C++ High Performance

Inner function

The inner function requires an atomic size_t that synchronizes the write positions. As the algorithm is recursive, it cannot store the atomic size_t itself; it requires an outer function to invoke the algorithm:

template <typename SrcIt, typename DstIt, typename Pred>auto _inner_par_copy_if_sync(  SrcIt first,  SrcIt last,  DstIt dst,  std::atomic_size_t& dst_idx,  Pred pred,  size_t chunk_sz) -> void {  auto n = std::distance(first, last);  if (n <= chunk_sz) {    std::for_each(first, last, [&](const auto& v) {      if (pred(v)) {        auto write_idx = dst_idx.fetch_add(1);        *std::next(dst, write_idx) = v;      }    });    return;  }  auto middle = std::next(first, n / 2);  auto future = std::async(    [first, middle, dst, chunk_sz, &pred, &dst_idx] {      return