C2594 : Ambiguous Conversion with COM Object

Question

I have an ATL class that I'm trying to implement a cast operator for each interface that it implements. The class is a mock representing a real object.

class CMock :
  public IDispatchImpl<Interface1 ...>
  public IDispatchImpl<Interface2 ...>

The ATL COM Mapping is as follows:

BEGIN_COM_MAP(CMock)
  // Resolve ambiguity by exposing Interface1 through Interface2
  COM_INTERFACE_ENTRY2(Interface1, Interface2)
  COM_INTERFACE_ENTRY(Interface)
END_COM_MAP()

I then have overloaded the cast operators for Interface1* and Interface2*

operator Interface1* () 
{ 
  Interface1* pInterface1;
  if(FAILED(this->QueryInterface(IID_Interface1, reinterpret_cast<void**>(&pInterface1)))
     return nullptr;
  return pInterface1;
}
// Repeat for IInterface2

Despite the overloaded cast operators, I'm still getting an ambiguous error when trying to cast to any interface that is exposed through another.

error C2594: 'argument' : ambiguous conversions from 'CMock *' to 'Interface1 *'

int main()
{
   CComPtr<CMock> mock = new CComObject<CMock>();

   Interface1* pInterface1 = mock; // Error C2594
   Interface2* pInterface2 = mock; // compiles OK.
   return 0;
}

I understand WHY the ambiguous conversions are there, but I'm confused as to why the compiler is even looking at those when I define a custom cast operator. Is there anyway to define the cast operator such that the compiler will ignore the other conversion possibilities?

Answer 1

Your defining cast operator does not help here since you are defining it on CMock class, while the problem happens with CComPtr<CMock>, which is a wrapper on top of CMock*. So the operator is not helpful, however it does not need to help.

Skipping other issues in your snippets (and there are a few of them), you eventually need the following:

If you have a raw pointer, such as CMock* or CComObject<CMock>* then you can cast using C++ cast operators, e.g.

Interface1* pInterface1 = (Interface*) (CMock*) mock;

If you have an interface pointer of your object, not the native pointer, then you can get your interface using QueryInterface or CComQIPtr.

CComQIPtr<Interface1> pInterface1 = mock; // or, "... = (Interface2*) mock;"
                                 // to possibly resolve `IUnknown*` ambiguities