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15.13 Unions

A union type defines alternative ways of looking at the same piece of memory. Each alternative view is defined with a data type, and identified by a name. A union definition looks like this:

union name
  alternative declarations

Each alternative declaration looks like a structure field declaration, except that it can’t be a bit field. For instance,

union number
  long int integer;
  double float;

lets you store either an integer (type long int) or a floating point number (type double) in the same place in memory. The length and alignment of the union type are the maximum of all the alternatives—they do not have to be the same. In this union example, double probably takes more space than long int, but that doesn’t cause a problem in programs that use the union in the normal way.

The members don’t have to be different in data type. Sometimes each member pertains to a way the data will be used. For instance,

union datum
  double latitude;
  double longitude;
  double height;
  double weight;
  int continent;

This union holds one of several kinds of data; most kinds are floating points, but the value can also be a code for a continent which is an integer. You could use one member of type double to access all the values which have that type, but the different member names will make the program clearer.

The alignment of a union type is the maximum of the alignments of the alternatives. The size of the union type is the maximum of the sizes of the alternatives, rounded up to a multiple of the alignment (because every type’s size must be a multiple of its alignment).

All the union alternatives start at the address of the union itself. If an alternative is shorter than the union as a whole, it occupies the first part of the union’s storage, leaving the last part unused for that alternative.

Warning: if the code stores data using one union alternative and accesses it with another, the results depend on the kind of computer in use. Only wizards should try to do this. However, when you need to do this, a union is a clean way to do it.

Assignment works on any union type by copying the entire value.

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