Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: C union problems (or is a pointer a pointer?)
Message-ID: <1989Apr26.215903.10906@utzoo.uucp>
Organization: U of Toronto Zoology
References: <15058@sequent.UUCP>
Date: Wed, 26 Apr 89 21:59:03 GMT

In article <15058@sequent.UUCP> bills@sequent.UUCP (Bill Sears) writes:
>The problem with this is that in order to declare a union, you must
>introduce a new variable into the structure (i.e. dummy).  Now my two
>pointers must be accessed as "a.dummy.menu" and "a.dummy.action",
>rather than (the preferable) "a.menu" and "a.action".  One way to 
>solve this is not to use a union.

Another way is to use the union but use #defines to hide the extra
name:

	union {
		MENU	*dumenu;
		ACTION	*duaction;
	} dummy;
	#define	menu	dummy.dumenu
	#define	action	dummy.duaction

so you can say "a.menu" and have it work.  It's a bit inelegant, but it
works very nicely.

>    char	*objptr;	/* or int * or long * */
>
>Now, by casting objptr to be the type of pointer that I am using
>at any given time, I can get rid of any compile errors, but is this
>always guaranteed to work?  In other words, is a pointer a pointer?

No; in the general case a cast from one type of pointer to another yields
machine-dependent and unpredictable results.  However, "void *" and (as a
grandfather clause) "char *" are exceptions:  they are guaranteed to be
able to hold any pointer, and conversions to and from them are guaranteed
safe.

>Although this is probably a matter of personal taste and programming
>style, which of the above implementations is the "most desirable"?

I would give the nod to union-plus-#define, which avoids waste of storage
and, at the cost of slight inelegance at the declaration, avoids notational
clumsiness at the points of use.

>Was the Pascal variant record syntax designed to overcome the above
>restrictions with the C union, or is it just coincidental that it does?

They're somewhat differently-flavored solutions to the same problem.
Pascal wins on this particular point of syntax-at-point-of-use, at the
cost of more complex structure-declaration syntax.
-- 
Mars in 1980s:  USSR, 2 tries, |     Henry Spencer at U of Toronto Zoology
2 failures; USA, 0 tries.      | uunet!attcan!utzoo!henry henry@zoo.toronto.edu