class MainWindow : TopWindow {
    SecondWin second;
    ThirdWin  third;
public:
    MainWindow();
}

class SecondWin : TopWindow {
public:
    SecondWin();
    SecondUpdate();
}

class ThirdWin : TopWindow {
public:
    ThirdWin();
    DoSomething();
}

and
...
MainWindow().Run();