1. connect(pushButton,SIGNAL(clicked()),this,SLOT(close()));
    2. connect(pushButton_2,SIGNAL(clicked()),this,SLOT(close()));
    3. connect(pushButton_3,SIGNAL(clicked()),this,SLOT(close()));
    4. //对象是pushButton,它产生了clicked的信号,被槽slot接受到了,触发了close的关闭事件