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From: bob@odi.COM (Bob Miner)
Newsgroups: comp.windows.x
Subject: Re: C++ and X: Member-Functions as Callbacks
Message-ID: <9103091835.AA05512@odi.com>
Date: 9 Mar 91 18:35:57 GMT
References: <1991Mar9.123358.14352@csn.org>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 79


     Howdy.
     Help requested from C++ and X gurus:

     	I am trying to set up a callback for an X widget.  I would LIKE
     	to have the callback be a member-function of a C++ class.
     	An example:

     		struct	blah
     		{
     			void	foo();
     		}

     		....

     		blah	instance;

     		XtAddCallback( ---- , ---- , instance.foo, ---- );
     	
     The problem(s):

     	When I try it (as in the example), foo() is not called
     	with the proper, hidden, THIS* argument.  THIS, in fact,
     	points to a mess.

     	I suspected that X is packing the normal callback arguments
     	where they shouldn't be (w, client_data, and call_data),
     	so I tried explicitly specifying the arguments:

     		struct	blah
     		{
     			void	foo(Widget, caddr_t, caddr_t);
     		}

     	But it still doesn't work.  THIS still points somewhere else.

     	Any ideas?  I'm sorry if this isn't more clear.  I'm rather new
     	to both X and C++.

     Jeff Rink
     (rink@tramp.colorado.EDU)

To the best of my knowledge, there's no way to make Xt directly call a
C++ member function.  If there is, I'd love to hear it.

The way to make Xt and C++ work together is to create a static C++ function
as the callback function for each callback.  This function receives events,
determines which C++ object should handle the event and then passes the
event to that C++ object.

Your callback function can determine which C++ object to call in at least
two ways.  One way is to, when you add the callback to the widget, set
the clientData (the second arg) to be a pointer to your C++ object.  Then,
when you get the callback, cast the clientData to a pointer to the
appropriate C++ type and call the appropriate event handling function on
the C++ object.

Another way which can be used for some toolkits (OSF/Motif at least) is to,
when creating the widget, set the userData resource on the widget to be a
pointer to the corresponding C++ object.  Then, when you get the callback,
get the userData resource from the given widget and cast the userData resource
value to the appropriate C++ type and call the appropriate event handling
function on that C++ object.

I've also heard of people using hash tables to map widgets to C++ objects,
but I'm not sure what advantage that has over the above two alternatives.

Bob Miner 

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