Error C2275 while creating a COM smart-pointer within "if" statement

Why can't i

if (IUnknownPtr p = anotherComPtr) {} //error C2275: 'IUnknownPtr' : illegal use of this type as an expression
 

while i can

if (int* a = anotherPointer) {}
 

IUnknownPtr is defined throught _COM_SMARTPTR_TYPEDEF(IUnknown, __uuidof(IUnknown)) (like any others smart pointers i use)

How can i create a com smartptr within if statement and verify is it valid or not? Thank you.

I use VS 2008

p.s. This is not about is it good way of coding or not, it's about error C2275.

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