Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!usc!polyslo!ttwang
From: ttwang@polyslo.CalPoly.EDU (Thomas Wang)
Newsgroups: comp.object
Subject: Re: Subclassing vs. subtyping
Message-ID: <1989Oct9.184205.20790@polyslo.CalPoly.EDU>
Date: 9 Oct 89 18:42:05 GMT
References: <33009@cornell.UUCP>
Reply-To: ttwang@polyslo.CalPoly.EDU (Thomas Wang)
Distribution: usa
Organization: Cal Poly State University -- San Luis Obispo
Lines: 44

wilk@cs.cornell.edu (Michael Wilk) writes:

>A "class" serves at least two purposes: the (usually run-time)
>generation of "instances" and the (usually compile-time) genera-
>tion of "subclasses."  If we think of classes as sets, then
>instances of that class refer to members of that set.

>The meaning of a subclass is more subtle.  In Smalltalk and CLOS
>(and probably others) a subclass represents a set of objects that
>are extensions of objects in the original class: they have the
>same instance variables and methods, and additional instance
>variables and methods can be added.

>This seems to me a programming construct that has no semantic
>correlation to anything outside of programming.  Let us consider
>an example below.  A rectangle is what you think it is; a
>venetian blind is a rectangular geometric figure with additional
>horizontal lines.

This is a very interesting point.  There are two kinds of inheritance.
The first kind is real ISA relationship, such as student-person.  You can say
with confidence that a student is a person.  The second kind is what I call
marriage of convenience, such as your example of venetian blind - rectangle.
Venetian blind is a shade, but is it a rectangle?  It is not so clear that
a venetian blind ISA a rectangle.

In this situation, the best thing to do is to allow venetian blind to
steal some functionalities from rectangle, but say that a venetian blind
is not ISA a rectangle.

I remembered someone on the net call them 'real inheritance', and
'user inheritance'.  I don't know if they are common terminalogies.

This is yet another example of an OO concept with multiple names.  We need
standard terminalogies fast!

>Michael Wilk (wilk@cs.cornell.edu)



 -Thomas Wang (There is a maniac in the bathtub!!
                            -Akane in "Ranma 1/2")

                                                     ttwang@polyslo.calpoly.edu