20) | `size_t` 类型常量，指明 `span` 拥有动态长度 (常量)
### 函数
as_bytes as_writable_bytes | 转换 `span` 为对其底层字节的视图 (函数模板)
##### 范围访问
begin cbegin (C++11)(C++14) | 返回指向容器或数组起始的迭代器 (函数模板)
end cend (C++11)(C++14) | 返回指向容器或数组结尾的迭代器 (函数模板)
rbegin crbegin (C++14) | 返回指向一个容器或数组的逆向迭代器 (函数模板)
rend crend (C++14) | 返回容器或数组的逆向尾迭代器 (函数模板)
size ssize (C++17)(C++20) | 返回容器或数组的大小 (函数模板)
empty (C++17) | 检查容器是否为空 (函数模板)
data (C++17) | 获得指向底层数组的指针 (函数模板)

### 概要

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

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

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

      // 对象表示的视图
      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;
    }

#### 类模板 std::span

    namespace std {
      template<class ElementType, size_t Extent = dynamic_extent>
      class span {
      public:
        // 常量与类型
        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 = /* 由实现定义 */;
        using reverse_iterator = std::reverse_iterator<iterator>;
        static constexpr size_type extent = Extent;

        // 构造函数、复制与赋值
        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 span(const span& other) noexcept = default;
        template<class OtherElementType, size_t OtherExtent>
          constexpr explicit(/* 见描述 */)
            span(const span<OtherElementType, OtherExtent>& s) noexcept;

        ~span() noexcept = default;

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

        // 子视图
        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;

        // 观察器
        constexpr size_type size() const noexcept;
        constexpr size_type size_bytes() const noexcept;
        [[nodiscard]] constexpr bool empty() const noexcept;

        // 元素访问
        constexpr reference operator[](size_type idx) const;
        constexpr reference front() const;
        constexpr reference back() const;
        constexpr pointer data() const noexcept;

        // 迭代器支持
        constexpr iterator begin() const noexcept;
        constexpr iterator end() const noexcept;
        constexpr reverse_iterator rbegin() const noexcept;
        constexpr reverse_iterator rend() const noexcept;

      private:
        pointer data_;              // 仅用于阐释
        size_type size_;            // 仅用于阐释
      };

      template<class It, class EndOrSize>
        span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>>;
      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>>>;
    }