create new QApplication in different thread

csab6597

running this very simple code

int main(int argc, char** argv) {
    qDebug() << "create app 1" << QThread::currentThreadId();
    QApplication* a = new QApplication(argc, argv);
    qDebug() << "delete app 1";
    delete a;

    qDebug() << "create app 2" << QThread::currentThreadId();
    QApplication* b = new QApplication(argc, argv);
    qDebug() << "delete app 2";
    delete b;

    auto func = [&] {
        qDebug() << "create app 3" << QThread::currentThreadId();
        QApplication* c = new QApplication(argc, argv);
        qDebug() << "delete app 2";
        delete c;
    };
    std::thread t(func);
    t.join();

    qDebug() << "done";
    return 0;
}

I get the output

create app 1 0x420c
delete app 1
create app 2 0x420c
delete app 2
create app 3 0x75fc
WARNING: QApplication was not created in the main() thread.
Exception thrown at 0x00007FFE915B7BC9 (Qt6Guid.dll)

Now I do not understand:
I read there can be only one instance of QApplication - Ok I take care of that
I read a QApplication can be created on a different thread, which then becomes sort of the application thread
Then why is this example not possible?

Qt 6.11.1, Windows 11

Ronel_qtmaster

@csab6597 this is bad coding . I think it is better to use QProcess as it seems you dont understand the purpose of your QApplication

JKSH

@csab6597 said in create new QApplication in different thread:

I read a QApplication can be created on a different thread, which then becomes sort of the application thread

Yes, that's right.

If you don't create a and b first, then your c will not produce any warnings and your func thread will be considered the "main thread".

Then why is this example not possible?

This is a current limitation in Qt. QGuiApplication creates some internal QObjects that never get destroyed (here is one example: https://qt-project.atlassian.net/browse/QTBUG-141714), so it is currently not possible to re-create QGuiApplication/QApplication in a different thread from your first instance.

In contrast, your example should work fine if you use QCoreApplication.

JKSH

@Ronel_qtmaster said in create new QApplication in different thread:

@csab6597 this is bad coding . I think it is better to use QProcess as it seems you dont understand the purpose of your QApplication

There are perfectly valid use-cases to destroy a QApplication and recreate it in another thread. For example, when Qt is used to implement a plugin for a non-Qt app.

TimTom420

Thanks for the clarification. That explains the behavior.

The limitation makes sense for my use case (Qt used as a plugin inside a non-Qt application). I had interpreted the documentation to mean that, as long as only one QApplication instance exists at a time, recreating it in a different thread would be supported.

Knowing that QGuiApplication leaves behind internal objects tied to the first thread, making cross-thread recreation currently impossible, answers my question. I'll adjust the design accordingly.

Thanks for the detailed explanation and the Qt bug reference.