From: "David A. Black" <dblack@...>
Date: 2008-03-21T10:49:38+09:00
Subject: Re: including instance methods and setting an instance variable

Hi --

On Fri, 21 Mar 2008, Leon Bogaert wrote:

> Hi all,
>
> I'm trying to include a module and in that module I want to set an
> instance variable. I have the following:
>
> class Queue
>  include LBo::InstanceMethods
> end
>
> module LBo
>  module InstanceMethods
>    attr_accessor :position
>    self.position = 1

self at this point is the Module object LBo::InstanceMethods. You're
calling the #position method on that Module object.

>  end
> end
>
> q = Queue.new
> q.position
>
> But I'm getting this error:
> undefined method `position=' for LBo::InstanceMethods:Module
> (NoMethodError)

See above.

> I'm feeling like a moron because I can't understand why this is not
> working.
> I tried setting the @position variable directly, but when I do
> q.position it gives back nil.
>
> Could somebody lend me a hand with this one?

It's mostly about 'self'. You have to be aware of what self is when
you use it. Inside a module definition, it's the Module object itself.
Inside an instance method, it's the instance.

The job of the module is (mainly) to define instance methods that will
be executed by objects yet unborn. The module itself has no direct
access to the instance variables of those objects. But it can define
methods that do things with them.

For example:

   module LBo
     module InstanceMethods
       attr_writer :position
       def position
         @position ||= 1
       end
     end
   end

   class C
     include LBo::InstanceMethods
   end

   C.new.position    # 1

Here, I'm writing an instance method #position, which when executed
with a C instance as 'self' will set the appropriate instance
variable if that variable isn't set already.


David

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