20 General utilities library [utilities]

20.7 Variants [variant]

20.7.4 variant helper classes [variant.helper]

template<class T> struct variant_size;

All specializations of variant_size meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, N> for some N.

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

Let VS denote variant_size<T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, VS::value>.

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

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

Let VA denote variant_alternative<I, T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17TransformationTrait requirements ([meta.rqmts]) with a member typedef type that names the type add_const_t<VA::type>.

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

Mandates:

I < sizeof...(Types)

Type: The type

T_{I}