type safe issue

[George2]

Hello everyone,


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;
2. Define a function in the template class called CreateInstance, the implementation is,

2.1 invoke Release(); // release previous held pointer
2.2 invoke CoCreateInstance (rclsid, pUnkOuter, dwClsContext, *iid, &m_pI); // m_pI the wrapped interface pointer member variable for the smart pointer template class.

I think why the solution works, is because in the creation of the template class, the interface type T and iid is matched (example, through constructor or assignment operator), so when using the internal variable *iid and m_pI to invoke CoCreateInstance, the type always match (Interface ID and Interface type).

Is my understanding correct?


thanks in advance,
George