# 标准库头文件 <string_view>

< cpp‎ | header

此头文件是字符串库的一部分。

### 包含
<compare> (C++20) | 三路比较运算符支持
### 类
basic_string_view (C++17) | 只读的字符串视图 (类模板)
std::string_view (C++17) | std::basic_string_view<char>
std::u8string_view (C++20) | std::basic_string_view<char8_t>
std::u16string_view (C++17) | std::basic_string_view<char16_t>
std::u32string_view (C++17) | std::basic_string_view<char32_t>
std::wstring_view (C++17) | std::basic_string_view<wchar_t>
std::hash<std::string_view>std::hash<std::wstring_view>std::hash<std::u8string_view>std::hash<std::u16string_view>std::hash<std::u32string_view> (C++17)(C++17)(C++20)(C++17)(C++17) | string_view 的散列支持 (类模板特化)
##### 前置声明
定义于头文件 `<functional>`
hash (C++11) | 散列函数对象 (类模板)
### 函数
operator==operator!=operator<operator>operator<=operator>=operator<=> (C++20 中移除)(C++20 中移除)(C++20 中移除)(C++20 中移除)(C++20 中移除)(C++20) | 以字典序比较两个字符串视图 (函数模板)
operator<< | 进行字符串视图的流输出 (函数模板)
swap | 交换两个对象的值 (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)
##### 字面量
定义于内联命名空间 `std::literals::string_view_literals`
operator""sv (C++17) | 创建一个字符数组字面量的字符串视图 (函数)

### 概要

    #include <compare>

    namespace std {
      // 类模板 basic_string_view
      template<class CharT, class Traits = char_traits<CharT>>
      class basic_string_view;

      template<class CharT, class Traits>
        inline constexpr bool ranges::enable_view<basic_string_view<CharT, Traits>> = true;
      template<class CharT, class Traits>
        inline constexpr bool ranges::enable_borrowed_range<basic_string_view<CharT, Traits>> =
          true;

      // 非成员函数
      template<class CharT, class Traits>
        constexpr bool operator==(basic_string_view<CharT, Traits> x,
                                  basic_string_view<CharT, Traits> y) noexcept;
      template<class CharT, class Traits>
        constexpr /* 见描述 */
          operator<=>(basic_string_view<CharT, Traits> x,
                      basic_string_view<CharT, Traits> y) noexcept;

      // 充足的比较函数额外重载

      // 插入器与提取器
      template<class CharT, class Traits>
        basic_ostream<CharT, Traits>&
          operator<<(basic_ostream<CharT, Traits>& os,
                     basic_string_view<CharT, Traits> str);

      // basic_string_view typedef 名
      using string_view    = basic_string_view<char>;
      using u8string_view  = basic_string_view<char8_t>;
      using u16string_view = basic_string_view<char16_t>;
      using u32string_view = basic_string_view<char32_t>;
      using wstring_view   = basic_string_view<wchar_t>;

      // 散列支持
      template<class T> struct hash;
      template<> struct hash<string_view>;
      template<> struct hash<u8string_view>;
      template<> struct hash<u16string_view>;
      template<> struct hash<u32string_view>;
      template<> struct hash<wstring_view>;

      inline namespace literals {
      inline namespace string_view_literals {
        // basic_string_view 字面量的后缀
        constexpr string_view    operator""sv(const char* str, size_t len) noexcept;
        constexpr u8string_view  operator""sv(const char8_t* str, size_t len) noexcept;
        constexpr u16string_view operator""sv(const char16_t* str, size_t len) noexcept;
        constexpr u32string_view operator""sv(const char32_t* str, size_t len) noexcept;
        constexpr wstring_view   operator""sv(const wchar_t* str, size_t len) noexcept;
      }
      }
    }

#### 类模板 std::basic_string_view

    namespace std {
      template<class CharT, class Traits = char_traits<CharT>>
      class basic_string_view {
      public:
        // 类型
        using Traits_type            = Traits;
        using value_type             = CharT;
        using pointer                = value_type*;
        using const_pointer          = const value_type*;
        using reference              = value_type&;
        using const_reference        = const value_type&;
        using const_iterator         = /* 由实现定义 */
        using iterator               = const_iterator;
        using const_reverse_iterator = reverse_iterator<const_iterator>;
        using reverse_iterator       = const_reverse_iterator;
        using size_type              = size_t;
        using difference_type        = ptrdiff_t;
        static constexpr size_type npos = size_type(-1);

        // 构造与赋值
        constexpr basic_string_view() noexcept;
        constexpr basic_string_view(const basic_string_view&) noexcept = default;
        constexpr basic_string_view& operator=(const basic_string_view&) noexcept = default;
        constexpr basic_string_view(const CharT* str);
        constexpr basic_string_view(nullptr_t) = delete;
        constexpr basic_string_view(const CharT* str, size_type len);
        template<class It, class End>
          constexpr basic_string_view(It begin, End end);
        template<class R>
          constexpr basic_string_view(R&& r);

        // 迭代器支持
        constexpr const_iterator begin() const noexcept;
        constexpr const_iterator end() const noexcept;
        constexpr const_iterator cbegin() const noexcept;
        constexpr const_iterator cend() const noexcept;
        constexpr const_reverse_iterator rbegin() const noexcept;
        constexpr const_reverse_iterator rend() const noexcept;
        constexpr const_reverse_iterator crbegin() const noexcept;
        constexpr const_reverse_iterator crend() const noexcept;

        // 容量
        constexpr size_type size() const noexcept;
        constexpr size_type length() const noexcept;
        constexpr size_type max_size() const noexcept;
        [[nodiscard]] constexpr bool empty() const noexcept;

        // 元素访问
        constexpr const_reference operator[](size_type pos) const;
        constexpr const_reference at(size_type pos) const;
        constexpr const_reference front() const;
        constexpr const_reference back() const;
        constexpr const_pointer data() const noexcept;

        // 修改器
        constexpr void remove_prefix(size_type n);
        constexpr void remove_suffix(size_type n);
        constexpr void swap(basic_string_view& s) noexcept;

        // 字符串操作
        constexpr size_type copy(CharT* s, size_type n, size_type pos = 0) const;

        constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;

        constexpr int compare(basic_string_view s) const noexcept;
        constexpr int compare(size_type pos1, size_type n1, basic_string_view s) const;
        constexpr int compare(size_type pos1, size_type n1, basic_string_view s,
                              size_type pos2, size_type n2) const;
        constexpr int compare(const CharT* s) const;
        constexpr int compare(size_type pos1, size_type n1, const CharT* s) const;
        constexpr int compare(size_type pos1, size_type n1, const CharT* s,
                              size_type n2) const;

        constexpr bool starts_with(basic_string_view x) const noexcept;
        constexpr bool starts_with(CharT x) const noexcept;
        constexpr bool starts_with(const CharT* x) const;
        constexpr bool ends_with(basic_string_view x) const noexcept;
        constexpr bool ends_with(CharT x) const noexcept;
        constexpr bool ends_with(const CharT* x) const;

        constexpr bool contains(basic_string_view x) const noexcept;
        constexpr bool contains(CharT x) const noexcept;
        constexpr bool contains(const CharT* x) const;

        // 搜索
        constexpr size_type find(basic_string_view s, size_type pos = 0) const noexcept;
        constexpr size_type find(CharT c, size_type pos = 0) const noexcept;
        constexpr size_type find(const CharT* s, size_type pos, size_type n) const;
        constexpr size_type find(const CharT* s, size_type pos = 0) const;
        constexpr size_type rfind(basic_string_view s, size_type pos = npos) const noexcept;
        constexpr size_type rfind(CharT c, size_type pos = npos) const noexcept;
        constexpr size_type rfind(const CharT* s, size_type pos, size_type n) const;
        constexpr size_type rfind(const CharT* s, size_type pos = npos) const;

        constexpr size_type find_first_of(basic_string_view s,
                                          size_type pos = 0) const noexcept;
        constexpr size_type find_first_of(CharT c, size_type pos = 0) const noexcept;
        constexpr size_type find_first_of(const CharT* s, size_type pos, size_type n) const;
        constexpr size_type find_first_of(const CharT* s, size_type pos = 0) const;
        constexpr size_type find_last_of(basic_string_view s,
                                         size_type pos = npos) const noexcept;
        constexpr size_type find_last_of(CharT c, size_type pos = npos) const noexcept;
        constexpr size_type find_last_of(const CharT* s, size_type pos, size_type n) const;
        constexpr size_type find_last_of(const CharT* s, size_type pos = npos) const;
        constexpr size_type find_first_not_of(basic_string_view s,
                                              size_type pos = 0) const noexcept;
        constexpr size_type find_first_not_of(CharT c, size_type pos = 0) const noexcept;
        constexpr size_type find_first_not_of(const CharT* s, size_type pos,
                                              size_type n) const;
        constexpr size_type find_first_not_of(const CharT* s, size_type pos = 0) const;
        constexpr size_type find_last_not_of(basic_string_view s,
                                           size_type pos = npos) const noexcept;
        constexpr size_type find_last_not_of(CharT c, size_type pos = npos) const noexcept;
        constexpr size_type find_last_not_of(const CharT* s, size_type pos,
                                             size_type n) const;
        constexpr size_type find_last_not_of(const CharT* s, size_type pos = npos) const;

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

      // 推导指引
      template<class It, class End>
        basic_string_view(It, End) -> basic_string_view<iter_value_t<It>>;

      template<class R>
        basic_string_view(R&&) -> basic_string_view<ranges::range_value_t<R>>;
    }