From: dblack@...
Date: 2006-03-20T04:38:52+09:00
Subject: Re: simple ruby language question

Hi --

On Mon, 20 Mar 2006, MichaelEconomy@gmail.com wrote:

> ok, i've got some module and it has some instance variables i want to
> be set by the classes extending the module, and aparently i'm not doing
> this correctly:
>
>
> module A
>  def test
>    raise 'ARG' if !@blah
>    @blah
>  end
> end
>
> class B
>  include A
>  extend A
>  @blah = 'YES!'
> end
>
>
>
> irb(main):025:0> b = B.new
> => #<B:0x136376c>
> irb(main):026:0> b.test
> RuntimeError: ARG
>        from (irb):9:in `test'
>        from (irb):26
>
>
> can anyone tell me what the syntax should be? how do i set @blah in the
> B class so that its available to the test method

When using instance variables, you have to match each one precisely to
the object whose instance variable it is.  The @blah in @blah = 'YES!'
belongs to the class object B.  The @blah inside test belongs (or will
belong, upon execution) to whatever object is calling "test".  Unless
that object is the class object B, its @blah will be different (even
if it's an instance of B).

So if you want them to match up, you have to call test on the class
object B:

   B.test

That will run "test" with B as the receiver, so you'll see B's
instance variable @blah.  (By the way, it's a good idea not to name
your test methods "test", because there's already a method with that
name in Kernel, so sometimes you'll get behavior you don't expect.
That's not the issue here, though.)


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

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