Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!cs.utexas.edu!uunet!munnari.oz.au!csc!bdm659
From: bdm659@csc.anu.oz
Newsgroups: comp.std.c
Subject: Re: Two questions about pointers
Message-ID: <849.2542f059@csc.anu.oz>
Date: 23 Oct 89 11:04:56 GMT
References: <1989Oct18.121516.10695@anucsd.oz> <1989Oct21.005145.23598@twwells.com>
Organization: Computer Services, Australian National University
Lines: 31

In article <1989Oct21.005145.23598@twwells.com>, bill@twwells.com (T. William Wells) writes:
> In article <1989Oct18.121516.10695@anucsd.oz> bdm@anucsd.oz (Brendan McKay) writes:
> : (1) Is it true that two pointers of the same type which point to the
> : same object must compare equal?
>
> Yes. See 3.3.9. Read further before disagreeing.
> You either have an old draft or you have not read yours carefully.
> From the May 88 draft: "If two pointers point to the same object
> or function, they compare equal."

Yes, alas, I have an old draft.  This addition places the semantics of
pointers on MUCH firmer footing.  Pointers to objects, anyway.
[Can someone please mail me the address for ordering the most recent draft?
 And the price.  Thanks muchly.]

> : (2) If two pointers of the same type compare equal, and each is cast
> : to a second pointer type, must the results compare equal?
>
> If there is an object there of that new type, yes. (E.g., unions.)
> Or, if both pointers were originally of the new type, yes.
> As near as I can tell, converting two pointers to a void* and then
> testing for equality does not have to work, unless both were
> void*'s to begin with.

Yes, that certainly follows from the sentence my copy doesn't have. Thanks.

As far as I can tell, my (old) draft contains absolutely nothing about the
meaning of the equality operators (==, !=) when applied to void* values.
Has that situation changed, or do we still have to guess?

Brendan McKay.  bdm@anucsd.oz  or  bdm@anucsd.oz.au