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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: discriminant questions
Date: Sat, 24 Sep 2011 13:54:26 +0100
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Date: 2011-09-24T13:54:26+01:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> Consider a generic implementation of removing all elements of the
> container using iterators:
>
>    This := Container.First;
>    while This /= Null_Iterator loop
>        Next := This.Next;
>        This.Delete;
>        This := Next;
>    end loop;
>
> This may work or not depending on the semantics of Delete. E.g. for a
> circular list, it will loop forever.

I'm not sure whether to be amused by the fact that for
BC.Containers.Rings (which are logically circular), which offer

   procedure Delete_Item_At (It : in out Ring_Iterator);

you would get an exception Not_Yet_Implemented :-)

Clearly no one has encountered this in practice; I don't feel especially
motivated to fix it (mainly because a Ring has the concept of Top and
Mark, and I don't have an intuitive grasp of what should happen to Top
or Mark if either happens to be the one that's deleted!)

This particular Container seems to me to be a prime example of adding a
feature to a library for theoretical reasons rather than because someone
has a reasonable need for it and therefore a use case. My own use of the
BC containers has involved Collections (like Vectors) (plain and
ordered), Maps, Queues (plain and ordered), Sets and on one occasion
Bags; no Deques, Rings, Stacks, Graphs, Trees of any flavour ...