# std::variant_size, std::variant_size_v

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< 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>; template <class T> class variant_size<volatile T>; template <class T> class variant_size<const volatile T>; | (3) | (since C++17)

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 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>>`

### 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 BaseCharacteristic `std::integral_constant<std::size_t, N>` for some `N`.

### Example

| This section is incomplete Reason: no example

### 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)
tuple_size | obtains the size of `tuple` at compile time (class template specialization)

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