static function
QUrl QFileDialog::getSaveFileUrl(QWidget * parent = 0, const QString & caption = QString(), const QUrl & dir = QUrl(), const QString & filter = QString(), QString * selectedFilter = 0, Options options = 0, const QStringList & supportedSchemes = QStringList()) [static]

when press cancel button, an empty path of url will be retrun. but, this is be described in Qt 5.3.0 Reference Documentation as follow:
This is a convenience static function that returns a file selected by the user. The file does not have to exist. If the user presses Cancel, it returns an empty url.