Standard library header <variant> (C++17)
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Standard library headers
<compare> (C++20)
<contracts> (C++26)
<coroutine> (C++20)
<cstdint> (C++11)
<source_location> (C++20)
<stdfloat> (C++23)
<version> (C++20)
<concepts> (C++20)
<debugging> (C++26)
<stacktrace> (C++23)
<system_error> (C++11)
<memory_resource> (C++17)
<scoped_allocator> (C++11)
<type_traits> (C++11)
<ratio> (C++11)
<any> (C++17)
<bit> (C++20)
<charconv> (C++17)
<expected> (C++23)
<format> (C++20)
<optional> (C++17)
<stdbit.h> (C++26)
<tuple> (C++11)
<typeindex> (C++11)
<variant> (C++17)
<array> (C++11)
<flat_map> (C++23)
<flat_set> (C++23)
<forward_list> (C++11)
<hive> (C++26)
<mdspan> (C++23)
<span> (C++20)
<unordered_map> (C++11)
<unordered_set> (C++11)
<generator> (C++23)
<ranges> (C++20)
<cuchar> (C++11)
<string_view> (C++17)
<codecvt> (C++11/17/26*)
<regex> (C++11)
<cfenv> (C++11)
<linalg> (C++26)
<numbers> (C++20)
<random> (C++11)
<simd> (C++26)
<stdckdint.h> (C++26)
<chrono> (C++11)
<ccomplex> (C++11/17/20*)
<ciso646> (until C++20)
<cstdalign> (C++11/17/20*)
<cstdbool> (C++11/17/20*)
<ctgmath> (C++11/17/20*)
<cinttypes> (C++11)
<filesystem> (C++17)
<print> (C++23)
<spanstream> (C++23)
<strstream> (C++98/26*)
<syncstream> (C++20)
<atomic> (C++11)
<barrier> (C++20)
<condition_variable> (C++11)
<future> (C++11)
<hazard_pointer> (C++26)
<latch> (C++20)
<mutex> (C++11)
<rcu> (C++26)
<semaphore> (C++20)
<shared_mutex> (C++14)
<stdatomic.h> (C++23)
<stop_token> (C++20)
<thread> (C++11)
<execution> (C++17)
This header is part of the general utility library.
Contents
Includes
Classes
(C++17)
variant of non-default-constructible types (class) [edit]
(C++17)
variant's list of alternatives at compile time(class template) (variable template)[edit]
obtains the type of the alternative specified by its index, at compile time
(class template) (alias template)[edit]
(class template) (alias template)[edit]
Constants
Functions
(C++17)
variants (function template) [edit]
(C++17)
(function template) [edit]
(C++17)
variant given the index or the type (if unique), returns null on error (function template) [edit]
(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)
variant objects as their contained values (function template) [edit]
[edit] Synopsis
// mostly freestanding #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> 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...>&); // freestanding-deleted template<size_t I, class... Types> constexpr variant_alternative_t<I, variant<Types...>>&& get( variant<Types...>&&); // freestanding-deleted template<size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>& get( const variant<Types...>&); // freestanding-deleted template<size_t I, class... Types> constexpr const variant_alternative_t<I, variant<Types...>>&& get( const variant<Types...>&&); // freestanding-deleted template<class T, class... Types> constexpr T& get(variant<Types...>&); // freestanding-deleted template<class T, class... Types> constexpr T&& get(variant<Types...>&&); // freestanding-deleted template<class T, class... Types> constexpr const T& get(const variant<Types...>&); // freestanding-deleted template<class T, class... Types> constexpr const T&& get(const variant<Types...>&&); // freestanding-deleted 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>; }
[edit] Class template 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 */); // visitation template<class Self, class Visitor> constexpr decltype(auto) visit(this Self&&, Visitor&&); template<class R, class Self, class Visitor> constexpr R visit(this Self&&, Visitor&&); }; }
[edit] Class std::monostate
namespace std { struct monostate{}; constexpr bool operator==(monostate, monostate) noexcept { return true; } constexpr strong_ordering operator<=>(monostate, monostate) noexcept { return strong_ordering::equal; } }
[edit] Class std::bad_variant_access
namespace std { class bad_variant_access : public exception { public: // see description for the specification of the special member functions constexpr const char* what() const noexcept override; }; }