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From: marc@dumbcat.UUCP (Marco S Hyman)
Newsgroups: comp.lang.c++
Subject: Re: What to do about generics and polymorphism in C++
Message-ID: <152@dumbcat.UUCP>
Date: 23 Mar 90 05:37:22 GMT
References: <16808@well.sf.ca.us>
Organization: MH Software, Hayward, Ca.
Lines: 50

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
is not performed when assigning to a void pointer.  When you take from a
void * a different type than was placed in the void pointer, things break.
Example:

	#include <stdio.h>
	class B1 {
	public:
		int	i1;
		B1(): i1(0) {};
	};
	class B2 {
	public:
		int	i2;
		B2(): i2(0) {};
	};
	class D: public B1, public B2 {
	public:
		int	d;
		D(): d(0) {};
	};

	main()
	{
	void * generic;
	D something;
	B1 * b1 = &something;
	B2 * b2;

	++b1->i1;
	generic = b1;	// no adjustment
	b2 = generic;	// no adjustment
	printf( "%d\n", b2->i2 );
	}

The output will be 1, not zero.  The compiler assumes you know what you're
doing when you use void *.  Ouch.

// marc
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