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From: maxtal@extro.ucc.su.OZ.AU (John MAX Skaller)
Subject: Re: Zero-length structures and pointer comparisons
Message-ID: <1992Dec15.162202.11231@ucc.su.OZ.AU>
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References: <1992Dec8.173855.18153@meaddata.com> <1992Dec10.121935.19315@ucc.su.OZ.AU> <1992Dec11.230534.10499@microsoft.com>
Date: Tue, 15 Dec 1992 16:22:02 GMT
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In article <1992Dec11.230534.10499@microsoft.com> jimad@microsoft.com (Jim Adcock) writes:
>In article <1992Dec10.121935.19315@ucc.su.OZ.AU> maxtal@extro.ucc.su.OZ.AU (John MAX Skaller) writes:
>|	The answer to this particular question is YES, they must
>|compare equal according to the ARM. And I think this is a problem
>|for OS design for the 386, for example, where the memory management
>|kernel will just have to ignore the ARM.
>
>No more nor less than any other virtual memory OS that allows
>multiply mapped pages.  Since there is no functionality in the
>language requiring such multiply mapped pages, you only get in
>this situation by invoking some system dependency, in which case
>you are making use of implementation dependencies.  If you will,
>a common C/C++ implementation defined extension to the language
>is to allow the use of OS calls in order to map various regions
>of memory to the same underlying page, allowing the same object
>to be accessed via differing addresses.  Implementation defined,
>no more nor less.
>

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.

So the question is really whether the requirement that pointers to
the same object compare equal should be left implementation defined.
Or perhaps more restrictions be made, like saying they must
compare equal if one is assigned from another, but not otherwise.



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