From: Chunyun Zhao <chunyun.zhao@...>
Date: 2006-12-22T06:58:44+09:00
Subject: Re: Class Variable Confusion

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Seems it works perfectly fine when evaluating the string instead of block:

  class A
    @@avar = "hello"
  end

  A.class_eval("puts @@avar")  # => hello
  A.class_eval { puts @@avar } #=> throws NameError

Very interestsing, anyone could explain it?

On 12/21/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
>
> Hi --
>
> On Fri, 22 Dec 2006, gwtmp01@mac.com wrote:
>
> > I generally avoid class variables but at the last meeting of the New
> Haven
> > Ruby Brigade a simple question about them led to a long discussion and
> irb
> > session that only served to confuse us all more.
> >
> > I'm hoping someone on the list can shed some light on a couple of issues
> that
> > were raised:
> >
> >>> class A
> >>> @@avar = 'hello'
> >>> end
> > => "hello"
> >>> A.class_variables
> > => ["@@avar"]
> >>> A.class_eval { puts @@avar }
> > NameError: uninitialized class variable @@avar in Object
> >       from (irb):5
> >       from (irb):5
> >>> class A
> >>> puts @@avar
> >>> end
> > hello
> > => nil
> >>> class A
> >>> def get_avar
> >>> @@avar
> >>> end
> >>> end
> > => nil
> >>> a = A.new
> > => #<A:0x4ba360>
> >>> a.get_avar
> > => "hello"
> >>>   a.instance_eval { puts @@avar }
> > NameError: uninitialized class variable @@avar in Object
> >       from (irb):16
> >       from (irb):16
> >>>
> >
> >
> > It seems like a block evaluated by class_eval should have access to  the
> > class variables. Similarly, it seems like if an instance
> method  (get_avar in
> > the example) has access to the class variable then the  variable should
> also
> > be visible via instance_eval.
> >
> > In both examples above (class_eval and instance_eval) it seems like  the
> > class variable @@avar is being looked up relative to the  top_level
> object
> > and not relative to the class and instance objects  respectively.
> >
> >
> > What am I missing?
>
> You had me at "I generally avoid class variables" :-)
>
> Here's another one you'll like:
>
>    @@avar = 1
>    class A
>      @@avar = "hello"
>    end
>    puts @@avar  # => hello
>
>    A.class_eval { puts @@avar }  # => hello
>
> I love Ruby madly, but (or "therefore"? :-) I would be happy never to
> see another class variable again.  The confusingness-to-usefulness
> ratio is extremely high.
>
>
> David
>
> --
> Q. What's a good holiday present for the serious Rails developer?
> A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
>     aka The Ruby book for Rails developers!
> Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
> A. Ruby Power and Light, LLC (http://www.rubypal.com)
>
>

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