From: ara.t.howard@...
Date: 2007-03-28T08:27:01+09:00
Subject: Re: dynamically adding class variables

On Wed, 28 Mar 2007, Siemen Baader wrote:

>
> Could anyone enlighten me on why class_variables finds the class variable in 
> the subclass and why the class methods act like they were invoked on the 
> parent class? Apparently class_variables finds the variables in a different 
> way than ruby does when it evaluates the method code...

@@vars are shared among all child classes.  @var belong to only one class even
though methods which access them might be inherited - in otherwords you must
initialize them in each child.  ruby provides not built-in way to do this.

> Also, has anyone a good idea how I can give each child class variables which
> are shared among all instances of the classes but isolated from parents and
> other children?

traits and attributes both solve this problem completely.  traits.rb is a bit
heavy-weight and offers many other features.  attributes.rb is only 42 lines
of code but solves the issue you are dealing with.  i reccomend using
attributes.rb

   # gem install attributes
   http://codeforpeople.com/lib/ruby/attributes/attributes-3.2.0/README

to use with classes just do

   class Base
     class << self
       attribute(:a){ 42 }
     end
   end

   class Derived < Base
   end

the will setup both classes so that the default value of 'a' is 42 and it is
not shared, but it can be overriden easily too

   Derived.a = 42.0

traits info

   # gem install trails
   http://codeforpeople.com/lib/ruby/traits/traits-0.9.2/README

regards.

-a
-- 
be kind whenever possible... it is always possible.
- the dalai lama