Unions

Union types are derived types sharing many of the syntactic and functional features of structure types. The key difference is that a union allows only one of its members to be "active" at any one time. The size of a union is the size of its largest member. The value of only one of its members can be stored at any time. In the following simple case,

Copy Code

union myunion {     /* union tag = myunion */
   int i;
   double d;
   char ch;
} mu, *muptr=&mu;

the identifier mu, of type union myunion, can be used to hold an int, an 8-byte double, or a single-byte char, but only one of these at the same time

Note: Unions correspond to the variant record types of Delphi.

sizeof(union myunion) and sizeof(mu) both return 8, but 4 bytes are unused (padded) when mu holds an int object, and 7 bytes are unused when mu holds a char. You access union members with the structure member selectors (. and ->), but care is needed:

Copy Code

mu.d = 4.016;
printf("mu.d = %f\n",mu.d); //OK: displays mu.d = 4.016printf("mu.i = %d\n",mu.i); //peculiar resultmu.ch = 'A';
printf("mu.ch = %c\n",mu.ch); //OK: displays mu.ch = Aprintf("mu.d = %f\n",mu.d); //peculiar resultmuptr->i = 3;
printf("mu.i = %d\n",mu.i); //OK: displays mu.i = 3

The second printf is legal, since mu.i is an integer type. However, the bit pattern in mu.i corresponds to parts of the double previously assigned, and will not usually provide a useful integer interpretation.

When properly converted, a pointer to a union points to each of its members, and vice versa.

What do you think about this topic? Send feedback!