std::variant_size, std::variant_size_v

std::variant_size, std::variant_size_v - cppreference.com

From cppreference.com

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>;`	(4)	(since C++17) (deprecated in C++20)
`template< class T > class variant_size<const volatile T>;`	(5)	(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)>

3-5) meets the UnaryTypeTrait requirements with a base characteristic of std::integral_constant<std::size_t, std::variant_size<T>::value>

Helper variable template

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

Inherited from std::integral_constant

Member constants

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

Member functions

	converts the object to `std::size_t`, returns `value` (public member function)
	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 <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() {}