//////////////////////////////////////////////////////////////////////

Recall the Macro definition of DECLARE_MESSAGE_MAP()

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


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)
};

struct AFX_MSGMAP
{
	const AFX_MSGMAP* (PASCAL* pfnGetBaseMap)();
	const AFX_MSGMAP_ENTRY* lpEntries;
};

typedef void (AFX_MSG_CALL CCmdTarget::*AFX_PMSG)(void);

//////////////////////////////////////////////////////////////////////

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

///////////////////////////////////////////////////////////////////////
Using this definition, then the following line:

   BEGIN_MESSAGE_MAP(CSketcherApp, CWinApp)

will become, after preprocessor, the following:
///////////////////////////////////////////////////////////////////////

const AFX_MSGMAP* PASCAL CSketcherApp::_GetBaseMessageMap()
{ 
    return &CWinApp::messageMap;
}


const AFX_MSGMAP* CSketcherApp::GetMessageMap() const
{ 
    return &CSketcherApp::messageMap;
}


AFX_COMDAT AFX_DATADEF const AFX_MSGMAP CSketcherApp::messageMap = 
	{ &CSketcherApp::_GetBaseMessageMap, &CSketcherApp::_messageEntries[0] };
	
	
AFX_COMDAT const AFX_MSGMAP_ENTRY CSketcherApp::_messageEntries[] =
{

//////////////////////////////////////////////////////////////////////
BEGIN_MESSAGE_MAP(CSketcherApp, CWinApp)
	//{{AFX_MSG_MAP(CSketcherApp)
	ON_COMMAND(ID_APP_ABOUT, OnAppAbout)
	ON_COMMAND(ID_FILE_NEW, CWinApp::OnFileNew)
	ON_COMMAND(ID_FILE_OPEN, CWinApp::OnFileOpen)
	ON_COMMAND(ID_FILE_PRINT_SETUP, CWinApp::OnFilePrintSetup)
END_MESSAGE_MAP()

where in the file afxmsg.h, we find

#define ON_COMMAND(id, memberFxn) \
	{ WM_COMMAND, CN_COMMAND, (WORD)id, (WORD)id, AfxSig_vv, (AFX_PMSG)&memberFxn },

and

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

This block of codes now looks like the following:
//////////////////////////////////////////////////////////////////////

const AFX_MSGMAP* PASCAL CSketcherApp::_GetBaseMessageMap()
{ 
    return &CWinApp::messageMap;
}


const AFX_MSGMAP* CSketcherApp::GetMessageMap() const
{ 
    return &CSketcherApp::messageMap;
}


AFX_COMDAT AFX_DATADEF const AFX_MSGMAP CSketcherApp::messageMap = 
	{ &CSketcherApp::_GetBaseMessageMap, &CSketcherApp::_messageEntries[0] };
	
	
AFX_COMDAT const AFX_MSGMAP_ENTRY CSketcherApp::_messageEntries[] =
{
	//{{AFX_MSG_MAP(CSketcherApp)
	{ WM_COMMAND, CN_COMMAND, (WORD)ID_APP_ABOUT, (WORD)ID_APP_ABOUT, AfxSig_vv, (AFX_PMSG)&OnAppAbout },
	{ WM_COMMAND, CN_COMMAND, (WORD)ID_FILE_NEW, (WORD)ID_FILE_NEW, AfxSig_vv, (AFX_PMSG)&CWinApp::OnFileNew },
	{ WM_COMMAND, CN_COMMAND, (WORD)ID_FILE_OPEN, (WORD)ID_FILE_OPEN, AfxSig_vv, (AFX_PMSG)&CWinApp::OnFileOpen },
	{ WM_COMMAND, CN_COMMAND, (WORD)ID_FILE_PRINT_SETUP, (WORD)ID_FILE_PRINT_SETUP, AfxSig_vv, (AFX_PMSG)&CWinApp::OnFilePrintSetup },
	{          0,          0,                         0,                         0, AfxSig_end, (AFX_PMSG)0 }
};