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From: rmartin@clear.com (Bob Martin)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Inheritance and Information Hiding
Message-ID: <1991Feb5.143726.17160@clear.com>
Date: 5 Feb 91 14:37:26 GMT
References: <485@eiffel.UUCP> <488@eiffel.UUCP> <27A86309.281A@tct.uucp>
Organization: Clear Communications, Inc.
Lines: 45

In article <27A86309.281A@tct.uucp> chip@tct.uucp (Chip Salzenberg) writes:
>According to bertrand@eiffel.UUCP (Bertrand Meyer):
>>A class POLYGON exists and has a procedure
>>``add_vertex'', which it exports. You want
>>to add a class RECTANGLE. What do you do?
>>
>>1. Decide not to use inheritance.
>>2. Redo the existing inheritance hierarchy.
>>3. Accept that RECTANGLE inherits from POLYGON and does not export
>>   ``add_vertex''. ... and disallow polymorphic assignments of the form
>>   p := r where p is of type POLYGON and r of type RECTANGLE.
>>4. Accept that RECTANGLE inherits from POLYGON and does not export 
>>   ``add_vertex''. Permit polymorphic assignments of the above form;
>>   but have the type checker reject as invalid any system which would
>>   contain both such an assignment and a call p.add_vertex (...), since
>>   it may lead to a run-time inconsistency.
>
>In C++, the add_vertex() member function would almost certainly be
>virtual; [...] add_vertex becomes a non-problem.  The
>implementor of RECTANGLE simply provides an appropriate definition of
>the RECTANGLE::add_vertex(): a do-nothing function body that returns a
>failure indication (if appropriate).
>
>In the real world, how is this solution deficient?

Assume that this example were part of a very large system which
added points to POLYGONs all over the place.  Assume also that
there had never before been a reason for add_point to fail.  Your
proposed solution _changes_ the interface to class POLYGON since
now some POLYGONs (or their derivatives i.e. RECTANGLEs) will 
issue failure for add_point.  Thus it breaks old working code.
>
>>As an aside (meant to be thought- rather than flame-provoking)
>>I have come to realize, although not early enough, that
>>information hiding is a greatly overrated goal.

This is not a flame, it is a provoked thought.  Some people may
overrate a goal.  That does not devalue the goal.  
>

-- 
+-Robert C. Martin-----+:RRR:::CCC:M:::::M:| Nobody is responsible for |
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+----------------------+:R::R::CCC:M:::::M:| the blame.  So there.     |