std::variant_size, std::variant_size_v

Since C++17

Header: <variant>

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

# Declarations

template< class T >
struct variant_size; /* undefined */

(since C++17)

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

(since C++17)

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

(since C++17)

template< class T >
class variant_size<volatile T>;

(since C++17) (deprecated in C++20)

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

(since C++17) (deprecated in C++20)

# 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() {}

# See also