- Solution

#include <algorithm>
#include <future>
#include <iostream>
#include <thread>
#include <deque>
#include <vector>
class SumUp{
public:
  SumUp(int b, int e): beg(b), end(e){}
  int operator()(){
    long long int sum{0};
    for (int i= beg; i < end; ++i ) sum += i;
    return sum;
  }
private:
    int beg;
    int end;
};
static const unsigned int hwGuess = 4;
static const unsigned int numbers = 10001;
int main(){
  std::cout << std::endl;
  unsigned int hw = std::thread::hardware_concurrency();
  unsigned int hwConcurr = (hw != 0)? hw : hwGuess;
  // define the functors
  std::vector<SumUp> sumUp;
  for (unsigned int i = 0; i < hwConcurr; ++i){
    int begin = (i*numbers)/hwConcurr;
    int end = (i+1)*numbers/hwConcurr;
    sumUp.push_back(SumUp(begin , end));
  }
  // define the tasks
  std::deque<std::packaged_task<int()>> sumTask;
  for (unsigned int i = 0; i < hwConcurr; ++i){
    std::packaged_task<int()> SumTask(sumUp[i]);
    sumTask.push_back(std::move(SumTask));
  }
  // get the futures
  std::vector< std::future<int>> sumResult;
  for (unsigned int i = 0; i < hwConcurr; ++i){
    sumResult.push_back(sumTask[i].get_future());
  }
  // execute each task in a separate thread
  while (! sumTask.empty()){
    std::packaged_task<int()> myTask = std::move(sumTask.front());
    sumTask.pop_front();
    std::thread sumThread(std::move(myTask));
    sumThread.detach();
  }
  // get the results
  int sum = 0;
  for (unsigned int i = 0; i < hwConcurr; ++i){
    sum += sumResult[i].get();
  }
  std::cout << "sum of 0 .. 100000 = " << sum << std::endl;
  std::cout << std::endl;
}