多线程创建对话框时Create出错

最新推荐文章于 2022-12-02 16:05:44 发布

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最新推荐文章于 2022-12-02 16:05:44 发布

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分类专栏：工具箱文章标签： AssertValid CHandleMap afxMapHWND Create MFC

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本文探讨了在MFC程序从单线程转换到多线程过程中遇到的CWnd::AssertValid()函数错误问题。详细分析了问题原因，并提供了解决方案，即通过线程间消息传递机制自定义消息来避免跨线程使用窗口对象。

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太牛逼了，也可看大牛原帖：http://blog.chinaunix.net/uid-24862988-id-3793502.html

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

<span style="color:#666666;">#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();<span style="white-space:pre">		</span>//此处问题
        ASSERT(pMap != NULL);<span style="white-space:pre">				</span>//此处问题

        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</span>

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

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;
}