我们在调用CWnd::GetDlgItem()函数时，MSDN告诉我们：The returned pointer may be temporary and should not be stored for later use.
中文意思就是：返回的指针可能是临时的并且最好不要保存起来放到以后用。
猜测：返回的指针既然可能是临时的，那么可能是非临时的（永久的），最好不要保存起来放到以后用（有时候可以保存起来）
源码面前，了无秘密。让我们深入MFC源代码去看个究竟。
先随便建立一个Dialog程序，然后在窗体上拉一个按钮，添加按钮事件，在按钮事件里写上如下代码：GetDlgItem(IDC_BUTTON1);然后给这行代码加上断点。好，我们开始进去看看，运行程序，按下按钮，程序就停在刚才的断点出，然后F11进去。
CWnd* CWnd::GetDlgItem(int nID) const
{
ASSERT(::IsWindow(m_hWnd));
if (m_pCtrlCont == NULL)
return CWnd::FromHandle(::GetDlgItem(m_hWnd, nID));
else
return m_pCtrlCont->GetDlgItem(nID);
}
再跟踪到红色代码中去：
CWnd* PASCAL CWnd::FromHandle(HWND hWnd)
{
CHandleMap* pMap = afxMapHWND(TRUE); //create map if not exist
ASSERT(pMap != NULL);
CWnd* pWnd = (CWnd*)pMap->FromHandle(hWnd);
#ifndef _AFX_NO_OCC_SUPPORT
pWnd->AttachControlSite(pMap);
#endif
ASSERT(pWnd == NULL || pWnd->m_hWnd == hWnd);
return pWnd;
}
再一次跟踪进入红色函数里去：
CHandleMap* PASCAL afxMapHWND(BOOL bCreate)
{
AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
if (pState->m_pmapHWND == NULL && bCreate)
{
BOOL bEnable = AfxEnableMemoryTracking(FALSE);
#ifndef _AFX_PORTABLE
_PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler);
#endif
pState->m_pmapHWND = new CHandleMap(RUNTIME_CLASS(CTempWnd),
offsetof(CWnd, m_hWnd));
#ifndef _AFX_PORTABLE
AfxSetNewHandler(pnhOldHandler);
#endif
AfxEnableMemoryTracking(bEnable);
}
return pState->m_pmapHWND;
}
看一下AFX_MODULE_THREAD_STATE的定义：
// AFX_MODULE_THREAD_STATE (local to thread *and* module)
class AFX_MODULE_THREAD_STATE : public CNoTrackObject
{
public:
AFX_MODULE_THREAD_STATE();
virtual ~AFX_MODULE_THREAD_STATE();
// current CWinThread pointer
CWinThread* m_pCurrentWinThread;
// list of CFrameWnd objects for thread
CTypedSimpleList<CFrameWnd*> m_frameList;
// temporary/permanent map state
DWORD m_nTempMapLock; // if not 0, temp maps locked
CHandleMap* m_pmapHWND;
CHandleMap* m_pmapHMENU;
CHandleMap* m_pmapHDC;
CHandleMap* m_pmapHGDIOBJ;
CHandleMap* m_pmapHIMAGELIST;
// thread-local MFC new handler (separate from C-runtime)
_PNH m_pfnNewHandler;
#ifndef _AFX_NO_SOCKET_SUPPORT
// WinSock specific thread state
HWND m_hSocketWindow;
#ifdef _AFXDLL
CEmbeddedButActsLikePtr<CMapPtrToPtr> m_pmapSocketHandle;
CEmbeddedButActsLikePtr<CMapPtrToPtr> m_pmapDeadSockets;
CEmbeddedButActsLikePtr<CPtrList> m_plistSocketNotifications;
#else
CMapPtrToPtr* m_pmapSocketHandle;
CMapPtrToPtr* m_pmapDeadSockets;
CPtrList* m_plistSocketNotifications;
#endif
#endif
};
看一下黄色的那一行代码，很明显，MFC说临时的map可以被锁定，先记住就可以。
看一下黄色下面的代码：
CHandleMap* m_pmapHWND;
CHandleMap* m_pmapHMENU;
CHandleMap* m_pmapHDC;
CHandleMap* m_pmapHGDIOBJ;
CHandleMap* m_pmapHIMAGELIST;
很明显，这里放了一些临时的映射，包括HWND到CWnd，HDC到CDC的，等等。
并且，MFC除了这些临时的映射表之外，还有永久的映射表。
也就是说，GetDlgItem以及FromHandle等函数返回的CWnd以及CDC等指针是可以保存的，不管是临时的map还是永久 map中的，我们都可以安全的保存，前提是把临时的map锁定，那么不管怎么样，返回的指针可以在任何时候都是安全的。默认的，MFC是在空闲时间里把临 时map里的东西清空掉的。
刚才说到在空闲时间MFC会把临时的map删除掉，我们在刚才那个按钮事件里添上如下代码：
AfxGetApp()->OnIdle(1);
然后运行并跟踪到如下函数：
BOOL CWinApp::OnIdle(LONG lCount)
{
if (lCount <= 0)
{
CWinThread::OnIdle(lCount);
// call doc-template idle hook
POSITION pos = NULL;
if (m_pDocManager != NULL)
pos = m_pDocManager->GetFirstDocTemplatePosition();
while (pos != NULL)
{
CDocTemplate* pTemplate = m_pDocManager->GetNextDocTemplate(pos);
ASSERT_KINDOF(CDocTemplate, pTemplate);
pTemplate->OnIdle();
}
}
else if (lCount == 1)
{
VERIFY(!CWinThread::OnIdle(lCount));
}
return lCount < 1; // more to do if lCount < 1
}
再跟踪进上面的红色函数中，整个函数如下：
BOOL CWinThread::OnIdle(LONG lCount)
{
ASSERT_VALID(this);
#if defined(_DEBUG) && !defined(_AFX_NO_DEBUG_CRT)
// check MFC's allocator (before idle)
if (_CrtSetDbgFlag(_CRTDBG_REPORT_FLAG) & _CRTDBG_CHECK_ALWAYS_DF)
ASSERT(AfxCheckMemory());
#endif
if (lCount <= 0)
{
// send WM_IDLEUPDATECMDUI to the main window
CWnd* pMainWnd = m_pMainWnd;
if (pMainWnd != NULL && pMainWnd->m_hWnd != NULL &&
pMainWnd->IsWindowVisible())
{
AfxCallWndProc(pMainWnd, pMainWnd->m_hWnd,
WM_IDLEUPDATECMDUI, (WPARAM)TRUE, 0);
pMainWnd->SendMessageToDescendants(WM_IDLEUPDATECMDUI,
(WPARAM)TRUE, 0, TRUE, TRUE);
}
// send WM_IDLEUPDATECMDUI to all frame windows
AFX_MODULE_THREAD_STATE* pState = _AFX_CMDTARGET_GETSTATE()->m_thread;
CFrameWnd* pFrameWnd = pState->m_frameList;
while (pFrameWnd != NULL)
{
if (pFrameWnd->m_hWnd != NULL && pFrameWnd != pMainWnd)
{
if (pFrameWnd->m_nShowDelay == SW_HIDE)
pFrameWnd->ShowWindow(pFrameWnd->m_nShowDelay);
if (pFrameWnd->IsWindowVisible() ||
pFrameWnd->m_nShowDelay >= 0)
{
AfxCallWndProc(pFrameWnd, pFrameWnd->m_hWnd,
WM_IDLEUPDATECMDUI, (WPARAM)TRUE, 0);
pFrameWnd->SendMessageToDescendants(WM_IDLEUPDATECMDUI,
(WPARAM)TRUE, 0, TRUE, TRUE);
}
if (pFrameWnd->m_nShowDelay > SW_HIDE)
pFrameWnd->ShowWindow(pFrameWnd->m_nShowDelay);
pFrameWnd->m_nShowDelay = -1;
}
pFrameWnd = pFrameWnd->m_pNextFrameWnd;
}
}
else if (lCount >= 0)
{
AFX_MODULE_THREAD_STATE* pState = _AFX_CMDTARGET_GETSTATE()->m_thread;
if (pState->m_nTempMapLock == 0)
{
// free temp maps, OLE DLLs, etc.
AfxLockTempMaps();
AfxUnlockTempMaps();
}
}
#if defined(_DEBUG) && !defined(_AFX_NO_DEBUG_CRT)
// check MFC's allocator (after idle)
if (_CrtSetDbgFlag(_CRTDBG_REPORT_FLAG) & _CRTDBG_CHECK_ALWAYS_DF)
ASSERT(AfxCheckMemory());
#endif
return lCount < 0; // nothing more to do if lCount >= 0
}
怎么样，当了老半天的间谍了，总算能看出点门道了吧？上面3行红色的代码说的很清楚，这里会把临时的map释放掉。
两个函数的代码如下：
AfxLockTempMaps();
AfxUnlockTempMaps();
void AFXAPI AfxLockTempMaps()
{
AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
++pState->m_nTempMapLock;
}
BOOL AFXAPI AfxUnlockTempMaps(BOOL bDeleteTemps)
{
AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
if (pState->m_nTempMapLock != 0 && --pState->m_nTempMapLock == 0)
{
if (bDeleteTemps)
{
if (bDeleteTemps != -1)
{
// allow COM libraries to be freed
CWinThread* pThread = AfxGetThread();
if (pThread != NULL && pThread->m_lpfnOleTermOrFreeLib != NULL)
(*pThread->m_lpfnOleTermOrFreeLib)(FALSE, FALSE);
}
// clean up temp objects
pState->m_pmapHGDIOBJ->DeleteTemp();
pState->m_pmapHDC->DeleteTemp();
pState->m_pmapHMENU->DeleteTemp();
pState->m_pmapHWND->DeleteTemp();
pState->m_pmapHIMAGELIST->DeleteTemp();
}
#ifndef _AFX_PORTABLE
CWinApp* pApp = AfxGetApp();
_AFX_THREAD_STATE* pThreadState = _afxThreadState.GetData();
// restore safety pool after temp objects destroyed
if (pApp != NULL &&
(pThreadState->m_pSafetyPoolBuffer == NULL ||
_msize(pThreadState->m_pSafetyPoolBuffer) < pApp->m_nSafetyPoolSize) &&
pApp->m_nSafetyPoolSize != 0)
{
// attempt to restore the safety pool to its max size
size_t nOldSize = 0;
if (pThreadState->m_pSafetyPoolBuffer != NULL)
{
nOldSize = _msize(pThreadState->m_pSafetyPoolBuffer);
free(pThreadState->m_pSafetyPoolBuffer);
}
// undo handler trap for the following allocation
BOOL bEnable = AfxEnableMemoryTracking(FALSE);
pThreadState->m_pSafetyPoolBuffer = malloc(pApp->m_nSafetyPoolSize);
if (pThreadState->m_pSafetyPoolBuffer == NULL)
{
TRACE1("Warning: failed to reclaim %d bytes for memory safety pool./n",
pApp->m_nSafetyPoolSize);
// at least get the old buffer back
if (nOldSize != 0)
{
//get it back
pThreadState->m_pSafetyPoolBuffer = malloc(nOldSize);
ASSERT(pThreadState->m_pSafetyPoolBuffer != NULL);
}
}
AfxEnableMemoryTracking(bEnable);
}
#endif // !_AFX_PORTABLE
}
// return TRUE if temp maps still locked
return pState->m_nTempMapLock != 0;
}
看到上面黄色的代码了吧，这时候你还敢保存GetDlgItem或者FromHandle返回的指针吗？
刚才你不是说可以锁定临时map的吗？是的，可以用AfxLockTempMaps锁定，但是必须与AfxUnlockTempMaps配对使用，其内部有一个计数变量。MSDN上没这个函数，最好不要用它，否则除了什么问题我可不保证哦J
永久的map存放的应该就是我们显示创建的例如CWnd此类的对象了。
其实很多函数如FromHandle、FindWindow等函数都是用到了这种技术，所以MSDN必定有这样类似这样的一句话：The returned pointer may be temporary and should not be stored for later use.