From: David Goodlad <dgoodlad@...>
Date: 2006-12-22T07:02:30+09:00
Subject: Re: Class Variable Confusion

On 12/21/06, Chunyun Zhao <chunyun.zhao@gmail.com> wrote:
> 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?

Since the block acts as a closure, the class variable references seem
to be bound to its original scope; iirc, the only variable types that
will change scope in the context of a class_eval (which changes the
value of 'self') are instance variables.  That's why the string will
evaluate correctly, while the block fails to find the class variable.

I'm not an expert, but this has been my experience.

Dave

>
> 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)
> >
> >
>
>


-- 
Dave Goodlad
dgoodlad@gmail.com or dave@goodlad.ca
http://david.goodlad.ca/