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From: gvc@ocsystems.com (G. Vincent Castellano)
Subject: Re: Ada/Motif problem...
Date: 1996/08/28
Message-ID: <1996Aug28.235051.12007@ocsystems.com>#1/1
X-Deja-AN: 177132272
organization: OC Systems, Inc.
newsgroups: comp.lang.ada
Date: 1996-08-28T00:00:00+00:00
List-Id: <comp.lang.ada>


> One of our students has run into problems writing a Motif routine.
> The callback data is to be a string as follows..
>
> Xt.Intrinsic.XtAddCallback(
>                                  pb,
>                                  Motif.xmstrdefs.XmNactivateCallback,
>                                  hello_dialog_pkg.popup'access,
>                                  "Hello World");

The specification of this routine is:

    procedure XtAddCallback(
                widget       : Xt.Intrinsic.Widget;
                callback_name: X.Strings.const_charp;
                callback     : XtCallbackProc;
                closure      : XtPointer);

The type XtPointer is not really a useful one, being a
private (access) type with no operations for setting it.
You are obliged to use Unchecked_Conversion to convert to
and from it.

A good choice in this case would be String_Access from the
package Ada.Strings.Unbounded.  (Any access type will
probably work, since access types are generally all the
same size.)

  function To_XtPointer is new Unchecked_Conversion(
              Source => Ada.Strings.Unbounded.String_Access,
              Target => Xt.Intrinsic.XtPointer );

  Hi_String:  constant Ada.Strings.Unbounded.String_Access
              := new String'("Hello World");

-- ...

  Xt.Intrinsic.XtAddCallback(    pb,
                                 Motif.xmstrdefs.XmNactivateCallback,
                                 hello_dialog_pkg.popup'access,
                                 To_XtPointer( Hi_String ) );

BUT:  be careful that the object you're specifying as callback
data is not local to a subprogram, else it will be garbage by
the time the callback is invoked.  Declare it in a package spec
or body to be safe.

Also consider whether callback data is really useful.  If it's
never going to change, you probably don't need it.  I prefer to
play it safe and design callbacks so that only integers are
passed around, which I can then use to index into a table.
 -----------------------------------------------------------------------
 -    G. Vincent Castellano, Sr. Software Engineer, OC Systems Inc     -
 -   gvc@ocsystems.com :: X/Ada WWW => http://www.ocsystems.com/xada   -
 -----------------------------------------------------------------------
 - "If virtual memory did not exist, it would                          -
 -   have become necessary for us to invent it."                       -
 -----------------------------------------------------------------------