Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!udel!rochester!kodak!uupsi!sunic!news.funet.fi!tukki.jyu.fi!sakkinen
From: sakkinen@tukki.jyu.fi (Markku Sakkinen)
Newsgroups: comp.object
Subject: Re: Void references
Message-ID: <1990Nov16.065609.15460@tukki.jyu.fi>
Date: 16 Nov 90 06:56:09 GMT
References: <454@eiffel.UUCP> <1990Nov15.011702.25087@Neon.Stanford.EDU> <11113@pt.cs.cmu.edu>
Reply-To: sakkinen@jytko.jyu.fi (Markku Sakkinen)
Organization: University of Jyvaskyla, Finland
Lines: 36

In article <11113@pt.cs.cmu.edu> ddean@rain.andrew.cmu.edu (Drew Dean) writes:
>From a relative neophyte,

Keep on asking these questions!  I suppose that the object-oriented people
(like many others) live under a small but constant threat of
"the emperor's new clothes" (H.C. Andersen).

>        if a subclass only inherits parts of its superclass
>(ie. some subset of the methods and/or variables), how can an instance
>of the subclass be used anywhere an instance of the superclass can be used ?
>In my (comparably primitive) understanding of OOP, this is important.  Without
>this property, what good are abstract classes, which seem to be a useful 
>organizational tool ?

Such partial inheritance can only be used for inheriting implementation
(what I called "incidental inheritance" in my ECOOP'89 paper),
thus more typically with concrete than abstract superclasses.
An instance of the subclass can _not_ be substituted for an instance
of the superclass.

In my opinion, inheriting only parts of superclass, say C,
into a subclass, say D, is a highly dubious idea even for this purpose.
One should rather have defined a fully-fledged class B containing
those parts first, then both C and D as subclasses of B.
It would be nice to have the kind of generalisation facilities
that Claus Pedersen suggested in his OOPSLA'89 paper:
that one could derive _super_classes approximately as easily
as existing languages allow us to derive _sub_classes.

Markku Sakkinen
Department of Computer Science and Information Systems
University of Jyvaskyla (a's with umlauts)
Seminaarinkatu 15
SF-40100 Jyvaskyla (umlauts again)
Finland
          SAKKINEN@FINJYU.bitnet (alternative network address)