1. Subclass QApplication
2. Add login() and cancel() signals to the login widget.
3. Connect the login() and cancel() signals of your login widget to the QApplication.
4. On the slot that handles login() and cancel() widgets, delete the login widget and setMainWidget to an instance your main window.
5. Call again exec() to the subclassed QApplication (internally, from the login slot handler).
6. Have fun