From: Jan Svitok <jan.svitok@...>
Date: 2006-08-20T08:34:53+09:00
Subject: Re: Class variables in Ruby

On 8/19/06, Paul <prcorcoran@comcast.net> wrote:
> Hi Gary,
>
> I'm not quite sure what you mean by using "an instance variable of the
> class object". I am familiar with class variables and instance
> variables. In my case I wanted to cache the data in a class variable
> because *any* instance could reference the single class variable. Is
> there yet another type of variable (other than temporary method scope
> variables) that one could define?
>
> Thanks,
> Paul

This comes from Ruby's object model (and this may sound familiar to
you as a smalltalker): in Ruby, everything is an object, so any class
is an object as well. A class is an instance of class Class. Therefore
any class can have its instance variables (because it is an object) in
addition to class variables (class' special behaviour).

Class variables (@@variable) are shared among all sublasses of the class.

Class instance variables are those, that are defined in class methods i.e.

class A
  def self.m()
     @a = 1 # <- HERE!
  end
end

or:

class A
  class << self
    def m()
        @a = 1 # <- HERE!
    end
  end
end

These are not shared among subclasses.

HTH,
J.