From: "David A. Black" <dblack@...>
Date: 2005-11-27T06:44:00+09:00
Subject: Re: Nubish questions about syntax and gems

Hi --

On Sun, 27 Nov 2005, Sean O'Halpin wrote:

> This is a little test that (to me) shows how wacky the scoping rules
> are for class variables:
>
> class A
>  @@foo = "class variable foo"
>  @foo = "class instance variable foo"
>  def self.evaluate(txt)
>    instance_eval txt
>  end
>  def self.eval_block(&block)
>    instance_eval &block
>  end
> end
>
> begin
>  p A.instance_eval { @@foo }
> rescue => e
>  puts e
> end
>
> @@foo = "hi"
>
> p A.instance_eval { [self, @@foo, @foo] }
> p A.evaluate("[self, @@foo, @foo]")
> p A.eval_block { [self, @@foo, @foo] }
>
> class B
>  @@foo = "B's class variable foo"
>  @foo = "B instance foo"
>  def self.eval_block(&block)
>    instance_eval &block
>  end
> end
>
> p B.eval_block { [self, @@foo, @foo] }
> __END__
> uninitialized class variable @@foo in Object
> [A, "hi", "class instance variable foo"]
> [A, "class variable foo", "class instance variable foo"]
> [A, "hi", "class instance variable foo"]
> [B, "B's class variable foo", "B instance foo"]
>
> I'd be grateful if anyone can explain this behaviour (i.e. why the
> call to B.eval_block gets B's class variable but the call to A gets
> the outer scope).

What you've got basically is:

   class A
     @@foo = "A's class variable foo"
   end
   @@foo = "Object's class variable foo"
   class B
     @@foo = "B's class variable foo"
   end

Since B is a subclass of Object, setting B's @@foo also sets Object's
@@foo.  The reason A's @@foo (when you see it via the instance eval)
does not change is that if you create a subclass's class variable
*first*, and then the superclass's (in this case, A and Object,
respectively), they are separate:

irb(main):002:0> class A; end; class B < A; @@foo = 1; end; class A;
@@foo = 2; puts @@foo; end
2
=> nil
irb(main):003:0> class B; puts @@foo; end
1


David

-- 
David A. Black
dblack@wobblini.net