Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!ocsmd!jim
From: jim@ocsmd.ocs.com (Jim Muth)
Newsgroups: comp.lang.c++
Subject: Re: pointer to function
Keywords: pointer function arguments
Message-ID: <861@ocsmd.ocs.com>
Date: 21 Mar 90 04:35:38 GMT
References: <1990Mar19.233906.23149@elroy.jpl.nasa.gov>
Reply-To: jim@ocsmd.ocs.com (Jim Muth)
Distribution: usa
Organization: Online Computer Systems, Inc.
Lines: 50

In article <1990Mar19.233906.23149@elroy.jpl.nasa.gov> richard@ben.Jpl.Nasa.Gov (Conan the Barbarian) writes:

>Yuck. That means the handler for a asynchronous server can
>not use C++ strategies unless you have all of the server
>source also. Double yuck. Who wants to re-write X, Motif,
>Xview, Sunview, etc? Triple yuck

Perhaps only a single yuck.  A yuck and a half?

There is a hack to get C++ member functions called from a system that uses
C callbacks, as long as there is a way to hang a pointer off of the
"server" object.  For example, in say you wanted to define a SunView class,
TkFrame, that could handle events:

    class TkFrame {
    public:
	TkFrame() { handle = 0; }
	Frame Create();
	void EventHandler(Event*, void*);
    private:
	Frame handle;
    };

The trick is to associate a pointer to an instance of TkFrame with the
SunView Frame, and to define an ordinary, non-member function as the
callback:

    Frame TkFrame::Create()
    {
	handle = window_create(NULL, FRAME,
		WIN_CLIENT_DATA, this,
		WIN_EVENT_PROC, ::EventHandler,
		0);
	return handle;
    }

The ordinary function just yanks the pointer to a TkFrame out of the
Frame and calls the member function:

    static void EventHandler(Frame handle, Event* event, void* arg)
    {
	TkFrame* frame = (TkFrame*) window_get(handle, WIN_CLIENT_DATA);
	frame->EventHandler(event, arg);
    }

Not pretty, but it gets the job done.

Jim Muth
Online Computer Systems, Inc
jim@ocsmd.ocs.com