From: Jim Menard <jimm@...>
Date: 2002-03-13T02:57:50+09:00
Subject: Re: Method type 'abstract'

Peter Hickman <peter@semantico.com> writes:

> Nat Pryce wrote:
> 
> >In Ruby *all* methods are abstract unless implemented by a class.
> >
> >Or to put it in a more useful way, abstract methods are not required in a
> >dynamically typed language such as Ruby, only in typed languages like Java
> >or C++.
> >
> Not quite what I had in mind. If I create a class I will provide various 
> methods that a subclass can override. But in many cases they will pick 
> up the super classes' methods. However in the class I have in mind there 
> is one method that must exist in the subclass for which there is no 
> meaningfull default action that it can undertake, the subclass must 
> override this method. In Java I would mark the method as abstract rather 
> than just public or private and when I subclass it the compiler would 
> choke if I did not provide the override in the subclass.

In Smalltalk it is common to implement this method in the superclass and
have it call "subclassResponsibility", a method defined in Object.

You could throw an exception. That's very useful for unit testing.

Jim
-- 
Jim Menard, jimm@io.com, http://www.io.com/~jimm/
"Alas! The onion you are eating is someone else's waterlily."
    -- A fortune cookie fortune