From: Joel VanderWerf <vjoel@...>
Date: 2006-10-17T22:56:13+09:00
Subject: Re: metaprogramming surprise?

Artur Merke wrote:
> Jan Svitok schrieb:
...
>> There's a difference between class variables (@@x) and singleton class
>> instance variables (self.class.@x). They are not the same thing.
> 
> OK, but why are they not the same???

A class variable is not an instance variable of any instance, not even a 
class. It has its own special semantics of sharing that is quite 
different from the privacy of an instance variable.

class A
   @@y = "A y"
end

class B < A
   @@x = "B x"
   @@y = "B y"
end

class A
   @@x = "A x"
end

class A
   p [@@x, @@y] # ==> ["A x", "B y"]
end

class B
   p [@@x, @@y] # ==> ["B x", "B y"]
end

Note that the fact that @@x was assigned first in a subclass makes it 
distinct in A and B, but the fact that @@y was assigned first in the 
superclass makes it shared.

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407