Compilation errors through incorrect use of CComPtr objects

Question

I have defined the following CComPtr object and method in my class:

private:

    CComPtr<IRawPdu>& getRawPdu();
    // Returns the RawPdu interface pointer from the mRawPdu data member.
    // mRawPdu is initialized, if necessary.

    CComPtr<IRawPdu> mRawPdu;
    // Initialized to 0 in the ctor.  Uses lazy evaluation via getRawPdu().

In the constructor of my class, I initialise mRawPdu to 0 via the initialisor list. The getRawPdu() method used lazy evaluation if mRawPdu has yet to be initialised.

When compiling the code, I get the following errors:

Compiling...
topport.cpp
C:\Program Files\Microsoft Visual Studio 8\VC\atlmfc\include\atlcomcli.h(295) : error C2664: 'ATL::AtlComPtrAssign' : cannot convert parameter 2 from 'const ATL::CComPtr<T>' to 'IUnknown *'
        with
        [
            T=IRawPdu
        ]
        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
        C:\Program Files\Microsoft Visual Studio 8\VC\atlmfc\include\atlcomcli.h(292) : while compiling class template member function 'IRawPdu *ATL::CComPtr<T>::operator =(const ATL::CComPtr<T> &) throw()'
        with
        [
            T=IRawPdu
        ]
        sessionutilities.h(186) : see reference to class template instantiation 'ATL::CComPtr<T>' being compiled
        with
        [
            T=IRawPdu
        ]
topglobals.cpp
C:\Program Files\Microsoft Visual Studio 8\VC\atlmfc\include\atlcomcli.h(295) : error C2664: 'ATL::AtlComPtrAssign' : cannot convert parameter 2 from 'const ATL::CComPtr<T>' to 'IUnknown *'
        with
        [
            T=IRawPdu
        ]
        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
        C:\Program Files\Microsoft Visual Studio 8\VC\atlmfc\include\atlcomcli.h(292) : while compiling class template member function 'IRawPdu *ATL::CComPtr<T>::operator =(const ATL::CComPtr<T> &) throw()'
        with
        [
            T=IRawPdu
        ]
        sessionutilities.h(186) : see reference to class template instantiation 'ATL::CComPtr<T>' being compiled
        with
        [
            T=IRawPdu
        ]

Any suggestions as to what could be causing this?

Answer 1

Based on the error given by the compiler it appears that it cannot infer a conversion between IRawPdu and IUnknown.

Does it actually inherit from IUnknown? If so then it's possibly an include ordering issue. Can you give more insight into the hierarchy of IRawPdu

Answer 2

Don't pass CComPtr<>'s around since there's no need, just return the pointer to the interface. For example:

  IRawPdu* getRawPdu() { return mRawPdu; }   // Does not add to the reference count

  HRESULT get_RawPdu(IRawPdu** ppPdu)   // Returns RawPdu, but add ref's it.
  {
     return mRawPdu.CopyTo(ppPdu);
  }

  CComPtr<IRawPdu> mRawPdu;
  // Initialized to 0 in the ctor.  Uses lazy evaluation via getRawPdu().

So, when it comes time to use it:

  IRawPdu* pTempRawPdu  = class->getRawPdu();
  // use pTempRawPdu in a temporary manner (since it's not add reffed)

But, better would be:

  CComPtr<IRawPdu> spRawPdu = class->getRawPdu();
  // the ctor of the local CComPtr<> calls AddRef() (and automagically Release's when done)