The following steps demonstrate how to aggregate a COM
object in an ATL project:
- Add an IUnknown pointer to your class object and initialize
it to NULL in the constructor.
- 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.
NOTE: If you are using Visual C++.NET the Wizard adds this for
you. - 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.
- Override FinalConstruct() to create the
aggregate.
- Override FinalRelease() to release the IUnknown
pointer.
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);
}
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
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