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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: discriminant questions
Date: Fri, 23 Sep 2011 17:42:39 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news1.google.com comp.lang.ada:18106
Date: 2011-09-23T17:42:39-05:00
List-Id: <comp.lang.ada>

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:xumuqaannoov$.18le1ojsi30l8.dlg@40tude.net...
> On Fri, 23 Sep 2011 05:23:38 -0400, Stephen Leake wrote:
>
>> "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.
>>
>> No, it will produce an invalid dereference; for a one element circular
>> list, This.Next = This, so Next is invalid after This.Delete.
>
> If the element is only in this list and freed upon delete, then Next is
> dangling.
>
> However a common schema is that elements of lists are actually never 
> freed,
> but rather moved to the list of unused element to reuse, when a new 
> element
> is needed. For this implementation the above loops.

In that case, the implementation of cursors is broken (IMHO) - dangling 
cursor detection should have been manditory, and surely should be in the 
case of element reuse. So Program_Error should be raised in either case, 
nothing erroneous or looping here.

[Note that we are talking about containers, so I'm assuming that these 
things are cursors. If you are using raw access types in the user interface 
of any sort of container, you are living in the 1990s and are purposely 
destroying all of the real advantages of container usage.]

                                   Randy.