Article #379: How Can I Use a Class Method as a Callback?

Created at 10:59 Jan 29, 2005 by mike

Because of the way C++ and FLTK work, you can only use static methods for callbacks, at least directly.

To get the this pointer for the class you can pass the pointer as the user_data argument for your callback. Typically this is then used to call a non-static method:

class MyClass {
  static static_callback(Fl_Widget* w, void* data) {
    ((MyClass*)data)->real_callback(w);
  }
  void real_callback(Fl_Widget* w) {
    ... this is the real callback ...
  }
public:
  MyClass() {
    ...
    // building the gui now:
    button = new Fl_Button(...);
    button->callback(static_callback, this);
  }
};

If you need the user_data pointer for something else, you can also set the user_data pointer in the top-most group/window in your class, and then use the Fl_Widget pointer that is passed to your callback to find the class pointer (this is the method used by FLUID):

class MyClass {
  static static_callback(Fl_Widget *w, void *data) {
    Fl_Widget *p = w->parent();
    while (p->parent()) p = p->parent();
    ((MyClass*)p->user_data())->real_callback(w, data);
  }
  void real_callback(Fl_Widget *w, void *data) {
    ... this is the real callback ...
  }
public:
  MyClass() {
    ...
    // building the gui now:
    window = new Fl_Window(...);
    window->user_data(this);
    button = new Fl_Button(...);
    button->callback(static_callback, ...);
  }
};

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From NULL, 06:42 Nov 13, 2005 (score=3)

static static_callback(Fl_Widget *w, void *data) {
    Fl_Widget *p = w->parent();
    while (p->parent()) p = p->parent();
    ((MyClass*)p->user_data())->real_callback(w, data);
  }

There is a bug in this method, if you use this on Fl_Window you will have seg fault. Fl_Widget *p = w; // should be used insted of Fl_Widget *p = w->parent();

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