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From: mario@cs.man.ac.uk (Mario Wolczko)
Newsgroups: comp.object
Subject: Re: A Hard Problem for Static Type Systems
Message-ID: <2499@m1.cs.man.ac.uk>
Date: 7 May 91 17:02:08 GMT
References: <554@eiffel.UUCP> <1991Apr26.203642.17387@leland.Stanford.EDU> <CIMSHOP!DAVIDM.91May5120037@uunet.UU.NET>
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Reply-To: mario@cs.man.ac.uk (Mario Wolczko)
Distribution: comp
Organization: Department of Computer Science, University of Manchester
Lines: 37

In article <CIMSHOP!DAVIDM.91May5120037@uunet.UU.NET>, cimshop!davidm@uunet.UU.NET (David S. Masterson) writes:
    do most people use inheritance to build down from some
    underspecified level to something more specific to the problem at
    hand or do most people build up from some highly specific to
    something that can be more easily generalized to later problems?
    If you say either-or (depending on the circumstances), what are
    the circumstances that make you choose one over the other?

I can only speak for myself.  When trying to solve a specific problem
I will try to identify aspects of that problem which are of a general
nature, and which could be reused.  (Example: in writing a compiler, I
have developed classes to support directed acyclic graphs in a general
way).  In some cases, there will be parts of a class that are very
specific to the problem at hand, and then I will try to factor the
class into two classes -- an "abstract", reusable superclass, and a
"concrete" subclass.  Sometimes this factoring can be repeated,
leading to a hierarchy of abstract classes, with concrete classes at
the leaves.

Sometimes the abstract class will already be there, and I just have to
provide a concrete subclass, which is usually easy.  (I do all my
serious OO programming in Smalltalk, which has lots of abstract
classes.)  Occasionally I find a concrete class which does part of
what I want, but perhaps something else which I don't, and then I try
to split that class into abstract and concrete pieces, such that I can
inherit from the abstract part to get what I want.  (Splitting an
existing class while remaining compatible can be very hard.)  In any
case I try to avoid having one concrete class inherit from another
concrete class.

Mario Wolczko

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