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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: 2012-11-19T12:52:17-08:00
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
>=20
>    procedure Print(Message : Message_Type);
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>=20
>=20
> end Some_Package;
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>=20
>=20
>=20
>=20
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> package body Some_Package is
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>=20
>=20
>    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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>=20
>=20
> end Some_Package;
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>=20
>=20
>=20
>=20
> Then it is possible to create the following Main application that compile=
s just fine:
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>=20
> with Some_Package;
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>=20
>=20
> procedure Main is
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>    Message : Some_Package.Message_Type;
>=20
> begin
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>    Message.Print;
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> end Main;
>=20
>=20
>=20
> Is it possible to create a parameterless access type to the Message.Print=
 procedure? The following application refuses to compile:
>=20
>=20
>=20
> with Some_Package;
>=20
>=20
>=20
> procedure Main is
>=20
>    Message : Some_Package.Message_Type;
>=20
>   =20
>=20
>    Method_Ref : access procedure :=3D Message.Print'Access;
>=20
> begin
>=20
>    Method_Ref;
>=20
> end Main;
>=20
>=20
>=20
> The error message is: no selector "Print" for type "Message_Type" defined=
 at some_package.ads:3
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>=20
>=20
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> It is possible to create a workaround:
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>=20
>=20
> with Some_Package;
>=20
>=20
>=20
> procedure Main is
>=20
>    Message : Some_Package.Message_Type;
>=20
>   =20
>=20
>    procedure Method_Ref is
>=20
>    begin
>=20
>       Message.Print;
>=20
>    end Method_Ref;
>=20
>   =20
>=20
> begin
>=20
>    Method_Ref;
>=20
> end Main;
>=20
>=20
>=20
> Is there a better way? Or is this the optimal way?
>=20
>=20
>=20
> Regards,
>=20
> =C5ke Ragnar Dahlgren

Actually I've been thinking. The workaround worked for the contrived exampl=
e above but not for my Gtkada application.

What inspired the Gtkada application I've been experimenting with is the ex=
planation given by Ada in Denmark on their wiki:

http://wiki.ada-dk.org/building_gui_with_glade_3

What one can see there is that a Builder instance is created and when the a=
pplication quits it is deallocated. This makes the code suitable for using =
Ada's object oriented features and to let the Builder instance be created i=
n the Initialize procedure and let it be deallocated in the Finalize proced=
ure.

Also it is possible to see that the callbacks are declared in the Simple_Ca=
llbacks package. Note that the methods are not instance member methods.

I hope I am wrong but it seems to me that since Ada lacks the ability to cr=
eate pointers (maybe access types are the right words) to instance member m=
ethods, it is not possible to take advantage of Ada's OOP features when dev=
eloping Glade3 applications.

Best regards,
=C5ke Ragnar Dahlgren