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From: "Matthew Heaney" <mheaney@on2.com>
Newsgroups: comp.lang.ada
Subject: Re: Impossible problem? A protected buffer to queue objects of a
 class-wide type
Date: 13 Apr 2007 10:26:16 -0700
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Date: 2007-04-13T10:26:16-07:00
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On Apr 13, 1:02 pm, "Matthew Heaney" <mhea...@on2.com> wrote:
> On Apr 11, 6:34 am, "Phil Slater" <phil.sla...@baesystems.com> wrote:
>
> > I've hit a brick wall. Every strategy I try doesn't work.

I probably should have pointed out that I my previous post, the Deque
function is implemented this way:

  function Deque (Q : not null access QT) return ET;

This will necessitate declaring queue objects as aliased, and then
using taking the 'Access of the object:

declare
  Q : aliased QT;
begin
  ...
  declare
     E : ET := Deque (Q'Access);
  begin
    ...
  end;
...
end;


If you don't like the syntactic overhead, there are a couple of things
you can do.  First is to declare the queue type as tagged:

  type QT is tagged limited private;

Ada05 allows you do use distinguished-receiver syntax, and will do an
implicit dereference is the object is aliased (or implicitly aliased),
e.g.

declare
  Q : aliased QT;  -- QT is tagged; explicitly aliased
begin
  ...
  declare
     E : ET := Q.Deque;
  begin
    ...
  end;
...
end;

--or--

procedure Op (Q : in out QT) is  -- implicitly aliased
begin
  ...
  declare
     E : ET := Q.Deque;
  begin
    ...
  end;
...
end Op;


So taggedness is one option.  Another option is to use the Rosen
Trick, which allows a function to modify its parameter, if it's a by-
reference type.  (Types whose full view is limited qualify, so we're
eligible.)  That allows us to declare the deque function like this:

  function Deque (Q : QT) return ET;

In the implementation of the QT, we simply need to add another
component:

private

  type HT (Q : not null access QT) is limited null record;
 ...
  type QT is limited record
     H : HT (QT'Access);
     B : BT;
  end record;

end Queues;

Now we just need to make a small change to the implementation of our
deque function:

   function Deque (Q : QT) return ET is
      Obj : ET_Access;

   begin
      Q.H.Q.B.Get (Obj);  -- manipulate buffer via handle (Rosen
Trick)

      return E : ET := Obj.all do
        Free (Obj);
      end return;
   end Deque;

Here I didn't bother making the queue type tagged, but if you like
distinguished-receiver syntax, then you can do that too.

Regards,
Matt

P.S. I'll be giving a tutorial in Geneva at the end of June, to
discuss these sorts of techniques.