Problem with "moveable-only types" in VC++ 2010
- by Luc Touraille
I recently installed Visual Studio 2010 Professional RC to try it out and test the few C++0x features that are implemented in VC++ 2010.
I instantiated a std::vector of std::unique_ptr, without any problems. However, when I try to populate it by passing temporaries to push_back, the compiler complains that the copy constructor of unique_ptr is private. I tried inserting an lvalue by moving it, and it works just fine.
#include <utility>
#include <vector>
int main()
{
typedef std::unique_ptr<int> int_ptr;
int_ptr pi(new int(1));
std::vector<int_ptr> vec;
vec.push_back(std::move(pi)); // OK
vec.push_back(int_ptr(new int(2)); // compiler error
}
As it turns out, the problem is neither unique_ptr nor vector::push_back but the way VC++ resolves overloads when dealing with rvalues, as demonstrated by the following code:
struct MoveOnly
{
MoveOnly() {}
MoveOnly(MoveOnly && other) {}
private:
MoveOnly(const MoveOnly & other);
};
void acceptRValue(MoveOnly && mo) {}
int main()
{
acceptRValue(MoveOnly()); // Compiler error
}
The compiler complains that the copy constructor is not accessible. If I make it public, the program compiles (even though the copy constructor is not defined).
Did I misunderstand something about rvalue references, or is it a (possibly known) bug in VC++ 2010 implementation of this feature?
