title: Undefined reference to in Constructor with QNetworkAccessManager derived class
body: <p>I am trying to read a html page using the following code which uses Qt libraries</p>
  
  <pre><code> #include &lt;QtWidgets/QApplication&gt;
      #include &lt;QtNetwork/QNetworkRequest&gt;
      #include &lt;QtNetwork/QNetworkReply&gt;
      #include &lt;QtNetwork/QNetworkAccessManager&gt;
      #include &lt;QtCore/QString&gt;
      #include &lt;iostream&gt;
  
      class SillyRequest : public QNetworkAccessManager
  
  {
      Q_OBJECT
      public:
          SillyRequest();
          QString getData();
      public slots:
          void replyFinished(QNetworkReply *);
      private:
         QString collectedData;
  };
  QString SillyRequest :: getData()
  {
      return collectedData;
  }
  SillyRequest :: SillyRequest()
  {
      connect(this, SIGNAL(finished(QNetworkReply*)), this, SLOT(replyFinished(QNetworkReply*)));
  }
  
  void SillyRequest :: replyFinished(QNetworkReply *reply) 
  {
      collectedData = reply-&gt;readAll();
  }
  
  int main(int argc, char **argv)
  {
      QApplication app(argc, argv);
  
      SillyRequest *network = new SillyRequest;
  
      QNetworkRequest request;
  
      request.setUrl(QUrl("http://qt.nokia.com"));
  
      request.setRawHeader("User-Agent", "MyOwnBrowser 1.0");
  
      QNetworkReply *reply = network-&gt;get(request);
  
      std :: cout &lt;&lt; network-&gt;getData().toStdString() &lt;&lt; std :: endl;
  
      return app.exec();
  
  }
  </code></pre>
  
  <p>I get the following error when I compile. Am not sure what I am not doing in the right way.</p>
  
  <pre><code>testing.cpp:(.text+0x73): undefined reference to `vtable for SillyRequest'
  collect2: error: ld returned 1 exit status
  Makefile:192: recipe for target 'network' failed
  make: *** [network] Error 1
  </code></pre>
  
  <p>I have inherited from QNetworkAccessManager so as to implement the 'replyFinished' slot. Signal 'finished' is already part of the 'QNetworkAccessManager'.</p>