Connect Qt signal and slot in a derived QObject constructor

Question

I am writing a simple EchoServer class, inheriting from QTcpServer. And when I connect signal and slot in the constructor, it does not go well.

class EchoServer : public QTcpServer {
  //Q_OBJECT
public:
  EchoServer(int listenling_port) {
    this->listen(QHostAddress(), listenling_port);
    connect(this, SIGNAL(newConnection()), this, SLOT(HandleIncomingConnection()));
  }
public slots:
  void HandleIncomingConnection() {
    auto echo_handler = EchoServerHandler(this->nextPendingConnection());
    echo_handler.Echo();
  }
private:
};

The application does listen to the port, and can be telneted. But the console displays

"QObject::connect: No such slot QTcpServer::HandleIncomingConnection()",

which seems it is recognizing this as a base class QTcpServer pointer.

Also if I leave Q_OBJECT in the code, it would not compile, saying

"error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall EchoServer::metaObject(void)const " (?metaObject@EchoServer@@UBEPBUQMetaObject@@XZ)",

are they related?

Answer 1

You need to leave Q_OBJECT in your code, and you need a destructor to go with your class.

You might need to run "Clean Project" on your code to fix that linker error.

Also check out Q_OBJECT linker error!

Hope that helps.

Answer 2

The link Q_OBJECT linker error! in phyatt's answer helps me find the problem. So I'll answer this myself.

The code in the question was put entirely into one main.cpp file, which prevents the meta processing of Qt. Put the class definition into a separate hpp header and re-compiles, the problem is gone. I am using Visual Studio 2012 + Qt 5.0.