How to initialize in constructor?

I have a class that I want to hold a value in like this -

Code:

class guiwindow {
public:
    guiwindow();
    ~guiwindow();

    Fl_Window& getWindow();
private:
    Fl_Window window;
};
#endif

Then I want to initialize it in the constructor like this -

Code:

guiwindow::guiwindow() {
    Fl_Window win( 300,200,"Testing");
    win.begin();
    Fl_Button but( 120, 90, 70, 30,"Click me" );
    win.end();
    but.callback(but_cb);
    win.show();
    Fl::run();
    window = win;   //HERE
}

Why won't this work? Is there a way to make it work without overloading the = operator or declaring it as a pointer in the header? Any help is appreciated.

Use the initializer list.

Code:

guiwindow::guiwindow(): window( 300,200,"Testing") {
    window.begin();
    Fl_Button but( 120, 90, 70, 30,"Click me" );
    window.end();
    but.callback(but_cb);
    window.show();
    Fl::run();
}