The QApplication class manages the application event queue. ... Example (a complete Qt application ): #include qapplication .h // defines QApplication #include qpushbutton.h // defines QPushButton int main( int argc, char **argv ) { QApplication app( argc, argv ); // create application object QPushButton hello( Hello, world! ); // create a push button app. setMainWidget ( hello ); // define as main ... Returns the main application widget, or 0 if there is not a defined main widget. ...