1. [my understanding of solution] How do you think of my understanding of
2. [solution itself] How do you think of the solution I posted above to
"George" <George@discussions.microsoft.com> wrote in message
news:5775D716-BD25-46A6-980D-E59E01741CDB@microsoft.com
My question is whether my understanding of the solution to type safe
issue in CoCreateInstance is correct. There is type safe issue in
CoCreateInstance,
http://msdn2.microsoft.com/en-us/library/ms686615.aspx
STDAPI CoCreateInstance(
REFCLSID rclsid,
LPUNKNOWN pUnkOuter,
DWORD dwClsContext,
REFIID riid,
LPVOID * ppv
);
which means the riid and ppv may not conform to each other, for
example, we have a IY* variable pIY and wants to get IZ type
interface, CoCreateInstance (rclsid, pUnkOuter, dwClsContext, IID_IZ,
&pIY);
To solve this issue, the solution from Inside COM is,
1. Define a template class smart pointer IComPtr<T, IID* iid>, where
T is type of interface to wrap, iid is its related interface IID;
I think why the solution works, is because in the creation of the
template class, the interface type T and iid is matched
Well, if you are hell-bent on breaking this solution, you can of course
write
IComPtr<IY, &IID_IZ> p;
p.CreateInstance(...);
which will end up calling CoCreateInstance in the same incorrect way
you've started with. Perhaps you are less likely to do that by accident,
since you need to specify interface name and its IID right next to each
other.
Note that ATL's CCom[QI]Ptr uses __uuidof facility so that you don't
need to explicitly specify the second parameter in most cases:
template <typename I, IID* iid = &__uuidof(I)>
class CComPtr {...};
This reduces still further the possibility of introducing a mismatch
accidentally.
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
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
overhead. -- RFC 1925