Visual Studio 2013 C++ - Passing std::unique_ptr to a bound function

Question

Using Visual Studio 2013 RC and C++, I\'m trying to pass an std::unique_ptr to a function that has been bound using std::bind. However, I\'m having

Answer 1

I had the same problem with VS 2012 recently. I believe this is a bug in MSVC; at least in MSVC++11 the pseudo-variadic expansion seems to forward the parameters by value to some internal function. Seems this hasn't been improved.
As a workaround, I'm using lambdas instead, but another hack is required to make it work:

std::function<void (std::unique_ptr<int>)> bound =
    [] (std::unique_ptr<int> arg) { func(std::move(arg)); };

still doesn't compile. But if you add any captured value (even one that isn't used), it compiles:

int x;
std::function<void (std::unique_ptr<int>)> bound =
    [x] (std::unique_ptr<int> arg) { func(std::move(arg)); };

Answer 2

You have to move the parameter into the bound call to func also. Not only in the invocation of bound

bound(std::move(ptr));

but also in the binding:

std::function<void(std::unique_ptr<int>)> bound =
    std::bind(func,
              std::bind(std::move<std::unique_ptr<int>&>,
                        std::placeholders::_1));

This is compiling in VS2013 (update 4) for me.

Answer 3

Functions bound with std::bind do not forward arguments, it copies them to the function. As a result, std::bind doesn't work with move-only types as of c++11. This problem is the idea behind proposals for "more perfect forwarding" (like this one). There's a newer one, but I can't seem to find it right now.