Path: utzoo!attcan!uunet!mcsun!hp4nl!sci.kun.nl!atcmpe!leo
From: leo@atcmp.nl (Leo  Willems)
Newsgroups: comp.lang.c++
Subject: polymorfism on objects, not pointers
Keywords: polymorfism dynamic binding
Message-ID: <631@atcmpe.atcmp.nl>
Date: 4 Jul 90 20:17:02 GMT
Organization: AT Computing, Nijmegen, The Netherlands
Lines: 107


In the example below ::g() accepts a class B variable (not a pointer/reference)
If g() is called with a D (derived from B) the virtual mechanism works!

	B bb;
	D dd;

	g(dd);		//see end of posting for complete code

In C++ 1.2 this didn't work (compile time error: can not make a B from a D)
but in 2.0 it is possible to make a B from a D but as I understand it that
should not alter the B behaviour, and so no runtime binding is expected.

What strikes me is the difference in behaviour: when assigning a D to a B:

	bb = dd;
	bb->f();		//B::f();	as exptected

there is no dynamic binding, when calling a function with a D which is expecting
a B however there is:

	g(dd);		// g(B x) { x.f(); }   ===>  D::f() !!!!!!

In both cases I thought that the B part of a D was taken and no dynamic binding
is expected.

To make it even more complicated, when giving class B a copy constructor

	B::B(B&) {}

the dynamic behaviour in the call g(dd) has disapeared!

Can someone enlighten me? To be specific:

	*) Is the virtual mechanism supposed to work on class variables
	   or only on pointer or references to classes?

	  ( I have never ever read about the posibility that it
	    should work on class variables. So if it is supposed to work
	    please mail a reference on the subject.)

	  ( if it should not work with class variables (as I think) then
	    our compiler may be the problem: it is Glsp 2.0 for sparc stations
            Is this a known problem?)

	  ( if it should not work with class variables (as I think) then
	    the example in G Booch's OODWA is wrong: (p 102)

		void sendTelemetryData(TelemetryData D){
			D.send();
		}

		sendTelemetryData(telemtery);
		sendTelemetryData(electrical);

		(sorry for all the white space between the characters:-)

	    Booch claims that here polymorfism is at work.
            If it is (and I am wrong) flame on me :-(
	  )		

Thanks,

Leo

======== Complete example for C++ 2.0 ======

#include <iostream.hxx>

class b {
public:
	virtual void f() { cout << "fb()\n"; }
};

class d: public b {
public:
	virtual void f() { cout << "fd()\n"; }
};

void
g(b x)
{
	x.f();
}

main()
{
	b bb;
	d dd;

	g(bb);		// fb()
	g(dd);		// fd()!!!!

	bb = dd;
	g(bb);		// fb()
}



============


 Leo Willems			Internet: leo@atcmp.nl
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