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!news.eternal-september.org!.POSTED!not-for-mail
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: strange cursor in nested containers
Date: Mon, 8 May 2023 22:56:30 -0500
Organization: A noiseless patient Spider
Message-ID: <u3cg9c$5ppl$1@dont-email.me>
References: <a346f13d-9876-488a-9472-4563de130311n@googlegroups.com> <6aa39f27-62ca-4486-9fc7-1c713da5b098n@googlegroups.com> <lyttxak3ag.fsf@pushface.org> <ed311a62-41ba-493e-a4cc-481402a1cc6bn@googlegroups.com> <u209li$37hob$1@dont-email.me> <f72b8843-6022-408d-bf90-56be02010b73n@googlegroups.com> <u2199a$3cm0v$1@dont-email.me> <9a1c3ba5-d5ae-41d3-9eab-2b8cd941bc66n@googlegroups.com>
Injection-Date: Tue, 9 May 2023 03:56:29 -0000 (UTC)
Injection-Info: dont-email.me; posting-host="36aee6c93b48d52a7a489ecbf0a77bfa";
	logging-data="190261"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/JSAqt4rW68GgU6mIiQNwwXw1i2Aa8vhw="
Cancel-Lock: sha1:yXdipKPa8+VNFpTK1w9ERBgGNHs=
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.7246
X-RFC2646: Format=Flowed; Original
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2900.5931
X-Priority: 3
Xref: news.eternal-september.org comp.lang.ada:65184
List-Id: <comp.lang.ada>

It's not required to detect such cursors because it is expensive to do so 
perfectly (essentially, each cursor has to be controlled and one has to keep 
a list of them associated with the container). There are imperfect solutions 
which are likely to be "good enough" in practice, but they still have a 
(small) cost which may be an issue for some uses. Thus, most implementers 
don't use them. I know the original GNAT containers had a debugging version 
that included such checks, but I don't know if they still exist or how one 
would enable them if they do.

            Randy.

"Mario Blunk" <mario.blunk.gplus@gmail.com> wrote in message 
news:9a1c3ba5-d5ae-41d3-9eab-2b8cd941bc66n@googlegroups.com...
>> > How could I solve the problem ? Not using "rename" ? Using an aliased 
>> > access type as John Barnes suggests in Ada2005 on page 275 ?
>> You have to avoid calling Element. Since you have a cursor, you can call
>> Query_Element, which should pass the actual element to the procedure you 
>> supply.
>
> Right, that solves the problem. For those interested please find a 
> comparison here:
>
> 1. The demo with a dangling (dangerous) cursor:
> https://github.com/Blunk-electronic/ada_training/blob/master/src/containers/demo/nested/nest_1.adb
>
> And the demo with the issue solved properly:
> https://github.com/Blunk-electronic/ada_training/blob/master/src/containers/demo/nested/nest_2.adb
>
> Still the question remains, why such a dangling cursor is not detected at 
> runtime.