Here's the problem:
Qt Code: 
  1. #include <QtGui>
  2.  
  3. class MainWindow : public QMainWindow
  4. {
  5. public:
  6.     MainWindow(QWidget* parent = 0) : QMainWindow(parent)
  7.     {
  8.         menuBar()->addMenu("File");
  9.         menuBar()->addMenu("Edit");
  10.         menuBar()->addMenu("Tools");
  11.         menuBar()->addMenu("Help");
  12.  
  13.         QWidget* widget = new QWidget(this);
  14.         // change the color just to be able to see the widget
  15.         widget->setStyleSheet("background: rgba(255,0,0,128)");
  16.         // uncomment the following line and see the difference
  17.         // setCentralWidget(widget);
  18.     }
  19. };
  20.  
  21. int main(int argc, char* argv[])
  22. {
  23.     QApplication app(argc, argv);
  24.     MainWindow window;
  25.     window.show();
  26.     return app.exec();
  27. }
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