Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!bellcore!spectral!sjs
From: sjs@spectral.ctt.bellcore.com (Stan Switzer)
Newsgroups: comp.windows.news
Subject: Re: Class vars or Instance vars
Message-ID: <14719@bellcore.bellcore.com>
Date: 14 Mar 89 15:10:12 GMT
References: <101*knut-skog@rglab.uit.uninett>
Sender: news@bellcore.bellcore.com
Reply-To: sjs@ctt.bellcore.com.UUCP (Stan Switzer)
Organization: Bellcore
Lines: 49

In article <101*knut-skog@rglab.uit.uninett>
knut-skog%rglab.uit.uninett@NORUNIX.BITNET (Knut Skog) writes:

> 				    .... In object oriented system design
> a class should be constructed based on the true nature of the attributes
> of the objects under consideration. Class variations should be taken care
> of by subclassing and truly private data of a class instance should be
> defined as instance vars. The fact that class-vars is  'turned into
> instance vars' (placed in Instance dict) as soon as their attributs are
> assigned dynamicaly and therby makes it preferable to write 'natural
> instance attributes' as class-vars, is a pecullarity of this system that
> class-writers must be aware of. Right?

This is indeed true (so long as you use "def" and not "store" to
change the values).  It is an interesting and useful feature of NeWS's
class system that default values for instance variables can simply be
declared as class variables.  Generally, with variables that reflect
user preferences, etc., and which don't change too much (i.e. aren't
*really* part of the object's dynamic state) I make them class
variables with the understanding that they can be overridden by
assigning to the instance.  Additionally, such variables can be
overridden in subclasses.  Although this feature is somewhat unusual
it is not unique to NeWS.  The language "Self" described in the '87 (I
think) OOPSLA has the same property.
 
In a "true" object-oriented system, ALL data would be objects.  In
this case, the distinction between shared and non-shared object
usually remains.  Generally, composite objects are shared and simple
objects (in SmallTalk, just the small ints) are not.  (The point is
somewhat moot since there are no operations that *change* atomic
objects, only operations that create new immutable objects, so you
really wouldn't know if they were shared or not.)  In any system with
class variables (or other means of namespace sharing) one must
distinguish between shared objects and shared references to objects.

> 
> In my 25-years as computer professional I have never had a more
> enjoyable and interesting  experiance than NeWS.

Likewise.

I haven't had as much enjoyment programming since I first got my hands
on LISP 1.5.  In the elegance of its conception and the virtuosity of
its execution NeWS is nearly unparalleled.

Stan Switzer  sjs@ctt.bellcore.com

   "NeWS -- The windowing system for the other half of your brain."
	-- Don Hopkins