NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/std/cplus/1916

Text File | 1992-12-24 | 2.1 KB | 66 lines

Newsgroups: comp.std.c++
Path: sparky!uunet!microsoft!hexnut!jimad
From: [e-mail] (Jim Adcock)
Subject: Re: Must derived class reserve space for an empty base class?
Message-ID: <[e-mail]>
Date: 24 Dec 92 20:23:39 GMT
Organization: Microsoft Corporation
References: <[e-mail]> <[e-mail]> <[e-mail]>
Lines: 55

In article <[e-mail]> [e-mail] (Jamshid Afshar) writes:
|The fact the following idiom works under all current C++
|implementations, and that it will probably continue to work no matter
|what ANSI says, is what most worries me and why I originally posted
|this question.  Programmers will continue to do things like
|    class Register {
|       Set<Register*> alive;
|    protected:
|       Register() { alive += this; }
|       virtual ~Register() { alive -= this; }
|       static void foo();  // do something to all registered objects
|    };
|
|If a special-case rule is not added making the above safe on *any* C++
|implementation, I hope more C++ literature will point out the fact
|that the above idiom is incorrect.

Given that you do not specify your implementation, such as telling us
what pointer comparisons you use or don't use, its hard to say
whether your example does or doesn't work under all current C++
implementations.  In the absence of evidence, I will hypothesize
the opposite, namely that your class DOESN'T work under all current
C++ implementations.

Below find an example that frequently catches programmers off guard.
Again, C++ pointers DO NOT model any notion of object identity.  If you
want your objects to maintain a notion of object identity [A good
idea, IMHO] then you must explicitly GIVE your objects a notion of object
identity.  C++ comes with no such notion built-in.

#include <iostream.h>

class A { char bsPadding; };
class B : public A {};
class C : public A {};
class D : public B, public C {};

main()
{
    D d;

    B* pb = &d;
    C* pc = &d;
    A* pa1 = pb;
    A* pa2 = pc;

    if (pa1 == pa2)
        cout << "pointers match\n";
    else
        cout << "pointers don't match -- even though of the same"
            " type and 'to the same object'!\n";

    return 0;
};