Subclassing

FROM : K. Darcy Otto
DATE : Fri Apr 18 23:56:42 2008

I am working on a program with a complex hierarchy of classes, and I 
want to subclass one of the objects a few steps down on that 
hierarchy.  Do I have to create a parallel hierarchy to do this?  Here 
is the problem:

AppController instantiates a Deduction object.
The Deduction object instantiates a DeductionLine object.
The DeductionLine object instantiates a Dependency object.

I want to subclass Dependency and override a few things.  Can I do 
this without modifying DeductionLine?  As it stands, I think I'm going 
to have to do this:

AppController instantiates a SDeduction object (where SDeduction is a 
subclass of Deduction).
The SDeduction object instantiates a SDeductionLine object (where 
SDeductionLine is a subclass of DeductionLine).
The SDeduction object instantiates a SDependency object (where 
SDependency is a subclass of Dependency).

The only reason to subclass Deduction (with SDeduction) and then 
DeductionLine (with SDeductionLine) is to get an SDependency object. 
Is there an easier way?  I suppose I could just copy the Dependency 
class files and then modify it for this particular project; but I was 
hoping for a different way.  Thanks.