Re: Clean Object Class Design -- Circle/Ellipse

>Since Smalltalk *has* instances that are values (the number
>5 comes mind)

Hmmm... I have been looking for Smalltalk's definition of the term "instance". So far, all of the definitions seem to equate the term instance with the concept of variable:

From http://www.wi2.uni-erlangen.de/sw/smalltalk/glossary.html

instance: An object that is a single occurrence of a particular class. An instance exists in memory or external media in persistent form.

From
http://wuarchive.wustl.edu/languages/smalltalk/Smalltalk/faq/Definitions%20o f%20Terms.html

[An instance] holds data and a pointer to the class which holds the code.

From http://www.objs.com/x3h7/smalltalk.htm

The instance of a class is created by sending a new message to the class which uniquely identifies the object instance and allocates space for its variable(s).

and

The state of a Smalltalk instance is represented by the values of its instance variables.

and

Smalltalk object instances are created with the "new" method. Each object instance is given a unique identifier called an object pointer or object reference.

From http://www.fbk.eur.nl/FBK/VG1/JVH/OO/Smalltalk.htm

all instances respond to the same set of messages and have the same number of named index variables

I know what it means for a variable to have a specific value, but I do not know what it means for a value to have variables. Values, after all, do not change. The value, 5, for instance, never changes and exists independent of Smalltalk.

One can represent a value in memory or in storage, but the value exists independent of the representation. Memory and storage are variable -- even hardwired ROMs are variable at some level.

You aren't, by any chance, confusing "value" with "constant" are you? The ISO standard vocabulary for programming languages basically defines a constant as a variable constrained to hold a single unchanging value.

In the sense of a variable constrained to a single unchanging value, an instance could be a constant but arguments surrounding setFoci and LSP would no longer apply. It would be an error either at compile time or at runtime to attempt to change the value of a constant using an update operation like setFoci. Received on Mon Aug 27 2001 - 06:28:03 CEST