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From: Brad Moore <brad.moore@shaw.ca>
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Subject: Re: Access to procedure and generics
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Date: Tue, 28 Aug 2012 21:52:42 -0600
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Date: 2012-08-28T21:52:42-06:00
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On 28/08/2012 9:57 AM, Adam Beneschan wrote:
> If you want a portable solution, you can use tagged types instead of access-to-procedure:
>
> package Callback is
>      type Callback_Object is interface;
>      type Callback_T is access all Callback_Object'Class;
>      procedure Handler (Obj : in out Callback_Object) is abstract;
>      procedure Register_Callback(Callback : Callback_T);
> end Callback;
>
> -- if Z is a registered Callback_T, Z.Handler will call the handler
>
> generic
> package USE_CALLBACK is
>      procedure HANDLER;
> end USE_CALLBACK;
> --
> with CALLBACK;
> package body USE_CALLBACK
> is
>      procedure HANDLER is
>      begin
>         ...
>      end;
>
>      type Call_Handler is new Callback.Callback_Object with null record;
>      overriding
>      procedure Handler (Obj : in out Call_Handler) is
>      begin
>          Handler; -- of course, this doesn't *have* to have the same name
>      end Handler;
>
>      X : aliased Call_Handler;
>
> begin
>      CALLBACK.REGISTER_CALLBACK(X'Unchecked_Access);
> end USE_CALLBACK;
>
> Now you can instantiate Use_Callback inside or outside a subprogram, and it should still work on any Ada implementation.
 >
 > (Credit to Brad Moore who came up with essentially this solution to a 
similar problem.)

I have to give credit where credit is due. It was Steve Baird who 
suggested to me that I should try to use tagged types as an approach to 
get around this issue.

Brad