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Showing content from https://cplusplus.com/reference/type_traits/is_union/ below:

class template

<type_traits>

std::is_union

template <class T> struct is_union;

Is union

Trait class that identifies whether T is a union type.

It inherits from integral_constant as being either true_type or false_type.

Template parameters

T: A type.

Member types Inherited from integral_constant:

Member constants Inherited from integral_constant:
member constant definition value either true or false
Member functions Inherited from integral_constant:

operator bool: Returns value (public member function)

Example

// is_union example
#include <iostream>
#include <type_traits>

union A { int i; float f; };        // named union
struct B {
  union { int foo; float bar; };    // anonymous member union
};

int main() {
  std::cout << std::boolalpha;
  std::cout << "is_union:" << std::endl;
  std::cout << "int: " << std::is_union<int>::value << std::endl;
  std::cout << "A: " << std::is_union<A>::value << std::endl;
  std::cout << "B: " << std::is_union<B>::value << std::endl;
  return 0;
}

Output:

is_union:
int: false
A: true
B: false