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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: <574@atcmpe.atcmp.nl>
Date: 25 Mar 90 16:51:53 GMT
References: <152@dumbcat.UUCP>
Organization: AT Computing, Nijmegen, The Netherlands
Lines: 40

From article <152@dumbcat.UUCP>, by marc@dumbcat.UUCP (Marco S Hyman):
> In article <16808@well.sf.ca.us> nagle@well.UUCP (John Nagle) writes:
>  >       Let's start a discussion about the problem of how to achieve
>  > generic objects (ones with types as parameters, at least at compile
>  > time) in C++.  Some possibilities:
>  > 
>  >       1.  Explicitly use "pointer to void".  Works, but violates
>  > 	  type safety and requires much repetitive writing.
> 
> 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.

Isn't this true in C++ 2.0? (I have a lot of code (of the preprocessor-
generic type) which will break if type-information is lost (using a cast)).

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 :-))


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