From: Toby Clemson <tobyclemson@...>
Date: 2008-08-27T18:08:43+09:00
Subject: Difficult Inheritance Problem

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.

Thanks in advance,
Toby