Re: compilation error when using CComPtr w/ CUnknown-derived objects

"Igor Tandetnik" <>
Fri, 26 Jan 2007 11:22:03 -0500
Nol Danjou <noeld@online.nospam> wrote:

// Example #1, CTest declared as:
// class CTest : public CUnknown, public IUnknown

CUnknown* punk = CTest::CreateInstance(NULL, &hr); // OK: punk =
0x003e5e50 {a=0x00000003 b=0x00000006 }

pTest1 = static_cast<CTest*>(punk); // Corrupted: pTest1 =
{0x003e5e60}, p = 0x00c15e60 {a=0xabababab b=0x00000000 }

Correct. Upon more careful examination, CComPtr<T>::operator= not only
assumes that T derives from IUnknown, but further assumes that
offsetofclass(IUnknown, T) == 0. In other words,

T* p = new T;
IUnknown* punk = p;
assert(static_cast<T*>(punk) == reinterpret_cast<T*>(punk));

CComPtr<T>::operator= assumes that the assertion holds.

As I said, CComPtr is designed to be parameterized by a COM interface.
It's a bad idea to use anything other than a COM interface as its
template parameter. You've just found it out the hard way.
With best wishes,
    Igor Tandetnik

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