Thanks for answering!
I have already tried hide() with no success.
The program has a QTimer. Signal/Slot is required to use the connect-function,which means that my class Client has to inherit QObject:
in the .h file:
class Client: public QDialog
{
Q_OBJECT

To inherit Q_OBJECT, Client has to subclass a GUI-class QDialog, QWidget or QMainWindow. If I try to include the QT=-GUI in the .pro-file or QApplication app(argc,argv,false) in the main.cpp the program ceases to work. Is it possible to subclass a noneGui class instead of QDialog?