// packagedTask.cpp
#include <utility>
#include <future>
#include <iostream>
#include <thread>
#include <deque>
class SumUp{
  public:
    int operator()(int beg, int end){
      long long int sum{0};
      for (int i = beg; i < end; ++i ) sum += i;
      return sum;
    }
};
int main(){
  std::cout << std::endl;
  SumUp sumUp1;
  SumUp sumUp2;
  SumUp sumUp3;
  SumUp sumUp4;
  // wrap the tasks
  std::packaged_task<int(int, int)> sumTask1(sumUp1);
  std::packaged_task<int(int, int)> sumTask2(sumUp2);
  std::packaged_task<int(int, int)> sumTask3(sumUp3);
  std::packaged_task<int(int, int)> sumTask4(sumUp4);
  // create the futures
  std::future<int> sumResult1 = sumTask1.get_future();
  //std::future<int> sumResult2 = sumTask2.get_future();
  auto sumResult2 = sumTask2.get_future();
  std::future<int> sumResult3 = sumTask3.get_future();
  //std::future<int> sumResult4 = sumTask4.get_future();
  auto sumResult4 = sumTask4.get_future();
  // push the tasks on the container
  std::deque<std::packaged_task<int(int,int)>> allTasks;
  allTasks.push_back(std::move(sumTask1));
  allTasks.push_back(std::move(sumTask2));
  allTasks.push_back(std::move(sumTask3));
  allTasks.push_back(std::move(sumTask4));
  
  int begin{1};
  int increment{2500};
  int end = begin + increment;
  // execute each task in a separate thread
  while (not allTasks.empty()){
    std::packaged_task<int(int, int)> myTask = std::move(allTasks.front());
    allTasks.pop_front();
    std::thread sumThread(std::move(myTask), begin, end);
    begin = end;
    end += increment;
    sumThread.detach();
  }
  // get the results
  auto sum = sumResult1.get() + sumResult2.get() + 
             sumResult3.get() + sumResult4.get();
  std::cout << "sum of 0 .. 10000 = " << sum << std::endl;
  std::cout << std::endl;
}