Path: utzoo!attcan!uunet!husc6!mailrus!cornell!rochester!rocksanne!entire!elt
From: elt@entire.UUCP (Edward L. Taychert)
Newsgroups: comp.lang.smalltalk
Subject: Re: Specify New Classes in Smalltalk/V
Summary: if you don't understand the issues, use subclass
	 if you do, pick the correct one.
Message-ID: <3273@entire.UUCP>
Date: 21 Nov 88 19:03:21 GMT
References: <1361@aucs.UUCP> <3269@entire.UUCP> <1766@imagine.PAWL.RPI.EDU>
Distribution: na
Organization: Entire Inc, East Rochester, NY
Lines: 55

>In article <1361@aucs.UUCP>, 831059l@aucs.UUCP (Langlois) writes:
>> In article <3269@entire.UUCP> elt@entire.UUCP (Edward L. Taychert) writes:
>> ...
>> use variableSubclass, this means that object instance variable can reference
>> any kind of object.
>
> This is not, in general, good advice.  For most classes, you should choose
> just plain subclass.  This means that the instance variables of the instances
> of the class will all have names.  These named instance variables can each
> reference any kind of object.
>
> Use the variable* forms only for things like arrays which are going to
> maintain a list of members.  Even then, consider whether it wouldn't be
> better to have, say, an instance variable which always contains an array
> instead of indexed instance variables.

In general, I think this is true. If your object will never be referred to
like an array, "subclass" is correct. I recommended "variableSubclass" 
because it is the most general type of class. 

Using "variableSubclass" means that the object will respond to "new:",
"at:" and "at:put:". Using "subclass", it will not. However, as 
Langlois points out, you may implement these methods yourself 
within the class. Doing so, however, adds a level of
indirection which will slow you down `a little bit'. 

According to Digitalk customer support, there is no penalty in speed
or memory for using "variableSubclass", even if you don't use its indexing
capabilities. There is a potential risk, however, for using it blindly:
If you implement a variable subclass of a object which is not variable
and the super class implements (let's say) at:, the superclass's at: 
will not be inherited. I'm told that V286 changed class OrderedCollection
from "variableSubclass" to "subclass". Creating a variable subclass
of V286's OrderedCollection would probably be an error.

If you use "variableSubclass", the object may contain named instance
variables (pointers) of any object type and its `built in' array may
contain objects of any type also.

If you use "variableWordSubclass" or "variableByteSubclass:" the object
cannot contain named instance variables and the array elements cannot
refer to general objects, only words or bytes. Methods other than at: and
at:put: will contain lines like "self at: 7" to operate
on its contents.


-- 

____________________________________________________________________________

Ed Taychert				Phone: USA (716) 381-7500
Entire Inc.				UUCP: rochester!rocksanne!entire!elt
445 E. Commercial Street
East Rochester, N.Y. 14445 
_____________________________________________________________________________