Union declaration

 

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A union is a special class type that stores all of its data members in the same memory location.

Unions cannot have virtual functions, be inherited or inherit other classes.

(until C++11) Unions can only contain POD (plain old data) types.

(since C++11) If a union contains a non-POD member, which has a user-defined special function (constructor, destructor, copy constructor or copy assignment) that function is deleted by default in the union and needs to be defined explicitly by the user.

Syntax

union name { member_declarations } object_list (optional) ;	(1)
union { member_declarations } ;	(2)

Explanation

1. Named union
2. Anonymous union

Anonymous unions

Members of an anonymous union are accessible from the enclosing scope as single variables.

Anonymous unions have further restrictions: they must have only public members and cannot have member functions.

Namespace-scope anonymous unions must be static.

Example

union foo
{
  int x;
  signed char y;
};
 
int main()
{
  foo.x = 128 + 896;
  std::cout << "as int: "  << (int)foo.x << '\n';
  std::cout << "as char: " << (int)foo.y << '\n';
  return 0;
}

Output:

as int: 1024
as char: 128

(for little-endian processors)