Потоки в stl C++11

int f(int * arr, int what_to_find, int start, int end)
{
    for (int i = start; int i < end; i++)
        if (arr[i] == what_to_find) return i;
    return -1;
}

#include <iostream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <functional>
#include <atomic>
#include <future>
#include <thread>
#include <chrono>
 
template<typename Iterator, typename MatchType>
std::pair<Iterator, bool> find(Iterator first, Iterator last, const MatchType& value, std::atomic<bool>& done)
{
    for (; first != last && !done.load(); ++first)
    {
        if (*first == value) // как только нашли, сразу поднимаем флаг и помечаем итератор как true
        {
           done = true;
           return std::make_pair(first, true);
        }
    }
    return std::make_pair(last, false); // не нашли, - возвращаем последний итератор и помечаем его как false
}
 
template<typename Iterator, typename MatchType>
Iterator find_parallel(Iterator first, Iterator last, MatchType match)
{
   auto length = std::distance(first, last);
   if (!length)
      return last;
 
   std::atomic<bool> done(false);
   std::vector<std::future<std::pair<Iterator, bool>>> futures(std::thread::hardware_concurrency());
 
   if (futures.size() == 1)
      return std::find(first, last, match);
 
   auto block_size = length / futures.size();
 
   Iterator block_start = first, block_end = first;
   for (int i = 0; i < futures.size()-1; ++i)
   {
      std::advance(block_end, block_size);
      futures[i] = std::async(std::launch::async, &find<Iterator, MatchType>, block_start, block_end, match, std::ref(done));
      block_start = block_end;
   }
   futures.back() = std::async(std::launch::async, &find<Iterator, MatchType>, block_end, last, match, std::ref(done));
 
   for (int i = 0; i < futures.size(); ++i)
   {
      auto result = futures[i].get();
      if (result.second)
         return result.first;
   }
   return last; // ничего не найдено
}
 
class long_compare
{
   int n;
public:
   long_compare(int n_) : n(n_) {}
   bool operator == (const long_compare& other)
   {
      std::this_thread::sleep_for(std::chrono::milliseconds(20));
      return n == other.n;
   }
};
 
int main()
{
   std::vector<long_compare> v(1000, 1);
   v[999] = 2;
 
   std::cout << "Parallel find:\n";
   auto start = std::chrono::high_resolution_clock::now();
   auto pos = find_parallel(v.begin(), v.end(), 2);
   auto stop = std::chrono::high_resolution_clock::now();
   std::cout << "Founded in posithion: [" << std::distance(v.begin(), pos) << "]\n";
   std::cout<<"Time: "<<std::chrono::duration_cast<std::chrono::seconds>(stop-start).count()
            <<" seconds\n";
 
   std::cout << "\nDefault std::find:\n";
   start = std::chrono::high_resolution_clock::now();
   pos = std::find(v.begin(), v.end(), 2);
   stop = std::chrono::high_resolution_clock::now();
   std::cout << "Founded in posithion: [" << std::distance(v.begin(), pos) << "]\n";
   std::cout<<"Time: "<<std::chrono::duration_cast<std::chrono::seconds>(stop-start).count()
            <<" seconds\n";
 
 
   std::cout << "\n\nDone!\n";
   return 0;
}