From: Paul <prcorcoran@...>
Date: 2006-08-21T06:25:08+09:00
Subject: Re: Class variables in Ruby

Jan and David:

Yes, this makes perfect sense now that you mention it. I had not seen
anything written up on it yet. Thanks for the examples.

Paul

Jan Svitok wrote:
> 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.