qt - Why does calling quit() before exec() not quit the application?

Question

Why does this program run normally and display the main window? I would expect it to exit since quit() is called in the constructor.

Main.cpp:

#include<QApplication>
#include"MainWindow.h"

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);
    MainWindow mainWindow;
    mainWindow.show();
    return app.exec();
}

MainWindow.cpp:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
   qApp->quit();
}

void MainWindow::closeEvent(QCloseEvent *)
{
    qDebug("Hello world!");
}

Answer 1

Since QCoreApplication::quit() is a no-op until the event loop has been started, you need to defer the call until it starts. Thus, queue a deferred method call to quit().

The following lines are functionally identical, either one will work:

QTimer::singleShot(0, qApp, &QCoreApplication::quit);
//or
QTimer::singleShot(0, qApp, SLOT(quit()));
// or - see https://stackoverflow.com/a/21653558/1329652
postToThread([]{ QCoreApplication::quit(); });
// or
QMetaObject::invokeMethod(qApp, "quit", Qt::QueuedConnection);