ATL COM - How To Reuse Code for Interface Method

Question

I have several interfaces, e.g. IA, IB, IC, and so on, that share common properties, e.g. Site. I would like to know how to reuse code for these interfaces (please save me the answer on COM aggregation).

Current implementation is as follows:

class CA
// ATL specific...
{
    STDMETHODIMP get_Site(...) {...}
    STDMETHODIMP put_Site(...) {...}
}

class BA
// ATL specific...
{
    STDMETHODIMP get_Site(...) {...}
    STDMETHODIMP put_Site(...) {...}
}

class CC
// ATL specific...
{
    STDMETHODIMP get_Site(...) {...}
    STDMETHODIMP put_Site(...) {...}
}

What I want to achieve (but cannot) is as follows.

template<typename T>
class SharedProperties
{
    STDMETHODIMP get_Site(...) {...}
    STDMETHODIMP put_Site(...) {...}
}

class CA :
// ATL specific...
SharedProperties<CA>
{
    // properties are inherited and are accessible from IC
}

class BA
// ATL specific...
SharedProperties<CB>
{
    // properties are inherited and are accessible from IB
}

class CC
// ATL specific...
SharedProperties<CC>
{
// properties are inherited and are accessible from IA
}

I came across this idea after reading up (http://vcfaq.mvps.org/com/7.htm) but the site does not have a working example and no matter how much I tried I could not get it to work. I keep getting "Cannot instantiate abstract class" because the pure virtual functions get_Site and put_Site are not implemented (as per the second snippet).

EDIT Do note that I am using VS2010. Sample implementation below:

class ATL_NO_VTABLE CArticle :
    public CComObjectRootEx<CComSingleThreadModel>,
    public CComCoClass<CArticle, &CLSID_Article>,
    public IDispatchImpl<IArticle, &IID_IArticle, &LIBID_GeodeEdiLib, /*wMajor =*/ 1, /*wMinor =*/ 0>
{
public:
    CArticle()
    {
    }

Answer 1

The compiler does not know that the methods get_Site and put_Siteimplement the methods from the interface. You need to inherit the SharedProperties class template from the corresponding interface. That's the argument to make SharedProperties a template at all.

interface IA
{
    STDMETHOD(get_Site)() = 0;
    STDMETHOD(put_Site)() = 0;
};

template<typename T>
class Sharedproperties : T
{
public:
    STDMETHODIMP get_Site() { return E_NOTIMPL; };
    STDMETHODIMP put_Site() { return E_NOTIMPL; };
};

class CAX : public IA
{
    STDMETHOD(other)() { return S_OK; }
}

class CA: public Sharedproperties<CAX>
{
public:
    CA() {}
};

Please note that the class CA is not directly inherited from the interface IA.

Edit: The VS2008 class wizard generates this inheritance for a simple ATL class object:

class ATL_NO_VTABLE CMyObject :
    public CComObjectRootEx<CComMultiThreadModel>,
    public CComCoClass<CMyObject, &CLSID_MyObject>,
    IMyObject

where IMyObject is the in the IDL defined interface. So in ATL context you just need to replace the IMyObject inheritance:

class ATL_NO_VTABLE CMyObject :
    public CComObjectRootEx<CComMultiThreadModel>,
    public CComCoClass<CMyObject, &CLSID_MyObject>,
    public Sharedproperties<MyIntermediateClass>