QEventLoop: Cannot be used without QApplication

qt, qt4, qtxml

Solution

`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";
}

Problem

Original source