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Ah, sorry, my message were coincidendtal, but this has nothing to do with Groups. This is purely a callback thing that is meant to give callback users a chance to deallocate user data that they may have allocated for just this widget. The Group thing is quite easy to resolve: we just need to make Fl_Group::insert(), remove(), and clear() virtual.
But now to explain this case. Assuming your app allocates and deletes widgets dynamically, let's say you need more data in a callback than just a single pointer. So you allocate a dataset when allocating the widget, but you can't make sure that the dataset is deleted whenever the widget is deleted by the system.
For example (not compilable code, grossly simplified):
struct {
int x;
int y;
} CallbackData;
void button_cb(Fl_Button *w, void *u) {
CallbackData *d = (CallbackData*)u;
if (Fl::event()==FL_DELETING)
delete d;
else
printf("I got %d and %d\n", d->x, d->y);
}
myButton = new Fl_Button(...)
myButton->callback(button_cb, new CallbackData(42, 128) ); myButton->when(FL_WHEN_CHANGED|FL_WHEN_DELETING);
This needs just a few lines of code to be implemented:
in Fl_Widget at the top: if (when() & FL_WHEN_DELETING) { int old_event = Fl::event(); Fl::event(FL_DELETING); do_callback(); Fl::event(old_event); }
plus the declaration of those constants and some Fluid changes.
There is an alternative: just as in Fl_Widget::label_copy(char*), we could add Fl_Widget::callback_data_copy(void*), set a flag, and call free(_user_data) when deleting, or call delete _user_data, so it would be a set-and-forget with less flexibility.
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