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From: riks@csl.sony.JUNET (Rik Smoody)
Newsgroups: comp.lang.c++,comp.lang.eiffel,comp.lang.smalltalk
Subject: Re: Use of inheritance for classification?
Keywords: metrics, oop
Message-ID: <10248@riks.csl.sony.JUNET>
Date: 15 May 89 01:45:43 GMT
References: <30582@apple.Apple.COM>
Reply-To: riks@riks. (Rik Smoody)
Followup-To: comp.lang.c++
Organization: Sony Computer Science Laboratories, Inc., Tokyo, Japan.
Lines: 61

In article <30582@apple.Apple.COM> kmcentee@Apple.COM (Kevin McEntee) writes:
>should take place only in the context of the is-a relationship.
>
>    e.g.  Every dog is a mammal.
But not all dogs are frisbeeCatchers. 
Do I need a special sub-class?
Most dogs who catch frisbees use their mouth.
Do we need special code for each dog or class who is to learn that trick?
How would the subclasses by abilities combine with the subclasses
such as Collie, AustralianShepherd, Spotted, LongHaired, FinickyEater,
MailManBiting, etc.
Soon, I might have an inordinately large set of classes, with such
taxonomic gems as 
Every
BurglarBarkingFrisbeeFetchingBlackAndWhiteLongHairedOmnivorousAustralianShepherdDog
is-a  FrisbeeFetchingBlackAndWhiteLongHairedOmnivorousAustralianShepherdDog
	(only slightly more sightly if I don't make the artificial
	restriction to single-inheritance)
>
>This seems to imply that inheritance is first a classification scheme and 
>only second a code sharing mechanism.  
The hard part about code sharing is knowing when.  That's true for
objects which belong to classes, and for procedures which get called
from different places with different arguments Fortran or C.
There's a baby in that bath-water.
Where does the "code" of tieShoeLaces get attached to the taxonomy of
humans?  (Oops... can't  a monkey learn how to tie shoes?  Two sparrows
in a Disney cartoon?)  Most of us do it the same way.  A good candidate
for code sharing.

>
>What are your thoughts on this?  Can code sharing outside of the natural 
>taxonomy of the problem space happen gracefully?
Don't blindly assume a natural taxonomy, even in nature.
Many great minds have argued inconclusively on the natural
taxonomy of biological systems.  Since we in CS are not bound by such
constraints as species development by evolution, it seems a long reach
to assert that any one taxonomy is comprehensive.

Taxonomizing (is that a word?) and Code Sharing are not equivalent.
We all have a blood type.  That's a taxonomy.  Exposure to certain
diseases is another taxonomy.  Profession is another.  People of
almost any profession (not the oldest), who do not possess any of the
forbidden risk factors, should all share the code and give blood
now and then.

"Most generalizations are pretty good"

Rik Fischer Smoody
Sony Computer Science Lab, Inc.
3-14-13 Higashigotanda, Shinagawa-ku	
Tokyo 141 Japan

phone: (03)448-4380
E-mail: riks@csl.sony.jp
or maybe: riks@csl.sony.co.jp
-- 
Rik Fischer Smoody
Sony Computer Science Lab, Inc.,  3-14-13 Higashigotanda
Shinagawa-ku, Tokyo 141 Japan
(03)448-4380