Path: utzoo!attcan!uunet!munnari.oz.au!uhccux!ames!pacbell!hoptoad!hsfmsh!dumbcat!marc
From: marc@dumbcat.UUCP (Marco S Hyman)
Newsgroups: comp.lang.c++
Subject: Re: What to do about generics and polymorphism in C++
Message-ID: <153@dumbcat.UUCP>
Date: 27 Mar 90 06:34:00 GMT
References: <152@dumbcat.UUCP> <574@atcmpe.atcmp.nl>
Organization: MH Software, Hayward, Ca.
Lines: 30

In article <574@atcmpe.atcmp.nl> leo@atcmp.nl (Leo  Willems) writes:
 > From article <152@dumbcat.UUCP>, by marc@dumbcat.UUCP (Marco S Hyman):
 > > 	void * generic;
 > > 	D something;
 > > 	B1 * b1 = &something;
 > > 	B2 * b2;
 > > 
 > > 	++b1->i1;
 > > 	generic = b1;	// no adjustment
 > > 	b2 = generic;	// no adjustment
 > 
 > If you apply a cast in the example does it work correct then?
 > (If it works, is it legal or just luck? Andrew Koenig told me a long time
 > ago it is legal, but that was in the 1.2 darkages, in which I still live :-))

G++ (the version I used to check the above example) does not require the
cast.  My cfront 2.0 hasn't arived for this machine yet so I can't test it,
but am under the impression that when you use a case you are only telling
the compiler that you know what you are doing.  If you specify

	b2 = (B2 *) generic;

you are telling the compiler that you know generic is a B2.  No adjustment
will be made and your code breaks.  The moral is *don't* use void* for
generics.

// marc
-- 
// {ames,decwrl,sun}!pacbell!dumbcat!marc
// pacbell!dumbcat!marc@lll-winken.llnl.gov