Create shared packaged_task that accepts a parameter with forwarding

Question

Hi I don't know how to write a proper bind to queue and execute a lambda expression passed to the method OCRQueue::enqueue()

// the task queue
std::queue< std::function<void(OCRK*)> > tasks;

// add new work item to the pool
template<class F>
auto OCRQueue::enqueue(F&& f)
-> std::future<typename std::result_of<F(OCRK*)>::type>
{
    using return_type = typename std::result_of<F(OCRK*)>::type;

    auto task = std::make_shared< std::packaged_task<return_type()> >
    (
        // how to initialize this task so that it can be called 
        // task(OCRK*) passing the parameter to f(OCRK*)
        std::bind
        (
            std::forward<F>(f)
        )
    );

    std::future<return_type> res = task->get_future();
    {
        std::unique_lock<std::mutex> lock(queue_mutex);

        // don't allow enqueueing after stopping the pool
        if (stop)
            throw std::runtime_error("enqueue on stopped thread_pool");

        // this fails because task does not accept parameters
        tasks.emplace([task](OCRK* ocr){ task(ocr); });
    }
    condition.notify_one();
    return res;
}

Currently this fails to compile on "auto task =" because f expects a parameter of type OCRK* with:

Error 6 error C2064: term does not evaluate to a function taking 0 arguments C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xrefwrap 58 1 Skilja.PR.API

On: 1> d:\sourcecode\skilja\alpr\alpr\skilja.pr.api.native\OCRQueue.h(51) : see reference to function template instantiation 'std::shared_ptr> std::make_shared,std::_Bind &,>>(std::_Bind &,> &&)' being compiled

The expected usage is like this:

OCRQueue ocrPool(std::thread::hardware_concurrency());
auto work = [](OCRK* ocr)
{
    return ocr->DoSomething();
};
future<DoSomethingResult> result = ocrPool.enqueue(work);

The OCRK* will be passed to the enqueued function when dequeued and executed in another thread.

Jonathan Wakely Jonathan Wakely · Accepted Answer · 2015-09-15T16:51:58

To create a callable object that accepts a single argument you need to tell bind to use a placeholder for the argument:

std::bind( std::forward<F>(f), std::placeholders::_1 )

This tells bind to leave an unbound argument, which must be supplied when you call the function object.

But I don't understand why you are using bind at all, you're not binding anything and f is already a callable object that accepts a single argument, why don't you just pass it to the packaged_task constructor?

There is another problem, which is that you say you want a task that accepts a single argument, but you have declared packaged_task<return_type()> which is a task that takes no arguments. I think you want:

auto task = std::make_shared< std::packaged_task<return_type(OCRK*)> >(std::forward<F>(f));

Also:

    tasks.emplace([task](OCRK* ocr){ task(ocr); });

that will still fail because task is a shared_ptr, so it is not callable. It should be:

    tasks.emplace([task](OCRK* ocr){ (*task)(ocr); });

Create shared packaged_task that accepts a parameter with forwarding

1 Answers