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();
  });