Question

I’m working on a chat program where the client is single-threaded but the server will start a new thread for each connected client. I believe my client code is solid but the server has me bewildered.

Right now, I have a derived QTcpSocket class that looks for incoming connections and when it see’s one, begins a new QThread. When the QThread runs, it creates an instance of a QMainWindow (which is the chat window) and shows it.

void secureserver::incomingConnection(int socketDescriptor)
{
    securethread *thread = new securethread(socketDescriptor, this);
    connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
    thread->start();
}

void securethread::run()
{
    serverwindow myServerWindow;
    myServerWindow.setSocketDescriptor(mySocket);
    myServerWindow.show();
}

I’ve been getting errors to stderror like the following, and the QMainWindow never appears so chatting is impossible at this point.

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QApplication(0xbf9e5358), parent's thread is QThread(0x98a54f0), current thread is securethread(0x99e9250)
QPixmap: It is not safe to use pixmaps outside the GUI thread

My questions are:

• Do I need to make the QThread a parent of the QMainWindow?
• Am I going about this in the completely wrong way?
• Any other general ideas on why this isn’t working as I expect it to
  or how to fix it would be greatly appreciated as well.

Answer 1

Yes, you are going about this in the wrong way. GUIs, due to platform limits, are single threaded systems. You cannot create, change and manage GUI objects on different threads – everything must be done on one thread (normally, the GUI thread).

Qt has two mechanisms for dealing with worker threads and the GUI: queued signals and slots, and the QCoreApplication::postEvent() handler.

More details are in the comprehensive Qt threading document: http://doc.qt.io/qt-5/thread-basics.html