std::variant_size, std::variant_size_v

< cpp‎ | utility‎ | variant
Utilities library
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Date and time
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Elementary string conversions
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 BaseCharacteristic of std::integral_constant<std::size_t, sizeof...(Types)>
3) meets the UnaryTypeTrait requirements with a BaseCharacteristic of std::integral_constant<std::size_t, variant_size<T>>


[edit] 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>

[edit] Notes

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

[edit] 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."); }


All static assertions passed.

[edit] See also

obtains the type of the alternative specified by its index, at compile time
(class template) (alias template) [edit]
obtains the size of tuple at compile time
(class template specialization) [edit]