parallel_for_each_listing_8.8.cpp

#include <algorithm> //std::min, std::for_each
#include <future> // std::future, std::async

template<typename Iterator, typename Func>
void parallel_for_each(Iterator first, Iterator last, Func f){
    unsigned long const length = std::distance (first, last);
    
    if(!length)
        return;
    
    unsigned long const min_per_thread = 25;
    
    if (length < (2*min_per_thread)){
        std::for_each(first, last, f);
    }
    else{
        Iterator const mid_point = first + length/2;
        std::future<void> first_half = 
           std::async(&parallel_for_each<Iterator, Func>,first, mid_point, f);
           
        /*Will test out below version to confirm if the above always avoids creating a new thread
          std::future<void> first_half = 
           std::async(std::launch::async , &parallel_for_each<Iterator, Func>,first, mid_point, f);
           */
        parallel_for_each(mid_point, last, f);
        first_half.get();
    }
}