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: subclasses of Float in Smalltalk/V
Summary: cannot do it!!
Message-ID: <8354@rayssd.ray.com>
Date: 22 Jun 89 17:08:49 GMT
References: <56225@linus.UUCP>
Sender: trr@rayssd.ray.com (Terry R. Raymond @ Raytheon Company, Portsmouth RI)
Organization: Raytheon Company, Portsmouth RI
Lines: 37


I tried to do it and this is what I found.  Apparently, the primitives
that perform the actual operations check the class of the object.  They
appear to test "is it a Float" not "is it kindof a Float" which would
permit subclasses.

This is what I did.

I created a class Degree as a variableByteSubclass of Float, just like
Float.  I created a class method "new:".

new: anInteger

^(Float fromInteger: anInteger) become: (self basicNew).

The method "become:" is in Behavior, it changes the class of the receiver
to the class of the argument.  The method "basicNew" is also in Behavior,
this method is never overridden by a subclass.  I then evaluated the 
following expressions.

(Degree new: 50) + 4.0      This produces a floating point exception error.

4.0 + (Degree new: 50)      This produces a stack overflow.

The stack over flow was produced because the "+" method trys to
convert the Degree instance to a float, i.e. it does not consider Degree
to be class Float.


I suggest that you look at the class Fraction and implement your class
in a similar way.
-- 
Terry Raymond
Raytheon Submarine Signal Division; Portsmouth RI; (401)-847-8000 x5597
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