// accumulate just the element of the range I've been asked to void operator()(std::size_t begin, std::size_t end) { value += std::accumulate(input.begin() + begin, input.begin() + end, 0.0); }
// accumulate just the element of the range I've been asked to void operator()(std::size_t begin, std::size_t end) { value += std::accumulate(input.begin() + begin, input.begin() + end, 0.0); }