Qt2 QRadioButton,点击选择按钮,连接的退出程序槽,怎么没反应啊?

定义了3个单选按钮,然后添加到按钮组,最后定义连接的槽,程序如下,请帮分析,哪没设置好?
    group = new QButtonGroup(tr("Please Select Display"), this);
    group->setGeometry(20, 50, 230, 120);
    b1 = new QRadioButton(tr("Form Display"), group);
    b1->move(30, 25);
    b2 = new QRadioButton(tr("Histogram Display"), group);
    b2->move(30, 50);
    b3 = new QRadioButton(tr("Curves Show"), group);
    b3->move(30, 75);
    group->insert(b1);
    group->insert(b2);
    group->insert(b3);

                     connect( b1, SIGNAL(ischecked()),  qApp,SLOT(quit()));