QT: how to exit application and close UI - c++

Question

I tried to use qApp->exit() to exit application and close UI. but I failed the UI is still there after qApp->exit() executed. Anyone can help to figure out why? thanks a lot.
#include "clsDownloadUpdateList.h"
#include <QApplication>
#include <qtranslator.h>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QTranslator translator;
translator.load("en-CN_upgrader");
qApp->installTranslator(&translator);
clsDownloadUpdateList w;
w.show();
return a.exec();
}
clsDownloadUpdateList::clsDownloadUpdateList(QWidget *parent) :
QMainWindow(parent),
_state(STOP),
ui(new Ui::clsDownloadUpdateList)
{
ui->setupUi(this);
this->setWindowTitle("GCS Upgrader");
// other code
// here comes the code to exit application
qApp->exit();
// but the UI is still there.
}

Answer 1

#thuga is right. The problem you have is caused by your wrong code: you call qApp->exit() before right in your constructor, where your application has not started it's message cycle yet (by a.exec()).
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QTranslator translator;
translator.load("en-CN_upgrader");
qApp->installTranslator(&translator);
clsDownloadUpdateList w; // <- you call qApp->exit() right here, before a.exec();
w.show();
return a.exec();
}

Answer 2

It will not help you in constructor, because of no event loop started yet.
In such case you can use QTimer::singleShot() with timeout equal zero. It will cause calling what you need when event loop started. Also it is good idea to use initialization method and check it in main:
Window w;
if ( !w.init() )
return 1;
w.show();
return a.exec();

Answer 3

Working code:
#include <QMetaObject>
//...
QMetaObject::invokeMethod(qApp, "quit",
Qt::QueuedConnection);
Or for widgets:
QMetaObject::invokeMethod(this, "close",
Qt::QueuedConnection);