Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!rayssd!trr
From: trr@rayssd.ray.com (Terry R. Raymond)
Newsgroups: comp.lang.smalltalk
Subject: Re: Smalltalk/V: adding new instance variable
Message-ID: <8353@rayssd.ray.com>
Date: 22 Jun 89 16:51:27 GMT
References: <17@piyopiyo.hatori.t.u-tokyo.JUNET>
Sender: trr@rayssd.ray.com (Terry R. Raymond @ Raytheon Company, Portsmouth RI)
Organization: Raytheon Company, Portsmouth RI
Lines: 24


If you look at the description of user-defined primitives in the
Accessing Object within primitives section of the manual, you will
see that it states that the instance variables in the object are in
the same order in which they are defined by the class definition. 
The primitives access the variables by knowing the "offset".  If you
add a new variable it could easily cause an error with a primitive that
uses an instance variable.

Compiled smalltalk code is "probably" very similar.  I assume that when
a method references an instance variable the compiler compiles the 
reference as an offset.  Therefore, when a method, that has been recompiled
against a class definition that has a new instance variable, references
an old instance, one without the new variable, the reference may be wrong.

The moral is it can be difficult to add a new instance variable to a class
that has instances, or to a class that has a primitive that references
instance variables.
-- 
Terry Raymond
Raytheon Submarine Signal Division; Portsmouth RI; (401)-847-8000 x5597
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