From: Artur Merke <am@...>
Date: 2006-10-18T00:05:09+09:00
Subject: Re: metaprogramming surprise?


Joel VanderWerf schrieb:

> 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
thanx, now I understand.

I also recovered my analogy

class E
  def show; print "\noutput show= ",@e; end
end

class F
  def self.show; print "\noutput F.show= ",@f; end
  def show
     print "\noutput @f= ",@f
  end
end

obj= E.new
class << obj
  attr_accessor :e
end

class << F
  attr_accessor :f
end

F.f= "should be 'F.@f', and really is"
obj.e= "should be @e, and really is"

obj.show
F.show
F.new.show

<<<<<<<<<<<<<<<<<<<

results in (the now expected)

output show= should be @e, and really is
output E.show= should be 'F.@f', and really is
output @f= nil


So one has to distinguish between using a 'class variable' and
'class instance variable':

class A
   @@class_variable
   @class_instance_variable
   def initialize
       @instance_variable
   end
end

I hope this is the correct naming for this variables