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From: ik@laic.UUCP (Ik Su Yoo)
Newsgroups: comp.lang.c
Subject: union or type casting, which to use?
Keywords: union, type casting, static initialization
Message-ID: <931@laic.UUCP>
Date: 27 Nov 90 00:26:35 GMT
Distribution: usa
Organization: Lockheed AI Center, Menlo Park
Lines: 44


(Excuse me if this question is covered in the FAQ list.)

Suppose I have the following typedefs:

  typedef struct _foo { /* foo def */ } foo;
  typedef struct _bar { /* bar def */ } bar;

I want to create a new typedef with a field to hold a pointer to
either `foo' or `bar'. Is there any pros/cons of using union vs.
(explicit) type casting? For instance, is

  typedef struct {
    int type; /* 0 for foo pointer, 1 for bar pointer */
    union {
      foo *f;
      bar *b;
    } item;
  } oneof1;

better (or worse) than

  #include <sys/types.h>

  typedef struct {
    int     type; /* 0 for foo pointer, 1 for bar pointer */
    caddr_t item;
  } oneof2;

In the second method, the `item' field will be type casted to (foo *)
or (bar *) as necessary.

One of the major need is to be able to statically initialize the
`item' field. If the first method is better, how do I initialize the
`item' field? I don't care much for inefficiency due to run-time type
casting.

Thanks in advance.

-- 
|  Ik Su Yoo                                 |  Office: (415) 354-5584        |
|  Scientist @ Lockheed AI Center            |  Fax:    (415) 354-5235        |
|  Orgn. 96-20, Bldg. 259, 3251 Hanover St.  |  E-mail: ik@laic.lockheed.com  |
|  Palo Alto, CA  94304-1191                 |          ...!leadsv!laic!ik    |