From: Frederick Cheung <frederick.cheung@...>
Date: 2008-08-27T19:08:32+09:00
Subject: Re: Difficult Inheritance Problem


On 27 Aug 2008, at 10:08, Toby Clemson wrote:
>
> 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.
>

two possible approaches are rails' class_inheritable_accessor and  
superclass_delegating_accessor.

Both lean on class instance variables. In the case of  
class_inheritable_accessor the attributes are store in a hash and  
self.inherited is hooked to copy that hash over when a subclass is  
created

superclass_delegating_accessor looks for an appropriate instance  
variable and if it does not exist calls the superclass (stopping when  
it gets to the the class that created the  
superclass_delegating_accessor). Writing however always writes to an  
instance variable of the class being changed. I put some more detail  
about them at http://www.spacevatican.org/2008/8/19/fun-with-class-variables

Fred
> Thanks in advance,
> Toby
>