COM组件开发实践（六）---From C++ to COM :Part 3-优快云博客

在上一篇文章《COM组件开发实践（五）---From C++ to COM :Part 2 》中，我们进展到使用COM库加载C++对象了，这一篇中我们将真正将C++对象变成 COM对象，而在下一篇中我们将为它添加多接口支持。

C++对象变成COM对象

要将一个C++对象变成一个真正的COM对象，只需要如下操作：

1）实现接口的引用计数。因此每个COM对象都需要有两个函数用于管理引用计数：

ULONG AddRef();

ULONG Release();

这两个函数不返回HRESULT,因为它们不可能失败，除非对象已经不存在，而且它们也不需要返回值，因为它们只是简单地加减引用计数。

2）对象允许实现多个接口。假如对象要为不同的客户返回不同的接口，则需要客户告诉对象它需要哪个接口。实际上前面已经运用了一种方法：DllGetClassObject函数的IID参数。如果对象有多个接口，则类工厂对象的CreateDB函数（也就是真正创建对象的函数）应该增加一个参数：接口ID,即IID。

假如我们的DB对象有多个接口，一个接口用于建表，一个用于读写，一个用于获得数据库信息。我们可以让DB对象提供一个接口查询函数，

HRESULT QueryInterface(RIID riid,void** ppObj);

3）类工厂对象使用标准的IClassFactory接口。4）使用_stdcall调用约定。

5）实现DLL动态卸载。DLL的卸载由COM负责，COM查询DLL看是否还在使用，它会调用DLL的引出函数DllCanUnloadNow来判断是否可以卸载DLL.如果客户程序想让COM卸载所有未使用的库，它会调用CoFreeUnusedLibraries函数。

6）实现对象自动注册。只需要DLL实现两个引出函数DllRegisterServer和DllUnregisterServer

1，修改接口文件

1）将IDB由IUnknown派生，删除Release成员函数声明，为IDB所有成员函数添加_stdcall（因为COM对象在win32下采用标准调用约定）。2)删除类工厂IDBSrvFactory声明，因为我们现在要使用标准类工厂接口IClassFactory，同时也删除IID_IDBSrvFactory的声明。3）声明外部变量IID_IDB,

typedef long HRESULT;
#define DEF_EXPORT__declspec(dllexport)
// {30DF3430-0266-11cf-BAA6-00AA003E0EED}
extern const GUIDCLSID_DBSAMPLE;
// {0x30df3430,0x266,0x11cf,{0xba,0xa6,0x0,0xaa,0x0,0x3e,0xe,0xed}};
// {30DF3432-0266-11cf-BAA6-00AA003E0EED}
extern const GUIDIID_IDB;
// {0x30df3432,0x266,0x11cf,{0xba,0xa6,0x0,0xaa,0x0,0x3e,0xe,0xed}};
class IDB: public IUnknown
{
// Interfaces
public :
// Interfacefordataaccess
virtual HRESULT_stdcallRead( short nTable, short nRow,LPWSTRlpszData) = 0 ;
virtual HRESULT_stdcallWrite( short nTable, short nRow,LPCWSTRlpszData) = 0 ;
// Interfacefordatabasemanagement
virtual HRESULT_stdcallCreate( short & nTable,LPCWSTRlpszName) = 0 ;
virtual HRESULT_stdcallDelete( short nTable) = 0 ;
// Interfaseparaobtenberinformacionsobrelabasededatos
virtual HRESULT_stdcallGetNumTables( short & nNumTables) = 0 ;
virtual HRESULT_stdcallGetTableName( short nTable,LPWSTRlpszName) = 0 ;
virtual HRESULT_stdcallGetNumRows( short nTable, short & nRows) = 0 ;
// virtualULONGRelease()=0;
};

2 ，修改对象程序

1)将CDBSrvFactory由IDBSrvFactory派生改为由IClassFactory派生。将CDBSrvFactory::CreateDB改为CDBSrvFactory:: CreateInstance,并添加一个成员函数CDBSrvFactory:: LockServer。为CDB和CDBSrvFactory都添加一个引用计数变量：ULONG m_dwRefCount；声明一个外部变量：extern ULONG g_dwRefCount;；为CDB和CDBSrvFactory加上QueryInterface， AddRef和Release三个成员函数

typedef long HRESULT;
class CDB: public IDB
{
// Interfaces
public :
// Interfacefordataaccess
HRESULT_stdcallRead( short nTable, short nRow,LPWSTRlpszData);
HRESULT_stdcallWrite( short nTable, short nRow,LPCWSTRlpszData);
// Interfacefordatabasemanagement
HRESULT_stdcallCreate( short & nTable,LPCWSTRlpszName);
HRESULT_stdcallDelete( short nTable);
// Interfaseparaobtenberinformacionsobrelabasededatos
HRESULT_stdcallGetNumTables( short & nNumTables);
HRESULT_stdcallGetTableName( short nTable,LPWSTRlpszName);
HRESULT_stdcallGetNumRows( short nTable, short & nRows);
HRESULT_stdcallQueryInterface(REFIIDriid, void ** ppObject);
ULONG_stdcallAddRef();
ULONG_stdcallRelease();
// Implementation
private :
CPtrArraym_arrTables; // ArrayofpointerstoCStringArray(the"database")
CStringArraym_arrNames; // Arrayoftablenames
ULONGm_dwRefCount;
public :
CDB();
~ CDB();
};
extern ULONGg_dwRefCount;
class CDBSrvFactory: public IClassFactory
{
// Interface
public :
HRESULT_stdcallQueryInterface(REFIIDriid, void ** ppObject);
ULONG_stdcallAddRef();
ULONG_stdcallRelease();
HRESULT_stdcallCreateInstance(IUnknown * pUnkOuter,REFIIDriid, void ** ppObject);
HRESULT_stdcallLockServer(BOOLfLock);
// Implementation
private :
ULONGm_dwRefCount;
public :
CDBSrvFactory();
};

2）为IID_IDB定义GUID,在CDB构造函数中将m_dwRefCount初始化为0，实现CDB的QueryInterface，AddRef，Release三个成员函数。

CDB实现文件
CDB::CDB()
{
m_dwRefCount=0;
}
HRESULTCDB::QueryInterface(REFIIDriid,void**ppObject)
{
if(riid==IID_IUnknown||riid==IID_IDB)
{
*ppObject=(IDB*)this;
}
else
{
returnE_NOINTERFACE;
}
AddRef();
returnNO_ERROR;
}
ULONGCDB::AddRef()
{
g_dwRefCount++;
m_dwRefCount++;
returnm_dwRefCount;
}
ULONGCDB::Release()
{
g_dwRefCount--;
m_dwRefCount--;
if(m_dwRefCount==0)
{
deletethis;
return0;
}
returnm_dwRefCount;
}

注：头文件中成员函数的声明顺序不影响vtable中的顺序，因为vtable是按照IDB的声明顺序来定义的。

3）删除IID_IDBSrvFactory定义；定义全局变量g_dwRefCount；在构造函数中将m_dwRefCount初始化为0；实现CDBSrvFactory的QueryInterface，AddRef，Release三个成员函数；将CDBSrvFactory的CreateDB函数修改为CreateInstance；实现CDBSrvFactory:: LockServer；添加 DllCanUnloadNow, DllUnregisterServer, DllRegisterServer三个成员函数。

CDBSrvFactory实现
ULONGg_dwRefCount=0;
//{30DF3430-0266-11cf-BAA6-00AA003E0EED}
staticconstGUIDCLSID_DBSAMPLE=
{0x30df3430,0x266,0x11cf,{0xba,0xa6,0x0,0xaa,0x0,0x3e,0xe,0xed}};
//Createanewdatabaseobjectandreturnapointertoit
HRESULTCDBSrvFactory::CreateInstance(IUnknown*pUnkOuter,REFIIDriid,void**ppObject)
{
if(pUnkOuter!=NULL)
{
returnCLASS_E_NOAGGREGATION;
}
CDB*pDB=newCDB;
if(FAILED(pDB->QueryInterface(riid,ppObject)))
{
deletepDB;
*ppObject=NULL;
returnE_NOINTERFACE;
}
returnNO_ERROR;
}
HRESULTCDBSrvFactory::LockServer(BOOLfLock)
{
if(fLock)
{
g_dwRefCount++;
}
else
{
g_dwRefCount--;
}
returnNO_ERROR;
}
CDBSrvFactory::CDBSrvFactory()
{
m_dwRefCount=0;
}
HRESULTCDBSrvFactory::QueryInterface(REFIIDriid,void**ppObject)
{
if(riid==IID_IUnknown||riid==IID_IClassFactory)
{
*ppObject=(IDB*)this;
}
else
{
returnE_NOINTERFACE;
}
AddRef();
returnNO_ERROR;
}
ULONGCDBSrvFactory::AddRef()
{
g_dwRefCount++;
m_dwRefCount++;
returnm_dwRefCount;
}
ULONGCDBSrvFactory::Release()
{
g_dwRefCount--;
m_dwRefCount--;
if(m_dwRefCount==0)
{
deletethis;
return0;
}
returnm_dwRefCount;
}
STDAPIDllGetClassObject(REFCLSIDrclsid,REFIIDriid,void**ppObject)
{
if(rclsid==CLSID_DBSAMPLE)
{
CDBSrvFactory*pFactory=newCDBSrvFactory;
if(FAILED(pFactory->QueryInterface(riid,ppObject)))
{
deletepFactory;
*ppObject=NULL;
returnE_INVALIDARG;
}
}
else
{
//hereyoucouldcheckforadditionalCLSID'syouDLLmayprovide
returnCLASS_E_CLASSNOTAVAILABLE;
}
returnNO_ERROR;
}
HRESULT_stdcallDllCanUnloadNow()
{
if(g_dwRefCount)
{
returnS_FALSE;
}
else
{
returnS_OK;
}
}
STDAPIDllRegisterServer(void)
{
HKEYhKeyCLSID,hKeyInproc32;
DWORDdwDisposition;
if(RegCreateKeyEx(HKEY_CLASSES_ROOT,_T("CLSID//{30DF3430-0266-11cf-BAA6-00AA003E0EED}"),NULL,_T(""),REG_OPTION_NON_VOLATILE,KEY_ALL_ACCESS,NULL,&hKeyCLSID,&dwDisposition)!=ERROR_SUCCESS)
{
returnE_UNEXPECTED;
}
if(RegSetValueEx(hKeyCLSID,_T(""),NULL,REG_SZ,(BYTE*)_T("DBSampleServer"),sizeof(_T("DBSampleServer")))!=ERROR_SUCCESS)
{
RegCloseKey(hKeyCLSID);
returnE_UNEXPECTED;
}
if(RegCreateKeyEx(hKeyCLSID,_T("InprocServer32"),NULL,_T(""),REG_OPTION_NON_VOLATILE,KEY_ALL_ACCESS,NULL,&hKeyInproc32,&dwDisposition)!=ERROR_SUCCESS)
{
RegCloseKey(hKeyCLSID);
returnE_UNEXPECTED;
}
HMODULEhModule=GetModuleHandle(_T("DB.DLL"));
if(!hModule)
{
RegCloseKey(hKeyInproc32);
RegCloseKey(hKeyCLSID);
returnE_UNEXPECTED;
}
TCHARszName[MAX_PATH+1];
if(GetModuleFileName(hModule,szName,sizeof(szName))==0)
{
RegCloseKey(hKeyInproc32);
RegCloseKey(hKeyCLSID);
returnE_UNEXPECTED;
}
if(RegSetValueEx(hKeyInproc32,_T(""),NULL,REG_SZ,(BYTE*)szName,sizeof(TCHAR)*(lstrlen(szName)+1))!=ERROR_SUCCESS)
{
RegCloseKey(hKeyInproc32);
RegCloseKey(hKeyCLSID);
returnE_UNEXPECTED;
}
RegCloseKey(hKeyInproc32);
RegCloseKey(hKeyCLSID);
returnNOERROR;
}
STDAPIDllUnregisterServer(void)
{
if(RegDeleteKey(HKEY_CLASSES_ROOT,_T("CLSID//{30DF3430-0266-11cf-BAA6-00AA003E0EED}//InprocServer32"))!=ERROR_SUCCESS)
{
returnE_UNEXPECTED;
}
if(RegDeleteKey(HKEY_CLASSES_ROOT,_T("CLSID//{30DF3430-0266-11cf-BAA6-00AA003E0EED}"))!=ERROR_SUCCESS)
{
returnE_UNEXPECTED;
}
returnNOERROR;
}

4）修改DEF文件，现在我们需要在db.def中引出DllCanUnloadNow，DllRegisterServer，DllUnregisterServer三个函数，如下所示：

EXPORTS
; WEP@ 1 RESIDENTNAME
DllGetClassObject
DllCanUnloadNow
DllRegisterServer
DllUnregisterServer

5）创建并注册DLL,编译生成DB.dll,在命令行中运行regsvr32对dll进行注册。

3，修改客户程序

1)在DBDoc.cpp中定义IID_IDB,并且删除以前定义的IID_IDBSrvFactory:

// {30DF3430-0266-11cf-BAA6-00AA003E0EED}
static const GUIDCLSID_DBSAMPLE =
{ 0x30df3430 , 0x266 , 0x11cf ,{ 0xba , 0xa6 , 0x0 , 0xaa , 0x0 , 0x3e , 0xe , 0xed }};
// {30DF3432-0266-11cf-BAA6-00AA003E0EED}
static const GUIDIID_IDB =
{ 0x30df3432 , 0x266 , 0x11cf ,{ 0xba , 0xa6 , 0x0 , 0xaa , 0x0 , 0x3e , 0xe , 0xed }};

2)以前创建对象的过程使用的是IDBFactory::CreateDB来创建CDB对象的，现在改为IClassFactory:: CreateInstance:

// 新建数据库对象
// m_pDB=newCDB;
// createadatabaseobjectthroughtheexportedfunction&classfactoryobject
IClassFactory * pDBFactory = NULL;
HRESULThRes;
hRes = CoGetClassObject(CLSID_DBSAMPLE,CLSCTX_SERVER,NULL,IID_IClassFactory,( void ** ) & pDBFactory);
if (FAILED(hRes))
{
CStringcsError;
csError.Format(_T( " Error%xobtainingclassfactoryforDBObject! " ),hRes);
AfxMessageBox(csError);
return FALSE;
}
hRes = pDBFactory -> CreateInstance(NULL,IID_IDB,( void ** ) & m_pDB);
if (FAILED(hRes))
{
CStringcsError;
csError.Format(_T( " Error%xcreatingDBObject! " ),hRes);
AfxMessageBox(csError);
return FALSE;
}
pDBFactory -> Release(); // donotneedthefactoryanymore

3)卸载不再使用的对象，通过调用CoFreeUnusedLibraries来确保任何不再使用的COM DLL对象及时卸载。

BOOLCDBApp::OnIdle(LONGlCount)
{
if (CWinApp::OnIdle(lCount))
{
return TRUE;
}
CoFreeUnusedLibraries();
return FALSE;
}

Ok,到此为止，我们已经将一个简单的C++对象真正转变为一个COM对象了。