CODEWITHSUNDEEP
c++multithreadingqt
JBL
JBL

Reputation: 12917

Issue with Qt thread affinity and moveToThread

I'm trying to use threads in Qt to delegate some work to a thread, but I can't get it to work. I have a class inheriting QMainWindow that have a member object that launch threads to do work. This object has the QMainwindow as parent. It contains and initialize notably another QObject, the m_poller, which I want to move to the thread I create :

m_pollThread = new QThread;
m_poller->moveToThread(m_pollThread);
//Bunch of connection
m_pollThread->start();

I followed the guidelines about how to manage thread in Qt without subclassing it (aka not doing it wrong), but I still get the following message in VS :

QObject::moveToThread: Current thread (0x2dfa40) is not the object's thread (0x120cf5c0). Cannot move to target thread (0x1209b520)

I found the following post that seems to deal with the same issue, but couldn't fix my code with the answer. I feel like I'm actually calling moveToThread correctly (as in I don't call it from within another thread to "pull" an object to it), but apparently I'm still missing something there: as the message hints, it seems there's already multiple thread and my call to moveToThread() seems to end up in the wrong one (though I admit I'm completely new to this and could figure this out completely wrong...)

So what could still be possibly wrong with the way I use Qt threads ?

Thanks !

Upvotes: 6

Views: 12745

Answers (4)

Mohammad Rahimi
Mohammad Rahimi

Reputation: 1123

If you are moving your object through signals and slots (you have created your m_poller in one thread and called a signal and passed it to a slot of another object which is not in caller thread) make sure to use Qt::DirectConnection type for your connect. In this way your slot will execute in the caller thread and calling moveToThread is in the caller thread.

Upvotes: 0

pPanda_beta
pPanda_beta

Reputation: 658

You can also move it to your thread by doing it from the object's owner thread.

#include <thread>
#include <memory>
#include <condition_variable>
#include <QTimer>
#include <QThread>
#include <QApplication>


template <typename Func>
inline void runOnThread(QThread *qThread, Func &&func)
{
    QTimer *t = new QTimer();
    t->moveToThread(qThread);
    t->setSingleShot(true);
    QObject::connect(t, &QTimer::timeout, [=]()
    {
        func();
        t->deleteLater();
    });
    QMetaObject::invokeMethod(t, "start", Qt::QueuedConnection, Q_ARG(int, 0));
}



void moveToThread(QObject *ptr, QThread *targetThrd=QThread::currentThread())
{
    std::mutex mt;
    std::condition_variable_any cv;
    runOnThread(ptr->thread(),[&]
    {
        ptr->setParent(NULL);
        ptr->moveToThread(targetThrd);
        cv.notify_one();
    });
    cv.wait(mt);
}

You just need to call

moveToThread( m_poller, m_pollThread);

Upvotes: 2

spiritwolfform
spiritwolfform

Reputation: 2293

You can only use moveToThread in case when

  • Your object has no parent (because otherwise the parent will have different thread affinity)
  • You are on the object's owner thread so you actually 'push' the object from current thread to another

So your error message says you're violating the second case. You should call moveToThread from the thread that created the object.
And according to you

This object has the QMainwindow as parent.

So moveToThread will not work, again. You should remove the parent from m_poller object

Upvotes: 9

hmn
hmn

Reputation: 746

I think the problem is with the initialization of m_poller, which according to the error message seems to be assigned to a different (third) thread from the one that is executing your code snippet.

Also, if this code is executed multiple times, it may work the first time but then fail on subsequent times as m_poller no longer belongs to the executing thread, but rather m_pollThread.

Upvotes: 1

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