Gotcha Ccomptrbase T Assignment

Today what started out as a crash due to a pure virtual call turned into finding a gotcha in CComPtrBase. Essentially the code in question boiled down to the following. Can you spot the problem?

void GetAStudent(CComPtrBase<T> &spStudent)
{
    CComPtr<Student> spLocal;
    // Do some operation to get a student
    spLocal = spStudent;
}

The problem isn’t apparent until you look at the definition for CComPtrBase::operater =. See the problem? Basically CComPtrBase::operator= isn't explicitly defined. This means that C++ will automatically implement [memberwise assignment.](http://msdn2.microsoft.com/en-us/library/x0c54csc\(VS.71\).aspx) The RHS of the operator= will be a "const CComPtrBase&".

CComPtr derives from CComPtrBase therefore it satisfies this and a memberwise assignment will occur. We now have two smart pointers on the same value. However the second smart pointer, CComPtrBase, did not perform an AddRef. So when both objects are destroyed there will be an extra Release and hopefully a crash.

The fix?

1. Use CComPtr or CComPtrEx instead of CComPtrBase
2. Less Optimal: call AddRef() on spLocal.