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std::variant_size, std::variant_size_v

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>;	(3)	(since C++17)
template <class T> class variant_size<volatile T>; template <class T> class variant_size<const volatile T>;	(3)	(since C++17) (deprecated in C++20)

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 base characteristic of std::integral_constant<std::size_t, sizeof...(Types)>

3) meets the UnaryTypeTrait requirements with a base characteristic 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 base characteristic std::integral_constant<std::size_t, N> for some N.

Example

#include <any>
#include <cstdio>
#include <variant>
 
static_assert(std::variant_size_v<std::variant<>> == 0);
static_assert(std::variant_size_v<std::variant<int>> == 1);
static_assert(std::variant_size_v<std::variant<int, int>> == 2);
static_assert(std::variant_size_v<std::variant<int, int, int>> == 3);
static_assert(std::variant_size_v<std::variant<int, float, double>> == 3);
static_assert(std::variant_size_v<std::variant<std::monostate, void>> == 2);
static_assert(std::variant_size_v<std::variant<const int, const float>> == 2);
static_assert(std::variant_size_v<std::variant<std::variant<std::any>>> == 1);
 
int main() { std::puts("All static assertions passed."); }

Output:

All static assertions passed.

variant_alternativevariant_alternative_t (C++17)	obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
std::tuple_size<std::tuple> (C++11)	obtains the size of `tuple` at compile time (class template specialization)