From: Brian Candler <B.Candler@...>
Date: 2007-04-11T05:33:19+09:00
Subject: Re: What happens to instance variables in modules when they are extended into classes

On Tue, Apr 10, 2007 at 11:25:06PM +0900, me@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.