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;
}