std::is_union

Defined in header <type_traits>

template< class T >
struct is_union;

(since C++11)

Checks whether T is an union type. Provides the member constant value which is equal to true, if T is an union type . Otherwise, value is equal to false.

Template parameters

a type to check

Helper variable template

template< class T >
constexpr bool is_union_v = is_union<T>::value;

(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

true if T is a union type , false otherwise
(public static member constant)

Member functions

operator bool	converts the object to `bool`, returns `value` (public member function)
operator() (C++14)	returns `value` (public member function)

Member types

Type	Definition
`value_type`	`bool`
`type`	`std::integral_constant<bool, value>`

Example

#include <iostream>
#include <type_traits>
  
struct A {};
  
typedef union {
    int a;
    float b;
} B;
  
struct C {
    B d;
};
  
int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_union<A>::value << '\n';
    std::cout << std::is_union<B>::value << '\n';
    std::cout << std::is_union<C>::value << '\n';
    std::cout << std::is_union<int>::value << '\n';
}

Output:

false
true
false
false

is_class (C++11)	checks if a type is a non-union class type (class template)
std::experimental::is_union_v (library fundamentals TS)	variable template alias of `std::is_union::value` (variable template)