Standard library header <variant>
From cppreference.com
Classes
(since C++17) |
a type-safe discriminated union (class template) |
(C++17) |
placeholder type for use as the first alternative in a variant of non-default-contructible types (class) |
(C++17) |
exception thrown on invalid accesses to the value of a variant (class) |
(C++17) |
obtains the size of the variant's list of alternatives at compile time (class template) (variable template) |
obtains the type of the alternative specified by its index, at compile time (class template) (alias template) | |
(C++17) |
specializes the std::hash algorithm (class template specialization) |
specializes the std::uses_allocator type trait (class template specialization) |
Helpers
(C++17) |
index of the variant in the invalid state (constant) |
Functions
(C++17) |
calls the provided functor with the arguments held by one or more variants (function template) |
(C++17) |
checks if a given type appears exactly once in a variant (function template) |
(C++17) |
reads the value of the variant given the index or the type (if the type is unique), throws on error (function template) |
(C++17) |
obtains a pointer to the value of a pointed-to variant given the index or the type (if unique), returns null on error (function template) |
compares variant objects as their contained values (function template) | |
(C++17) |
specializes the std::swap algorithm (function) |
Synopsis
namespace std { // 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> struct variant_size<volatile T>; template <class T> struct variant_size<const volatile 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> struct variant_alternative<I, volatile T>; template <size_t I, class T> struct variant_alternative<I, const volatile 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...>&); // visitation template <class Visitor, class... Variants> constexpr /*see definition*/ visit(Visitor&&, Variants&&...); // class monostate struct monostate; // monostate relational operators constexpr bool operator<(monostate, monostate) noexcept; constexpr bool operator>(monostate, monostate) noexcept; constexpr bool operator<=(monostate, monostate) noexcept; constexpr bool operator>=(monostate, monostate) noexcept; constexpr bool operator==(monostate, monostate) noexcept; constexpr bool operator!=(monostate, monostate) noexcept; // specialized algorithms template <class... Types> void swap(variant<Types...>&, variant<Types...>&) noexcept(/*see definition*/); // 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>; // allocator-related traits template <class T, class Alloc> struct uses_allocator; template <class... Types, class Alloc> struct uses_allocator<variant<Types...>, Alloc>; }
Class template std::variant
template <class... Types> class variant { public: // constructors constexpr variant() noexcept(/*see definition*/); variant(const variant&); variant(variant&&) noexcept(/*see definition*/); template <class T> constexpr variant(T&&) noexcept(/*see definition*/); 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&&...); // allocator-extended constructors template <class Alloc> variant(allocator_arg_t, const Alloc&); template <class Alloc> variant(allocator_arg_t, const Alloc&, const variant&); template <class Alloc> variant(allocator_arg_t, const Alloc&, variant&&); template <class Alloc, class T> variant(allocator_arg_t, const Alloc&, T&&); template <class Alloc, class T, class... Args> variant(allocator_arg_t, const Alloc&, in_place_type_t<T>, Args&&...); template <class Alloc, class T, class U, class... Args> variant(allocator_arg_t, const Alloc&, in_place_type_t<T>, initializer_list<U>, Args&&...); template <class Alloc, size_t I, class... Args> variant(allocator_arg_t, const Alloc&, in_place_index_t<I>, Args&&...); template <class Alloc, size_t I, class U, class... Args> variant(allocator_arg_t, const Alloc&, in_place_index_t<I>, initializer_list<U>, Args&&...); // destructor ~variant(); // assignment variant& operator=(const variant&); variant& operator=(variant&&) noexcept(/*see definition*/); template <class T> variant& operator=(T&&) noexcept(/*see definition*/); // modifiers template <class T, class... Args> void emplace(Args&&...); template <class T, class U, class... Args> void emplace(initializer_list<U>, Args&&...); template <size_t I, class... Args> void emplace(Args&&...); template <size_t I, class U, class... Args> void emplace(initializer_list<U>, Args&&...); // value status constexpr bool valueless_by_exception() const noexcept; constexpr size_t index() const noexcept; // swap void swap(variant&) noexcept(/*see definition*/); };
Class std::monostate
struct monostate{};
Class std::bad_variant_access
class bad_variant_access : public exception { public: bad_variant_access() noexcept; virtual const char* what() const noexcept; };