Re: Access type to member procedure of instance (Object Oriented programming in Ada)

[ake.ragnar.dahlgren@gmail.com]

On Monday, November 19, 2012 10:59:42 AM UTC+1, ake.ragna...@gmail.com wrote:
> 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;
> 
>    
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>    procedure Print(Message : Message_Type);
> 
> 
> 
> end Some_Package;
> 
> 
> 
> 
> 
> 
> 
> package body Some_Package is
> 
> 
> 
>    procedure Print (Message : Message_Type) is
> 
>    begin
> 
>       null;
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>    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 procedure? The following application refuses to compile:
> 
> 
> 
> with Some_Package;
> 
> 
> 
> procedure Main is
> 
>    Message : Some_Package.Message_Type;
> 
>    
> 
>    Method_Ref : access procedure := Message.Print'Access;
> 
> begin
> 
>    Method_Ref;
> 
> end Main;
> 
> 
> 
> The error message is: no selector "Print" for type "Message_Type" defined at some_package.ads:3
> 
> 
> 
> 
> 
> It is possible to create a workaround:
> 
> 
> 
> with Some_Package;
> 
> 
> 
> procedure Main is
> 
>    Message : Some_Package.Message_Type;
> 
>    
> 
>    procedure Method_Ref is
> 
>    begin
> 
>       Message.Print;
> 
>    end Method_Ref;
> 
>    
> 
> begin
> 
>    Method_Ref;
> 
> end Main;
> 
> 
> 
> Is there a better way? Or is this the optimal way?
> 
> 
> 
> Regards,
> 
> Åke 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 define pointers to member methods of class instances. And the answer is no. To add this as a feature to Ada might be useful. But of course this needs careful 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      => Builder,
    Handler_Name => "File_Quit",
    Handler      => On_Quit'Access);

where the On_Quit procedure is declared as follows:

procedure On_Quit (Object : access Gtkada.Builder.Gtkada_Builder_Record'Class) 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 instance. In this case there is a Controller instance whose method should be called. It is declared as:

procedure On_Quit (This   : Controller_Type;
                   Object : access Gtkada.Builder.Gtkada_Builder_Record'Class);

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

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

This did not compile and thereof the question.

Additional information if needed: The spec. of the Register_Handler procedure 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,
Åke Ragnar Dahlgren