命名空间
变体 (Variants)
视图 (Views)
操作 (Actions)

标准库头文件 <variant> (C++17)

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< cpp‎ | header
 
C++
 
标准库头文件
 

此头文件是 通用工具 库的一部分。

目录

包含 (Includes)

(C++20)
 三向比较运算符 (Three-way comparison operator) 支持[编辑]

类 (Classes)

(C++17)
 类型安全的区分联合 (a type-safe discriminated union)
(类模板) [编辑]
(C++17)
 用作非默认可构造类型的 variant 中第一个候选项的占位符类型 (placeholder type for use as the first alternative in a variant of non-default-constructible types)
(类) [编辑]
(C++17)
 当无效访问 variant 的值时抛出的异常 (exception thrown on invalid accesses to the value of a variant)
(类) [编辑]
(C++17)
 在编译时获取 variant 的候选项列表的大小 (obtains the size of the variant's list of alternatives at compile time)
(类模板) (变量模板)[编辑]
(C++17)
 在编译时获取由其索引指定的候选项的类型 (obtains the type of the alternative specified by its index, at compile time)
(类模板) (别名模板)[编辑]
(C++17)
 对 std::variant 的哈希支持 (hash support for std::variant)
(类模板特化) [编辑]

常量 (Constants)

(C++17)
 variant 在无效状态下的索引 (index of the variant in the invalid state)
(常量) [编辑]

函数 (Functions)

(C++17)
 使用一个或多个 variant 保存的参数调用提供的仿函数 (calls the provided functor with the arguments held by one or more variants)
(函数模板) [编辑]
(C++17)
 检查 variant 当前是否持有给定类型 (checks if a variant currently holds a given type)
(函数模板) [编辑]
(C++17)
 读取变体的值，给定索引或类型（如果类型是唯一的），错误时抛出异常 (reads the value of the variant given the index or the type (if the type is unique), throws on error)
(函数模板) [编辑]
(C++17)
 获取指向被指向的 variant 值的指针，给定索引或类型（如果唯一），错误时返回 null (obtains a pointer to the value of a pointed-to variant given the index or the type (if unique), returns null on error)
(函数模板) [编辑]
(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
 比较 variant 对象作为它们包含的值 (compares variant objects as their contained values)
(函数模板) [编辑]
(C++17)
 特化 std::swap 算法 (specializes the std::swap algorithm)
(函数模板) [编辑]

[编辑] 概要 (Synopsis)

#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>;
}

[编辑] 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 */);
  };
}

[编辑] 类 std::monostate

struct monostate{};

[编辑] 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;
};
检索自“https://cppreference.cn/mwiki/index.php?title=cpp/header/variant&oldid=178525”