How to use QTcpSocket instance multiple times with

Question

I have to repeat the same request with QTcpSocket multiple times with a couple seconds intervals. Easy task but I can't get it to work with only one instance of object. How to connect to server again using the same instance of QTcpSocket? I've tried reset(), resume(), flush(), open() in many diffrent combinations and it has done nothing. I'm unable to connectToHost() for second time...

Answer 1

All you have to do is to connectToHost(), use the connection, then disconnectFromHost(). That's it. Nothing more, nothing less.

The below example illustrates the reuse of both client and server socket instances. The server instances are kept in a pool that only grows when necessary. It's written for Qt 5 and uses C++11.

// https://github.com/KubaO/stackoverflown/tree/master/questions/multisocket-22726075
#include <QtNetwork>

class EchoServer : public QTcpServer
{
   QStack<QTcpSocket*> m_pool;
   void incomingConnection(qintptr descr) Q_DECL_OVERRIDE {
      if (m_pool.isEmpty()) {
         auto s = new QTcpSocket(this);
         QObject::connect(s, &QTcpSocket::readyRead, s, [s]{
            s->write(s->readAll());
         });
         QObject::connect(s, &QTcpSocket::disconnected, this, [this, s]{
            m_pool.push(s);
         });
         m_pool.push(s);
      }
      m_pool.pop()->setSocketDescriptor(descr, QTcpSocket::ConnectedState);
   }
public:
   ~EchoServer() { qDebug() << "pool size:" << m_pool.size(); }
};

void setupEchoClient(QTcpSocket & sock)
{
   static const char kByteCount[] = "byteCount";
   QObject::connect(&sock, &QTcpSocket::connected, [&sock]{
      auto byteCount = 64 + qrand() % 65536;
      sock.setProperty(kByteCount, byteCount);
      sock.write(QByteArray(byteCount, '\x2A'));
   });
   QObject::connect(&sock, &QTcpSocket::readyRead, [&sock]{
      auto byteCount = sock.property(kByteCount).toInt();
      if (byteCount) {
         auto read = sock.read(sock.bytesAvailable()).size();
         byteCount -= read;
      }
      if (byteCount <= 0) sock.disconnectFromHost();
      sock.setProperty(kByteCount, byteCount);
   });
}

int main(int argc, char *argv[])
{
   QCoreApplication a(argc, argv);
   QHostAddress addr("127.0.0.1");
   quint16 port = 5050;

   EchoServer server;
   if (! server.listen(addr, port)) qFatal("can't listen");

   QTcpSocket clientSocket;
   setupEchoClient(clientSocket);

   auto connectsLeft = 20;
   auto connector = [&clientSocket, &addr, port, &connectsLeft]{
      if (connectsLeft--) {
         qDebug() << "connecting" << connectsLeft;
         clientSocket.connectToHost(addr, port);
      } else
         qApp->quit();
   };
   // reconnect upon disconnection
   QObject::connect(&clientSocket, &QTcpSocket::disconnected, connector);
   // initiate first connection
   connector();

   return a.exec();
}