HOWTO: Aggregate a COM Object with ATL

• The Microsoft Active Template Library (ATL) 2.1 included with: - Microsoft Visual C++, 32-bit Editions, version 5.0

SUMMARY

The following steps demonstrate how to aggregate a COM object in an ATL project:

1. Add an IUnknown pointer to your class object and initialize it to NULL in the constructor.

2. Add the DECLARE_PROTECT_FINAL_CONSTRUCT macro to protect your object from being deleted if (during FinalConstruct) the internal aggregated object increments the reference count then decrements the count to 0.

3. Use the IUnknown pointer you defined as the second parameter to the COM_INTERFACE_ENTRY_AGGREGATE macros. The first parameter is the IID of the interface of the inner object that you want to expose.

4. Override FinalConstruct() to create the aggregate.

5. Override FinalRelease() to release the IUnknown pointer.

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
   };

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

      }

REFERENCES

Visual C++ Books Online: Visual C++ Books; C/C++ Language and C++ Library; Active Template Library; Articles; Introduction to COM and ATL; Introduction to COM; Aggregation

Visual C++ Books Online: Visual C++ Books; C/C++ Language and C++ Library; Active Template Library; Articles; Fundamentals of ATL COM Objects; Creating an Aggregate

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Chuck Bell, Microsoft Corporation