From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
	ip-172-31-65-14.ec2.internal
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=3.0 tests=BAYES_00,T_SCC_BODY_TEXT_LINE,
	XPRIO autolearn=ham autolearn_force=no version=3.4.6
Path: eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Problems using Generic_Dispatching_Constructor
Date: Wed, 1 Jun 2022 16:25:16 -0500
Organization: A noiseless patient Spider
Message-ID: <t78lfu$rve$1@dont-email.me>
References: <9a5e9c8b-c263-4ac7-86f8-97a8ee1fdb99n@googlegroups.com> <t77mrv$1e20$1@gioia.aioe.org>
Injection-Date: Wed, 1 Jun 2022 21:25:18 -0000 (UTC)
Injection-Info: reader02.eternal-september.org; posting-host="e091d6e4097c0c85b8fa2a28aaea7cae";
	logging-data="28654"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18woE4jAfneyuAs8RlmFi9RxnoXG0M/GE8="
Cancel-Lock: sha1:JzjoWPnJ6bS/S9GTfA/WmxjXTWU=
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.7246
X-RFC2646: Format=Flowed; Response
X-Newsreader: Microsoft Outlook Express 6.00.2900.5931
X-Priority: 3
X-MSMail-Priority: Normal
Xref: reader02.eternal-september.org comp.lang.ada:63904
List-Id: <comp.lang.ada>

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:t77mrv$1e20$1@gioia.aioe.org...
> On 2022-06-01 13:36, Mark Lorenzen wrote:
>
>> Why are access types used at all?

We needed this usable to implement dispatching stream attributes (the 
generic dispatching constructor was intended to be a user-definable 
generalization of the mechanism of the class-wide stream attribute). The 
stream attributes probably used access types because "in out" parameters 
were not allowed for functions when they were invented. (So mistakes piled 
on mistakes. :-)

> Parameters are kind of factory object, you want to have the factory 
> mutable.

Right. For instance, consider a factory where each object gets a unique ids 
while being constructed. You would want to update the Next_Id component at 
the end of each construction.

>> Is there another workaround?
>
> In my practice I never had a case when I could obtain the tag needed for 
> generic dispatching constructor. All my designs ended up with a mapping
>
>    key -> constructing function
>
> with an explicit registering the type in the mapping.

Right. Generally, one uses a mapping of some sort of key or menu choice or 
whatever to tags. If you aren't adverse to a giant case statement, then you 
might as well call the constructor directly. (And if you are willing to use 
access-to-functions, you don't need OOP at all.) So this "factory" is mostly 
a bone for OOP purists.

The one exception is the case where you have an external tag as the key, 
since you can get the tag from that directly. But even that is really a 
mapping (one built by the implementation).

                          Randy.