[Last modified: October 93] Answer: There are three common user problems with C++ callbacks. First, make sure you use the correct function prototype for the function declarations. Second, the callback function must be declared as a static member of the class. Third, when registering it with XtAddCallback(), you must use its full signature. For example: (Ken Lee klee@synoptics.com) class MyClass { ... void createWidgets(); static void myButtonCB(Widget, XtPointer, XtPointer); ... }; void MyClass::createWidgets() { ... w = XtCreatePushButton(...); XtAddCallback(w, XmNactivateCallback, &MyClass::myButtonCB, (XtPointer) this); ... } void myButtonCB(Widget w, XtPointer clientData, XtPointer callData) { MyClass *myclass = (MyClass *) clientData; ... } Note that the "this" pointer is used as the client data. This technique is popular, but not required. Motif++ has a nice tutorial summarising mechanisms (this is available separately by email from Ronald van Loon (rvloon@motif.hacktic.nl)). Doug Young's book deals extensively with one of these. The problem is that you don't get the object when you just use the function as a callback. You need to pass the object as a pointer through as the client_data. (use "this" as the client_data.) Then you can retrieve the object's address, and dereference from there. For example (Leo O'Donnell, Email: leo@avs.com), class MyButton { public: MyButton (Widget parent, const char *name) { _button = XtVaCreateManagedWidget ( name, xmPushButtonWidgetClass, parent, NULL, 0); XtAddCallback ( _button, XmNactivateCallback, &MyButton::activateCB, (XtPointer) this); } ~MyButton () { XtDestroyWidget (_button); } private: Widget _button; static void activateCB (Widget, XtPointer, XtPointer); }; void MyButton::activateCB (Widget, XtPointer thisBtn, XtPointer) { MyButton *btn = (MyButton *) thisBtn; // OK you've got the button instance now. Do some stuff with it! }Go Back Up