QObject::~QObject: Timers cannot be stopped from another thread - c++

Question

I have this very simple Qt code:
void thread_func()
{
int a1 = 1;
const char* a2[] = { "dummy_param" };
QApplication app(a1, (char**)a2);
QMessageBox msg(QMessageBox::NoIcon, "MyTitle", "Foo bar Foo bar", QMessageBox::Ok);
msg.exec();
}
If I call the above function from my main in a std::thread, it brings up the dialog:
int main()
{
std::thread t(thread_func);
t.join();
}
...but when I close it, I get the warning message:
QObject::~QObject: Timers cannot be stopped from another thread
I've checked that the thread affinity of both QApplication instance and msg is the same. Calling the thread_func function directly from my main() (without creating a std::thread) removes that message.
I am using Qt 5.15.1 on Windows 10.
What am I missing here? Thanks

Answer 1

It's not allowed to operate Qt GUI directly outside the main thread(GUI thead). You can emit signals.

Answer 2

The warning message says it all. Use a signal/slot mechanism to accomplish the same thing.
#include <QApplication>
#include <QMessageBox>
#include <QObject>
#include <QThread>
#include <QWidget>
class Widget : public QWidget
{
Q_OBJECT
public:
Widget(){}
public slots:
void displayMessageBox()
{
QMessageBox msg(QMessageBox::NoIcon, "MyTitle", "Foo bar Foo bar", QMessageBox::Ok);
msg.exec();
this->close();
}
};
class Worker : public QObject
{
Q_OBJECT
public:
explicit Worker() {}
void start() { emit askForMessageBox(); }
signals:
void askForMessageBox();
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget *widget;
Worker *worker;
QThread *thread(nullptr);
widget = new Widget();
worker = new Worker();
thread = new QThread(nullptr);
QObject::connect(worker, &Worker::askForMessageBox, widget, &Widget::displayMessageBox);
QObject::connect(thread, &QThread::started, worker, &Worker::start);
widget->show();
worker->moveToThread(thread);
thread->start();
return a.exec();
}