ok, I have finally managed. Heres the code:
//class decl
class LocationListener: public nsIWebProgressListener, public nsSupportsWeakReference
{
public:
LocationListener();
virtual ~LocationListenerW();
NS_DECL_ISUPPORTS
NS_DECL_NSIWEBPROGRESSLISTENER
};
//class impl
NS_IMPL_ISUPPORTS2(LocationListener, nsIWebProgressListener, nsISupportsWeakReference)
NS_IMETHODIMP LocationListener::OnXXChange(...)
{
return NS_OK;
}
//and class usage. plistener must be created non-locally
LocationListener * listener = new LocationListener();
nsCOMPtr<nsISupports> plistener = static_cast<nsIWebProgressListener *>(listener);
nsCOMPtr<nsIWeakReference> weakref = do_GetWeakReference(plistener, &retval);