QT信槽编程，QObject::connect: Cannot connect (null)报错的两种成因

{	
	connect(
		ui.btnHelp, SIGNAL(clicked()), this, SLOT(OnBtnHelp())
		);
	connect(
		ui.btnHelp, SIGNAL(toggled(bool)), this, SLOT(OnBtnHelpChanged(bool))
		);
	ui.setupUi(this);
}

上面这段代码报错如下:

QObject::connect: Cannot connect (null)::clicked() to Test5_4::OnBtnHelp()
QObject::connect: Cannot connect (null)::toggled(bool) to Test5_4::OnBtnHelpChanged(bool)

这是ui没有被提前初始化的原因，把最后一段代码前置就可以了，原因是：（quote自http://blog.csdn.net/oowgsoo/article/details/1529411#comments）

即编译器未能识别到信号发送者，sender为零时，报错NULL。

 // 不允许空输入
    if (sender == 0 || receiver == 0 || signal == 0 || method == 0) {
#ifndef QT_NO_DEBUG
        qWarning("Object::connect: Cannot connect %s::%s to %s::%s",
                 sender ? sender->metaObject()->className() : "(null)",
                 signal ? signal+1 : "(null)",
                 receiver ? receiver->metaObject()->className() : "(null)",
                 method ? method+1 : "(null)");
#endif
        return false;
    }

发送和接收者定义重叠，也会保类似错误，详见：

http://stackoverflow.com/questions/8493506/qobjectconnect-cannot-connect-null