Create a QObject in another thread, retrieve it to the current thread and set a parent = ASSERT failure in Debug on msvc16

NiHoTleHot

Hi everyone,
I am just trying to set a parent on a QObject created in another thread (the object is of course moved to the parent thread before), that's all !

#include <QApplication>
#include <QDebug>
#include <QThread>

class Thread : public QThread //Just a convenient Class using a lambda
{
public:
    Thread::Thread(QObject *parent = nullptr) : QThread(parent){}
    std::function<void()> todo;
protected:
    virtual void run() override{
        todo();
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    ///////////////////////////////////////////////
    // Change this flag to switch behaviours
    bool tryToSetParentInThread = true;
    ///////////////////////////////////////////////

    QObject mainObj;
    QObject *dummy; //Just to get back obj created in thread;

    Thread thread;
    thread.todo = [&](){
        //QObject *obj = new QObject(&mainObj); //"QObject: Cannot create children for a parent that is in a different thread." ! Of course!

        // So we try this
        QObject *obj = new QObject;
        dummy = obj;

        qDebug()<<obj->thread();
        obj->moveToThread(mainObj.thread());
        qDebug()<<obj->thread(); //Check that the Thread affinity change is done

        if(tryToSetParentInThread)
            obj->setParent(&mainObj);

        QObject::connect(obj, &QObject::destroyed, [](){ //Parent mecanism is OK
            qDebug()<<"Child destroyed";
        });
    };
    thread.start();
    thread.wait();

    if(!tryToSetParentInThread)
        dummy->setParent(&mainObj);

    return 0; //No need for an event loop here
}

This example is working fine in release, but if you try to launch this code in debug with mscv16:

Maybe the call of obj->setParent(&mainObj) doesn't like that mainObj is not in the thread calling the method ..?

Christian Ehrlicher

You're accessing a QObject and doing a lot of stuff with it inside another thread - that this will crash sooner or later should be clear.

Christian Ehrlicher

You can't set a parent which is not in the same thread.

NiHoTleHot

@Christian-Ehrlicher Of course ! This is not what I am doing here !
The objects have the same thread affinity when I do the setParent()
You can see a commented line of what I am not doing :-D

Christian Ehrlicher

But you're calling the QObject function from another thread which creates an event in the thread where the object does not life in and therefore the assert

NiHoTleHot

@Christian-Ehrlicher That is what I supposed "Maybe the call of obj->setParent(&mainObj) doesn't like that mainObj is not in the thread calling the method ..?"
So obj1->setParent(obj2) triggers a sendEvent() ? No mention of that in documentation !!
But if it does, it is weird to not check thread affinity and replace the sendEvent() by a postEvent() :-(

Edit :
@Christian-Ehrlicher This really ugly but indeed it worked, I had to force the execution of the setParent() in the main thread via a queued connection in the thread. And of course start the eventloop in the main thread

#include <QApplication>
#include <QDebug>
#include <QThread>
#include <QTimer>

class Thread : public QThread //Just a convenient Class using a lambda
{
public:
    Thread::Thread(QObject *parent = nullptr) : QThread(parent){}
    std::function<void()> todo;
protected:
    virtual void run() override{
        todo();
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    ///////////////////////////////////////////////
    // Change this flag to switch behaviours
    bool tryToSetParentInThread = true;
    ///////////////////////////////////////////////

    QObject mainObj;

    Thread thread;
    thread.todo = [&](){
        //QObject *obj = new QObject(&mainObj); //"QObject: Cannot create children for a parent that is in a different thread." ! Of course!

        // So we try this
        QObject *obj = new QObject;

        qDebug()<<obj->thread();
        obj->moveToThread(mainObj.thread());
        qDebug()<<obj->thread(); //Check that the Thread affinity change is done


       QObject::connect(QThread::currentThread(), &QThread::finished, obj, [obj,&mainObj](){
           qDebug()<<"Most fucked up code I wrote";
           obj->setParent(&mainObj);
        });



        QObject::connect(obj, &QObject::destroyed, [](){ //Parent mecanism is OK
            qDebug()<<"Child destroyed";
        });
    };
    thread.start();
    thread.wait();

    QTimer::singleShot(0,[](){
        qApp->quit();
    });

    return a.exec(); //Add an event loop for the connect (queued) from the thread -> setParent() triggers a SendEvent() in the main thread where the two object now live
}

Christian Ehrlicher

You're accessing a QObject and doing a lot of stuff with it inside another thread - that this will crash sooner or later should be clear.

NiHoTleHot

So I debugged qt lib, the problem comes from setParent() doing a sendEvent() without checking the thread affinity. This is simply a bug.
Depending on the calling thread it should make a postevent() instead

I ve just replace obj->setParent(&mainObj); by

QMetaObject::invokeMethod(&mainObj, [&mainObj, obj](){
    obj->setParent(&mainObj);
});

The invokeMethod() execute setParent in the correct.

Christian Ehrlicher

@NiHoTleHot said in Create a QObject in another thread, retrieve it to the current thread and set a parent = ASSERT failure in Debug on msvc16:

This is simply a bug.

No, you're modifying an object which lives in another thread - it may work or it may crash.