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"Team Ada: Ada Advocacy Issues (83 & 95)" <[log in to unmask]>
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Date:
Wed, 21 Feb 2001 09:54:50 +0000
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>> Does anyone know how to call an Ada 95 function by address
>> (if possible)? I
>> know you can
>> in C/C++.
>    type Trig_Spec is access function(Theta : Radians) return Float;
>    pragma Convention(StdCall, Trig_Spec); -- StdCall or whatever
>    A_Function : Trig_Spec;
>    ...
>    A_Function := -- get a pointer from C program, "access xyz",
>                  -- System.Address_To_Access_Conversions, or whatever
>    Y := A_Function(X);

You appear to have assumed that the original question related to calling
imported non-Ada functions by address.

If what you are trying to do is within Ada, you can simplify the code above by
missing out the pragma Convention, and using a different method to assign a
value to A_Function. A simple, but slightly more complete example would be:

package Callback_List is

   type Callback is access procedure; -- Access to procedure with no paramters

   type Callback_Index is range 1 .. 10;
   type Callback_List_Type is array (Callback_Index) of Callback;

   procedure Add_Callback (Index        : in Callback_Index,
                           The_Callback : in Callback);

   procedure Execute_Callbacks;

end Callback_List;

package body Callback_List is

   The_Callback_List : Callback_List_Type := (others => null);

   procedure Add_Callback (Index        : in Callback_Index,
                           The_Callback : in Callback) is
   begin
      The_callback_List (Index) := The_Callback;
   end Add_Callback;

   procedure Execute_Callbacks is
   begin
      for Index in Callback_Index loop
         if The_Callback_List (Index) /= null then
            The_Callback_List (Index).all; -- Calls the operations listed
         end if;
      end loop;
   end Execute_Callbacks;
end Callback_List;

package Use_Callback_List is

   procedure Update; -- This will be used as a callback

end Use_Callback_List;

with Callback_List;
with Ada.Text_IO;
package body Use_Callback_List is

   procedure Update is
   begin
      Ada.Text_IO.Put_Line ("Update Called");
   end Update;

begin
   Callback_List.Add_Callback (Index        => 1,
                               The_Callback => Update'Access);

   Callback_List.Execute_Callbacks;
end Use_Callback_List;

Hope this helps.
John



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