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.


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

(public static member constant)

Member functions

operator std::size_t
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>


All specializations of std::variant_size satisfy UnaryTypeTrait with base characteristic std::integral_constant<std::size_t, N> for some N.


#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."); }


All static assertions passed.

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)

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