条件查找映射
MFC中大量使用了BEGIN_XXX_MAP这样的宏，以及映射进行查找优化，例如消息映射，OLE命令映射，以及接口等等。每个映射包含一个指向基类的映射的指针。这样，当一个类需要根据一定的条件查找一个对象时，它会查找本类对象，如果没有找到，那么会查找基类，直到根基类。这类查找包含Windows消息，命令，事件和OLE命令的分发，和对象实现的接口的查询等等。
下面是函数BOOL CWnd::OnWndMsg(UINT message, WPARAM wParam, LPARAM lParam, LRESULT* pResult)的部分代码，演示了如何根据消息的ID查找处理函数。
const AFX_MSGMAP* pMessageMap; pMessageMap = GetMessageMap();
UINT iHash; iHash = (LOWORD((DWORD_PTR)pMessageMap) ^ message) & (iHashMax-1);
AfxLockGlobals(CR99v_WINMSGCACHE);
AFX_MSG_CACHE* pMsgCache; pMsgCache = &_afxMsgCache[iHash];
const AFX_MSGMAP_ENTRY* lpEntry;
if (message == pMsgCache->nMsg && pMessageMap == pMsgCache->pMessageMap)
{
// cache hit
lpEntry = pMsgCache->lpEntry;
AfxUnlockGlobals(CR99v_WINMSGCACHE);
if (lpEntry == NULL)
return FALSE;
// cache hit, and it needs to be handled
if (message < 0xC000)
goto LDispatch;
else
goto LDispatchRegistered;
}
else
{
// not in cache, look for it
pMsgCache->nMsg = message;
pMsgCache->pMessageMap = pMessageMap;
#ifdef _AFXDLL
for (/* pMessageMap already init'ed */; pMessageMap->pfnGetBaseMap != NULL;
pMessageMap = (*pMessageMap->pfnGetBaseMap)())
#else
for (/* pMessageMap already init'ed */; pMessageMap != NULL;
pMessageMap = pMessageMap->pBaseMap)
#endif
{
// Note: catch not so common but fatal mistake!!
// BEGIN_MESSAGE_MAP(CMyWnd, CMyWnd)
#ifdef _AFXDLL
ASSERT(pMessageMap != (*pMessageMap->pfnGetBaseMap)());
#else
ASSERT(pMessageMap != pMessageMap->pBaseMap);
#endif
if (message < 0xC000)
{
// constant window message
if ((lpEntry = AfxFindMessageEntry(pMessageMap->lpEntries,
message, 0, 0)) != NULL)
{
pMsgCache->lpEntry = lpEntry;
AfxUnlockGlobals(CR99v_WINMSGCACHE);
goto LDispatch;
}
}
else
{
// registered windows message
lpEntry = pMessageMap->lpEntries;
while ((lpEntry = AfxFindMessageEntry(lpEntry, 0xC000, 0, 0)) != NULL)
{
UINT* pnID = (UINT*)(lpEntry->nSig);
ASSERT(*pnID >= 0xC000 || *pnID == 0);
// must be successfully registered
if (*pnID == message)
{
pMsgCache->lpEntry = lpEntry;
AfxUnlockGlobals(CR99v_WINMSGCACHE);
goto LDispatchRegistered;
}
lpEntry++; // keep looking past this one
}
}
}