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 and cannot be inherited.

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

(since C++11) Unions can contains non-trivial types provided that eventual non-trivial constructors are defined by the user.

Syntax

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

Explanation

1. Named union
2. Unnamed union

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)