Xref: utzoo comp.lang.eiffel:1426 comp.object:2596
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From: caseau@maya.bellcore.com (Yves J Caseau)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Inheritance and Information Hiding
Message-ID: <1991Feb5.130359.9735@bellcore.bellcore.com>
Date: 5 Feb 91 13:03:59 GMT
References: <488@eiffel.UUCP> <27A86309.281A@tct.uucp> <1991Feb1.135956.12425@tukki.jyu.fi> <27ADD06B.46A3@tct.uucp>
Sender: usenet@bellcore.bellcore.com (Poster of News)
Reply-To: caseau@maya.UUCP (Yves J Caseau)
Organization: Bell Communications Research, Morristown NJ
Lines: 35

Here is my $0.02 contribution to the problem:
=============================================

1. This is a well-known and common problem. In the type theory and object-oriented
   database community, the example is a class point with an instance variable x,
   and the subclass is the set of point for which x is between 0 and 10. So if we
   change the instance variable, the "class" to which the object belongs must
   change, or an error must be detected.

   The question is: given a class A, and a subclass B which has a characteristic
   property, how to represent it into an object-oriented system?

   Obviously it depends if your system supports such characteristic properties.
   If it does (Eiffel with invariants, Semantic networks with specialisation
   predicates (eg CLASSIC), LORE with selection sets, FOL-based object-oriented 
   systems) you have a "nice" solution: inheritance with automatic error detection
   (Eiffel: the invariant is violated) or reclassification.

   If the system does not support such properties, you have to implement them
   yourself, copying one of the two previous solutions. This means that each
   method that returns an object from B must be specialized (if it is inherited)
   so as to check the characteristic property (and decide between error or 
   re-classification, according to the language features (C++ vs. CLOS)

2. My solution to the POLYGON/RECTANGLE problem.

   I tend to be simple minded:
      a. a RECTANGLE is a POLYGON (thefore there must be inheritance)
      b. Adding a vertex to a polygon produces ANOTHER polygon (a different
         mathematical object)
      c. Thus, rectangle inherit the property add_vertex, which produces
         a pentagone from a rectangle

   Don't flame me on this, I know this is cheating ... but it makes a lot of
   sense.


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