Subject: how can i declare & define Browser callback?
I haven't found any examples that use a subclassed Browser with .h & .cpp files.

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Hold_Browser.H>

// Demonstrate subclassing Fl_Hold_Browser and using callback -erco 6/18/21

class MyBrowser : public Fl_Hold_Browser {
    void MyCallback2(Fl_Widget *w) {
        void *i = selection();
        if ( i ) printf("item selected is '%s'..\n", item_text(i));
    }

    static void MyCallback(Fl_Widget *w, void *data) {
        MyBrowser *brow = static_cast<MyBrowser*>(data);
	brow->MyCallback2(w);
    }
public:
    MyBrowser(int X,int Y,int W,int H) : Fl_Hold_Browser(X,Y,W,H) {
        when(FL_WHEN_CHANGED);
        callback(MyCallback, (void*)this);
    }
};

int main() {
    Fl_Window win(300,500);
    MyBrowser brow(10,10,300-20,500-20);
    brow.add("aaa"); brow.add("bbb"); brow.add("ccc");
    win.show();
    return Fl::run();
}