Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.lang.c
Subject: Re: initialization of unions
Message-ID: <8864@utzoo.UUCP>
Date: Sat, 31-Oct-87 19:20:08 EST
Article-I.D.: utzoo.8864
Posted: Sat Oct 31 19:20:08 1987
Date-Received: Sat, 31-Oct-87 19:20:08 EST
References: <165600017@uiucdcsb> <4237@ccv.bbn.COM>, <1936@killer.UUCP>
Organization: U of Toronto Zoology
Lines: 45

> > ...unions are initialized
> > as if you were initializing their first element...
> 
> I've never understood why this is the case.  Is there some reason I'm
> overlooking why the compiler couldn't let you cast it to one of the
> following elements?

Oh oh.  This is another one of those subjects that tends to produce long
flaming debates, with increasingly elaborate "solutions" proposed and
vigorously championed by people who have never used or implemented them.

As I understand it (I do not speak for X3J11), the main reason that the
ANSI draft says *anything* about the matter is a desire to have some sort
of well-defined value in a static union variable at startup time.  While
there is general agreement that a more flexible approach could be useful
at times, it falls to the usual objections:  there is no dire need for it
and no implementation experience with it.  The first-element approach, by
the way, *is* used in some existing implementations and has worked all
right (i.e. it's better than nothing).

Using casts for this is thoroughly ugly, incidentally -- it may *look*
like a minimal solution, but it introduces a major special case into the
language because it is now the only place where casts are mandatory and
the usual implicit conversions are not applicable.  (Relaxing either of
these rules leads to other problems.)

If you have a wonderful idea on the matter, remember that ANSI standards
do come up for revision regularly.  You are much too late to influence the
current standardization effort, but it's time to get moving if you hope to
get your idea into the next revision (which is maybe 5 years away).  What
you should do is:

	(a) implement your idea in your favorite C compiler
	(b) get a fair number of users using the resulting compiler
	(c) get feedback from them on how useful the feature is and
		whether it causes other problems
	(d) using that information as ammunition, prepare a formal
		proposal in time for the revision process

No, you cannot omit step (a); nobody will take the idea seriously unless
you have DEMONSTRATED its usefulness (as opposed to speculating about it).
In C, not some other language.  Better start now, (a) and (b) will take time.
-- 
PS/2: Yesterday's hardware today.    |  Henry Spencer @ U of Toronto Zoology
OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry