std::variant_size, std::variant_size_v

 

C++

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std::variant

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variant_npos

 

Defined in header <variant>
template <class T> struct variant_size; /* undefined */	(1)	(since C++17)
template <class... Types> struct variant_size<std::variant<Types...>> : std::integral_constant<std::size_t, sizeof...(Types)> { };	(2)	(since C++17)
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>;	(3)	(since C++17)

Provides access to the number of alternatives in a possibly cv-qualified variant as a compile-time constant expression.

Formally,

2) meets the UnaryTypeTrait requirements with a BaseCharacteristic of std::integral_constant<std::size_t, sizeof...(Types)>

3) meets the UnaryTypeTrait requirements with a BaseCharacteristic of std::integral_constant<std::size_t, variant_size<T>>

Helper variable template

template <class T> inline constexpr std::size_t variant_size_v = std::variant_size<T>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

sizeof...(Types) (public static member constant)

Member functions

operator std::size_t	converts the object to std::size_t, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
value_type	std::size_t
type	std::integral_constant<std::size_t, value>

Notes

All specializations of std::variant_size satisfy UnaryTypeTrait with BaseCharacteristic std::integral_constant<std::size_t, N> for some N.

See also

variant_alternativevariant_alternative_t (C++17)	obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
tuple_size	obtains the size of tuple at compile time (class template specialization)