People's failures at using COM

What does MSDN say about the ppvObject parameter in IUnknown::QueryInterface?

ppvObject [out]

The address of a pointer variable that receives the interface pointer requested in the riid parameter. Upon successful return, *ppvObject contains the requested interface pointer to the object. If the object does not support the interface, *ppvObject is set to NULL.

I made the relevant bit a bit more prominent.

We now have the following code, written by X:

#define COM_QI_BEGIN() HRESULT STDMETHODCALLTYPE QueryInterface(REFIID iid,void ** ppvObject) { if (ppvObject == NULL) return E_INVALIDARG;
#define COM_QI_ENTRY(IWhat) { if (iid == IID_##IWhat) {IWhat * temp = this; temp->AddRef(); * ppvObject = temp; return S_OK;} }
#define COM_QI_END() return E_NOINTERFACE; }

COM_QI_BEGIN()
COM_QI_ENTRY(IUnknown)
COM_QI_ENTRY(IDataObject)
COM_QI_END()

This expands to:

HRESULT STDMETHODCALLTYPE QueryInterface(REFIID iid,void ** ppvObject)
{
if (ppvObject == NULL) return E_INVALIDARG;
{ if (iid == IID_IUnknown) {IUnknown * temp = this; temp->AddRef(); * ppvObject = temp; return S_OK;} }
{ if (iid == IID_IDataObject) {IDataObject * temp = this; temp->AddRef(); * ppvObject = temp; return S_OK;} }
return E_NOINTERFACE;
}

Does that look like it sets *ppvObject to NULL on failure?

More on the general subject here.