How to create ATL COM objects in Visual C++

MORE INFORMATION

ATL 2.x

The following code illustrates how to create an instance of your ATL COM object without using the APIs, as in the COM creation mechanism:

Sample Code

   // For CComObject you can use its CreateInstance() member.
   CComObject<CMyClass>* pMyClass;
   // Note that at this point the ref count for the object is 0 (zero).
   HRESULT hRes = CComObject<CMyClass>::CreateInstance(&pMyClass);

   // DECLARE_AGGREGATABLE or no macro in CMyClass indicates that the COM
   // object is based on CComAggObject. DECLARE_POLY_AGGREGATABLE macro in
   // CMyClass indicates that the COM object is based on CComPolyObject.
   // The ref count of this object will be 1.
   LPUNKNOWN m_pInnerUnk;
   // m_pOuterUnk is the IUnknown of the outer object. It can be NULL if
   // the COM object is based on CcomPolyObject.
   CMyClass::_CreatorClass::CreateInstance(m_pOuterUnk, IID_IUnknown,
      (LPVOID*)&m_pInnerUnk);
				

Alternatively you can create an instance of your class using the "new" operator:

Sample Code

   // Use CComObject, CComPolyObject, and CComAggObject as appropriate.
   // In this case, the FinalConstruct() member is not called.
   // Note that at this point the ref count for the object is 0 (zero).
   // The CComAggObject constructor takes outer unknown as an argument.
   // The CComPolyObject constructor may or may not take outer unknown as
   // an argument depending on whether it is being aggregated or not.
   CComObject<CMyClass>* pMyClass = new CComObject<CMyClass>();
   HRESULT hr = pMyClass->FinalConstruct();
				

ATL 3.0

In ATL 3.0, you can use the following alternatives to create COM objects. Just as CComObject, CcomAggObject, and CComPolyObject also have static CreateInstance() methods:

   CComAggObject<CMyClass>* pMyClass;

   // pUnkOuter is IUnknown of the aggregate object, FinalConstruct() is
   // called and the ref count is 0 (zero).
   HRESULT hRes = CComAggObject<CMyClass>::CreateInstance(m_pUnkOuter,
   &pMyClass);
				

In ATL 3.0, the _CreateClass:CreateInstace() function is wrapped by the CreateInstance() method of CcomCoClass:

   // The following calls increment the ref count to 1 and call
   // FinalConstruct.
   // If CMyClass is aggregateable, either DECLARE_AGGREGATABLE or
   // DECLARE_POLY_AGGREGATABLE.
   // m_pOuterUnk, the outer unknown can be NULL for the poly
   // aggregateable case.
   LPUNKNOWN m_pInnerUnk;
   CComCoClass<CMyClass>::CreateInstance(m_pOuterUnk, &m_pInnerUnk);

   // If CMyClass uses the DECLARE_NOT_AGGREGATABLE macro, use following
   // version:
   CComCoClass<CMyClass>::CreateInstance(&m_pInnerUnk);
				

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Jaganathan Thangavelu, Microsoft Corporation.