# std::variant_size, std::variant_size_v

From cppreference.com
< cpp‎ | utility‎ | variant

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.

Formally,

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

value [static] | `sizeof...(Types)` (public static member constant)

### Member functions

operator std::size_t | converts the object to std::size_t, returns value (public member function)
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

Run this code

    #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."); }

Output:

    All static assertions passed.

### See also

variant_alternative variant_alternative_t (C++17) | obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
std::tuple_size<std::tuple> (C++11) | obtains the size of `tuple` at compile time (class template specialization)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/variant/variant_size&oldid=123919"