Re: COM Interface intialization

"Igor Tandetnik" <>
Mon, 15 Jan 2007 14:14:49 -0500
"Steven Edison" <EdisonCPP@newsgroups.nospam> wrote in message

What I'd like to do is something like this:

STDMETHODIMP CAnotherInterfaceImpl::GetItemInfo(IUnknwn** pUnk)
    CItemInfo* pInfo = new CItemInfo; //above impl class
    pInfo->SetSomeData(0x42); //exposed through impl. class, but
not the interface
                                                    //so caller can't
set "somedata", but it can be done here.

The usual idiom goes like this:

CComObject<CItemInfo>* pInfo;
HRESULT hr = pInfo->QueryInterface(IID_IUnknown, (void**)pUnk);
return hr;

//final use:
CComPtr<IItemInfo> pInfo = NULL;
hr = pAnotherI->GetItemInfo((IUnknown**)&pInfo);

This is illegal. GetItemInfo returns an IUnknown*, you can't just place
it into IItemInfo*. This amounts to a downcast. Instead, you have to
retrieve into IUnknown* pointer, then QueryInterface for IItemInfo. Or,
have GetItemInfo return IItemInfo* in the first place.
With best wishes,
    Igor Tandetnik

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