Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!uxc!uxc.cso.uiuc.edu!m.cs.uiuc.edu!p.cs.uiuc.edu!johnson
From: johnson@p.cs.uiuc.edu
Newsgroups: comp.lang.c++
Subject: Re: Functions not defined in base class
Message-ID: <77300022@p.cs.uiuc.edu>
Date: 5 May 89 13:41:00 GMT
References: <897@servax0.ESSEX.AC.UK>
Lines: 21
Nf-ID: #R:servax0.ESSEX.AC.UK:897:p.cs.uiuc.edu:77300022:000:1056
Nf-From: p.cs.uiuc.edu!johnson    May  5 08:41:00 1989


Given the choice of
	1) defining a virtual function with a null body in the base class
and	2) casting a pointer
I would ALWAYS pick 1.  One of the characteristic features of an abstract
class is that it relies on code that is defined by subclasses.  Virtual
methods provide this mechanism in C++.  Abstract classes are one of the
key ideas in object-oriented design and the most important functions
in an abstract class are usually the ones that are defined in the
subclasses.  An abstract class essentially contains the instructions
for deriving subclasses from it, because you just define the functions
that are left to the subclasses.  C++ requires that virtual functions be 
defined in the base class, so you have to provide a dummy body or the 
loader will complain.

This idea is certainly not specific to C++, but is a feature of all
object-oriented languages.  In Smalltalk, the base of most large
class hierarchies is abstract, such as Collection, Number, and DisplayObject.
When it isn't, such as class View, it usually should be.

Ralph Johnson