From: Joel VanderWerf <vjoel@...>
Date: 2003-04-08T09:54:58+09:00
Subject: Re: @@class variables and subclasses

Ryan Pavlik wrote:
> OK, this has been causing me problems:
> 
>     class A
>         @@index = Index.new(...)
>     end #c:A
> 
> 
>     class B < A
>         @@index = Index.new(...)
>     end #c:B
> 
> This has the unfortunate problem of causing @@index in class A to be
> replaced with the new one in class B, because @@class variables are
> shared between classes.  This is annoying.  In some cases I can use
> @instance variables for a class, but in this case I need to access the
> class index (@@index) in instance methods, and it's not an instance
> variable.  (I can't access class @instance variables from instance
> functions that I know of.)
> 
> Ideas?
> 

This is one approach, but it makes everything public, unlike @@index. 
(You could make the attr_accessor protected, though.)

class A
   class <<self; attr_accessor :index; end
   @index = "A's index"
end

class B < A
   @index = "B's index"
end

A.index
B.index

class A
   def index
     self.class.index
   end
end

A.new.index
B.new.index