23 General utilities library [utilities]

23.7 Variants [variant]

23.7.4 variant helper classes [variant.helper]

template <class T> struct variant_size;

Remarks: All specializations of variant_­size shall meet the UnaryTypeTrait requirements with a base characteristic of integral_­constant<size_­t, N> for some N.

template <class T> class variant_size<const T>; template <class T> class variant_size<volatile T>; template <class T> class variant_size<const volatile T>;

Let VS denote variant_­size<T> of the cv-unqualified type T. Then each of the three templates shall meet the UnaryTypeTrait requirements with a base characteristic of integral_­constant<size_­t, VS​::​value>.

template <class... Types> struct variant_size<variant<Types...>> : integral_constant<size_t, sizeof...(Types)> { };

template <size_t I, class T> class variant_alternative<I, const T>; template <size_t I, class T> class variant_alternative<I, volatile T>; template <size_t I, class T> class variant_alternative<I, const volatile T>;

Let VA denote variant_­alternative<I, T> of the cv-unqualified type T. Then each of the three templates shall meet the TransformationTrait requirements with a member typedef type that names the following type:

  • for the first specialization, add_­const_­t<VA​::​type>,

  • for the second specialization, add_­volatile_­t<VA​::​type>, and

  • for the third specialization, add_­cv_­t<VA​::​type>.

variant_alternative<I, variant<Types...>>::type

Requires: I < sizeof...(Types).

Value: The type .