A variant object holds and manages the lifetime of a value. If the variant holds a value, that value's type has to be one of the template argument types given to variant. These template arguments are called alternatives.
namespace std { // [variant.variant], variant template <class... Types> class variant; // [variant.helper], 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> 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...>>; constexpr size_t variant_npos = -1; // [variant.get], 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; // [variant.relops], 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...>&); // [variant.visit], visitation template <class Visitor, class... Variants> constexpr see below visit(Visitor&&, Variants&&...); // [variant.monostate], class monostate struct monostate; // [variant.monostate.relops], 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; // [variant.specalg], specialized algorithms template <class... Types> void swap(variant<Types...>&, variant<Types...>&) noexcept(see below); // [variant.bad.access], class bad_variant_access class bad_variant_access; // [variant.hash], hash support template <class T> struct hash; template <class... Types> struct hash<variant<Types...>>; template <> struct hash<monostate>; // [variant.traits], allocator-related traits template <class T, class Alloc> struct uses_allocator; template <class... Types, class Alloc> struct uses_allocator<variant<Types...>, Alloc>; }