QEventLoop: Cannot be used without QApplication

Question

I'm trying to validate an xml file against a specific schema.
So I'm loading the schema into the QXmlSchema object. But I get some strange errors.
My code looks like:

int main() {

QUrl url("http://www.schema-example.org/myschema.xsd");

QXmlSchema schema;
if (schema.load(url) == true)
    qDebug() << "schema is valid";
else
    qDebug() << "schema is invalid";

return 1;
}

When I try to run the above piece of code, Qt errors out saying:

QEventLoop: Cannot be used without QApplication
QDBusConnection: system D_Bus connection created before QCoreApplication.
Application may misbehave.
QEventLoop: Cannot be used without QApplication

My Qt Designer version: qt4-designer 4:4.8.1-0ubuntu4.1
My Qt Creator version : qtcreator 2.4.1-0ubuntu2

Could someone please help me to solve this problem.
Thanks

Answer 1

QXmlSchema creates, among other things, a message handler which inherits from QObject. Since this message handler will be using Qt's event system, an event loop (the structure which handles queueing and routing of events) is required. As the error messages tell you, the main event loop is created along with your QApplication.

If you're creating a GUI application generally you should have a small amount of code in your main() function, something like:

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  MainWindow w;
  w.show();

  return a.exec();
}

Start your code off in, say, the constructor of MainWindow:

MainWindow::MainWindow(QWidget *parent) :
  QMainWindow(parent),
  ui(new Ui::MainWindow)
{
  ui->setupUi(this);

  QUrl url("http://www.schema-example.org/myschema.xsd");

  QXmlSchema schema;
  if (schema.load(url) == true)
    qDebug() << "schema is valid";
  else
    qDebug() << "schema is invalid";
}