扩展代码，

#include <future>
#include <iostream>
#include <numeric>
#include <thread>
#include <vector>

using namespace std;

double accum(double* beg, double* end, double init) { return accumulate(beg, end, init); }

double comp2(vector<double>& v) {
    using Task_type = double(double*, double*, double);  // type of task

    packaged_task<Task_type> pt0{accum};  // package the task (i.e., accum)
    packaged_task<Task_type> pt1{accum};

    future<double> f0{pt0.get_future()};  // get hold of pt0's future
    future<double> f1{pt1.get_future()};  // get hold of pt1's future

    double* first = &v[0];
    thread t1{move(pt0), first, first + v.size() / 2, 0};             // start a thread for pt0
    thread t2{move(pt1), first + v.size() / 2, first + v.size(), 0};  // start a thread for pt1

    // ...

    t1.join();  // wait for the thread to complete
    t2.join();  // wait for the thread to complete

    return f0.get() + f1.get();  // get the results
}

int main() {
    vector<double> v{1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5};

    double result = comp2(v);

    cout << "Result: " << result << endl;

    return 0;
}