Move packaged_task to lambda

Question

I want to move and call boost :: packaged_task inside lambda.

However, I cannot find an elegant solution.

eg. This will not compile.

        template<typename Func>
        auto begin_invoke(Func&& func) -> boost::unique_future<decltype(func())> // noexcept
        {   
            typedef boost::packaged_task<decltype(func())> task_type;

            auto task = task_type(std::forward<Func>(func));
            auto future = task.get_future();

            execution_queue_.try_push([=]
            {
                try{task();}
                catch(boost::task_already_started&){}
            });

            return std::move(future);       
        }

    int _tmain(int argc, _TCHAR* argv[])
    {
        executor ex;
        ex.begin_invoke([]{std::cout << "Hello world!";});
       //error C3848: expression having type 'const boost::packaged_task<R>' would lose some const-volatile qualifiers in order to call 'void boost::packaged_task<R>::operator ()(void)'
//          with
//          [
//              R=void
//          ]
        return 0;
    }

My pretty ugly solution:

    struct task_adaptor_t
    {
        // copy-constructor acts as move constructor
        task_adaptor_t(const task_adaptor_t& other) : task(std::move(other.task)){}
        task_adaptor_t(task_type&& task) : task(std::move(task)){}
        void operator()() const { task(); }
        mutable task_type task;
    } task_adaptor(std::move(task));

    execution_queue_.try_push([=]
    {
        try{task_adaptor();}
        catch(boost::task_already_started&){}
    });

What is the “right” way to move packed_task to the lambda that calls it?

Answer 1

With the correct implementation of std :: bind (or something equivalent with respect to types with movement support) you should be able to combine bind and C ++ 0x lambda as follows:

task_type task (std::forward<Func>(func));
auto future = task.get_future();

execution_queue_.try_push(std::bind([](task_type const& task)
{
    try{task();}
    catch(boost::task_already_started&){}
},std::move(task)));

return future;

btw: std:: move around future, - . , , , "". std:: move /.

Answer 2

, . ++ 0x . , , - - .

template<typename T, typename F> class move_capture_proxy {
    T t;
    F f;
public:
    move_capture_proxy(T&& a, F&& b) 
        : t(std::move(a)), f(std::move(b)) {}
    auto operator()() -> decltype(f(std::move(b)) {
        return f(std::move(b));
    }
};
template<typename T, typename F> move_capture_proxy<T, F> make_move_proxy(T&& t, F&& f) {
    return move_capture_proxy<T, F>(std::move(t), std::move(f));
}

execution_queue.try_push(make_move_proxy(std::move(task), [](decltype(task)&& ref) {
    auto task = std::move(ref);
    // use task
});

, , , MSVC10 , .