Description of an alternative implementation of ATL singleton in Visual C++ 6.0

MORE INFORMATION

To use this version of a singleton, copy and paste the following into a header file in your project:

/********** Start of singleton code **********/ 
template <class Base>
class  CMyComObject: public CComObject<Base>
{
public:
   // Variable that lets class factory know COM object is deleted.
   CMyComObject** m_ppObj;

   STDMETHOD_(ULONG, Release)()
   {
      ULONG l = InternalRelease();
      if (l == 0)
      {
         // Set to NULL so class factory knows to recreate us.
         *m_ppObj = NULL;
         delete this;
      }
      return l;
   }

   static HRESULT WINAPI CreateInstance(CMyComObject<Base>** pp)
   {
      ATLASSERT(pp != NULL);
      HRESULT hRes = E_OUTOFMEMORY;
      CMyComObject<Base>* p = NULL;
      ATLTRY(p = new CMyComObject<Base>())
      if (p != NULL)
      {
         p->SetVoid(NULL);
         p->InternalFinalConstructAddRef();
         hRes = p->FinalConstruct();
         p->InternalFinalConstructRelease();
         if (hRes != S_OK)
         {
            delete p;
            p = NULL;
         }
      }
      *pp = p;
      // Initialize m_ppObj to CComObject<> pointer in class factory.
      p->m_ppObj = pp;
      return hRes;
   }
};

template <class T>
class CMyComClassFactorySingleton : public CComClassFactory
{
public:
   CMyComObject<T>* m_pObj;   // actual COM object
   CComAutoCriticalSection m_sec;  // A critical section is needed
   // because class factory is stored in CComObjectCached. All clients
   // get the same raw pointer to the class factory. Any instance
   // data in this class factory must be synchronized.
   // IClassFactory
   STDMETHOD(CreateInstance)(LPUNKNOWN pUnkOuter, REFIID riid,
                             void** ppvObj)
   {
      if (NULL == ppvObj)
         return E_POINTER;
      *ppvObj = NULL;
      // Aggregation is not supported in Singletons.
      ATLASSERT(pUnkOuter == NULL);
      if (pUnkOuter != NULL)
         return CLASS_E_NOAGGREGATION;
      HRESULT hRes = E_FAIL;
      // Lock critical section.
      m_sec.Lock();
      // Create COM object.
      if (NULL == m_pObj)
      {
         hRes = CMyComObject<T>::CreateInstance(&m_pObj);
         if (SUCCEEDED(hRes))
         {
            // Free marshaled interface and release stream.
            if (m_pStream)
            {
               AtlFreeMarshalStream(m_pStream);
               m_pStream = NULL;
            }
            // Marshal interface into a stream.
            // Can't use AtlMarshalPtrInProc since we need to use
            // MSHLFLAGS_TABLEWEAK.
            hRes = CreateStreamOnHGlobal(NULL, TRUE, &m_pStream);
            if (SUCCEEDED(hRes))
            {
               hRes = CoMarshalInterface(m_pStream, IID_IUnknown,
                      m_pObj, MSHCTX_INPROC, NULL, MSHLFLAGS_TABLEWEAK);
               if (FAILED(hRes))
               {
                  m_pStream->Release();
                  m_pStream = NULL;
               }
            }
         }
         if (FAILED(hRes))
         {
            // Unlock critical section.
            m_sec.Unlock();
            return hRes;
         }
      }
      CComPtr<IUnknown> spUnk;
      // Unmarshal interface from stream.
      hRes = AtlUnmarshalPtr(m_pStream, IID_IUnknown, &spUnk);
      // unlock critical section
      m_sec.Unlock();
      if (FAILED (hRes))
         return hRes;
      // Call QI on unmarshaled interface.
      hRes = spUnk->QueryInterface(riid, ppvObj);
         return hRes;
   }
   LPSTREAM m_pStream; // IStream* used for storing marshaled interface.

   void FinalRelease()
   {
      // Free marshaled interface and release stream.
      if (m_pStream)
         AtlFreeMarshalStream (m_pStream);
   }

   CMyComClassFactorySingleton() : m_pObj(NULL), m_pStream(NULL)
   {
   }
};

#define DECLARE_CLASSFACTORY_MYSINGLETON(obj)
    DECLARE_CLASSFACTORY_EX(CMyComClassFactorySingleton<obj>)
/********** End of singleton code **********/ 
				

Instead of DECLARE_CLASSFACTORY_SINGLETON, use the DECLARE_CLASSFACTORY_MYSINGLETON() macro in your CComCoClass-derived class:

class CMyComponent : public CComCoClass<...>, ...
{
   DECLARE_CLASSFACTORY_MYSINGLETON(CMyComponent)
   ...
}