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Xref: sparky comp.std.c:2589 comp.std.c++:1134
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Path: sparky!uunet!munnari.oz.au!cs.mu.OZ.AU!munta.cs.mu.OZ.AU!fjh
From: fjh@munta.cs.mu.OZ.AU (Fergus James HENDERSON)
Subject: Zero-length structures and pointer comparisons
Message-ID: <9225302.22791@mulga.cs.mu.OZ.AU>
Sender: news@cs.mu.OZ.AU
Organization: Computer Science, University of Melbourne, Australia
Date: Tue, 8 Sep 1992 16:14:16 GMT
Lines: 41

First, a question for comp.std.c:
is the following a legal (strictly conforming) program?

	struct S { unsigned:0; }
	int main() { return sizeof(S); }

Secondly, some comments about C++.
In ANSI C, pointer comparisons between pointers to unrelated objects (ie.
objects that are not both members of the same aggregate) cause undefined
behavior, I believe. For example, the following program
	#include <assert.h>
	int main() {
		int x,y;
		assert(&x != &y);
	}
should cause undefined behavior. [Is this correct?]

Now the C++ reference manual (5.9) says basically the same thing, with the
further constraints that comparisons with pointers to static members are
undefined, as are comparisons between pointers to members whose declarations
are separated by an access-class specifier (public/protected/private).

Section 5.9 says that "Two pointers to the same object compare equal",
but it does NOT say that "pointers to distinct objects compare unequal" -
instead it says "... Other pointer comparisons are implementation dependent."

Given that this is the case, it seems contradictory that repeated calls to
operator new(0) are required to return pointers to "distinct objects".
How could a conforming program possibly detect whether this requirement
was actually met? Surely the following program
	#include <assert.h>
	int main() {
		assert(operator new(0) != operator new(0));
	}
is not strictly conforming, is it?

-- 
Fergus Henderson             fjh@munta.cs.mu.OZ.AU      
This .signature virus is a self-referential statement that is true - but 
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