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From: hansen@pegasus.att.com (Tony L. Hansen)
Newsgroups: comp.lang.c++
Subject: Re: Comparison functions for qsort() and bsearch()
Summary: qsort and MI
Keywords: Sun, C++
Message-ID: <1991Jan6.050801.20821@cbnewsk.att.com>
Date: 6 Jan 91 05:08:01 GMT
References: <3069@lupine.NCD.COM> <277640DD.4A70@tct.uucp> <1990Dec26.160636.15566@clear.com>
Sender: hansen@cbnewsk.att.com (tony.l.hansen)
Organization: AT&T Bell Laboratories
Lines: 49

<< From: chip@tct.uucp (Chip Salzenberg)
<< Both qsort() and bsearch() require a pointer to a function which does
<< *actually* take two parameters of type pointer-to-void.  The fact that
<< the function in question will almost immediately cast those pointers
<< into pointer-to-T is utterly irrelevant.

< From: rmartin@clear.com (Bob Martin)
< I dissagree.  As Ron said in his article.  It is always possible to
< cast a pointer to anything to a pointer to void.  But it is not
< guaranteed that you can cast a pointer to void back to its original.
< Case in point is a class derived from multiple bases:
<	class AB : public A, public B;
< Now take the following variable:
<	AB *ab;
< The following two casts work just fine:  
<	(A *)ab;  (B *)ab;
< But the following, though it may compile, is likely not to work
< properly because it miscalculates the address of at least one of the
< base classes.
<	(A *)(void *)ab; or (B *)(void *)ab;
< Yet if qsort is to sort objects of type AB, then this is precicely what
< the void* implementation of compar would be asked to do!  Worse yet the
< compiler can't catch this error and will thus allow corrupted pointers
< loose in your program.  Amost certainly the program will crash.

Bob, you're seriously mixing apples and oranges, and getting caught in the
middle. You're right, you cannot take a void* created from an AB* and cast it
directly to a B* and expect things to work out right. SO DON'T DO IT! No
comparison function passed to qsort() should ever do that! You're passing an
AB* cast to a void*, so cast the void* back to an AB*!  If you choose to go
on from there to an A* or a B*, fine. But don't go directly from the void* to
an A* or B*. In other words, declare your comparison function like this:

	int comparison(const void *a, const void *b)
	{
	    const AB *ABa = (const AB*)a;
	    const AB *ABb = (const AB*)b;
	    // compare *ABa with *ABb
	}

If you do so, everything works out just fine.

See another article of mine on a brief description of a template definition
of qsort().

					Tony Hansen
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				    hansen@pegasus.att.com
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