标准库标头 <span> (C++20)

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C++
 
标准库头
 

此头文件是容器库的一部分。

类

(C++20)
 连续的对象序列上的无所有权视图
(类模板) [编辑]

常量

(C++20)
 std::size_t 类型的常量，指明 span 拥有动态长度
(常量) [编辑]

函数

(C++20)
 转换 span 为对其底层字节的视图
(函数模板) [编辑]
范围访问
(C++11)(C++14)
 返回指向容器或数组起始的迭代器
(函数模板) [编辑]
(C++11)(C++14)
 返回指向容器或数组结尾的迭代器
(函数模板) [编辑]
(C++14)
 返回指向一个容器或数组的逆向迭代器
(函数模板) [编辑]
(C++14)
 返回容器或数组的逆向尾迭代器
(函数模板) [编辑]
(C++17)(C++20)
 返回容器或数组的大小
(函数模板) [编辑]
(C++17)
 检查容器是否为空
(函数模板) [编辑]
(C++17)
 获得指向底层数组的指针
(函数模板) [编辑]

概要

// 大多数为独立
namespace std {
  // 常量
  inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();

  template<class T>
  concept /*integral-constant-like*/ =                    // 仅用于阐释
    is_integral_v<remove_cvref_t<decltype(T::value)>> &&
    !is_same_v<bool, remove_cvref_t<decltype(T::value)>> && /*constexpr-wrapper-like*/<T>;

  template<class T>
  constexpr size_t /*maybe-static-ext*/ = dynamic_extent; // 仅用于阐释
  template</*integral-constant-like*/ T>
  constexpr size_t /*maybe-static-ext*/<T> = { T::value };

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

  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;

  // 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 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;

    // 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;

    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, /* 见描述 */> 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;
    constexpr bool empty() const noexcept;

    // 元素访问
    constexpr reference operator[](size_type idx) const;
    constexpr reference at(size_type idx) const; // 独立环境中删除
    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 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_*/;   // 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>>, /*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>>>;
}