Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!samsung!munnari.oz.au!metro!extro!dpg
From: dpg@extro.ucc.su.OZ.AU (D P Gilbert)
Newsgroups: comp.lang.c++
Subject: polymorphism on base objects?
Summary: polymorphism on base objects or compiler bug
Keywords: C++, polymorphism, references, base objects, bug
Message-ID: <dpg.674226630@extro>
Date: 14 May 91 13:10:30 GMT
Sender: news@metro.ucc.su.OZ.AU
Distribution: comp
Organization: Sydney University Computing Service, Sydney, NSW, Australia
Lines: 54
Nntp-Posting-Host: extro.ucc.su.oz.au

I was experimenting with polymorphism to see if it works through a
function that takes a reference to a base class and got the following
surprises. 2 compilers (SUN C++ and Comeau on ix/386) based on cfront
2.0 gave the same results. The following sections in the ARM throw
some light on this (but I'm still confused): 4.7 (reference conversion),
5.17 (assignment operator), and 10.2 (Virtual functions).


#include <iostream.h>

class Base {
public:
    virtual void className() const { cout << "Class name: Base\n"; }
};
class D1 : public Base {
public:
    void className() const { cout << "Class name: D1\n"; }
};
class D2 : public Base {
public:
    void className() const { cout << "Class name: D2\n"; }
};

void test_a(const Base &bas) { bas.className(); }   // expect polymorphism
void test_b(const Base bas) { bas.className(); }    // don't expect ...

main()
{
    Base    b;
    D1      d1;
    D2      d2;
    Base    bb(d2);
                                        // Hoped for:
    cout << "test_a(b) : "; test_a(b);      //  Class name: Base
    cout << "test_a(d1) : "; test_a(d1);    //  Class name: D1
    cout << "test_a(bb) : "; test_a(bb);    //  Class name: Base
    cout << "test_b(b) : "; test_b(b);      //  Class name: Base
    cout << "test_b(d1) : "; test_b(d1);    //  Class name: Base
    cout << "test_b(bb) : "; test_b(bb);    //  Class name: Base
}

Got the following output:

test_a(b) : Class name: Base
test_a(d1) : Class name: D1
test_a(bb) : Class name: D2         ???
test_b(b) : Class name: Base
test_b(d1) : Class name: D1         ???
test_b(bb) : Class name: D2         ???

These last 2 results seem to indicate I can get polymorphism via
base objects? Apologies is this is a stupid question.

Thanks   Doug Gilbert