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From: sdm@cs.brown.edu (Scott Meyers)
Newsgroups: comp.lang.c++
Subject: Re: references to dereferenced null pointers
Message-ID: <33188@brunix.UUCP>
Date: 18 Mar 90 06:56:59 GMT
References: <51083@microsoft.UUCP> <25EB8EE8.8462@paris.ics.uci.edu> <16179@haddock.ima.isc.com> <10582@alice.UUCP>
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Reply-To: sdm@cs.brown.edu (Scott Meyers)
Organization: Brown University Department of Computer Science
Lines: 40

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.

    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().  If you let the evaluation
        semantics get any lazier, then you have to worry about this
        problem:

            int z = 1;                // global variable
            int fse() { z = 0; }      // function with side effects
            void foo(int y) {         // function that doesn't use its argument
              if (z) doSomething();
              else doSomethingElse();
            }

            int x;
            foo( fse() );

        When do you evaluate fse()?  If you don't initialize y with its
        value at the beginning of foo(), then when?  You've got to do it
        before z is evaluated...


Scott
sdm@cs.brown.edu