From: me@...
Date: 2007-04-11T17:05:07+09:00
Subject: Re: What happens to instance variables in modules when they are extended into classes

On Apr 10, 9:33 pm, Brian Candler <B.Cand...@pobox.com> wrote:
> On Tue, Apr 10, 2007 at 11:25:06PM +0900, m...@mikehogan.net wrote:
> > Why is it that an instance method of the module gets mixed into the
> > class, but instance variables of the module do not get mixed in?
>
> > Whats the best strategy for writing instance methods in a module that
> > depend on some state in the mixee?
>
> > Thanks very much,
> > Mike.
>
> > And now here is the code snippet:
>
> > module M
> >   @module_instance_variable = 'miv'
>
> >   def module_instance_method
> >     @module_instance_variable
> >   end
> > end
>
> > class C
> >   extend M
> > end
>
> Presumably you understand why the following code prints 'nil':
>
> class C
>   @foo = "hello"
>
>   def bar
>     puts @foo
>   end
> end
> C.new.bar
>
> So I'm not sure what you're trying to achieve with your code snippet. Do you
> want @module_instance_variable to be an instance variable of module M, or an
> instance variable of class C, or an instance variable of instances of class
> C?
>
> What you have at the moment is an instance variable of M, as shown by:
>
> puts M.module_eval { @module_instance_variable }
>
> If you want to set up instance variables of class C itself (remembering that
> many different C's may each extend M), then one approach is
>
> module M
>   def self.extended(obj)
>     puts "hello!"
>     obj.instance_eval { @miv = "miv" }
>   end
> end
>
> class C
>   extend M
> end
>
> puts C.instance_variables
>
> HTH,
>
> Brian.

Ok - yes thats twigged it for me.  Thanks very much Brian.

Mike