标准库头文件 <variant> (C++17)

 

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标准库头文件

语言支持
<cstddef> <cstdlib> <version> (C++20) <limits> <climits>	<cfloat> <cstdint> (C++11) <stdfloat> (C++23) <new> <typeinfo>	<source_location> (C++20) <exception> <initializer_list> (C++11) <compare> (C++20)	<coroutine> (C++20) <csignal> <csetjmp> <cstdarg>
概念
<concepts> (C++20)
诊断
<stdexcept> <stacktrace> (C++23)	<cassert>	<cerrno>	<system_error> (C++11)
内存管理
<memory>	<memory_resource> (C++17)	<scoped_allocator> (C++11)
元编程
<type_traits> (C++11)	<ratio> (C++11)
通用实用工具
<utility> <tuple> (C++11) <optional> (C++17) <variant> (C++17)	<any> (C++17) <debugging> (C++26) <expected> (C++23) <bitset>	<functional> <typeindex> (C++11) <execution> (C++17)	<charconv> (C++17) <format> (C++20) <bit> (C++20)
字符串
<string_view> (C++17) <string>	<cctype> <cwctype>	<cstring> <cwchar>	<cuchar> (C++11)
容器
<array> (C++11) <deque> <forward_list> (C++11) <list>	<vector> <map> <set> <unordered_map> (C++11)	<unordered_set> (C++11) <queue> <stack> <flat_map> (C++23)	<flat_set> (C++23) <span> (C++20) <mdspan> (C++23) <inplace_vector> (C++26)
迭代器
<iterator>
范围
<ranges> (C++20)	<generator> (C++23)
算法
<algorithm>	<numeric>
数值
<cfenv> (C++11) <complex>	<random> (C++11) <valarray>	<cmath> <linalg> (C++26)	<numbers> (C++20)
时间
<chrono> (C++11)	<ctime>
本地化
<locale>	<clocale>	<codecvt> (C++11/17/26*)	<text_encoding> (C++26)
输入/输出
<iosfwd> <iostream> <ios> <streambuf>	<istream> <ostream> <iomanip> <print> (C++23)	<sstream> <spanstream> (C++23) <fstream> <syncstream> (C++20)	<filesystem> (C++17) <cstdio> <cinttypes> (C++11) <strstream> (C++98/26*)
正则表达式
<regex> (C++11)
并发支持
<stop_token> (C++20) <thread> (C++11) <atomic> (C++11) <rcu> (C++26)	<stdatomic.h> (C++23) <mutex> (C++11) <shared_mutex> (C++14)	<condition_variable> (C++11)   <semaphore> (C++20) <latch> (C++20)	<barrier> (C++20) <future> (C++11) <hazard_pointer> (C++26)
C 兼容性
<cstdbool> (C++11/17/20*)   <ccomplex> (C++11/17/20*)	<ctgmath> (C++11/17/20*)	<cstdalign> (C++11/17/20*)	<ciso646> (直到 C++20)

 

此头文件是 通用实用工具 库的一部分。

包含
<compare> (C++20)	三路比较运算符 支持
类
variant (C++17)	类型安全的区分联合 (类模板)
monostate (C++17)	占位符类型，用作非默认可构造类型 variant 中的第一个备选方案 (类)
bad_variant_access (C++17)	在对variant的值进行无效访问时抛出的异常 (类)
variant_sizevariant_size_v (C++17)	在编译时获取variant的备选方案列表的大小 (类模板) (变量模板)
variant_alternativevariant_alternative_t (C++17)	在编译时获取由其索引指定的备选方案的类型 (类模板) (别名模板)
std::hash<std::variant> (C++17)	对std::variant的哈希支持 (类模板特化)
常量
variant_npos (C++17)	variant处于无效状态时的索引 (常量)
函数
visit (C++17)	使用一个或多个variant持有的参数调用提供的函数对象 (函数模板)
holds_alternative (C++17)	检查variant当前是否持有给定的类型 (函数模板)
get(std::variant) (C++17)	根据索引或类型（如果类型是唯一的）读取变体的值，在错误时抛出异常 (函数模板)
get_if (C++17)	根据索引或类型（如果唯一）获取指向指向的variant的值的指针，在错误时返回 null (函数模板)
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)	将variant对象与其包含的值进行比较 (函数模板)
std::swap(std::variant) (C++17)	特化了std::swap算法 (函数模板)

概要

#include <compare>
 
namespace std {
  // class template variant
  template<class... Types>
    class variant;
 
  // variant helper classes
  template<class T> struct variant_size;                        // not defined
  template<class T> struct variant_size<const T>;
  template<class T>
    inline constexpr size_t variant_size_v = variant_size<T>::value;
 
  template<class... Types>
    struct variant_size<variant<Types...>>;
 
  template<size_t I, class T> struct variant_alternative;       // not defined
  template<size_t I, class T> struct variant_alternative<I, const T>;
  template<size_t I, class T>
    using variant_alternative_t = typename variant_alternative<I, T>::type;
 
  template<size_t I, class... Types>
    struct variant_alternative<I, variant<Types...>>;
 
  inline constexpr size_t variant_npos = -1;
 
  // value access
  template<class T, class... Types>
    constexpr bool holds_alternative(const variant<Types...>&) noexcept;
 
  template<size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>&);
  template<size_t I, class... Types>
    constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&&);
  template<size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&
      get(const variant<Types...>&);
  template<size_t I, class... Types>
    constexpr const variant_alternative_t<I, variant<Types...>>&&
      get(const variant<Types...>&&);
 
  template<class T, class... Types>
    constexpr T& get(variant<Types...>&);
  template<class T, class... Types>
    constexpr T&& get(variant<Types...>&&);
  template<class T, class... Types>
    constexpr const T& get(const variant<Types...>&);
  template<class T, class... Types>
    constexpr const T&& get(const variant<Types...>&&);
 
  template<size_t I, class... Types>
    constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
      get_if(variant<Types...>*) noexcept;
  template<size_t I, class... Types>
    constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
      get_if(const variant<Types...>*) noexcept;
 
  template<class T, class... Types>
    constexpr add_pointer_t<T>
      get_if(variant<Types...>*) noexcept;
  template<class T, class... Types>
    constexpr add_pointer_t<const T>
      get_if(const variant<Types...>*) noexcept;
 
  // relational operators
  template<class... Types>
    constexpr bool operator==(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator!=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator<(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator>(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator<=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types>
    constexpr bool operator>=(const variant<Types...>&, const variant<Types...>&);
  template<class... Types> requires (three_way_comparable<Types> && ...)
    constexpr common_comparison_category_t<compare_three_way_result_t<Types>...>
      operator<=>(const variant<Types...>&, const variant<Types...>&);
 
  // visitation
  template<class Visitor, class... Variants>
    constexpr /* see description */ visit(Visitor&&, Variants&&...);
  template<class R, class Visitor, class... Variants>
    constexpr R visit(Visitor&&, Variants&&...);
 
  // class monostate
  struct monostate;
 
  // monostate relational operators
  constexpr bool operator==(monostate, monostate) noexcept;
  constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
 
  // specialized algorithms
  template<class... Types>
    constexpr void swap(variant<Types...>&,
                        variant<Types...>&) noexcept(/* see description */);
 
  // class bad_variant_access
  class bad_variant_access;
 
  // hash support
  template<class T> struct hash;
  template<class... Types> struct hash<variant<Types...>>;
  template<> struct hash<monostate>;
}
 
// deprecated
namespace std {
  template<class T> struct variant_size<volatile T>;
  template<class T> struct variant_size<const volatile T>;
 
  template<size_t I, class T> struct variant_alternative<I, volatile T>;
  template<size_t I, class T> struct variant_alternative<I, const volatile T>;
}

类模板 std::variant

namespace std {
  template<class... Types>
  class variant {
  public:
    // constructors
    constexpr variant() noexcept(/* see description */);
    constexpr variant(const variant&);
    constexpr variant(variant&&) noexcept(/* see description */);
 
    template<class T>
      constexpr variant(T&&) noexcept(/* see description */);
 
    template<class T, class... Args>
      constexpr explicit variant(in_place_type_t<T>, Args&&...);
    template<class T, class U, class... Args>
      constexpr explicit variant(in_place_type_t<T>, initializer_list<U>, Args&&...);
 
    template<size_t I, class... Args>
      constexpr explicit variant(in_place_index_t<I>, Args&&...);
    template<size_t I, class U, class... Args>
      constexpr explicit variant(in_place_index_t<I>, initializer_list<U>, Args&&...);
 
    // destructor
    constexpr ~variant();
 
    // assignment
    constexpr variant& operator=(const variant&);
    constexpr variant& operator=(variant&&) noexcept(/* see description */);
 
    template<class T> constexpr variant& operator=(T&&) noexcept(/* see description */);
 
    // modifiers
    template<class T, class... Args>
      constexpr T& emplace(Args&&...);
    template<class T, class U, class... Args>
      constexpr T& emplace(initializer_list<U>, Args&&...);
    template<size_t I, class... Args>
      constexpr variant_alternative_t<I, variant<Types...>>& emplace(Args&&...);
    template<size_t I, class U, class... Args>
      constexpr variant_alternative_t<I, variant<Types...>>&
        emplace(initializer_list<U>, Args&&...);
 
    // value status
    constexpr bool valueless_by_exception() const noexcept;
    constexpr size_t index() const noexcept;
 
    // swap
    constexpr void swap(variant&) noexcept(/* see description */);
  };
}

类 std::monostate

struct monostate{};

类 std::bad_variant_access

class bad_variant_access : public exception {
public:
  // see [exception] for the specification of the special member functions
  const char* what() const noexcept override;
};