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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Amount of copying on returned constant objects
Newsgroups: comp.lang.ada
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Date: Wed, 20 Jun 2007 09:34:06 +0200
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Date: 2007-06-20T09:31:17+02:00
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On Tue, 19 Jun 2007 16:33:26 -0500, Randy Brukardt wrote:

> "Alex R. Mosteo" <devnull@mailinator.com> wrote in message
> news:5dpp8pF35116bU1@mid.individual.net...
>> Randy Brukardt wrote:
> ...
>>> What really would help would be a way for the container to know when the
>>> access was destroyed, but there isn't any obvious way to do that in Ada.
>>
>> I guess then that some reference counting companion type (or maybe making
>> Cursors tagged and more heavyweight) was discarded because the distributed
>> overhead?
> 
> Cursors can be tagged if the implementation so choses, but that doesn't have
> an effect on the element access problem. For that you need something that
> allows direct dereferencing, and in Ada as it stands, that can only be an
> access type.
> 
> Having a companion type would work if it was impossible to separate the
> access from the reference counter. But there is no way in Ada to have a
> visible access type that cannot be assigned out of its surrounding wrapper.
> (Which is why I said that Dmitry would say that redefinition of ".all" for a
> private type could solve the problem.)

Actually it would be only a half of the solution. The access type for which
.all gets overridden is an implementation of the cursor. The public view
of, should be a derived type of Element:

                      --- This is far not Ada!
   type Cursor is new Element with private;
      -- I can do with a Cursor anything I could do with an Element

   function Get (Collection : Container, What : Key) return Cursor;
      -- Factory

private
   type Cursor is access Element with interface Element;
      -- Privately Cursor is an access type that implements the interface
      -- of Element. It could be also a fat record rather than access, with
      -- a reference to the container and an index within. Whatever, it
      -- cannot be seen from outside.

   function Is_Nice (Item : Cursor) return Boolean;
      -- Implementation of Element's interface by Cursor

   --  + Getter/Setter, of course

Here the referential nature of Cursor is hidden and Cursor is a full
substitute of Element

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de