Move constructor (rvalue reference) in implicit conversion - rvalue-reference

Question

I am upgrading a C++ project from MSVC 2008 to 2010, and because of the new CComBSTR move constructor [CComBSTR( CComBSTR&& )], I am getting a compiler error because of an ambiguous call.
Essentially, we have a String class, very similar to std::wstring that have a cast operator to CComBSTR. This is similator to the following code:
class CString {
public:
// ...
operator CComBSTR() {
CComBSTR temp;
/* Encoding conversion here */
return temp;
}
}
class CObjectConfig {
public:
CString GetName() const { return m_name; }
private:
CString m_name;
}
Now, at some places in the code, we do the following:
CObjectConfig config = GetObjectConfig();
CComBSTR objectName( config.GetName() );
In VS2008, this would work because the CString object would be implicitly converted to a CComBSTR rvalue and the copy constructor of CComBSTR (taking a const CComBSTR&) would be called to construct objectName.
In VS2010 with C++0x however, the compiler gives an ambiguous call error because CComBSTR rvalue seems to fit both the copy constructor and the move constructor.
While a bit clumsy, my solution to this problem is to static_cast the call to GetName:
CComBSTR objectName( static_cast<const CComBSTR&>( config.GetName() ) );
// or
CComBSTR objectName( static_cast<CComBSTR&&>( config.GetName() ) );
Both lines compile without error, but I need your advice on whether this is illegal, bad practice or undefined. Thank you.

Answer 1

This looks like a VC2010 bug to me. Either that, or I've incorrectly emulated your situation on my computer (I don't have VC2010). Here's what I'm doing:
#include <iostream>
class CComBSTR
{
public:
CComBSTR() {std::cout << "CComBSTR()\n";}
CComBSTR(const CComBSTR&) {std::cout << "CComBSTR(const CComBSTR&)\n";}
CComBSTR(CComBSTR&&) {std::cout << "CComBSTR(CComBSTR&&)\n";}
};
class CString {
public:
// ...
operator CComBSTR() {
CComBSTR temp;
/* Encoding conversion here */
return temp;
}
};
class CObjectConfig {
public:
CString GetName() const { return m_name; }
private:
CString m_name;
};
CObjectConfig GetObjectConfig()
{
return CObjectConfig();
}
int main()
{
CObjectConfig config = GetObjectConfig();
CComBSTR objectName( config.GetName() );
}
For me on g++-4.4 and clang (with -std=c++0x), this compiles fine. And it either calls or elides a call to CComBSTR(CComBSTR&&). My recommendation for working around this suspected bug is simply:
CComBSTR objectName( CComBSTR(config.GetName()) );
This is equivalent to your:
CComBSTR objectName( static_cast<CComBSTR&&>( config.GetName() ) );
but not as scary looking (and just as efficient). If you want to stay with the static_cast, then go with cast to CComBSTR&& as this will probably be more efficient than construction from a const lvalue.