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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Getting the index for an element in mutually referencing
 containers
Date: Sat, 11 Mar 2017 20:05:09 +0000
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Date: 2017-03-11T20:05:09+00:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On 2017-03-11 12:21, Simon Wright wrote:
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>>
>>> This looks like a clear case for reference counted objects. A handle
>>> to Person to be put into containers (for search) and into other
>>> objects for referencing. No problem with limited types. Weak handles
>>> are for backward referencing and so on.
>>
>> Does that mean reference-counted _handles_?
>
> Objects are reference-counted. Handles are not.
>
>> If you have multiple handles
>> to a particular Person object, which gets deleted, you'd want all those
>> handles to be invalidated.
>
> It is in essence the difference between weak and strong references. If
> you want a reference invalidated when the object vanishes that would
> be a weak reference. In order to access object through weak reference
> you first elevate it to a strong one. You drop the strong reference
> once you used the object.
>
> The semantics of "object deleted" is in question. Usually it is merely
> throwing it out of some list (container) and thus decreasing object's
> reference count. The object stays alive until nobody else uses it =
> nobody holds a strong reference to it.

That seems to be a very computer-oriented viewpoint. If your objects
represent real-world objects (or roles, or events - OOA view) then you
wouldn't want them to hang around! If you decide to abort an engagement
and therefore delete the Missile object, whose finalization sends a
self-destruct command to the corresponding missile in flight, you want
it to happen Right Now.