gistfile1.cpp

static void GenericCallback(Fl_Widget * widget, void * data) {
    auto cb = static_cast<std::function<void(Fl_Widget *)> *>(data);
    (*cb)(widget);
}
 
template<typename cb_t>
void cb(Fl_Widget * widget, const cb_t & cb) {
    auto wrapped = new std::function<void(Fl_Widget *)>;
    *wrapped = cb;
    widget->callback(GenericCallback, wrapped);
}
 
Usage:
 
button = new Fl_Button(20, 150, 60, 20, "A button");
int times = 0;
cb(button, [&times](Fl_Widget * widget) {
    cout << (format("button pressed %d times\n")% ++times);
});