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=-0.9 required=5.0 tests=BAYES_00,FORGED_GMAIL_RCVD,
	FREEMAIL_FROM autolearn=no 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!newsfeed.fsmpi.rwth-aachen.de!newsfeed.straub-nv.de!news-1.dfn.de!news.dfn.de!news.informatik.hu-berlin.de!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: Mart van de Wege <mvdwege@gmail.com>
Newsgroups: comp.lang.ada
Subject: Getting the index for an element in mutually referencing containers
Date: Thu, 09 Mar 2017 14:45:09 +0100
Message-ID: <86o9xa36oq.fsf@gaheris.avalon.lan>
Mime-Version: 1.0
Content-Type: text/plain
X-Trace: individual.net CotyNzPEWouAKNll/VeK+wDh6S8mAlnOQAn1mFCRvw567WlFu/
X-Orig-Path: gaheris.avalon.lan!not-for-mail
Cancel-Lock: sha1:B+G/wKqYhTvoC+XFFmevY7Up10g=
 sha1:aKCBUKDUst6BXODQeUaO9+AG4cc=
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)
Xref: news.eternal-september.org comp.lang.ada:33508
Date: 2017-03-09T14:45:09+01:00
List-Id: <comp.lang.ada>


I am missing another thing: I managed to boil down the problem to the
PoC code below. I want to have two objects, each with a container as a
record attribute, referring to each other, and then merely get the index
value of one of the two containers. Yet if I do, I get a Storage_Error
exception.

Code:

with Ada.Containers.Indefinite_Vectors;
with Ada.Containers.Indefinite_Doubly_Linked_Lists;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
procedure Test_Container is
   type Abstract_Person is abstract tagged null record;
   package Offspring is new Ada.Containers.Indefinite_Vectors
     (Index_Type => Positive,
      Element_Type => Abstract_Person'Class);
   package Group is new Ada.Containers.Indefinite_Doubly_Linked_Lists
     (Element_Type => Abstract_Person'Class);
   type Person is new Abstract_Person with record
      Children : Offspring.Vector := Offspring.To_Vector(4);
      Parents : Group.List;
   end record;
   Father : Person;
   Child : Person;
begin
   Child.Parents.Prepend(Father);
   Father.Children.Append(Child);
   Put(Integer(Father.Children.Find_Index(Child)));
end Test_Container;

This compiles OK, but on runtime throws an exception:

raised STORAGE_ERROR : stack overflow or erroneous memory access

Now, cyclic dependencies are tricky, so I am sure I am doing something
wrong, but I can't for the life of me see what.

Mart

-- 
"We will need a longer wall when the revolution comes."
    --- AJS, quoting an uncertain source.