1. Create a new Qt Widgets Application using the wizard.
2. In the main function, select the line

   w.show();
   

3. From the context menu, select "Refactor" -> "Extract Function".
4. Type a name for the new function and click "OK".
   You'll either get a function with the required parameter:

   void newFunction(MainWindow w)
   {
       w.show();    
   }
   

   or a function without the parameter which will not compile:

   void newFunction()
   {
       w.show();    
   }
   

5. Undo the refactoring.
6. Repeat steps 2 to 4.
   Sometimes you'll get the correct function, sometimes the compile error.

Refactoring should always result in the same correct code.