今天在QMainWindow添加控制，无法显示，加上布局管理器后也一样，到底为何？
首先，对于QDialog而言，添加控件过程：
1、创建子窗口部件，如
QRadioButton *radio1 = new QRadioButton(tr("&Radio button 1"));
2、创建一个布局
QVBoxLayout *vbox = new QVBoxLayout;
3、将子窗口部件添加到布局中去
vbox->addWidget(radio1);
4、将布局设置到QDialog中去
this->setLayout(vbox);
这样，在Qdialog就可以看到子窗口部件了；但在QMainWindow中看不到？？？
求助google、百度，从QT Assistant 中找到这句话：
Note: Creating a main
window without a central widget is
not supported. You must have a central widget even if it is just a placeholder.
原来mainwindow必须要有个centralWidget.于是便给他一个widget。示例代码如下：
QWidget *centerWindow = new QWidget;
this->setCentralWidget(centerWindow);
button1 = new QPushButton("One");
button2 = new QPushButton("Two");
button3 = new QPushButton("Three");
button4 = new QPushButton("Four");
button5 = new QPushButton("Five");
layout = new QVBoxLayout;
layout->addWidget(button1);
layout->addWidget(button2);
layout->addWidget(button3);
layout->addWidget(button4);
layout->addWidget(button5);
QGroupBox *groupBox = new QGroupBox(tr("登录"));
groupBox->setLayout(layout);
QVBoxLayout *mainLayout = new QVBoxLayout;
mainLayout->addWidget(groupBox);
centerWindow->setLayout(mainLayout);