Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!gatech!mcnc!thorin!luther!kelleymt
From: kelleymt@luther.cs.unc.edu (Michael T. Kelley)
Newsgroups: comp.lang.c++
Subject: Implementing virtual functions that return reference to self
Keywords: virtual functions, derived classes
Message-ID: <8975@thorin.cs.unc.edu>
Date: 26 Jul 89 16:13:42 GMT
Sender: news@thorin.cs.unc.edu
Lines: 37

I've been working on a set of classes for manipulating graphics
primitives.  For many of the sub-classes, there are members I don't
want to "stub-out" in the parent class.  The problem comes when
grouping redefined functions in the sub-class with functions
not stubbed out in the parent into a single expression:

#include <stream.h>

class Base {
public:
	Base() { }
	virtual Base& hello() { cout << "hello "; return *this; }
};

class Derived : public Base {
public:
	Derived() { }
	Base& hello() { cout << "g'day "; return *this; } // ugh.
	void goodbye() { cout << "see ya!\n" }
};

main()
{
	Derived d;
	d.hello().goodbye();	// ERROR: goodbye undefined
}


In this case, I know I'm dealing with a Derived, so I'd like to be able
to use goodbye().  Can someone explain the harm in allowing hello()
to return a Derived& in the Derived class?  Or is there an 
implementation issue lurking underneath?

Michael T. Kelley
University of North Carolina at Chapel Hill
kelleymt@cs.unc.edu				uunet!mcnc!unc!kelleymt
(919) 962-1761