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From: johnson@p.cs.uiuc.edu
Newsgroups: comp.object
Subject: Re: Evaluating "Object-Oriented" Progra
Message-ID: <135300024@p.cs.uiuc.edu>
Date: 15 Jan 90 01:38:47 GMT
References: <638@ajpo.sei.cmu.edu>
Lines: 23
Nf-ID: #R:ajpo.sei.cmu.edu:638:p.cs.uiuc.edu:135300024:000:1094
Nf-From: p.cs.uiuc.edu!johnson    Jan 13 09:39:00 1990


Markku Sakkinen wrote:
>Question: However, isn't it actually bogus that all fundamental things
>in _Smalltalk_ are first-class objects? For instance, how do you define
>a use a meaningful subclass of SmallInteger, adding at least one
>instance variable?

SmallIntegers are implemented by a hack (use of a tag bit) so, as far as
I know, none of the Smalltalks let you subclass SmallInteger.  However,
this is not a problem in practice, since it is easy to subclass Integer,
and all you have to do is make a new kind of Integer whose value is a
SmallInteger and that delegates a few important operations (+,-,/,*,<,=)
to its value.  Such a subclass will work like a SmallInteger and you can
easily add new methods or variables to it without interfering with
regular SmallIntegers.

This is an extreme example of a general property of inheritance: it is
usually a lot cleaner to inherit from abstract classes than from concrete
classes.  It is quite common for a subclass to need a different data
representation, but inheritance brings along the superclass's data
representation.  

Ralph Johnson