The barrier class is a
          simple barrier for N threads. The number of participating
          threads is specified in the constructor. The first N
          threads to call either std::experimental::barrier::arrive_and_wait or std::experimental::barrier::arrive_and_drop form the set of
          participating threads.
        
class barrier{ public: explicit barrier(ptrdiff_t num_threads); ~barrier(); barrier(barrier const&)=delete; barrier& operator=(barrier const&)=delete; void arrive_and_drop(); void arrive_and_wait(); };
          #include <experimental/barrier>