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Date: Mon, 19 Nov 2012 01:59:42 -0800 (PST)
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Subject: Access type to member procedure of instance (Object Oriented
 programming in Ada)
From: ake.ragnar.dahlgren@gmail.com
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Date: 2012-11-19T01:59:42-08:00
List-Id: <comp.lang.ada>

Not sure if this has already been discussed but: Is it possible to define a=
 pointer to a member procedure of an instance?

This problem has it's origin in the implementation of the MVC pattern for a=
 GtkAda application. Consider the following:

package Some_Package is

   type Message_Type is tagged null record;
  =20
   procedure Print(Message : Message_Type);

end Some_Package;



package body Some_Package is

   procedure Print (Message : Message_Type) is
   begin
      null;
   end Print;

end Some_Package;


Then it is possible to create the following Main application that compiles =
just fine:

with Some_Package;

procedure Main is
   Message : Some_Package.Message_Type;
begin
   Message.Print;
end Main;

Is it possible to create a parameterless access type to the Message.Print p=
rocedure? The following application refuses to compile:

with Some_Package;

procedure Main is
   Message : Some_Package.Message_Type;
  =20
   Method_Ref : access procedure :=3D Message.Print'Access;
begin
   Method_Ref;
end Main;

The error message is: no selector "Print" for type "Message_Type" defined a=
t some_package.ads:3


It is possible to create a workaround:

with Some_Package;

procedure Main is
   Message : Some_Package.Message_Type;
  =20
   procedure Method_Ref is
   begin
      Message.Print;
   end Method_Ref;
  =20
begin
   Method_Ref;
end Main;

Is there a better way? Or is this the optimal way?

Regards,
=C5ke Ragnar Dahlgren