From: Artur Merke <am@...>
Date: 2006-10-17T22:00:12+09:00
Subject: Re: metaprogramming surprise?


Jan Svitok schrieb:

> On 10/17/06, Artur Merke <am@artbot.de> wrote:
> > Here comes some simple meta code
> >
> > >>>>>>>>>>>>>>>>>>>>>>>>>
> > class D
> >   @@e=5
> >   @@f = 10
> >   @@g = 20
> >   class << self
> >     attr_reader :e
> >     attr_accessor :g
> >   end
> >   def self.f
> >     @@f
> >   end
> >
> >   def show
> >     print "\nshow @@e=",@@e, " @@f=", @@f, " @@g=",@@g
> >     print "\nshow self.class.e=", self.class.e
> >     print "\nshow self.class.f=", self.class.f
> >     print "\nshow self.class.g=", self.class.g
> >   end
> > end
> >
> > D.g= 40
> > print "\nD.e= ",D.e
> > print "\nD.g= ",D.g
> > print "\nD.f= ",D.f
> > D.new.show
> > <<<<<<<<<<<<<<<<<<<<<<<<<<<
> >
> > here is the result
> >
> > D.e= nil
> > D.g= 40
> > D.f= 10
> > show @@e=5 @@f=10 @@g=20
> > show self.class.e=nil
> > show self.class.f=10
> > show self.class.g=40
> >
> > I'would expect
> > D.e= 5
> > D.g= 40
> > D.f= 10
> > show @@e=5 @@f=10 @@g=40
> > show self.class.e=nil
> > show self.class.f=10
> > show self.class.g=40
> >
> > so what's wrong with my understanding, that
> >
> > class D
> >   @@e=5
> >   class << self
> >     attr_reader :e   # <- why this does not refer to @@e
> >   end
> > end
>
> 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???

I'm missing some kind of analogy, see the following code:

>>>>>>>>>>>>>>>>>>>

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

class F
  def show; print "\noutput @@ff= ",@@ff; end
end

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

class << F
  attr_accessor :ff
end


F.ff= "should be @@ff?, but is not"
obj.e= "should be @e, and really is"

obj.show
F.new.show

output @e= should be @e, and really is
test.rb:33:in `show': uninitialized class variable @@ff in F
(NameError)
	from test.rb:50