How MFC uses event handling and what is the utility of DECLARE_MESSAGE_MAP(), BEGIN_MESSAGE_MAP() and END_MESSAGE_MAP() macro?

MFC Event handling

MFC uses very limited number of callback events via virtual functions. A wide range of events can be implemented by event message array. There is a static array of type AFX_MSGMAP_ENTRY named as _ messageEntries. AFX_MSGMAP_ENTRY is a structure which has all necessary members needed to represent a windows message.

Message Data Structure

struct AFX_MSGMAP_ENTRY
{
  UINT nMessage;   /* windows message */
  UINT nCode;      /* control code or WM_NOTIFY code */
  UINT nID;        /* control ID (or 0 for windows messages) */
  UINT nLastID;    /* used for entries specifying a range of control id's */
  UINT nSig;       /* signature type (action) or pointer to message # */
  AFX_PMSG pfn;    /* routine to call (or special value) */
};

MFC Message Map

User declares this array with DECLARE_MESSAGE_MAP and defines this with starting with BEGIN_MESSAGE_MAP and ends with END_MESSAGE_MAP. BEGIN_MESSAGE_MAP declares array name and opens static array entries. User defines event maps one by one. Then this array declaration is closed with END_MESSAGE_MAP which is an array element where all the member elements are zeros. MFC framework iterates through these array elements and calls the callback event function when a proper match is found. It stops if it reaches till the last element.

#define DECLARE_MESSAGE_MAP() \
private: \
  static const AFX_MSGMAP_ENTRY _messageEntries[]; \
protected: \
  static AFX_DATA const AFX_MSGMAP messageMap; \
  virtual const AFX_MSGMAP* GetMessageMap() const; \

#define BEGIN_MESSAGE_MAP(theClass, baseClass) \
  const AFX_MSGMAP* theClass::GetMessageMap() const \
    { return &theClass::messageMap; } \
  AFX_COMDAT AFX_DATADEF const AFX_MSGMAP theClass::messageMap = \
  { &baseClass::messageMap, &theClass::_messageEntries[0] }; \
  AFX_COMDAT const AFX_MSGMAP_ENTRY theClass::_messageEntries[] = \
  { \

#define END_MESSAGE_MAP() \
    {0, 0, 0, 0, AfxSig_end, (AFX_PMSG)0 } \
  }; \

#define ON_WM_PAINT() \
  { WM_PAINT, 0, 0, 0, AfxSig_vv, \
    (AFX_PMSG)(AFX_PMSGW)(void (AFX_MSG_CALL CWnd::*)(void))&OnPaint },

Message Map Example

/* Use of MESSAGE_MAP */
class CMyDialog : public CDialog
{
  CMyDialog();
  void OnPaint();
  DECLARE_MESSAGE_MAP()
};

BEGIN_MESSAGE_MAP(CMyDialog, CDialog)
ON_WM_PAINT()
END_MESSAGE_MAP()
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Further readings

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Basic steps in Win32 GUI Application with source code.

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