Top Document: Motif FAQ (Part 7 of 9) Previous Document: 231) How can I avoid C++ String class and typedef char *String Next Document: 233) Is there a Common Lisp binding for Motif? See reader questions & answers on this topic! - Help others by sharing your knowledge [Last modified: Oct 94] 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: (from Ken Lee, http://www.rahul.net/kenton/) class MyClass { void createWidgets(); static void myButtonCB(Widget, XtPointer, XtPointer); }; void MyClass::createWidgets() { w = XmCreatePushButton(...); 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 summarizing mechanisms (Ronald van Loon, rvloon@motif.xs4all.nl). See his articles in the September, 1994 and Nov/December, 1994 issues of C++ Report. 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! } User Contributions:Top Document: Motif FAQ (Part 7 of 9) Previous Document: 231) How can I avoid C++ String class and typedef char *String Next Document: 233) Is there a Common Lisp binding for Motif? Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Part9 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: kenton@rahul.net (Ken Lee)
Last Update March 27 2014 @ 02:11 PM
|
Comment about this article, ask questions, or add new information about this topic: