22 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&&...);
constexpr ~variant();
constexpr variant& operator=(const variant&);
constexpr variant& operator=(variant&&) noexcept(see below);
template<class T> constexpr variant& operator=(T&&) noexcept(see below);
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&&...);
constexpr bool valueless_by_exception() const noexcept;
constexpr size_t index() const noexcept;
constexpr void swap(variant&) noexcept(see below);
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&&);
};
}
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 nested within (
[intro.object]) the
variant object
.A program that instantiates the definition of
variant with
no template arguments is ill-formed
.If a program declares an explicit or partial specialization of
variant,
the program is ill-formed, no diagnostic required
.