轉載：http://blog.csdn.net/xyyangkun/article/details/7345423

You must call QWidget::show to show the label since you add it after the parent widget has already been shown.

QLabel* l = new QLabel;
l->setText("Hello World!");
l->setParent(w);
l->setGeometry(0,0,100,100);
l->show();

An alternative solution is to show the parent after all the child widgets are already added. You don't need to allocate anything explicitly the heap:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWidget w;
    QLabel l("Hello World!", &w);
    l.setGeometry(0,0,100,100);
    w.show();
    return a.exec();
}