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From: stefanmairhofer@gmx.at
Newsgroups: comp.lang.ada
Subject: Observer Pattern / Concurrency with ObjectAda
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Date: Mon, 19 Nov 2007 03:07:34 -0800 (PST)
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Date: 2007-11-19T03:07:34-08:00
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Hello!

I'm currently working on a software-pattern based project with
ObjectAda and I would require to implement an Observer pattern for my
program.

I've stumbled upon an example of the "Observer Pattern with an
Concurrent Subject" from Matthew Heaney
http://www.adapower.com/index.php?Command=Class&ClassID=Patterns&CID=287

And I was not able to run it on ObjectAda 7.2.2. Embedded (I get
access-errors, constraint-errors and sometimes deadlocks). When I
tried the program using MinGW with gccada it worked perfectly.

Apparently the Attach and Detach functions do not work properly and in
the main procedure Test_Observers the Clock declaration results in an
Constraint_Error / access error

procedure Test_Observers is

   Timer : aliased Clock_Timer;
   Clock : Digital_Clock (Timer'Access);  -- <-- Constraint_Error

begin
...

(little help : Timer = subject, which includes a task that informs the
observers via notify(),
 Clock = observer, which has the update() method)

Any ideas why it works with GNAT, but not with ObjectAda?

Thank you for your help!
Best regards
Mairhofer Stefan