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

Question

I am using Qt Creator 1.3.0 with Qt 4.6.0 (32 bit) under Windows 7 Ultimate. Consider the following QThread:

Header

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? Thanks in advance.

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 neccessary send a pointer to your ResultThread as a parameter with your emit function to gain access to member funtions/variables.