Visual C++ and MFC Programming 2nd Edition

Visual C++ and MFC Fundamentals Chapter 15: Fundamental Controls

CEdit m_RectHeight;

CEdit m_RectPerimeter;

CEdit m_RectArea;

};

8. Save All

14.1.3..The Control’s Data Exchange...........................................................

After declaring the variable, you must “map” it to the control it is referring to, otherwise

the variable would behave just like any other and it cannot directly access the control you

wanted it to refer to. To specify what control your variable refers to, you must call the

DDX_Control() framework function. The syntax of this function is:

void AFXAPI DDX_Control(CDataExchange* pDX, int nIDC, CWnd& rControl);

The first argument, pDX, is a pointer to CDataExchange. The nIDC argument is the

identifier of the control your variable will refer to. The rControl argument is the name

you gave to your variable. An example of calling this function would be:

DDX_Control(pDX, IDC_STATIC_ADV, stcAdvanced);

The pDX argument in reality handles the mapping. It creates the relationship between

your rControl variable and the nIDC control that rControl must refer to. Besides that,

pDX insures that information can flow easily between both entities. The CDataExchange

class, to which the pDX argument points, is a parent-less class, meaning it is based

neither on CObject nor on CWnd.

The DDX_Control() function can be called for each variable you intend to map to a

control. When calling any of these functions, the mappings must be performed in the

CWnd::DoDataExchange() event. Its syntax is:

virtual void DoDataExchange(CDataExchange* pDX);

As you can see, this event is passed a CDataExchange pointer. This pointer in turn will

become the first argument to the DDX_Control() function. Behind the scenes, this allows

the dialog box, the parent of the controls to better manage the exchange of information

between the application and the controls.

This review helps to have an idea of how variables would be declared and associated with

the intended controls. In reality, this job is usually, and should always be, handled by

Visual C++ for you. When you create a dialog-based object, such as a dialog box, a form,

a property sheet, or a property page, the CDialog::DoDataExchange() event is created

and made ready for you. To declare a variable you want to associate to a control, unless

you have a good reason to proceed manually, use either the ClassWizard in MSVC 6 or

the Add Member Variable Wizard in MSVC 7 to add the variable. When you do this, the

wizard will take care of all the mapping for you.

After adding a variable using a wizard and once the variable mapping has been

performed, if you change the name of the variable in the header file, you must manually

change its name in the DoDataExchange() event.