Visual Studio 2013 C ++ - 将std :: unique_ptr传递给绑定函数

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 trouble because VS doesn't seem to like it when I try this. Here's what I'm trying to compile:

#include <memory>
#include <iostream>
#include <functional>

void func(std::unique_ptr<int> arg)
{
    std::cout << *arg << std::endl;
}

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

    std::unique_ptr<int> ptr(new int(42));
    bound(std::move(ptr));

    return 0;
}

This compiles in GCC 4.8.1, but not in VS2013 RC. I've always had problems with move semantics in VS, but I'd really like to use std::unique_ptr instead of std::shared_ptr or raw pointers.

One workaround I've found is to change the function signature to accept an std::unique_ptr& , which does compile in VS and GCC, but doesn't make the intent of func taking ownership of the std::unique_ptr particularly clear, and also prevents me from safely asynchronously calling the function unless I do something particularly ugly:

#include <memory>
#include <iostream>
#include <functional>
#include <future>
#include <string>

void func(std::unique_ptr<int>& arg)
{
    std::cout << *arg << std::endl;
}

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

    std::unique_ptr<int> ptr(new int(42));
    std::promise<void> prom;
    std::async(
        [&bound, &ptr, &prom]
        {
            std::unique_ptr<int> movedPtr = std::move(ptr);
            prom.set_value();

            bound(std::move(movedPtr));
        });

    prom.get_future().wait();

    // Wait here
    std::string dummy;
    std::cin >> dummy;
}

Is there a way to get around this without changing func 's signature?

Thanks!

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.