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COM+ Release Method Not Dropping to 0

I wish to share with you this problem I'm facing. Long story short, I have this little code (test purpose only):

    int main ()
    {
        IXMLDOMDocument *pDoc(nullptr);
        CoCreateInstance(CLSID_DOMDocument, nullptr, CLSCTX_ALL, IID_IXMLDOMDocument, reinterpret_cast<LPVOID*>(&pDoc));
        DWORD d = pDoc->AddRef();
        std::cout << "pDoc: add ptr=" << pDoc << " d=" << d << std::endl;
        d = pDoc->Release();
        std::cout << "pDoc: rel ptr=" << pDoc << " d=" << d << std::endl;

        IUnknown *pUnk(nullptr);
        pDoc->QueryInterface(IID_IUnknown, reinterpret_cast<LPVOID*>(&pUnk));
        d = pUnk->AddRef();
        std::cout << "pUnk: add ptr=" << pUnk << " d=" << d << std::endl;
        d = pUnk->Release();
        std::cout << "pUnk: rel ptr=" << pUnk << " d=" << d << std::endl;

        /*Release objects*/
        d = pUnk->Release();
        std::cout << "pUnk: rel ptr=" << pUnk << " d=" << d << std::endl;
        d = pDoc->Release();
        std::cout << "pDoc: rel ptr=" << pDoc << " d=" << d << std::endl;

        return 0;
    }

I'm expecting that the last 2 cout print 0 as the returned count, in their place I'm seeing:

pDoc: add ptr=004A4628 d=2
pDoc: rel ptr=004A4628 d=1
pUnk: add ptr=004A3A10 d=4
pUnk: rel ptr=004A3A10 d=3
pUnk: rel ptr=004A3A10 d=2
pDoc: rel ptr=004A4628 d=0

Why QueryInterface returned me an IUnknown which internal count begins in 3? Why last Release method of the IUnknown object isn't returning 0 as excepted?

What I'm might be missing?

Solution

  • According to the MDSN documention of AddRef and Release, the return value is intended to only be used for test purposes. It may not be an accurate reflection of the actual number of references; and in particular testing it against 0 is not a guarantee that the object is finished.

    Under what conditions will the IUnknown::AddRef method return 0?.