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From: johnson@m.cs.uiuc.edu
Newsgroups: comp.object
Subject: Re: Void references
Message-ID: <77500065@m.cs.uiuc.edu>
Date: 21 Nov 90 12:11:00 GMT
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Nf-From: m.cs.uiuc.edu!johnson    Nov 21 06:11:00 1990


pcg@cs.aber.ac.uk said:

>Abstract (deferred) classes are no good of course. They are just a hack
>to decouple a bit the algebra of interfaces from that of
>implementations, which are otherwise coupled in inheritance.

I am shocked!  What can this mean?  Abstract classes are one of
the most important design techniques in object-oriented programming!

Reading on, I decided that Piercarlo and I have different definitions
of abstract classes.  I think that he defines an abstract class as
one that has NO implementation, in which every method is deferred.
However, my definition of an abstract class is that it is a class
that does not have a COMPLETE definition, in which there is SOME
method that is deferred.  Abstract classes according to my definition
are a crucial technique for reusing classes.  An abstract class is a 
template for its subclasses, and replaces other ways of reusing design 
such as code skeletons.  I agree with Piercarlo that abstract classes
should not be used just to represent types (though of course if you
are programming in C++ then you have no other choice).  Thus, we
probably don't really disagree.

Ralph Johnson -- University of Illinois at Urbana-Champaign