20 General utilities library [utilities]

20.7 Variants [variant]

20.7.3 Class template variant [variant.variant]

20.7.3.1 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 ~variant(); // [variant.assign], assignment constexpr variant& operator=(const variant&); constexpr variant& operator=(variant&&) noexcept(see below); template<class T> variant& operator=(T&&) noexcept(see below); // [variant.mod], modifiers template<class T, class... Args> T& emplace(Args&&...); template<class T, class U, class... Args> T& emplace(initializer_list<U>, Args&&...); template<size_t I, class... Args> variant_alternative_t<I, variant<Types...>>& emplace(Args&&...); template<size_t I, class U, class... Args> 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 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.
The contained value shall be allocated in a region of the variant storage suitably aligned for all types in Types.
All types in Types shall meet the Cpp17Destructible requirements (Table 34).
A program that instantiates the definition of variant with no template arguments is ill-formed.