NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/std/cplus/1915

Text File | 1992-12-24 | 1.5 KB | 60 lines

Newsgroups: comp.std.c++
Path: sparky!uunet!microsoft!hexnut!jimad
From: [e-mail] (Jim Adcock)
Subject: Re: Pointer comparisons
Message-ID: <[e-mail]>
Date: 24 Dec 92 20:08:40 GMT
Organization: Microsoft Corporation
References: <[e-mail]> <[e-mail]> <[e-mail]>
Lines: 49

In article <[e-mail]> [e-mail] (Stuart MacMartin) writes:
|2.  Perhaps there is a philosophical argument that might resolve this issue.
|    If two objects have the same interface but no state (or constant state), and
|    they have the same lifetime, are they in fact the same object?  Or, perhaps,
|    can they be treated as if they are the same object?

Here's a simple counterexample. b and b.a are clearly different objects.
They have different type, and they respond differently to a given method
call.

HOWEVER, they can both be legally used to initialized an A*, and
in which case the pointers compare equal, and the two different objects
THEN respond the same to a given method.


#include <iostream.h>

class A
{
public:
    static void print() { cout << "A\n"; }
};

class B : public A
{
public:
    A a;
    static void print() { cout << "B\n"; }
};

main()
{
    B b;
    b.print();
    b.a.print();

    A* pb = &b;
    A* pa = &(b.a);

    if (pa == pb)
        cout << "addresses match\n";
    else
        cout << "addresses don't match\n";

    pb->print();
    pa->print();

    return 0;
}