Signals and slots are used for communication between objects. The signal/slot mechanism is a central feature of Qt and probably the part that differs most from other toolkits. ... All classes that inherit from QObject or one of its subclasses (e.g. QWidget) can contain signals and slots. ... The preprocessor changes or removes the signals , slots and emit keywords so the compiler won't see anything it can't digest. Run the moc on class definitions that contains signals or slots. ... Q_OBJECT . ...