Re: CComPtr strange behaviour
On Mon, 27 Jul 2009 12:59:48 -0400, Igor Tandetnik wrote:
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).
There is more than one IUnknown? Huh? You've reached the limit of my COM
knowledge? Why would there be more than one? I thought the IUnkown* was
used to determine object identity (i.e. safe to use == on) so how can an
object have more than one?
Also, I didn't think I was doing _any_ casting. As far as I was aware I
was doing everything through QueryInterface. I've never understood when
plain-old casting of COM interface pointer was allowed and when not so I
always stuck to QI. Am I casting somewhere without realising?
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)
I thought I was. Do you mean that I pass pointers directly as smart
pointers rather than in an out parameter and I use exceptions? Is this
fundamentally non-COM?
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?
By QueryInterface.
If by a downcast, then there goes your type safety.
True.
If by QueryInterface, then what parameters should QueryInterface
itself take?
IUnknown* p_object = NULL;
HRESULT hr = p_you_there->GetMeAnObjectThatImplements(
IID_IWantedInterface, &p_object);
if (SUCCEEDED(hr))
{
IWantedInterface* p_wanted = NULL;
hr = p_object->QueryInterface(IID_IWantedInterface, &p_wanted);
if (SUCCEEDED(hr)
do_your_thing(p_wanted)
}
QueryInterface is the one function that should be allowed to take a void**
as it is effectively doing a checked cast to your desired interface type.
If all others take an IUnknown** and then the caller QIs the result, it
will be safe, if less efficient.
I'm sure the above only demonstrates my lack of COM understanding but I'm
very keen to learn more. I own (and have read cover to cover) three books
on COM and still don't seem to know enough.
Eagerly awaiting enlightenment.
Alex