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From: sakkinen@tukki.jyu.fi (Markku Sakkinen)
Newsgroups: comp.object
Subject: Re: Inheritance Question
Message-ID: <1991Mar8.064345.6404@tukki.jyu.fi>
Date: 8 Mar 91 06:43:45 GMT
References: <13349@ccncsu.ColoState.EDU>
Reply-To: sakkinen@jytko.jyu.fi (Markku Sakkinen)
Organization: University of Jyvaskyla, Finland
Lines: 61

In article <13349@ccncsu.ColoState.EDU> sharif@lamar.colostate.edu (Thomas Abdallah) writes:
>I have a question regarding inheritance.
>
>In all my readings I have not found a decisive answer to the question can
>objects inherit an attribute through more than one channel?
>
>Say that I have the following construct:
>
>				  Water
>         +-----------------------+ | | +----------------+
>         |              +----------+ +--+               |
>         v              v               v               v
>	Ocean		Lake		Beer <--------- Hops
>
>
>Base class WATER has an attribute, say w' which is inherited by Derived classes
>OCEAN, LAKE, and BEER, and HOPS.  Now BEER also inherits the attribute w' from
>HOPS.  Is this a legal OO construct?  If not, why not?

Several people have already answered, most of them pointing out that
the inheritance relations are not very sensible if the names of these
classes are to be taken literally.  It is indeed very common that one
confuses an inheritance hierarchy (is kind of) with a part hierarchy
(is part of).  I'll omit this part of the problem and continue the
discussion without reference to the supposed real-world semantics
of these classes.

In principle, if Beer is a subclass of Hops which is a subclass of Water,
then Beer is also a subclass of Water;  thus there is no idea in declaring
Beer also an immediate subclass of Water.  However, I see you are using
C++ terminology -- in C++ Hops may inherit Water privately and this
kind of hierarchy could then be sensible; but in that case Hops is
not regarded as a _subtype_ of Water.

A certainly sensible and interesting situation occurs if you have
a "fork-join" kind of hierarchy, the simplest possible being
        A
      /   \
    B       C
      \   /
        D
I dislike the way this is handled in Bertrand Meyer's "Object-oriented
Software Construction" (a book that I hold in high esteem in general).
I agree better, but not completely, with Jorgen Lindskov Knudsen's
paper in the ECOOP'88 proceedings.  While writing my own paper
"Disciplined inheritance" for ECOOP'89, I totally agreed with the way
that the multiple-inheritance addition to C++ (a language that I do _not_
hold in high esteem in general) was designed.

More recently I have found that there is at least one clear problem
even in the C++ approach.  Essentially, the "virtuality" of a base class
can be "too strong" (too global) in more complex situations where
some of the derivations are private.

Markku Sakkinen
Department of Computer Science and Information Systems
University of Jyvaskyla (a's with umlauts)
PL 35
SF-40351 Jyvaskyla (umlauts again)
Finland
          SAKKINEN@FINJYU.bitnet (alternative network address)