Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker!bloom-beacon!EXPO.LCS.MIT.EDU!kit
From: kit@EXPO.LCS.MIT.EDU (Chris D. Peterson)
Newsgroups: comp.windows.x
Subject: Re: Need help with class data of widgets
Message-ID: <9002212222.AA03015@expo.lcs.mit.edu>
Date: 21 Feb 90 22:22:17 GMT
References: <47393@lll-winken.LLNL.GOV>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 77


> I'm having trouble understanding the use and access of class data for
> widgets.  Basically, I would like to have a function that is common to
> a bunch of different widgets that are of different types.  For
> example, I have widgets which are subclasses of command widgets and
> widgets which are subclasses of AsciiText.  I would like to define
> some sort of generic procedure for both types of widgets so that I can
> nuse the same call for both.  I.e.:

>	mycommonroutine(object,data);

> where object is either the command widget or the text widget and data
> is just some pointer.

> I'm thinking of using the class data to hold this routine, but then I
> don't know how to access it or even declare it.  

In each widget's Private Header file add the function definition

		void   (*mycommonroutine)();

to the class part structure of your new subclasses.  Then when you 
statically define your widgetclass you need to add something like the 
following to your widgets .c file.


extern Void __CommonRoutine();	/* Tell the compiler that it really exists. */

NewWidgetClassRec newWidetClassRec = {
  {
    (WidgetClass) SuperClass,		/* superclass		  */	

    .
    .
    .

    XtInheritQueryGeometry,		/* query_geometry	  */
    XtInheritDisplayAccelerator,	/* display_accelerator	  */
    NULL				/* extension		  */
  },  /* CoreClass fields initialization */
  {
    XtInheritChangeSensitive		/* change_sensitive	*/
  },  /* SimpleClass fields initialization */
  {
    0,                                     /* field not used    */
  },  /* LabelClass fields initialization */
  {
    0,                                     /* field not used    */
  },  /* CommandClass fields initialization */
  {
    __CommonRoutine,			   /* common routine used by
					      many widgets.
  },  /* NewWidgetClass fields initialization */
};


The to call the routine use this code:

    (XtClass(w)->new_class_class.mycommonroutine) (w, data);


Kind 'o yucky huh?  It is not clear that this really saves anything over
defining the function externally.  The only time this is really useful is if you
want a subclass to be able to substitute in a new routine.  Otherwise you are
probabally better off just using a global externa function with a unique name.

The other reason to do this is if you control a common superclass.  You can
the make this routine static to that superclass, and all subclasses of this
widget have access to that staticly defined function.


						Chris D. Peterson     
						MIT X Consortium 

Net:	 kit@expo.lcs.mit.edu
Phone:   (617) 253 - 9608	
Address: MIT - Room NE43-213