QThread delete on application close?

[CJha]

Hi, I am using the void QObject::moveToThread(QThread *targetThread) approach. My thread is not printing the debug output from its destructor on application close.

MyThread class: h file

class MyThread : public QThread
{
	Q_OBJECT

public:
    const quint64 idx; // to keep a track of the number of threads created
    MyThread(); // to debug output on construction
    ~MyThread() override; // to debug output on destruction

private:
    inline static quint64 count{0};
};

MyThread class: cpp file

MyThread::MyThread() : idx{++count} { qDebug() << "MyThread" << idx; }

MyThread::~MyThread() { qDebug() << "~MyThread" << idx; }

MyObj class: h file

class MyObj : public QObject
{
    Q_OBJECT

public:
    const quint64 idx; // to keep a track of the number of threads created
    MyObj();
    ~MyObj() override;

public slots:
    void useMultiThreading(bool state);

private:
    inline static quint64 count{0};
    QPointer<MyThread> myThread{nullptr};
};

MyObj class: cpp file

MyObj::MyObj() : idx{++count}
{
    qDebug() << "MyObj" << idx;
    myThread = new MyThread;
    connect(myThread, &QThread::finished, myThread, &QThread::deleteLater);
    this->moveToThread(myThread);
    myThread->start();
}

MyObj::~MyObj()
{
    if(!myThread.isNull())
        myThread->quit();
    qDebug() << "~MyObj" << idx;
}

void MyObj::useMultiThreading(bool state)
{
    if(state) {
        if(myThread.isNull()) {
            myThread = new MyThread;
            connect(myThread, &MyThread::finished, myThread, &MyThread::deleteLater);
            this->moveToThread(myThread);
            myThread->start();
        }
        else {
            if(this->thread() != myThread)
                this->moveToThread(myThread);
            if(myThread->isFinished())
                myThread->start();
        }
    }
    else {
        if(this->thread() != qApp->thread()) {
            this->moveToThread(qApp->thread());
            if(!myThread.isNull())
                myThread->quit();
        }
    }
}

MainWindow: h file

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget* parent = nullptr);
    ~MainWindow();

signals:
    void useMultiThreading(bool state);

private:
    Ui::MainWindowClass ui;
    QPointer<MyObj> myObj{nullptr};

private slots:
    void on_useMultiThreading_chk_clicked(bool state);
    void on_useMyObj_chk_clicked(bool state);
};

MainWindow: cpp file

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    ui.setupUi(this);
    myObj = new MyObj;
    connect(this, &MainWindow::useMultiThreading, myObj, &MyObj::useMultiThreading, Qt::QueuedConnection);
}

MainWindow::~MainWindow()
{
    if(!myObj.isNull())
        myObj->deleteLater();
}

void MainWindow::on_useMultiThreading_chk_clicked(bool state) {emit useMultiThreading(state);}

void MainWindow::on_useMyObj_chk_clicked(bool state)
{
    if(state) {
        if(myObj.isNull()) {
            myObj = new MyObj;
            connect(this, &MainWindow::useMultiThreading, myObj, &MyObj::useMultiThreading, Qt::QueuedConnection);
        }
    }
    else {
        if(!myObj.isNull())
            myObj->deleteLater();
    }
}

The slot void MyObj::useMultiThreading(bool state) is connected to void MainWindow::useMultiThreading(bool state) as Qt::QueuedConnection. A checkbox in the main window sends the signal out.

When I send out the signal useMultiThreading(false) the MyObj::myThread is deleted and prints the debug output in its destructor and if I send out signal useMultiThreading(true) and a new MyThread object is created and prints the debug output in its constructor.

If I delete the myObj class object in my MainWindow it calls the destructor of MyThread class and the debug output from both classes destructors are printed. And if I create a new MyObj class object in my MainWindow then again the debug outputs in both MyObj and MyThread class constructors are printed.

But on application close only the debug output from the destructor of MyObj class is printed, not from the MyThread class even if MyObj::myThread is not a nullptr. Why is this? Is my MyObj::myThread object getting destroyed properly on application close?

[Christian Ehrlicher]

@CJha said in QThread delete on application close?:

Why is this? Is my MyObj::myThread object getting destroyed properly on application close?

Correct because of deleteLater(). I would guess the event loop is already gone when the second deleteLater() is called.