MFC源码学习，为什么_AFX_THREAD_STATE *pState=_afxThreadState

        看过深入浅出MFC，但还是一知半解，有些细节还需得自已敲下代码，才能理解得深刻，这两天看MFC源代码，看到_AFX_THREAD_STATE *pState=_afxThreadState 时觉得很奇怪，怎么_AFX_THREAD_STATE类型指针能等于一个CThreadLocal类对象呢？

        _afxThreadState是由EXTERN_THREAD_LOCAL(_AFX_THREAD_STATE, _afxThreadState)   

#define EXTERN_THREAD_LOCAL(class_name, ident_name) extern CThreadLocal<class_name> ident_name;等宏、类定义语句得到的一个CThreadLocal类型的对象

       最后先是自已敲了代码，就跟想象的一样，编译出错，提示类弄转换出错，后来把MFC源代码删减了一些，复制到我的工程里，咦，很奇怪的事发生了，编译成功了。经过翻看c++primer 找出了原因。还是要把基础打好了，把c++ primer 多看几遍，光看c++ primer不行，还得和MFC源代码一起看，并敲下代码，才有好的效果，

MFC源代码从下图语句开始：         

extern "C" int WINAPI_tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,_In_ LPTSTR lpCmdLine, int nCmdShow)#pragma warning(suppress: 4985){// call shared/exported WinMainreturn AfxWinMain(hInstance, hPrevInstance, lpCmdLine, nCmdShow);}

到

int AFXAPI AfxWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,_In_ LPTSTR lpCmdLine, int nCmdShow){ASSERT(hPrevInstance == NULL);int nReturnCode = -1;CWinThread* pThread = AfxGetThread();CWinApp* pApp = AfxGetApp();// AFX internal initializationif (!AfxWinInit(hInstance, hPrevInstance, lpCmdLine, nCmdShow))goto InitFailure;// App global initializations (rare)if (pApp != NULL && !pApp->InitApplication())goto InitFailure;// Perform specific initializationsif (!pThread->InitInstance()){if (pThread->m_pMainWnd != NULL){TRACE(traceAppMsg, 0, "Warning: Destroying non-NULL m_pMainWnd\n");pThread->m_pMainWnd->DestroyWindow();}nReturnCode = pThread->ExitInstance();goto InitFailure;}nReturnCode = pThread->Run();InitFailure:#ifdef _DEBUG// Check for missing AfxLockTempMap callsif (AfxGetModuleThreadState()->m_nTempMapLock != 0){TRACE(traceAppMsg, 0, "Warning: Temp map lock count non-zero (%ld).\n",AfxGetModuleThreadState()->m_nTempMapLock);}AfxLockTempMaps();AfxUnlockTempMaps(-1);#endifAfxWinTerm();return nReturnCode;}

_AFXWIN_INLINE CWinApp* AFXAPI AfxGetApp(){ return afxCurrentWinApp; }

#define afxCurrentWinApp    AfxGetModuleState()->m_pCurrentWinApp

AFX_MODULE_STATE* AFXAPI AfxGetModuleState(){_AFX_THREAD_STATE* pState = _afxThreadState;ENSURE(pState);AFX_MODULE_STATE* pResult;if (pState->m_pModuleState != NULL){// thread state's module state serves as overridepResult = pState->m_pModuleState;}else{// otherwise, use global app statepResult = _afxBaseModuleState.GetData();}ENSURE(pResult != NULL);return pResult;}

CThreadLocal是一个模板类，_afxThreadState是它的一个对象

CThreadLocal类的定义和声明如下：

template<class TYPE>class CThreadLocal : public CThreadLocalObject{// Attributespublic:AFX_INLINE TYPE* GetData(){TYPE* pData = (TYPE*)CThreadLocalObject::GetData(&CreateObject);ENSURE(pData != NULL);return pData;}AFX_INLINE TYPE* GetDataNA(){TYPE* pData = (TYPE*)CThreadLocalObject::GetDataNA();return pData;}AFX_INLINE operator TYPE*(){ return GetData(); }AFX_INLINE TYPE* operator->(){ return GetData(); }// Implementationpublic:static CNoTrackObject* AFXAPI CreateObject(){ return new TYPE; }};

              一直到这里，可以看出_AFX_THREAD_STATE *pState=_afxThreadState，就是_AFX_THREAD_STATE类型指针等于一个CThreadLocal类对象??为什么会是这样的？？

             带着疑问，自已敲了如下段代码在VC++里，看能不能编译通过：

#include <iostream>using namespace std;class a{public:a();virtual ~a();int i;};a::a(){i=1;cout<<"a构造函数"<<endl;}a::~a(){}template<class TYPE>class b{// Attributespublic:b(){cout<<"b 构造函数"<<endl;}TYPE* GetData(){TYPE* p = CreateObject();return p;}public:static TYPE * CreateObject(){ return new TYPE; }};b<a> _c;void main(){a* pState =_c;}

提示出错：cannot convert from 'class b<class a>' to 'class a *'，不能把class b<class a> 转换为class a

最后复制MFC源码，并删减了一些到自已的工程里如下：

#include <iostream>using namespace std;class _AFX_THREAD_STATE{public:_AFX_THREAD_STATE();virtual ~_AFX_THREAD_STATE();int m_pWndInit;};_AFX_THREAD_STATE::_AFX_THREAD_STATE(){m_pWndInit=1;cout<<"_AFX_THREAD_STATE 构造函数"<<endl;}_AFX_THREAD_STATE::~_AFX_THREAD_STATE(){}template<class TYPE>class CThreadLocal{// Attributespublic:CThreadLocal(){cout<<"CThreadLocal 构造函数"<<endl;}TYPE* GetData(){TYPE* pData = CreateObject();return pData;}operator TYPE*(){ return GetData(); }TYPE* operator->(){ return GetData(); }// Implementationpublic:static TYPE * CreateObject(){ return new TYPE; }};//#define THREAD_LOCAL(class_name, ident_name) \//CThreadLocal<class_name> ident_name;//THREAD_LOCAL(_AFX_THREAD_STATE, _afxThreadState)CThreadLocal<_AFX_THREAD_STATE> _afxThreadState;void main(){_AFX_THREAD_STATE* pState = (_AFX_THREAD_STATE*)_afxThreadState;//cout<<pState->m_pWndInit<<endl;}

这份代码与我的代码有一点不同就是有operator type *(){return GetData();}

带着疑问，翻开了c++ primer 类类型转换一节，仔细看了一遍，原来是类里定义的operator type *()是类类型转换，这样做可以使类对象用于需要type*类型的任何地方。