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From: Natasha Kerensikova <lithiumcat@instinctive.eu>
Newsgroups: comp.lang.ada
Subject: Re: Should weak counted reference be able to be strengthened?
Date: Sat, 22 Nov 2014 09:57:05 +0000 (UTC)
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Date: 2014-11-22T09:57:05+00:00
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On 2014-11-21, Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
>>>>>> As a side note, I'm a bit surprised by how easy weak/strong reference
>>>>>> seem to be: I think I would only need to store two counters instead of
>>>>>> one, one for the strong references and one for all references,
>>>>>> deallocating the object when the strong reference counter reaches
>>>>>> zero, and freeing the counters when the total counter reaches zero -- as
>>>>>> if each weak reference was a strong reference to the counters. Then it's
>>>>>> only a matter of adding an atomic increase-if-not-zero to the strong
>>>>>> reference counter to build accessors from weak references.
>>>>>> Is there a subtle trap I'm missing?
>>>> 
>>>> [...]
>> 
>> In the example in the innermost quote, there is no direct notification,
>> in that there is no way to run arbitrary code when the referred object
>> disappear. However the information about object disappearance is
>> accessible to the weak reference: whenever the strong reference count is
>> zero,
>
> The reference count is a part of the object. When the object is gone, there
> is no reference count.
>
> Of course, you could add some levels of indirection to keep counters off
> the objects, but then you would have serious problems, e.g. like Ada tasks
> have when allocated and deallocated dynamically.

Having the counters separate from the object is actually the defining
point of implementation, and the only reason it exists in the first
place:

generic
   type Held_Data (<>) is limited priavte;
package Natools.References is
   [...]
private
   type Data_Access is access Held_Data;
   type Counter_Access is access Counter;

   type Reference is new Ada.Finalization.Controlled with record
      Count : Counter_Access := null;
      Data : Data_Acess := null;
   end record;
end Natools.References;

As far as I understand, this makes it possible to instantiate with
literally any Ada type, except incomplete ones. This makes it useful in
a niche not covered by any other implementation (that I know of).

However, what are the serious problems I'm supposed to have?
I have used this implementation with task types, but I have yet to
encounter any.


Natasha