How To Aggregate a COM Object with ATL

MORE INFORMATION

If you use and release an interface from the aggregate during FinalConstruct, you should add the DECLARE_PROTECT_FINAL_CONSTRUCT macro to the definition of your class object.

Here is a sample with all four of the steps done in the class definition. This is a simple object, COutObj, which aggregates another simple object, CInnObj.

Sample Code

   class ATL_NO_VTABLE COutObj :
         public CComObjectRootEx<CComSingleThreadModel>,
         public CComCoClass<COutObj, &CLSID_OutObj>,
         public IDispatchImpl<IOutObj, &IID_IOutObj, &LIBID_OUTEROBJLib>
   {
   public:
      COutObj() : m_pInnerUnk(NULL) // Step 1
      {
      }
      DECLARE_GET_CONTROLLING_UNKNOWN()
      DECLARE_REGISTRY_RESOURCEID(IDR_OUTOBJ)
      DECLARE_PROTECT_FINAL_CONSTRUCT() // Step 2

      BEGIN_COM_MAP(COutObj)
         COM_INTERFACE_ENTRY(IOutObj)
         COM_INTERFACE_ENTRY(IDispatch)
         COM_INTERFACE_ENTRY_AGGREGATE(IID_IInnObj, m_pInnerUnk) // Step 3
      END_COM_MAP()

      // Start of step 4
      HRESULT FinalConstruct()
      {
         HRESULT hr;
         CLSID   clsidInner;

         hr = CLSIDFromProgID(L"InnObj.InnObj.1", &clsidInner);
         if (hr == S_OK)
            hr = CoCreateInstance(clsidInner, GetControllingUnknown(),
                                  CLSCTX_INPROC_SERVER, IID_IUnknown,
                                  (void**)&m_pInnerUnk);
         return hr;
      } // End of step 4

      void FinalRelease(){m_pInnerUnk->Release();} // Step 5

      // IOutObj
   public:
      STDMETHOD(Test)();
   private:
      LPUNKNOWN m_pInnerUnk; // Step 1
   };
				

If the inner object is in the same ATL server, then you can use the following code to create the inner object without CoCreateInstance:

   HRESULT FinalConstruct()
      {
         return  CInnObj::_CreatorClass::CreateInstance(
             GetControllingUnknown(), IID_IUnknown,
             (void**)&m_pInnerUnk);

      }