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From: leo@atcmp.nl (Leo  Willems)
Newsgroups: comp.lang.c++
Subject: Re: What to do about generics and polymorphism in C++
Message-ID: <576@atcmpe.atcmp.nl>
Date: 27 Mar 90 21:39:00 GMT
References: <574@atcmpe.atcmp.nl>
Organization: AT Computing, Nijmegen, The Netherlands
Lines: 53

From article <574@atcmpe.atcmp.nl>, by leo@atcmp.nl (Leo  Willems):
> From article <152@dumbcat.UUCP>, by marc@dumbcat.UUCP (Marco S Hyman):

stuff deleted

>> Doesn't always work.  In the case of multiple inheritance pointer adjustment
>> void * a different type than was placed in the void pointer, things break.
> 
> Stuff deleted
> 
>> 	void * generic;
>> 	D something;
>> 	B1 * b1 = &something;
>> 	B2 * b2;
>> 
>> 	++b1->i1;
>> 	generic = b1;	// no adjustment
>> 	b2 = generic;	// no adjustment
> 
> Under C++ 1.2 the last sentence will give a "bad assigment type" error.
> As I understand it, assignment to void* must be casted in C++ (which
> is different in ANSI-C) thereby bringing type-information back.
> 
Oops, I spoke to soon: casting a B1* to B2* is nonsense (most of the time)

	b2 = (B2*) generic;	// most of the time wrong.

However, even if probably wrong, leaving out the cast will genete a compiler
error.

What I had in mind (and of which I think is legal in C++ 1.2 and 2.0):

class b{ ... };
class bd : b { .... };

	bd bd_o;

	void *g;
	b *bp;

	g = &bd_o;

	bp = (b*) g;	// not casting here will give an error

Sorry about that.


Leo Willems			Internet: leo@atcmp.nl
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