How to pass data via QThreads correctly?

Kirill Helm

Recently I write little project. I use QTcpServer which creates QTcpSockets in new QThreads after each new incomming connection. The QTcpSocket instance, next I will name it 'ConnectionEntry'. Connection entry should request from the database some info via signal/slot connection. The database it is the separate essence which running in it own QThread. So, I create the slot for the database and the signal for the connection entry.

Where I should connect database slot and connection entry signal?

I'm connecting it in QTcpServer when I create the instance of ConnectionEntry. Everything works but I get next message in console:

QObject: Cannot create children for a parent that is in a different thread. (Parent is QNativeSocketEngine(0x7f1070003290), parent's thread is CConnectionEntry(0x5631beb596f0), current thread is QThread(0x5631beb3bfd0)

It's little bit annoying me. Please, help if you know where I should connect these essences or which essences I should make parent for another.

Thank you in advance!

P.S. You can found some code below if you need ;)

CConnectionCtrl (Server)

class CConnectionCtrl : public QTcpServer
                      , public IServiceBase
                      , public IConnectionService
{
   Q_OBJECT

   public:

      explicit CConnectionCtrl(QObject* parent = nullptr);

      virtual void incomingConnection(qintptr handle) override final;

      ... ... ...

      virtual void init() override final;

      ... ... ...

   private:

      bool             mbIsServiceAvailable;
      int              mCountOfPendingConnections;
      unsigned int     mPort;
      unsigned int     mMaxConnectionCount;
      IDatabaseBroker* mDatabaseBrokerPtr;
      std::unordered_map<qintptr, CConnectionEntry*> mActiveConnectionsList;
}

CConnectionCtrl::CConnectionCtrl(QObject* parent)
: QTcpServer(parent)
, mbIsServiceAvailable(false)
, mCountOfPendingConnections(0)
, mPort(13666)
, mMaxConnectionCount(15)
{
   FNC_MSG();
}

void CConnectionCtrl::init()
{
   qRegisterMetaType<myDatabase::SRequestsQueueEntry>("myDatabase::SRequestsQueueEntry");

   setMaxPendingConnections(mMaxConnectionCount);
   if ( listen(QHostAddress::Any, mPort) )
   {
      DBG_MSG("Server is listening on PORT: [%d], max users count is [%d]", mPort, mMaxConnectionCount);
      mbIsServiceAvailable = true;
   }
   else
   {
      ERR_MSG("Server can not listen!");
      mbIsServiceAvailable = false;
   }
}

void CConnectionCtrl::incomingConnection(qintptr handle)
{
   ++mCountOfPendingConnections;
   DBG_MSG("Detected new connection, handle ID: [%d], Current connection count: [%d]", handle, mCountOfPendingConnections);
   CConnectionEntry *connectionEntry = new CConnectionEntry(handle, mDatabaseBrokerPtr, this);
   connect(connectionEntry, SIGNAL(finished()), connectionEntry, SLOT(deleteLater()));
   connect(connectionEntry, SIGNAL(clientIsDisconnected(int)), this, SLOT(disconected(int)) );
   connect(connectionEntry, SIGNAL(passRequestToBroker(myDatabase::SRequestsQueueEntry)), mDatabaseBrokerPtr, SLOT(addRequest(myDatabase::SRequestsQueueEntry)));
   connectionEntry->start();
   connectionEntry->setParent(this);
   mActiveConnectionsList.insert(std::pair<qintptr, CConnectionEntry*>(handle, connectionEntry));
}

CConnectionEntry (QTcp socket created by server)

class IConnectionEntry : public QThread
{
   Q_OBJECT

   public:

      explicit IConnectionEntry(QObject* parent = nullptr) : QThread(parent) { }

   public slots:

      virtual void sendReply(const SReply& replyStruct) = 0;
};

class CConnectionEntry : public IConnectionEntry
{
   Q_OBJECT

   public:

      explicit CConnectionEntry(const qintptr& handleID, IDatabaseBroker* brokerPtr, QObject* parent = 0);

   public slots:

      virtual void sendReply(const SReply& replyStruct) override final;
      void readyRead();

   protected:

      void run();

   signals:


      void passRequestToBroker(const myDatabase::SRequestsQueueEntry& request);

      ... ... ...

   private:

       QTcpSocket*      mSocketPtr;
       QDataStream      mDataStreamIn;
       QDataStream      mDataStreamOut;
       QByteArray       mSendedDataBlock;
       qintptr          mHandleID;
       IDatabaseBroker* mDatabaseBrokerPtr;
};

CConnectionEntry::CConnectionEntry(const qintptr& handleID, IDatabaseBroker* brokerPtr, QObject* parent)
: IConnectionEntry(parent)
, mHandleID(handleID)
, mDatabaseBrokerPtr(brokerPtr)
, mDataStreamOut(&mSendedDataBlock, QIODevice::WriteOnly)
{

}

void CConnectionEntry::sendReply(const SReply& replyStruct)
{
   //Some code with QTcpSoket
}

void CConnectionEntry::readyRead()
{
   ... ... ...

   if (isRecivedStructValid)
   {
      DBG_MSG("Handle ID: [%d] Recived data is valid. Request: [%s]", mHandleID, toString(recivedStruct.requestName).c_str());

      emit passRequestToBroker(myDatabase::SRequestsQueueEntry(recivedStruct, eProcessingPriority::PRIORITY_MIDDLE, this));
   }

   ... ... ...
}

void CConnectionEntry::run()
{
    DBG_MSG("Handle ID: [%d] Start working thread.", mHandleID);

    mSocketPtr = new QTcpSocket();
    if(!mSocketPtr->setSocketDescriptor(this->mHandleID))
    {
        emit error(mSocketPtr->error());
        return;
    }

    connect(mSocketPtr, SIGNAL(readyRead()), this, SLOT(readyRead()), Qt::DirectConnection);
    connect(mSocketPtr, SIGNAL(disconnected()), this, SLOT(disconnected()), Qt::DirectConnection);

    mDataStreamIn.setDevice(mSocketPtr);
    mDataStreamOut.setDevice(mSocketPtr);
    mDataStreamIn.setVersion(QDataStream::Qt_4_0);
    mDataStreamOut.setVersion(QDataStream::Qt_4_0);

    DBG_MSG("Handle ID: [%d] Client is connected.", mHandleID);

    exec();
}

CDatabaseBroker (This is the database which I described)

class IDatabaseBroker : public QThread
{
   Q_OBJECT

   public:

      explicit IDatabaseBroker(QObject* parent = nullptr) : QThread(parent) { }

   public slots:

      virtual void addRequest(const myDatabase::SRequestsQueueEntry& request) = 0;
};

class CDatabaseBroker : public IDatabaseBroker
                      , public IServiceBase
                      , public IDatabaseClient
                      , public IConnectionClient
{
   Q_OBJECT

      ... ... ...

   public slots:

      virtual void addRequest(const myDatabase::SRequestsQueueEntry& request) override final;

      ... ... ...

   private:

      void onRequestIsProcessed(const SReply& reply);

      ... ... ...

   signals:

      void replyOnRequest(const SReply&);

   private:

      ... ... ...

      QMutex              mThreadSaver;
      bool                mbIsBrockerAvailable;
      IDatabaseService*   mDatabasePtr;
      IConnectionService* mConnectionPtr;
      std::priority_queue<SRequestsQueueEntry, std::vector<SRequestsQueueEntry>, CPriorityPairComparision> mRequestsQueue;
};

void CDatabaseBroker::addRequest(const myDatabase::SRequestsQueueEntry& request)
{
   mThreadSaver.lock();

   DBG_MSG("Adding new request [%s] with priority [%s].", toString(request.request.requestName).c_str(), toString(request.priority).c_str());

   if (mbIsBrockerAvailable)
   {

      mRequestsQueue.push(request);
   }

   mThreadSaver.unlock();
}

void CDatabaseBroker::onRequestIsProcessed(const SReply& reply)
{
   IConnectionEntry* receiverPtr = mRequestsQueue.top().responseReceiverPtr;

   if (nullptr != receiverPtr)
   {
      connect(this, SIGNAL(replyOnRequest(SReply)), receiverPtr, SLOT(sendReply(SReply)));
      emit replyOnRequest(reply);
      disconnect(this, SIGNAL(replyOnRequest(SReply)), receiverPtr, SLOT(sendReply(SReply)));
   }
   else
   {
      WRG_MSG("nullptr == receiverPtr!");
   }

   mRequestsQueue.pop();
}

Also, if somebody knows the way to emit signal for specific client (if I have it pointer on this client) please tell in comments. Because now I connect/disconnect broker with connection entry each time when I want to sent message to certain connection entry.