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From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: references to dereferenced null pointers
Message-ID: <10595@alice.UUCP>
Date: 18 Mar 90 17:19:38 GMT
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <33188@brunix.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 49

In article <33188@brunix.UUCP>, sdm@cs.brown.edu (Scott Meyers) writes:
> In article <10582@alice.UUCP> shopiro@alice.UUCP (Jonathan Shopiro) writes:
> >	void   h(int& r)
> >	{
> >		if (&r == 0) ...
> >	}
> >
> >	int*   x = 0;
> >	h(*x);
> >I think the fundamental issue here is when is a pointer dereferenced?
> >(Since it is clearly illegal to dereference the null pointer).  I don't
> 
> Two comments:
> 
>     1.  In h(), r is a reference to an object.  By definition, no object
>         can have an address that is equal to 0, so (&r == 0) should always
>         fail.
But the question is, must a reference actually refer to an object?  Just as
a pointer can point to an object or to no-object, a reference could refer to
an object or to no-object.  Then, if a reference referred to no-object,
(&r == 0) would succeed.

Note that this has nothing whatsoever to do with the representation of a
null pointer.  If the reference r refers to no-object, then &r is the null
pointer (however it is represented).  It does have something to do with the
representation of a reference.  The representation would have to be capable
of storing a reference to no-object, as well as a reference to any object.
Since references are generally represented as pointers, this shouldn't be
a problem.

>     2.  In the call h(*x), we don't know precisely when *x is evaluated,
>         but since r is initialized with that value, it must occur prior to
>         executing any of the body of h().
The real question is not _when_ is *x evaluated, but __how_.  I claim that
*x is evaluated differently in

	int   i = *x;      // eval *x for rvalue

than in

	int*  p = &*x;     // eval *x for lvalue

and if x is the null pointer, the first is illegal and the second should
be legal, since x is never dereferenced.  The call h(*x) is analogous
to the second example.
-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Warren, NJ  07060-0908
		research!shopiro   (201) 580-4229