Question

I’m trying to wrap std::packaged_task inside another class in order to be used together with a task scheduler.

At the moment I got it all working except std::future support. To get std::future support I figured out I need to use std::packaged_task for the get_future() function that it provides.

I’ve been trying whole day all sorts of ways to get this to work, but I seem to be unable to properly declare and initialise the packaged_task using the return value from a std::bind. I have tried to decipher the implementations of all the related libstdc++ functions such as std::async, std::future, std::thread etc but with no luck.

The following code is the implementation of both the not working version and the working one. To get it to work uncomment the two /* — WORKS*/ and comment the other related line.

#include <vector>
#include <deque>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <iostream>
#include <chrono>
#include <functional>

#include <windows.h>

class task
{
private:
    struct task_implementation_base
    {
        virtual void execute() = 0;
    };

    template <class callable>
    struct task_implementation : public task_implementation_base
    {
        task_implementation(callable&& f) : /*m_task(std::forward<callable>(f)) WORKS*/m_task(f) { }
        void execute() { m_task(); }

        //callable m_task; // WORKS
        std::packaged_task<typename result_of<callable>::type> m_task;
    };

    template <class callable>
    std::shared_ptr<task_implementation<callable>> make_routine(callable&& f)
    {
        return std::make_shared<task_implementation<callable>>(std::forward<callable>(f));
    }

public:
    template <class callable, class... arguments>
    task(callable&& f, arguments&&... args) : m_function(make_routine(std::bind(std::forward<callable>(f), std::forward<arguments>(args)...))) {}

    void operator()() { run(); }

    void run() { m_function->execute(); }

private:
    std::shared_ptr<task_implementation_base> m_function;

};

int testint(int i)
{
    std::cout << "test6" << " :: ran from thread " << std::this_thread::get_id() << "\n";
    fflush(stdout);
    return i+100;
}

void test(const char* text)
{
    std::cout << text << " :: ran from thread " << std::this_thread::get_id() << "\n";
    fflush(stdout);
}

class testclass
{
public:
    void print1() { test("test3"); }
    void print2() { test("test4"); }
    void print3(const char* text) { test(text); }

};

int main()
{
    testclass testclass1;
    testclass* testclass2 = new testclass;

    task test1(test, "test1");
    task test2([]() { test("test2"); });
    task test3(&testclass::print1, &testclass1);
    task test4(&testclass::print2, &*testclass2);
    task test5(&testclass::print3, &*testclass2, "test5");
    task test6(&testint, 1);

    test1();
    test2();
    test3();
    test4();
    test5();
    test6();

    Sleep(2000);

    return 0;
}

I’m thinking the problem is typename result_of<callable>::type. I’m guessing it doesn’t properly evaluates to the return type of the callable function.

I’m using c++ (Built by MinGW-builds project) 4.8.0 20121225 (experimental) on a Windows 8 64bit. I’m suspecting the errors are irrelevant since I guess I’m just simply trying to get this work the wrong way but here is a pastebin for the errors anyway: errors

Answer 1

std::packaged_task not only takes the result type of the invoked function as a template argument but also the types of the arguments you are passing to the to be invoked function.

You can define them as follows:

// somewhere
int foo(bool, int);

// somewhere else
std::packaged_task<int(bool, int)> p(foo);

To fix your code you need to add two empty parenthesis pairs. What I explained above also applies to std::result_of.

std::packaged_task<typename std::result_of<callable()>::type()> m_task;