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From: jhd@herold.franken.de (Joachim Durchholz)
Subject: Re: Interesting but sensitive topic to discuss (HELP: - OOP and CASE
 t
Date: 1996/11/08
Message-ID: <6KVOxOkk3RB@herold.franken.de>#1/1
X-Deja-AN: 195472480
references: <32813322.41C6@kyebek3.kjist.ac.kr>
newsgroups: 
 comp.object,comp.ai,comp.lang.ada,comp.lang.c++,comp.lang.smalltalk
Date: 1996-11-08T00:00:00+00:00
List-Id: <comp.lang.ada>


alovejoy@concentric.net wrote 08.11.96:

> > I think this is a bit misleading.

Seems I got mislead my self a bit :)

> execution of a program.  And in a Self program, classes never exist at all
> (not even in the program text).

Well, I don't know Self (or any other prototype language), so I'm a bit  
bewildered about what you say about this statement.
Could anybody post a short characterisation of prototype languages?

> > We don't specify a single objects when
> > writing code for a class; instead, we specify the behaviour of a set of
> > objects, of which there may be none, one, or an unlimited number present
> > at run-time.
>
> To the extent that this is true, this is an artifact of using classes as an
> abstraction mechanism to specify the behavior of objects.

I don't think it is too artificial to define a class, even if it is for a  
single object. At least not if the class specification isn't longer or  
more complicated than the specification for the single object.
I do see some overhead in that it is necessary to declare an attribute  
that is instantiated with the constructor of the class. Given the size of  
class texts, I doubt that this overhead is a problem.

> > > Class inheritance is an abstraction mechanism for code sharing.
> >
> > It is much more. It is a mechanism for specifying similarities, and in
> > this respect goes far beyond mere code sharing.
>
> A class specifies the internal mechanisms of an object.  Class inheritance
> therefore specifies the commonality of implementation mechanisms.
> ...
> This is actually what I was trying to get at
> in saying that "class inheritance is an abstraction mechanism for code
> [implementation mechanism] sharing."

Not in all cases. Most OO languages have abstract/virtual/deferred classes  
that primarily serve to specify external behaviour, not implementation.

I definitely agree that inheritance can be used for code sharing, but it  
is also useful for object classification.

> The type/interface hierarchy is an important topic in design, and hence is
> an important issue that should be addressed by a design methodology.
> ...

Agreed. (As with the rest of your post :)


-Joachim

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