import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

ApplicationWindow
{
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

}

Your suggestion:

QMetaObject::invokeMethod(qApp, "quit", Qt::QueuedConnection);

seems to work. Can you tell me what it does different of qApp->quit ?
I guess it quits cleanly ?

Thank you !

Alex

When I run this main.cpp code, even with "qApp->quit();", the program does not quit (the QML window remains open).

Then there's something fishy. Either some thread is hanging or your QML code is hanging. As a side note, how do you ensure the threads had quit before returning and how do you start them (there's nothing even remotely resembling this in the C++ code you pasted).

By the way

qApp->quit();

does nothing here, there's no event loop running. You could try:

QMetaObject::invokeMethod(qApp, "quit", Qt::QueuedConnection);

instead.

I tried "exit(1)" which seems to work but I am not sure it exits cleanly.

Never do that without a good reason. ::exit's purpose is to terminate the application in extreme circumstances, not whenever one feels like it. For one it will not call any of your object's (app and engine) destructors, which is bad enough by itself and has serious implications.

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    qApp->quit();

    return app.exec();
}

The QML code is a simple empty window.

When I run this main.cpp code, even with "qApp->quit();", the program does not quit (the QML window remains open).

From this forum, there seems to be other solutions. I tried "exit(1)" which seems to work but I am not sure it exits cleanly.
http://stackoverflow.com/questions/10038372/how-to-terminate-a-qt-programming-if-initialization-failed

Thank you again for your time !

Alex

Thank you very much,

Alex

Hence my suggestion to store the return value from app.exec(), then quit your threads and then return the value you stored before.

Or do you want your threads to quit your application ?

*main.cpp
int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    return app.exec();
}

This main.cpp program will run and execute "return app.exec();" and the QML UI will remain.
In QML, the function "Qt.quit()" closes everythin but I don't know how to do the same through the main.cpp.

Thank you for your help !

Alex

What about :

QGuiApplication app(argc, argv);

// your stuff

int ret = app.exec();

// Stop your threads cleanly

return ret;