Re: CComPtr strange behaviour

"Igor Tandetnik" <>
Mon, 27 Jul 2009 12:59:48 -0400
Alexander Lamaison <> wrote:

On Mon, 27 Jul 2009 08:56:31 -0400, Igor Tandetnik wrote:

What happens to the pointer _after_ folder_item_object returns? I bet
the caller just casts it to the expected interface type, without
using QI.

Ah, indeed:

CFolder::GetUIObjectOf(..., REFIID riid, void** ppv) {
    CComPtr<IUnknown> object;
    object = folder_item_object(..., riid);
    *ppv = object.Detach();

GetUIObjectOf takes an IUnknown* pointer it got from
folder_item_object, and tells the caller that it's really a pointer
of type riid (which amounts to a downcast). Due to a happy
coincidence of implementation details, this does happen to be the
case when you use ".p" form in folder_item_object, but not when you
use assignment without ".p".

Aha! I think I see what you mean: so you're saying my implementation
of GetUIObjectOf has basically lied to the caller? The caller said
"hey CFolder, pass me a pointer to some object where the _pointer_ is
of type riid" in other words it wants a guarantee that it is safe to
cast the void** pointer without a QI? And I've lied to it by passing
a pointer of type IUnknown.

Correct. Now, it's possible that the problem is not with
CFolder::GetUIObjectOf, but with folder_item_object. There might be an
unspoken contract, which GetUIObjectOf relies on, that
folder_item_object returns not just any IUnknown, but specifically an
IUnknown that is an upcast from the interface identified by riid (so
that the correct pointer can be obtained with a downcast). In this case,
the non-".p" version of folder_item_object violates that contract. It's
hard to decide who to blame, since you are not using common COM
argument-passing conventions (which imply a well-known set of rules),
nor documenting your contracts explicitly.

It is precisely to avoid this kind of misunderstanding that methods like
QueryInterface or GetUIObjectOf define their [out] parameters as void**,
and not IUnknown**.

If my understanding is correct, this is the proper way to do it:

CFolder::GetUIObjectOf(..., REFIID riid, void** ppv) {
    CComPtr<IUnknown> object;
    object = folder_item_object(..., riid);
    return object->QueryInterface(riid, ppv);

Is that right?

Well, that should work. Whether it's "right" is for you to decide.

This highlights to me the absence of type safety in a
void** out-param. Out of curiosity, why do these functions ask for
void** rather than IUnknown**?

How is that more type-safe? If the caller does want to get
IExtractIcon*, how would it get it from that IUnknown* pointer? If by a
downcast, then there goes your type safety. If by QueryInterface, then
what parameters should QueryInterface itself take?
With best wishes,
    Igor Tandetnik

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necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
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