From: Martin Boese <boesemar@...>
Date: 2008-08-27T18:46:08+09:00
Subject: Re: Difficult Inheritance Problem

Hi,

I am using something like below for my program. I trick is to use methods 
instead of class variables as they persist through inheritance. 

Martin


class A
   def self.define_attr_method(name, value=nil)
     sing = class << self; self; end
     sing.class_eval "def #{name}; #{value.inspect}; end"
   end

   # Sets and/or returns current difference_mapping
   def self.difference_mapping(value = nil)
     self.define_attr_method('difmap', value) unless value.nil?
     self.difmap
   end
end

class B < A
  difference_mapping 'class B'
end

class C < B
end

class D < B
 difference_mapping 'class D'
end

puts B.difference_mapping
puts C.difference_mapping
puts D.difference_mapping


This will print: 
class B
class B
class D



On Wednesday 27 August 2008 09:08:43 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.
>
> Thanks in advance,
> Toby