QDialog don't display in the middle of the parent window

I create two QMessageBox(QDialog as the same).One of them has a parent.

QMessageBox msgWithParent(this);
msgWithParent.setText("msgWithParent");
msgWithParent.exec();

QMessageBox msgWithoutParent;
msgWithoutParent.setText("msgWithoutParent");
msgWithoutParent.exec();

But when I run the application,both of the two dialog display in the middle of the screen.