I'm seeing what I believe to be an erroneous/incorrect compiler error using the Visual Studio 2010 compiler. I'm in the process of up-porting our codebase from Visual Studio 2005 and I ran across a construct that was building correctly before but now generates a C2248 compiler error.

Obviously, the code snippet below has been generic-ized, but it is a compilable example of the scenario. The ObjectPtr<T> C++ template comes from our codebase and is the source of the error in question. What appears to be happening is that the compiler is generating a call to the copy constructor for ObjectPtr<T> when it shouldn't (see my comment block in the SomeContainer::Foo() method below). For this code construct, there is a public cast operator for SomeUsefulData * on ObjectPtr<SomeUsefulData> but it is not being chosen inside the true expression if the ?: operator. Instead, I get the two errors in the block quote below.

Based on my knowledge of C++, this code should compile. Has anyone else seen this behavior? If not, can someone point me to a clarification of the compiler resolution rules that would explain why it's attempting to generate a copy of the object in this case?

Thanks in advance,
Dylan Bourque

Visual Studio build output:

c:\projects\objectptrtest\objectptrtest.cpp(177): error C2248: 'ObjectPtr::ObjectPtr' : cannot access private member declared in class 'ObjectPtr'
with
[ T=SomeUsefulData ]
c:\projects\objectptrtest\objectptrtest.cpp(25) : see declaration of 'ObjectPtr::ObjectPtr'
with
[ T=SomeUsefulData ]
c:\projects\objectptrtest\objectptrtest.cpp(177): error C2248: 'ObjectPtr::ObjectPtr' : cannot access private member declared in class 'ObjectPtr'
with
[ T=SomeUsefulData ]
c:\projects\objectptrtest\objectptrtest.cpp(25) : see declaration of 'ObjectPtr::ObjectPtr'
with
[ T=SomeUsefulData ]

Below is a minimal, compilable example of the scenario:

#include <stdio.h>
#include <tchar.h>
template<class T>
class ObjectPtr {
public:
   ObjectPtr<T> (T* pObj = NULL, bool bShared = false) :
      m_pObject(pObj), m_bObjectShared(bShared)
   {}
   ~ObjectPtr<T> ()
   {
      Detach();
   }
private:
   // private, unimplemented copy constructor and assignment operator
   // to guarantee that ObjectPtr<T> objects are not copied
   ObjectPtr<T> (const ObjectPtr<T>&);
   ObjectPtr<T>& operator = (const ObjectPtr<T>&);
public:
   T * GetObject ()
      { return m_pObject; }
   const T * GetObject () const
      { return m_pObject; }
   bool HasObject () const
      { return (GetObject()!=NULL); }
   bool IsObjectShared () const
      { return m_bObjectShared; }
   void ObjectShared (bool bShared)
      { m_bObjectShared = bShared; }
   bool IsNull () const
      { return !HasObject(); }
   void Attach (T* pObj, bool bShared = false)
   {
      Detach();
      if (pObj != NULL) {
         m_pObject = pObj;
         m_bObjectShared = bShared;
      }
   }
   void Detach (T** ppObject = NULL)
   {
      if (ppObject != NULL) {
         *ppObject = m_pObject;
         m_pObject = NULL;
         m_bObjectShared = false;
      }
      else {
         if (HasObject()) {
            if (!IsObjectShared())
               delete m_pObject;
            m_pObject = NULL;
            m_bObjectShared = false;
         }
      }
   }
   void Detach (bool bDeleteIfNotShared)
   {
      if (HasObject()) {
         if (bDeleteIfNotShared && !IsObjectShared())
            delete m_pObject;
         m_pObject = NULL;
         m_bObjectShared = false;
      }
   }
   bool IsEqualTo (const T * pOther) const
      { return (GetObject() == pOther); }
public:
   T * operator -> ()
      { ASSERT(HasObject()); return m_pObject; }
   const T * operator -> () const
      { ASSERT(HasObject()); return m_pObject; }
   T & operator * ()
      { ASSERT(HasObject()); return *m_pObject; }
   const T & operator * () const
      {  ASSERT(HasObject()); return (const C &)(*m_pObject); }
   operator T * ()
      { return m_pObject; }
   operator const T * () const
      { return m_pObject; }
   operator bool() const
      { return (m_pObject!=NULL); }
   ObjectPtr<T>& operator = (T * pObj)
      { Attach(pObj, false); return *this; }
   bool operator == (const T * pOther) const
      { return IsEqualTo(pOther); }
   bool operator == (T * pOther) const
      { return IsEqualTo(pOther); }
   bool operator != (const T * pOther) const
      { return !IsEqualTo(pOther); }
   bool operator != (T * pOther) const
      { return !IsEqualTo(pOther); }
   bool operator == (const ObjectPtr<T>& other) const
      { return IsEqualTo(other.GetObject()); }
   bool operator != (const ObjectPtr<T>& other) const
      { return !IsEqualTo(other.GetObject()); }
   bool operator == (int pv) const
      { return (pv==NULL)? IsNull() : (LPVOID(m_pObject)==LPVOID(pv)); }
   bool operator != (int pv) const
      { return !(*this == pv); }
private:
   T * m_pObject;
   bool m_bObjectShared;
};

// Some concrete type that holds useful data
class SomeUsefulData {
public:
   SomeUsefulData () {}
   ~SomeUsefulData () {}
};

// Some concrete type that holds a heap-allocated instance of
// SomeUsefulData
class SomeContainer {
public:
   SomeContainer (SomeUsefulData* pUsefulData)
   {
      m_pData = pUsefulData;
   }
   ~SomeContainer ()
   {
      // nothing to do here
   }
public:
   bool EvaluateSomeCondition ()
   {
      // fake condition check to give us an expression
      // to use in ?: operator below
      return true;
   }
   SomeUsefulData* Foo ()
   {
      // this usage of the ?: operator generates a C2248
      // error b/c it's attempting to call the copy
      // constructor on ObjectPtr<T>
      return EvaluateSomeCondition() ? m_pData : NULL;
      /**********[ DISCUSSION ]**********
      The following equivalent constructs compile
      w/out error and behave correctly:

      (1) explicit cast to SomeUsefulData* as a comiler hint
      return EvaluateSomeCondition() ? (SomeUsefulData *)m_pData : NULL;

      (2) if/else instead of ?:
      if (EvaluateSomeCondition())
         return m_pData;
      else
         return NULL;

      (3) skip the condition check and return m_pData as a
          SomeUsefulData* directly
      return m_pData;
      **********[ END DISCUSSION ]**********/
   }
private:
   ObjectPtr<SomeUsefulData> m_pData;
};

int _tmain(int argc, _TCHAR* argv[])
{
   return 0;
}