From: "David A. Black" <dblack@...>
Date: 2008-08-27T20:37:48+09:00
Subject: Re: Difficult Inheritance Problem

Hi --

On Wed, 27 Aug 2008, Toby Clemson wrote:

> Hi all,
>
> I have a problem I can't think of a solution to with regard to
> inheritence.
> I have an abstract class that others inherit from:
>
> class DataObject
>  class << self
>    attr_accessor :difference_mapping
>
>    ...
>  end
>  ...
> end
>
> The descendents of this class each define their own difference
> mapping:
>
> class Page < DataObject
>  self.difference_mapping = {:blah => :blah}
> end
>
> I would now like to inherit from the Page class but use its
> difference_mapping. Obviously this does not work because the
> attr_accessor in the DataObject uses a class instance variable that is
> unique to each class. I tried using a proper class variable
> (@@difference_mapping) but that is the same for all descending
> classes.
>
> Is there a way to define a class level variable that descends down the
> inheritance tree unless it is overridden in the same way as this is
> possible for methods? I mean I could wrap the data structure in a
> method but that does not seem like a very clean solution.

The easiest way I can think of is:

   class Page
     class << self
       attr_accessor :difference_mapping
     end
     def self.inherited(c)
       c.difference_mapping = difference_mapping
     end
   end

   Page.difference_mapping =  { :blah => :blah }

   class Next < Page
   end

   p Next.difference_mapping   # { :blah => :blah }


David

-- 
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