22 General utilities library [utilities]

22.6 Variants [variant]

22.6.3 Class template variant [variant.variant]

22.6.3.1 General [variant.variant.general]

namespace std { template<class... Types> class variant { public: // [variant.ctor], constructors constexpr variant() noexcept(see below); constexpr variant(const variant&); constexpr variant(variant&&) noexcept(see below); template<class T> constexpr variant(T&&) noexcept(see below); 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&&...); // [variant.dtor], destructor constexpr ~variant(); // [variant.assign], assignment constexpr variant& operator=(const variant&); constexpr variant& operator=(variant&&) noexcept(see below); template<class T> constexpr variant& operator=(T&&) noexcept(see below); // [variant.mod], 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&&...); // [variant.status], value status constexpr bool valueless_by_exception() const noexcept; constexpr size_t index() const noexcept; // [variant.swap], swap constexpr void swap(variant&) noexcept(see below); }; }
Any instance of variant at any given time either holds a value of one of its alternative types or holds no value.
When an instance of variant holds a value of alternative type T, it means that a value of type T, referred to as the variant object's contained value, is allocated within the storage of the variant object.
Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.
All types in Types shall meet the Cpp17Destructible requirements (Table 35).
A program that instantiates the definition of variant with no template arguments is ill-formed.