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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Newbie question: Implementing a callback mechanism with Ada 83
Newsgroups: comp.lang.ada
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Date: Sat, 10 Feb 2007 21:40:07 +0100
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Date: 2007-02-10T21:39:48+01:00
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On 10 Feb 2007 11:38:19 -0800, benibilme@gmail.com wrote:

> I need to develop an Ada application which must conform to Ada83. It
> will be part of legacy system. I am very much new in Ada. The
> application I have to develop is very much event driven. As an old
> time C programmer I wanted to implement an event handling module which
> you can subscribe callback functions and trigger the events when
> necessary and unfortunately I have realized that Ada 83 does not
> support pointer to subprograms. I have read about generics with which
> defining subprograms as parameters of subprograms seems possible. On
> the other hand, I could not figure out how to do it.
> 
> I simply want to store function pointers to some data structure for
> some certain events and call them with dynamic data when the event is
> triggered.
> 
> How can I do it in Ada 83 without the support of pointers to
> subprograms. It is clear how to do this Ada95 as there is a direct
> support for pointer to subprograms.
> 
> Any help is much appreciated.

I remember that once I used access-to-task to emulate access-to-subprogram.
You can make handlers tasks and serve events in rendezvous.

Another [tedious] possibility could be a proxy subprogram:

type Call_Back_Id is (Foo_Id, Bar_Id, ...);

procedure Call_Back (Id : Call_Back_Id, <parameters>) is
begin
   case Id is
      when Foo_Id => Foo (<parameters>);
      when Bar_Id => Bar  (<parameters>);
      ...
   end case;
end Call_Back;

In the data structure you store Id.

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