8）事例部分代码对比说明：
代码一：
CDataExchange dx(this,TRUE);
DDX_Text(&dx,IDC_DAY,m_sDay);
DDX_Text(&dx,IDC_MONTH,m_sMonth);
DDX_Text(&dx,IDC_YEAR,m_sYear);
说明一：
CDataExchange类构造函数：（注，在MFC|SRC|AFXWIN.H中可以看到其构造函数声明，在MFC|SRC|WINCORE.CPP文件中可以看到其构造函数的定义。）
原型：CDataExchange::CDataExchange(CWnd* pDlgWnd, BOOL bSaveAndValidate);
定义：
CDataExchange::CDataExchange(CWnd* pDlgWnd, BOOL bSaveAndValidate)
{
ASSERT_VALID(pDlgWnd);
m_bSaveAndValidate = bSaveAndValidate;
m_pDlgWnd = pDlgWnd;
m_hWndLastControl = NULL;
}
//其中m_pDlgWnd和m_bSaveAndValidate是CDataExchange数据成员,可以通过这中方式给它们赋值。
m_pDlgWnd：The dialog box or window where the data exchange takes place.
m_bSaveAndValidate Flag for the direction of DDX and DDV. 详见说明二。
说明二：
//CDataExchange does not have a base class.
//The CDataExchange class supports the dialog data exchange (DDX) and dialog data validation (DDV) routines used by the Microsoft Foundation classes. Use this class if you are writing data exchange routines for custom data types or controls, or if you are writing your own data validation routines.
//A CDataExchange object provides the context information needed for DDX and DDV to take place. The flag m_bSaveAndValidate is FALSE when DDX is used to fill the initial values of dialog controls from data members. The flag m_bSaveAndValidate is TRUE when DDX is used to set the current values of dialog controls into data members and when DDV is used to validate the data values. If the DDV validation fails, the DDV procedure will display a message box explaining the input error. The DDV procedure will then call Fail to reset the focus to the offending control and throw an exception to stop the validation process.
代码二：（是为了比较代码一做些说明的）
void CActiveXDialog::DoDataExchange(CDataExchange* pDX)
{
CDialog::DoDataExchange(pDX);//调用基类的DoDataExchange
//{{AFX_DATA_MAP(CActiveXDialog)
DDX_Control(pDX, IDC_CALENDAR1, m_calendar);
DDX_Text(pDX, IDC_DAY, m_sDay);
DDX_Text(pDX, IDC_MONTH, m_sMonth);
DDX_Text(pDX, IDC_YEAR, m_sYear);
//}}AFX_DATA_MAP
}
说明一：
//DoDataExchange Called by the framework to exchange and validate dialog data.
//DoDataExchange Never call this function directly. It is called by the UpdateData member function. Call UpdateData to initialize a dialog box’s controls or retrieve data from a dialog box.
说明二：
//在MFC|SRC|WINCORE.CPP文件中可以看到UpdateData函数的定义
BOOL CWnd::UpdateData(BOOL bSaveAndValidate)
{ ...
CDataExchange dx(this, bSaveAndValidate);//创建了一个CDataExchange对象，与当前窗口相关联
...
DoDataExchange(&dx); //注意：DoDataExchange是个虚函数。子类中如果有重写了，则调用子类的。
...
}
说明三：
//在MFC|Include|AFXWIN2.INL文件中可有看到CWnd::DoDataExchange的如下定义（内联）：
// CWnd dialog data support
_AFXWIN_INLINE void CWnd::DoDataExchange(CDataExchange*)
{ } // default does nothing
由此可见代码二中CDialog::DoDataExchange(pDX)调用好象是个‘摆设’，不起做任何事情。框架设置DoDataExchange函数目的是在我们子窗口类（这里是对话筐）中重写它，添加代码完成子窗口类（这里是对话筐）中数据成员与对话筐上控件的交互。
说明四：
如果结合UpdateData和DoDataExchange两函数整体来看，应该体会到这里代码一与代码二实质上是一会事情。代码二只是借助了框架兜了些圈子。
说明四：
摘录MSDN中Dialog Data Exchange一些E文段落对上讨论做个总结：
Dialog Data Exchange
If you use the DDX mechanism, you set the initial values of the dialog object’s member variables, typically in your OnInitDialog handler or the dialog constructor. Immediately before the dialog is displayed, the framework’s DDX mechanism transfers the values of the member variables to the controls in the dialog box, where they appear when the dialog box itself appears in response to DoModal or Create. The default implementation of OnInitDialog in CDialog calls the UpdateData member function of class CWnd to initialize the controls in the dialog box.
The same mechanism transfers values from the controls to the member variables when the user clicks the OK button (or whenever you call the UpdateData member function with the argument TRUE). The dialog data validation mechanism validates any data items for which you specified validation rules.
UpdateData works in both directions, as specified by the BOOL parameter passed to it. To carry out the exchange, UpdateData sets up a CDataExchange object and calls your dialog class’s override of CDialog’s DoDataExchange member function. DoDataExchange takes an argument of type CDataExchange. The CDataExchange object passed to UpdateData represents the context of the exchange, defining such information as the direction of the exchange.
