Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!pdn!tscs!tct!chip
From: chip@tct.com (Chip Salzenberg)
Newsgroups: comp.lang.c++
Subject: Re: How to define a List class
Message-ID: <28539115.4D91@tct.com>
Date: 10 Jun 91 14:47:49 GMT
References: <350001@hpgnd.grenoble.hp.com>
Organization: Teltronics/TCT, Sarasota, FL
Lines: 23

According to jr@hpgnd.grenoble.hp.com (Jean-Ren BOUVIER):
>I'm looking for ideas in the classical area of "list"  packages.  ...
>
>I DON'T want to  consider  solutions  based on  templates  (List<Object>
>types) or generic classes - as I don't want to replicate code.

Well, then, you've got very few alternatives.  In C++, you cannot have
all three of minimal-code polymorphism, type safety and involiate base
classes.  That is because the three approaches to the downcasting
problem fall short in one way or another:

  1. Templates (sometimes implemented with the preprocessor) almost
     always result in some duplicate code.

  2. Brute-force downcasting completely trashes type safety.

  3. Virtual downcast functions require modifying the base class
     for each new derivation.

There is no magic bullet.  Choose your poison and make it work.
-- 
Chip Salzenberg at Teltronics/TCT     <chip@tct.com>, <uunet!pdn!tct!chip>
          perl -e 'sub do { print "extinct!\n"; }   do do()'