Creating an Error Logging Class

First, declare a class that will implement error logging. The class inherits the IAMErrorLog interface. It contains declarations for the three IUnknown methods, and for the single method in IAMErrorLog. The class declaration is as follows:

class CErrReporter : public IAMErrorLog
{
protected:
    long    m_lRef; // Reference count.

public:
    CErrReporter() { m_lRef = 0; }

    // IUnknown
    STDMETHOD(QueryInterface(REFIID, void**));
    STDMETHOD_(ULONG, AddRef());
    STDMETHOD_(ULONG, Release());

    // IAMErrorLog
    STDMETHOD(LogError(LONG, BSTR, LONG, HRESULT, VARIANT*));
};

The only member variable in the class is m_lRef, which holds the object's reference count.

Next, define the methods in IUnknown. The following example shows a standard implementation for these methods:

STDMETHODIMP CErrReporter::QueryInterface(REFIID riid, void **ppv)
{
    if (ppv == NULL) return E_POINTER;

    *ppv = NULL;
    if (riid == IID_IUnknown)
        *ppv = static_cast<IUnknown*>(this);
    else if (riid == IID_IAMErrorLog)
        *ppv = static_cast<IAMErrorLog*>(this);
        
    else 
    return E_NOINTERFACE;

    AddRef();
    return S_OK;
}

STDMETHODIMP_(ULONG) CErrReporter::AddRef()
{
    return InterlockedIncrement(&m_lRef);
}

STDMETHODIMP_(ULONG) CErrReporter::Release()
{
    // Store the decremented count in a temporary
    // variable. 
    ULONG uCount = InterlockedDecrement(&m_lRef);
    if (uCount == 0)
    {
        delete this;
    }
    // Return the temporary variable, not the member
    // variable, for thread safety.
    return uCount;
}

With the COM framework in place, you can now implement the IAMErrorLog interface. The next section describes how to do this.