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Newsgroups: comp.std.c++
Path: sparky!uunet!microsoft!hexnut!jimad
From: jimad@microsoft.com (Jim Adcock)
Subject: Re: Zero-length structures and pointer comparisons
Message-ID: <1992Dec16.201044.2968@microsoft.com>
Date: 16 Dec 92 20:10:44 GMT
Organization: Microsoft Corporation
References: <1992Dec10.121935.19315@ucc.su.OZ.AU> <1992Dec11.230534.10499@microsoft.com> <1992Dec15.162202.11231@ucc.su.OZ.AU>
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In article <1992Dec15.162202.11231@ucc.su.OZ.AU> maxtal@extro.ucc.su.OZ.AU (John MAX Skaller) writes:
|But Jim, it is not implementation defined, it is required that 
|pointers to the same object compare equal. It is therefore
|legal to have a function:
|
|	f(X* a, X*b) {
|	if(a==b) { /* special case, copy b object */ ... }
|	else *a+=*b;
|	}
|
|where the += operation is destructive and wont work if a and b point
|to the same object. The above code is guarranteed in C++, but
|will fail on a suitably organised system. It is therefore not
|allowed to do this sort of memory mapping in C++ behind the scenes,
|which is what I'm concerned with...we actually want to.

If a strictly conforming implementation is presented with a strictly
conforming program, then the above works subject to some additional 
constraints, such as a and b refering to disjoint objects.

However, if a strictly conforming implementation is presented with a 
non-strictly conforming program, such as a program that calls OS
virtual memory mapping adjustment routines, or programs that send
pointers off the ends of arrays, or programs who don't properly point
at objects that correspond to the types of objects they pretend to point
at .... in all such cases all bets are off.