20 General utilities library [utilities]
namespace std {
template<class... Types>
class variant {
public:
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();
constexpr variant& operator=(const variant&);
constexpr variant& operator=(variant&&) noexcept(see below);
template<class T> variant& operator=(T&&) noexcept(see below);
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&&...);
constexpr bool valueless_by_exception() const noexcept;
constexpr size_t index() const noexcept;
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
32)
.A program that instantiates the definition of
variant with
no template arguments is ill-formed
.