TITLE: anonymous unions 

(Newsgroups: comp.lang.c++.moderated, 31 July 99)

DEROCCO: Paul D. DeRocco <pderocco@ix.netcom.com>

>I read the standard the same way, but with respect to _anonymous_
>unions, I can't see any use for them, except to be able to stuff
>something in via one name, and pull it out again via another name. Can
>you?


KOENIG: Andrew Koenig <ark@research.att.com>

Sure.  Typical use is in conjunction with another variable that
tells the program which component is in use.  For example:

 class Number {
 // ...
 private:
 union {
 int i;
 NumberRep* p;
 };
 bool is_int;
 // ...
 };

This example might be part of a class that can represent any of
several kinds of numbers.  To avoid an indirection in the common
case, we store an integer right in the object; any other kind of
number is stored in dynamically allocated memory.

Another possibility:

 class String {
 // ...
 private:
 unsigned size;
 union {
 char s[8];
 char* p;
 };
 // ...
 };

Here, we store the string right in the object if it contains 8
characters or fewer; otherwise we store them in dynamically
allocated memory.  We use the size to decide which union
component is active.