QObject: Cannot create children for a parent that is in a different thread

Question

I am using Qt 4.6.0 (32 bit) under Windows 7 Ultimate. Consider the following QThread:

Interface

class ResultThread : public QThread { Q_OBJECT      QString _post_data;     QNetworkAccessManager _net_acc_mgr;  signals:     void onFinished(QNetworkReply* net_reply);  private slots:     void onReplyFinished(QNetworkReply* net_reply);  public:     ResultThread();      void run();     void setPostData(const QString& post_data); }; 

Implementation

ResultThread::ResultThread() : _net_acc_mgr(this) {     connect(&_net_acc_mgr, SIGNAL(finished(QNetworkReply*)),             this, SLOT(onReplyFinished(QNetworkReply*))); }  void ResultThread::onReplyFinished(QNetworkReply* net_reply) {     emit onFinished(net_reply); }  void ResultThread::setPostData(const QString& post_data) {     _post_data = post_data; }  void ResultThread::run() {     _net_acc_mgr.post(QNetworkRequest(QUrl("http://[omitted]")),                       QByteArray(_post_data.toStdString().c_str())); } 

Whenever _net_acc_mgr.post() is executed in ResultThread::run(), I got the following Application Output in Qt Creator:

QObject: Cannot create children for a parent that is in a different thread.

(Parent is QNetworkAccessManager(0x22fe58), parent's thread is QThread(0x9284190), current thread is ResultThread(0x22fe48)

What does this mean? How to solve it?

Answer 1

The run() member function is executed in a different thread, rather than the thread where QNetworkRequestManager object was created.

This kind of different-thread problems happen all the time with Qt when you use multiple threads. The canonical way to solve this problem is to use signals and slots.

Create a slot in the object where QNetworkRequestManager belongs to, create a signal in ResultThread and connect both of the somewhere, the constructor of ResultThread would be a good place.

The code which is currently in ResultThread::run() goes to the new slot, and is replaced by a emit(yourSignal()). If necessary send a pointer to your ResultThread as a parameter with your emit function to gain access to member functions/variables.