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From: rink@tramp.colorado.edu (Jeff)
Newsgroups: comp.windows.x
Subject: C++ and X: Member-Functions as Callbacks
Message-ID: <1991Mar9.123358.14352@csn.org>
Date: 9 Mar 91 12:33:58 GMT
Sender: news@csn.org (news)
Organization: University of Colorado, Boulder
Lines: 58
Originator: rink@tramp.Colorado.EDU
Nntp-Posting-Host: tramp.colorado.edu



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)