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From: cjmchale@cs.tcd.ie (Ciaran McHale)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Inheritance and Information Hiding
Message-ID: <1991Feb10.203124.456@cs.tcd.ie>
Date: 10 Feb 91 20:31:24 GMT
References: <1991Feb8.171341.19413@cs.tcd.ie> <2140@cs.rit.edu> <498@eiffel.UUCP>
Organization: DSG, Dept. of Comp. Sci., Trinity College, Dublin.
Lines: 61

This discussion has used a concrete example (involving
polygons, rectangles and an add_vertex operation) to
discuss an issue. From the top of my head I can recall
the following approaches being suggested:

	o Having the add_vertex operation in polygon makes
	  that class too specialised to be used as a
	  superclass for rectangle. Reorganise the class
	  hierarchy a bit to solve the problem.
	o The problem can be solved if the language allows
	  an object to change its type at run-time. Thus
	  applying add_vertex to a rectangle changes that
	  object into a (pentagon or a) polygon.
	o Rectangle.add_vertex should be a null operation.
	o Rectangle.add_vertex should raise an exception.
	o Have very fine grain multiple inheritance.
	o Have a read_only interface to each class. The
	  read_only interface for polygon would exclude
	  add_vertex, and this resultant class is then
	  suitable as a superclass for rectangle.

(I'm sure that other approaches were discussed too.) Then I
add in my $0.02 worth suggesting:

	o If a language has separate type and implementation
	  hierarchies then rectangle can reuse code from
	  polygon without having to support add_vertex in
	  its type.

Bertrand Meyer (article <498@eiffel.UUCP>) replies with:
>This is simply to point out that the issue of ``implementation
>reuse'' versus ``specification reuse'' has nothing to do
>with this whole discussion.

Why not? It _is_ an approach to solving the problem at hand.
It may not be the best approach but I don't see why you
are so eager to dismiss it out of hand.

>The separation between concept and implementation in software is
>an historical evil, not a desirable situation. With a good notation we
>can remove the difference.
>[...]
>One day we will stop being ashamed of implementation,
>not because we will have removed the need for implementation
>but for exactly the opposite reason: with us, everything
>is the implementation.

I don't know how (in)correct you are. Only time will
tell. But just because you feel that way does not
mean that others share your beliefs. So until
separation-between-type-and-implementation is widely
accepted to be a bad idea, why not consider it where it
might be useful?


Ciaran.
-- 
Ciaran McHale		"Verbosity says it all"			      ____
Department of Computer Science, Trinity College, Dublin 2, Ireland.   \  /
Telephone: +353-1-772941 ext 1538	FAX: +353-1-772204	       \/
Telex: 93782 TCD EI			email: cjmchale@cs.tcd.ie