Access to primitive operations of tagged types

Access to primitive operations of tagged types

[Nigel J. Tracey]

Hi,

I am having a go at writing a class hierarchy to interface to X-Windows/
Motif. My idea was to have tagged types for each of the widgets in
Motif with various functions. Among these functions would be null functions
for the callbacks. The user could then derive there own type from the
widget-tagged type and over-ride these callbacks with there own custom
functions.

My problem is however that I cannot pass an access to the callback functions
when installing the callback routine in X.
The Ada RM states that a dereference of an access-to-subprogram value is
never a dispatching call.

Does this mean I cannot do the above?

With GNAT 3.09 (i686/Linux 2.0.29) and the bindings to X11/Motif
from Intermetrics. I get the following error

primitive-label-pushbutton.adb:27:10:
  subprogram has invalid convention for context

The code is as follows :-

package Primitive is

   type Primitive_Widget is tagged private;

private

   type Primitive_Widget is tagged
      record
         Widget : Xt.Intrinsic.Widget;
      end record;

end Primitive;


package Primitive.Label is

      type Label_Widget is new Primitive_Widget with private;

private

   type Label_Widget is new Primitive_Widget with null record;

end Primitive.Label;


package Primitive.Label.PushButton is

   type PushButton_Widget is new Label_Widget with private;

   procedure Initialize ...

   procedure ActivateCallback ...
   
private

   type PushButton_Widget is new Label_Widget with null record;

end Primitive.Label.PushButton;


package body Primitive.Label.PushButton is

   ...

   procedure Initialize
               (Widget : in out Label_Widget;
                Parent : in     Xt.Intrinsic.Widget;
                Name   : in     String;
                Args   : in     Xt.Intrinsic.ArgList;
                Num    : in     Xt.Intrinsic.Cardinal)
   is
   begin
      Widget.Widget := Xt.Intrinsic.XtCreateWidget
                         (X.Strings.New_String (Name),
                          Motif.PushB.xmPushButtonWidgetClass_obj,
                          Parent,
                          Args,
                          Num);

      Xt.Intrinsic.XtAddCallback
        (Widget.Widget, Motif.XmStrDefs.XmNactivateCallback,
         ActivateCallback'Access,                  -- Line 27 Here
         null);

  end Initialize;

   procedure ActivateCallback
               (Widget      : in Xt.Intrinsic.Widget;
                Client_Data : in Xt.Intrinsic.XtPointer;
                Call_Data   : in Xt.Intrinsic.XtPointer)
   is
   begin
      Ada.Text_IO.Put_Line ("Activate Putton - Standard");
   end ActivateCallback;

   ...
   
end Primitive.Label.PushButton;


Thanks for any clues,

Nigel

--------------------
Nigel J. Tracey MEng, Research Associate (High Integrity Systems Group).
Department of Computer Science,   Office: X/D016
University of York,               Tel   : [0|+44]1904 432769
York, England.                    E-Mail: njt@minster.york.ac.uk
YO1 5DD.

Re: Access to primitive operations of tagged types

[Jon S Anthony]

In article <hkbmg5.2h2.ln@localhost> njt@sv1pc161.cs.york.ac.uk (Nigel J. Tracey) writes:

> My problem is however that I cannot pass an access to the callback
> functions when installing the callback routine in X.  The Ada RM
> states that a dereference of an access-to-subprogram value is never
> a dispatching call.

Make the "access" registered functions non dispatching, make them
private (or better yet in a body), and have them _call_ their
appropriate dispatchable operation.

/Jon
-- 
Jon Anthony
Organon Motives, Inc.
Belmont, MA 02178
617.484.3383
jsa@organon.com

Re: Access to primitive operations of tagged types

[Norman H. Cohen]

Nigel J. Tracey wrote:

> I am having a go at writing a class hierarchy to interface to X-Windows/
> Motif. My idea was to have tagged types for each of the widgets in
> Motif with various functions. Among these functions would be null functions
> for the callbacks. The user could then derive there own type from the
> widget-tagged type and over-ride these callbacks with there own custom
> functions.
> 
> My problem is however that I cannot pass an access to the callback functions
> when installing the callback routine in X.
> The Ada RM states that a dereference of an access-to-subprogram value is
> never a dispatching call.
> 
> Does this mean I cannot do the above?

Just pass an access value designating a nondispatching "wrapper
subprogram" whose body consists of a single dispatching call.  (Of
course the proper OO style would be to pass an access value designating
the classwide type of your root widget class, but Xlib wasn't designed
that way.)

-- 
Norman H. Cohen
mailto:ncohen@watson.ibm.com
http://www.research.ibm.com/people/n/ncohen