From: dblack@...
Date: 2006-09-27T08:44:28+09:00
Subject: Re: Modules, instance methods and class methods

Hi --

On Wed, 27 Sep 2006, Vincent Fourmond wrote:

>
>  Hello !
>
>  I have a rather tortured architecture where several classes should
> share code for some class methods and for some instance methods. To fix
> the ideas, I need that some class I define have:
>
> def ThisClass.desc
>  return @desc
> end
>
> def desc
>  return self.class.desc
> end
>
>  To do that, I define two modules
>
> module BaseInclude
>  def desc
>    return self.class.desc
>  end
> end
>
> module BaseExtend
>  def desc
>    return @desc
>  end
> end
>
>  and I include them with
>
> class ThisClass
>  include BaseInclude
>  extend BaseExtend
> end
>
>  That works perfectly fine. My question, then, is : is there a simpler
> way ? I can't afford to have a shared ancestor for these classes.

Personally, I like your approach: you've grouped methods according to
plan, and then carried out the plan.  There are more elaborate ways,
but this seems to work for you, and it's very clear.

Just to point you to a couple of possible variations:

You could make the module that's destined for the extend operation a
nested module inside the other.  In fact, you'll sometimes see this:

   module Something
     def a_method
     end

     module ClassMethods
       def some_class_level_method
       end
     end
   end

followed by:

   class MyClass
     include Something
     extend Something::ClassMethods
   end

(And you can even hook the extend operation into the include operation
by defining an appropriate self.included hook method in Something, so
that you don't have to have a separate extend call.)

I use the name "ClassMethods" advisedly: it's a popular choice.  It
has the disadvantage of being inaccurate, in the sense that the
methods inside it are instance methods.  But as long as they're
destined to be class methods (which, after all, are really instance
methods of the singleton class of a class!), it might be a reasonable
name.


David

-- 
                   David A. Black | dblack@wobblini.net
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org