QThread, signals and lambdas

[Jeff Barnes]

It took me a while to figure out that this causes a crash. Linguistically, it is opaque to me. Could someone please tell me why this isn't good, please?

MyClass::MyClass
    :   QWidget()
    ,   myObject(new MyObject)
{
    // Causes memory corruption, intermittent crashes, QTimer warnings, etc.
//    QObject::connect(myObject, MyObject::mySignal, []() {
//        QMessageBox mb;
//        mb.setText("Hello world");
//        mb.exec();
//    });
    //this works fine
    QObject::connect(myObject, MyObject::mySignal, this, &MyClass::handleMySignal);
    QThread *thr = new QThread;
    QObject::connect(myObject, MyObject::destroyed, thr, QThread::quit);
    QObject::connect(thr, &QThread, &QThread::finished, thr, &QThread::deleteLater);
    myObject->moveToThread(thr);
    thr->start();
}

void MyClass::handleMySignal()
{
    QMessageBox mb;
    mb.setText("Hello world");
    mb.exec();
}

[Christian Ehrlicher]

This is because the lambda version is executed in the thread, not in the main thread which is not allowed for gui operations.
Pass 'this' as third parameter so the lambda gets executed in the main thread (= where 'this' lives in)

[Jeff Barnes]

@Christian-Ehrlicher Thanks

[KroMignon]

@Jeff-Barnes To solve the issue, you should use this as receiver of the signal, so the lambda will be executed in the right thread:

QObject::connect(myObject, MyObject::mySignal, this, []() {
      QMessageBox mb;
      mb.setText("Hello world");
      mb.exec();
  });