Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!bellcore!texbell!uhnix1!sugar!ficc!cliff
From: cliff@ficc.uu.net (cliff click)
Newsgroups: comp.lang.smalltalk
Subject: Re: Smalltalk/V: adding new instance variable
Summary: "Has instance"
Message-ID: <4630@ficc.uu.net>
Date: 20 Jun 89 13:15:07 GMT
References: <17@piyopiyo.hatori.t.u-tokyo.JUNET>
Organization: Ferranti International Controls
Lines: 25

In article <17@piyopiyo.hatori.t.u-tokyo.JUNET>, ott@piyopiyo.hatori.t.u-tokyo.JUNET (ott) writes:
> Using Smalltalk/V. Why is it not possible to add a new instance
> variable to a class when it has already some?
> 
> I get the error message 'Has instance' ...

If you have any instances of that class floating around and your trying to
add or delete fields from the instance Smalltalk would have to find each
occurance of objects of this class and "edit" them somehow to reflect the
new declaration for the class.  IMHO it should do this, keeping any 
original data that it can, tossing deleted fields and adding the new 
fields with nils - this won't be a fast operation, but it would be a darn 
sight more convenient than "Has instance".  

There is a way to get Smalltalk to put all of the objects which have a 
pointer to these instances in a dictionary.  You can then search each 
object in the dictionary looking for instances of your class in them, and 
replacing those instances with nil.  When you are done you will have 
destroyed all references to the class's objects, so those objects will go 
away, and you will no longer "have instances".

-- 
Cliff Click, Software Contractor at Large
Business: uunet.uu.net!ficc!cliff, cliff@ficc.uu.net, +1 713 274 5368 (w).
Disclaimer: lost in the vortices of nilspace...       +1 713 568 3460 (h).