Как распараллелить цикл с помощью thread?

#include <iostream>
#include <vector>
#include <algorithm>
#include <thread>
#include <atomic>

template <typename RAIter>
int parallel_sum(RAIter begin, RAIter end)
{
    std::atomic<int> result;
    result = 0;
    int length = end - begin;
    if(length < 1000)
        return std::accumulate(begin, end, 0);

    std::vector<std::thread> threads;
    unsigned num_of_threads = std::thread::hardware_concurrency();
    threads.reserve(num_of_threads);

    int values_per_thread = length / num_of_threads;

    for(RAIter i = begin; i < end; i += values_per_thread) {
        RAIter j = i + values_per_thread;
        if(j > end) {
            j = end;
        }
        threads.push_back(std::thread([&result](RAIter begin, RAIter end){
            result += std::accumulate(begin, end, 0);
        }, i, j));
    }

    for(auto &thread : threads)
        thread.join();

    return result;
}

int main()
{
    std::vector<int> v(12002, 1);

    int sum = parallel_sum(v.begin(), v.end());
    std::cout << "The sum is " << sum << '\n';
}