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From: chip@tct.com (Chip Salzenberg)
Newsgroups: comp.lang.c++
Subject: Re: About Lists and things...
Message-ID: <286B8088.54A4@tct.com>
Date: 28 Jun 91 18:31:35 GMT
References: <19131@prometheus.megatest.UUCP> <28692A4A.59B7@tct.com> <1991Jun27.095856.2@minerva.inesc.pt>
Organization: TC Telemanagement, Clearwater, FL
Lines: 41

According to fmhv@minerva.inesc.pt (Fernando Manuel Vasconcelos):
>In article <28692A4A.59B7@tct.com> chip@tct.com (Chip Salzenberg) writes:
>
>>Of course, [a CircleList] may not be practical, perhaps because there are
>>other non-Circle objects in the ShapeList.  If so, congratulations!
>>You have just discovered that the function in question is actually a
>>Shape function in disguise.  You should therefore add it to the Shape
>>interface, presumably as a new virtual function.
>
>1. A pratical one: You may not be able to change shape.h because it belongs
>to a library which you have only in binary ( of course you have the .h's but
>you can't change them ... )

The best way to solve this problem is to avoid it: get source code.
My personal choice is never to derive from a class unless I have
source code for it.  Instead, I compose (create new classes with the
binary-only classes as members).

>2. A conceptual one: That means the interface of a base class depends on the
>interface of the derived classes.

My policy simply acknowledges this obvious fact: as a programmer gains
experience in deriving from class X, her insight into the nature of
class X deepens.  Such insight can lead her to change class X in
accord with her newly gained experience.

To return to Shapes and Circles: A ShapeList can contain anything
derived from Shape, including classes not yet invented at compile
time.  Any code that works with a ShapeList of necessity deals only
with the Shape interface.

Therefore, for you to perform a particular operation on the elements
of a ShapeList, it is apparent that the operation in question must (or
should) be a part of the Shape interface.  If such is not the case,
then the code needs to be recast so the operation in question is not
applied to a ShapeList, but rather to a more specialized collection
such as a CircleList.
-- 
Chip Salzenberg at Teltronics/TCT     <chip@tct.com>, <uunet!pdn!tct!chip>
 "I want to mention that my opinions whether real or not are MY opinions."
             -- the inevitable William "Billy" Steinmetz