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
	autolearn=ham autolearn_force=no version=3.4.6
Path: eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: strange cursor in nested containers
Date: Wed, 19 Apr 2023 16:07:00 +0100
Organization: A noiseless patient Spider
Message-ID: <lyttxbc3mz.fsf@pushface.org>
References: <a346f13d-9876-488a-9472-4563de130311n@googlegroups.com>
MIME-Version: 1.0
Content-Type: text/plain
Injection-Info: dont-email.me; posting-host="40e9c7e81895f811b59dc5c65377c5ce";
	logging-data="107378"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+rWU677ltPiDnS9nSYQhlkPmNcTlsD9Pg="
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.1 (darwin)
Cancel-Lock: sha1:Y0Dr5fCRr7N9Qi2azng//+/uFf8=
	sha1:4vvgL6cLWmLfzn0kVJ+fbYWaMHo=
Xref: news.eternal-september.org comp.lang.ada:65114
List-Id: <comp.lang.ada>

Mario Blunk <mario.blunk.gplus@gmail.com> writes:

> I've put together a demo to reproduce a strange behavior of a cursor.
>
> https://github.com/Blunk-electronic/ada_training/blob/master/src/containers/demo/nested/nest_1.adb
>
> Description:
> Inside the procedure "get_line" the cursor "result" is assigned only
> once (on match). Since there is no further match, in the course of the
> procedure, I assume that "result" is never changed. So the expected
> output should be:
>
> net: A
> L1 S: 2.00000E+00 E: 3.00000E+00
> L2 S: 2.00000E+00 E: 3.00000E+00
> net: B
> L2 S: 2.00000E+00 E: 3.00000E+00
> L3 S: 2.00000E+00 E: 3.00000E+00
>
> but the actual output is:
>
> net: A
> L1 S: 2.00000E+00 E: 3.00000E+00
> L2 S: 2.00000E+00 E: 3.00000E+00
> net: B
> L2 S: 1.20000E+01 E: 1.30000E+01
> L3 S: 7.47956E-39 E: 0.00000E+00 -- changes randomly

This is a puzzler.

To simplify matters (for me, anyway) I changed your doubly linked list
to a vector. Now, instead of junk, I get an access violation.

'result' isn't changed, but a Cursor contains a pointer to the container
and an index; using gdb to look at the container via the unchanged
pointer, it's been overwritten with garbage. However, looping with this
(after the Nets.Iterate call, i.e. after Result has been damaged)

      for N of Nets loop
         for TL of N.Lines loop
            Put_Line (To_String (TL));
         end loop;
      end loop;

works just fine.

My head hurts.