std::variant_size, std::variant_size_v

Defined in header <variant>.

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

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

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

pub

value [static]

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

Member functions

pub	operator std::size_t	converts the object to std::size_t, returns value (public member function)
pub	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."); }

Result

All static assertions passed.