NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/std/cplus/1145

Text File | 1992-09-09 | 1.6 KB | 42 lines

Newsgroups: comp.std.c++
Path: sparky!uunet!munnari.oz.au!cs.mu.OZ.AU!munta.cs.mu.OZ.AU!fjh
From: [e-mail] (Fergus James HENDERSON)
Subject: Re: Zero-length structures and pointer comparisons
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: Computer Science, University of Melbourne, Australia
References: <[e-mail]> <[e-mail]>
Date: Thu, 10 Sep 1992 04:37:29 GMT
Lines: 30

[e-mail] (Johan Bengtsson) writes:

>If sizeof(long) >= sizeof(void*), then you should be able to
>test like this:
>
>assert( (long)p1 != (long)p2 );
>
>since a pointer stored in an integer variable must keep all information
>needed to restore the pointer (if the integer type is large enough).

I don't think that this is the case.

If the integer variable is *larger* than the pointer, then some of the
bits in the integer need not be used when converting from integer to pointer.
I do not see any restriction in the reference manual that would require
the opposite conversion to initialize those bits.
This would then mean that (long) p1 != (long) p2 might return true even
when p1 == p2, because the uninitialized padding bits might be different.

[This means that ((long) p1) may behave "non-deterministically", although
((void *)(long)p1) is guaranteed to evaluate to p1 provided that "long"
is sufficiently large.]

Is that all correct?

--
Fergus Henderson             [e-mail]
This .signature virus is a self-referential statement that is true - but
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