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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: put of access type
Date: Thu, 20 Aug 2009 08:18:04 +0200
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Date: 2009-08-20T08:18:04+02:00
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Randy Brukardt schrieb:
> "Adam Beneschan" <adam@irvine.com> wrote in message 
> news:c9aec6d6-4e9b-4bf6-9586-68a237175c9d@i18g2000pro.googlegroups.com...
> ...
>> Also, there's no rule saying that an access value has to be an address
>> at all.  It's certainly conceivable that an access value may be
>> implemented as a reference to some storage pool and an offset into
>> that pool, allowing for the possibility that the memory management
>> system may just decide to pick up the whole pool and move it to some
>> other address, without making any of the access values invalid.
> 
> I think the Ada 95 definition of storage pools would make this 
> implementation hard to make work. (Which is unfortunate, it would have 
> worked fine in Ada 83).

It still works:

type Simple_Access is record
  Offset : Integer;
end record;

type Access_All (Pool_Access : Boolean) is record
  case Pool_Access is
     true =>
       Pool    : access Pool_Type;
       Offset  : Integer;
     false =>
       Address : System.Address;
  end case;
end record;

Since a simple access type always points into the pool only the Offset
is needed. since an access all can point anywhere more info is needed.
This is why I always advice against use of access all when not needed.

Martin
-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com