std::variant_size, std::variant_size_v

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< cpp‎ | utility‎ | variant
 
 
 
 
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.

Example

See also

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