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Date: Mon, 19 Nov 2012 12:22:21 -0800 (PST)
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Subject: Re: Access type to member procedure of instance (Object Oriented
 programming in Ada)
From: ake.ragnar.dahlgren@gmail.com
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Date: 2012-11-19T12:22:21-08:00
List-Id: <comp.lang.ada>

On Monday, November 19, 2012 10:59:42 AM UTC+1, ake.ragna...@gmail.com wrot=
e:
> Not sure if this has already been discussed but: Is it possible to define=
 a pointer to a member procedure of an instance?
>=20
>=20
>=20
> This problem has it's origin in the implementation of the MVC pattern for=
 a GtkAda application. Consider the following:
>=20
>=20
>=20
> package Some_Package is
>=20
>=20
>=20
>    type Message_Type is tagged null record;
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>   =20
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>    procedure Print(Message : Message_Type);
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> end Some_Package;
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> package body Some_Package is
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>    procedure Print (Message : Message_Type) is
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>    begin
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>       null;
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>    end Print;
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> end Some_Package;
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> Then it is possible to create the following Main application that compile=
s just fine:
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> with Some_Package;
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> procedure Main is
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>    Message : Some_Package.Message_Type;
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> begin
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>    Message.Print;
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> end Main;
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> Is it possible to create a parameterless access type to the Message.Print=
 procedure? The following application refuses to compile:
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>=20
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> with Some_Package;
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>=20
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> procedure Main is
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>    Message : Some_Package.Message_Type;
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>   =20
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>    Method_Ref : access procedure :=3D Message.Print'Access;
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> begin
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>    Method_Ref;
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> end Main;
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> The error message is: no selector "Print" for type "Message_Type" defined=
 at some_package.ads:3
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> It is possible to create a workaround:
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> with Some_Package;
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>=20
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> procedure Main is
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>    Message : Some_Package.Message_Type;
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>   =20
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>    procedure Method_Ref is
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>    begin
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>       Message.Print;
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>    end Method_Ref;
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>   =20
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> begin
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>    Method_Ref;
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> end Main;
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> Is there a better way? Or is this the optimal way?
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> Regards,
>=20
> =C5ke Ragnar Dahlgren

Thanks everybody for posting. Adam you answered my question perfectly since=
 my question was a pure Ada question whether or not it is possible to defin=
e pointers to member methods of class instances. And the answer is no. To a=
dd this as a feature to Ada might be useful. But of course this needs caref=
ul investigation.

I do not mean to make this a Gtkada issue but I have received requests for =
clarifying the origin of the question.

The Gtk application uses Glade and contains the following procedure call:

Gtkada.Builder.Register_Handler
   (Builder      =3D> Builder,
    Handler_Name =3D> "File_Quit",
    Handler      =3D> On_Quit'Access);

where the On_Quit procedure is declared as follows:

procedure On_Quit (Object : access Gtkada.Builder.Gtkada_Builder_Record'Cla=
ss) is
begin
  -- Code is omitted
end On_Quit;

When On_Quit is defined as above everything compiles just fine. The problem=
 is if one would like the On_Quit method to be a member method of an instan=
ce. In this case there is a Controller instance whose method should be call=
ed. It is declared as:

procedure On_Quit (This   : Controller_Type;
                   Object : access Gtkada.Builder.Gtkada_Builder_Record'Cla=
ss);

Using the Controller's On_Quit procedure one would like to write:

Gtkada.Builder.Register_Handler
   (Builder      =3D> Builder,
    Handler_Name =3D> "File_Quit",
    Handler      =3D> Controller.On_Quit'Access);

This did not compile and thereof the question.

Additional information if needed: The spec. of the Register_Handler procedu=
re in the Gtkada.Builder package is declared as:

procedure Register_Handler
  (Builder      : access Gtkada_Builder_Record'Class;
   Handler_Name : String;
   Handler      : Builder_Handler);

The Builder_Handler is a pointer to a method:

type Builder_Handler is access procedure
  (Builder : access Gtkada_Builder_Record'Class);

Best regards,
=C5ke Ragnar Dahlgren