std::variant_size, std::variant_size_v
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Common vocabulary types
std::variant
(C++26)
variant_size
Defined in header
<variant>
template< class T >
struct variant_size; /* undefined */
(1)
(since C++17)
struct variant_size; /* undefined */
template< class... Types >
(2)
(since C++17)
struct variant_size<std::variant <Types...>>
template< class T >
class variant_size<const T>;
(3)
(since C++17)
class variant_size<const T>;
template< class T >
class variant_size<volatile T>;
(4)
(since C++17) class variant_size<volatile T>;
(deprecated in C++20)
template< class T >
class variant_size<const volatile T>;
(5)
(since C++17) class variant_size<const volatile T>;
(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)>
3-5) meets the UnaryTypeTrait requirements with a base characteristic of std::integral_constant <std::size_t, std::variant_size<T>::value>
Contents
[edit] Helper variable template
template< class T >
constexpr std::size_t variant_size_v = std::variant_size<T>::value;
(since C++17)
constexpr std::size_t variant_size_v = std::variant_size<T>::value;
Inherited from std::integral_constant
Member constants
value
[static]
sizeof...(Types) (public static member constant)
Member functions
operator()
(C++14)
(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 base characteristic std::integral_constant <std::size_t, N> for some N.
[edit] Example
Run this code
#include <any> #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() {}
[edit] See also
obtains the type of the alternative specified by its index, at compile time
(class template) (alias template)[edit]
(class template) (alias template)[edit]
(C++11)
a tuple
(class template specialization) [edit]