From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!news.eternal-september.org!feeder.eternal-september.org!nntp-feed.chiark.greenend.org.uk!ewrotcd!newsfeed.xs3.de!io.xs3.de!news.jacob-sparre.dk!franka.jacob-sparre.dk!pnx.dk!.POSTED.rrsoftware.com!not-for-mail
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Smart Pointers and Tagged Type Hierarchies
Date: Mon, 31 Jul 2017 22:43:37 -0500
Organization: JSA Research & Innovation
Message-ID: <olotda$sqc$1@franka.jacob-sparre.dk>
References: <2017072417413775878-contact@flyx.org>
Injection-Date: Tue, 1 Aug 2017 03:43:38 -0000 (UTC)
Injection-Info: franka.jacob-sparre.dk;
 posting-host="rrsoftware.com:24.196.82.226";
	logging-data="29516"; mail-complaints-to="news@jacob-sparre.dk"
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2900.5931
X-RFC2646: Format=Flowed; Response
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.7246
Xref: news.eternal-september.org comp.lang.ada:47536
Date: 2017-07-31T22:43:37-05:00
List-Id: <comp.lang.ada>

"Felix Krause" <contact@flyx.org> wrote in message 
news:2017072417413775878-contact@flyx.org...
...
> A problem I encounter is how this can be used with type hierarchies i.e. I 
> have a smart pointer managing a tagged type, and I want to be able to 
> derive from that tagged type and still be able to use my smart pointer 
> with that new type.

What's needed is something like "co-derivation", where multiple types are 
derived in lock-step. Sadly, this is effectively impossible in an OOP 
environment, because dispatching calls would have the wrong parameter types 
for any co-derived types. (I've spent quite a bit of effort in trying to 
make such a solution work, and have enlisted others to help, but the 
conclusion was that it wasn't promising.)

One would have to have some form of multiple dispatch to get around that, 
but that's a bridge too far for a language that's mainly used in embedded, 
safety-critical systems.

One could make it work for non-tagged types, but of course that prevents 
dispatching and extension. So it's probably not worth the effort to design.

                                       Randy.