Qt tr for internationalisation does not work in main function?

Qt's translation function tr does not work in the main function but works fine in a QWidget member function. Why is that?

int main(int argc, char *argv[])
{

    QApplication a(argc, argv);
    QDialog dialog; 
    QString temp = tr("dadasda");//error:tr was not declared in this scope
    dialog.show();
    return a.exec();
}