#include <iostream>
#include <vector>
#include <thread>
using namespace std;


void square(int x, int &sum) {
    sum += x * x;
}

int main() {

    int sum = 0;

    vector<thread> allthread;
    for (int i = 1; i <= 20; i++) {
        allthread.push_back(thread(&square, i, ref(sum)));
	// need extra ref(..) wrapper here due to multiple function calls
    }

    for (auto& th : allthread) {
        th.join();
    }
    cout << "sum = " << sum << endl;
    return 0;
}