From: Gary Wright <gwtmp01@...>
Date: 2007-03-28T08:37:57+09:00
Subject: Re: dynamically adding class variables


On Mar 27, 2007, at 6:50 PM, Siemen Baader wrote:
> I am writing an object relational mapper for an educational project  
> where I need to add new class variables to classes in an  
> inheritance hierarchy. The variables hold class-specific  
> information such as  which tables are mapped and validation logic  
> so they cannot be shared among the hierarchy like inherited class  
> variables from the superclass would. Because of that I could not  
> declare them in the parent and just initialize them in the  
> subclasses. Instead, I decided to add and set them at runtime after  
> subclassing.
>
> However, i think ruby behaves a bit odd here. In class methods  
> inherited from the superclass I can see the class variables through  
> the method class_variables, but to use them in the child class I  
> have to re-open the children and re-define the class methods which  
> use the variables!

Don't use Ruby 'class variables' use Ruby 'class instance variables'.

Programmers coming to Ruby and expecting per class state often gravitate
towards the mis-named 'class variables' when what they *really* want are
'class instance variables'.

I fault the misleading name and explanations in Pickaxe and The Ruby Way
for this never ending source of confusion.  For example on p. 33 of
the Pickaxe:

"Sometimes classes themselves need to have their own states.  This is
where class variables come in."

And on page 3 and 4 of The Ruby Way is a several paragraph  
description of
per class state that consistently uses the term 'class variable' when,
with respect to Ruby, 'class instance variable' is more applicable.

Gary Wright