CWnd::AssertValid()函数执行出错---MFC多线程

1. 问题

在修改单线程MFC程序为多线程时，遇到了CWnd::AssertValid()函数执行出错问题。主要表现是在执行下面代码中绿色语句时出错

点击(此处)折叠或打开#ifdef _DEBUG

void CWnd::AssertValid() const

{

if (m_hWnd == NULL)

return; // null (unattached) windows are valid

// check for special wnd??? values

ASSERT(HWND_TOP == NULL); // same as desktop

if (m_hWnd == HWND_BOTTOM)

ASSERT(this == &CWnd::wndBottom);

else if (m_hWnd == HWND_TOPMOST)

ASSERT(this == &CWnd::wndTopMost);

else if (m_hWnd == HWND_NOTOPMOST)

ASSERT(this == &CWnd::wndNoTopMost);

else

{

// should be a normal window

ASSERT(::IsWindow(m_hWnd));

// should also be in the permanent or temporary handle map

CHandleMap* pMap = afxMapHWND();

ASSERT(pMap != NULL);

CObject* p=NULL;

if(pMap)

{

ASSERT( (p = pMap->LookupPermanent(m_hWnd)) != NULL ||

(p = pMap->LookupTemporary(m_hWnd)) != NULL);

}

ASSERT((CWnd*)p == this); // must be us

// Note: if either of the above asserts fire and you are

// writing a multithreaded application, it is likely that

// you have passed a C++ object from one thread to another

// and have used that object in a way that was not intended.

// (only simple inline wrapper functions should be used)

//

// In general, CWnd objects should be passed by HWND from

// one thread to another. The receiving thread can wrap

// the HWND with a CWnd object by using CWnd::FromHandle.

//

// It is dangerous to pass C++ objects from one thread to

// another, unless the objects are designed to be used in

// such a manner.

}

}

#endif

2. 原因

通过在网上查询资料发现问题的原因：在B线程中，直接通过A线程的窗口实例指针调用窗口关联函数。在MFC中，窗口类的事件映射表是和线程相关联，故只有在本线程中才能通过窗口实例指针调用该窗口关联函数。下面这段代码是afxMapHWND函数的实现，其中绿色部分是线程相关的变量的结构体的获取，事件映射表就是这个结构体的成员变量
点击(此处)折叠或打开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(CWnd),

ConstructDestruct<CWnd>::Construct, ConstructDestruct<CWnd>::Destruct,

offsetof(CWnd, m_hWnd));

#ifndef _AFX_PORTABLE

AfxSetNewHandler(pnhOldHandler);

#endif

AfxEnableMemoryTracking(bEnable);

}

return pState->m_pmapHWND;

}

下面的一段代码是函数定义和函数调用。CMainFrame类的实例指针mainFrame是主线程(A线程)的窗口实例指针，该指针在工作线程(B线程)中被用来调用类的成员函数getHtmlView，该函数是窗口关联函数(获取窗口中的视窗)。当在工作线程中获取窗口视窗时，就会调用函数CWnd::AssertValid()，执行该函数时会调用上面的afxMapHWND函数。很显然上述代码中绿色中的结构体AFX_MODULE_THREAD_STATE获取的是当前线程(即工作线程)的结构体，而非主线程的结构体，故出现了标题1中描述的问题

点击(此处)折叠或打开 // 在B线程中执行下述代码CMainFrame *mainFrame = (CMainFrame *)AfxGetApp()->GetMainWnd();

utHtmlView *htmlView = (utHtmlView *)mainFrame->getHtmlView();

// 函数定义CView *CMainFrame::getHtmlView( void )

{

if(!m_nMultiInterface)

return GetActiveView();

return (CView *)m_wndSplitter.GetPane(0, 1);

}

3. 解决

1) 方法一

CWnd::AssertValid只会在Debug下有效，而上述问题只会在多线程程序中出现。简单的解决方法是在Release下运行程序，就可以避免上述问题

2)方法二

用 FromHandle 返回临时对象的指针，就可以调用各种类的方法。临时对象会在 OnIdle 中销毁。这里对 FromHandle 的实现原理从源码上进行解析
下面代码是 CWnd 的 FromHandle 方法，大致的意思为从 CHandleMap 中获取临时 CWnd 对象的指针
点击(此处)折叠或打开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;

}

点击(此处)折叠或打开

// // 2 // 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; }再看#define offsetof(s,m) (size_t)&(((s *)0)->m)继续// // 3 // CObject* CHandleMap::FromHandle(HANDLE h) { ASSERT(m_pClass != NULL); ASSERT(m_nHandles == 1 || m_nHandles == 2); if (h == NULL) return NULL; CObject* pObject = LookupPermanent(h); if (pObject != NULL) return pObject; // return permanent one else if ((pObject = LookupTemporary(h)) != NULL) { HANDLE* ph = (HANDLE*)((BYTE*)pObject + m_nOffset); ASSERT(ph[0] == h || ph[0] == NULL); ph[0] = h; if (m_nHandles == 2) { ASSERT(ph[1] == h || ph[1] == NULL); ph[1] = h; } return pObject; // return current temporary one } // This handle wasn't created by us, so we must create a temporary // C++ object to wrap it. We don't want the user to see this memory // allocation, so we turn tracing off. BOOL bEnable = AfxEnableMemoryTracking(FALSE); #ifndef _AFX_PORTABLE _PNH pnhOldHandler = AfxSetNewHandler(&AfxCriticalNewHandler); #endif CObject* pTemp = NULL; TRY { pTemp = m_pClass->CreateObject(); if (pTemp == NULL) AfxThrowMemoryException(); m_temporaryMap.SetAt((LPVOID)h, pTemp); } CATCH_ALL(e) { #ifndef _AFX_PORTABLE AfxSetNewHandler(pnhOldHandler); #endif AfxEnableMemoryTracking(bEnable); THROW_LAST(); } END_CATCH_ALL #ifndef _AFX_PORTABLE AfxSetNewHandler(pnhOldHandler); #endif AfxEnableMemoryTracking(bEnable); // now set the handle in the object HANDLE* ph = (HANDLE*)((BYTE*)pTemp + m_nOffset); // after CObject ph[0] = h; if (m_nHandles == 2) ph[1] = h; return pTemp; }
对标题2中的程序进行修改如下，就可以避免出现上面的问题。首先获取到主窗口实例指针，然后根据主窗口实例指针的窗口句柄获取到临时主窗口实例指针，最后就可以通过临时实例指针操作该类中的各种方法了
点击(此处)折叠或打开CWnd *mWnd = AfxGetApp()->GetMainWnd();

CMainFrame *mainFrame = (CMainFrame *)(mWnd->FromHandle(mWnd->GetSafeHwnd()));

utHtmlView *htmlView = (utHtmlView *)mainFrame->getHtmlView();

3)方法三

方法二虽然解决了上面遇到的问题，但是不能完全解决。比如在主窗口下还有子窗口，通过上述方法主窗口可以调用类的各种方法，但是通过类方法再去调用子窗口的窗口关联函数时还是会出现问题。比如在下面的代码中，在主窗口的方法中，调用子窗口的窗口方法还是会遇到类似问题点击(此处)折叠或打开CView *CMainFrame::getHtmlView( void )

{

if(!m_nMultiInterface)

return GetActiveView();

return (CView *)m_wndSplitter.GetPane(0, 1);

}

要彻底解决这个问题，那么就需要通过多线程的线程间消息机制解决上述问题，MFC多线程消息机制
///////////////////////////////////////////////////////////////////////////////////////////////////////////
A. 声明与定义
(1) 消息声明
#define WM_MY_MESSAGE WM_USER + 100
(2) 类声明中添加以下声明
DECLARE_MESSAGE_MAP()

afx_msg LRESULT OnMyMessage(WPARAM wParam,LPARAM lParam);

(3) 类实现中添加下一代码
BEGIN_MESSAGE_MAP(MYCLASS, MYPARENTCLASS)
ON_MESSAGE(WM_MY_MESSAGE, OnMyMessage)
END_MESSAGE_MAP()

LRESULT MYCLASS::OnMyMessage(WPARAM wParam,LPARAM lParam)
{
return S_OK;
}

////////////////////////////////////////////////////////////////////////////
B. 工作线程代码实现
MYCLASS *myClass;
同步消息传递
HRESULT hr = myClass->SendMessage(WM_MY_MESSAGE, wParam, lParam);
异步消息传递
HRESULT hr = myClass->PostMessage(WM_MY_MESSAGE, wParam, lParam);

转载自: http://blog.chinaunix.net/uid-24862988-id-3793502.html