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From: gt0460d@prism.gatech.EDU (DSOUZA,ROSARIO JOHN)
Newsgroups: comp.lang.eiffel
Subject: a problem in programming with Eiffel
Message-ID: <20022@hydra.gatech.EDU>
Date: 21 Jan 91 23:02:12 GMT
Sender: gt0460d@prism.gatech.EDU
Organization: Georgia Institute of Technology
Lines: 27

I have a curious problem. I am writing an application in Eiffel that
makes use of the graphics library. In particular, I need to use objects
that are similar to the class COMPLEX_FIG in the library, yet a little
different from it. My class is similar in the sense that it is to have
a list of figures and the whole list has to behave like a figure. But it
is different because the figures in the list are really going to be a
special type of figure that is a descendant of class FIGURE.

At first I thought that I could make my class a descendent of the class
COMPLEX_FIG and proceed from there. This is the most natural solution.
However, it turns out that COMPLEX_FIG inherits from FIG and LINKED_LIST,
and the generic parameter of the linked list is FIGURE. This causes a
problem because I need the generic parameter to be something that is 
capable of responding to feature calls of my special figure. This 
seemingly cannot be done because the generic parameter is hardwired,
so to speak, to the class FIGURE.

My question is: short of duplicating the code for COMPLEX_FIG with the
generic parameter of the inherited linked list now as my special figure,
is there any other better way to solve the problem?

I suspect that the problem is not just a matter of good library design,
but also a problem of language design. Could someone enlighten me?

..Ross D'Souza
Georgia Tech.
ross@cc.gatech.edu