Example of returning values from threads

return_value.cpp

/**
 * Example of returning a value from threads
 */

#include <iostream>
#include <vector>
#include <mutex>
#include <thread>

using namespace std;

// global mutex
mutex process_mutex;


struct S {
    size_t x;
};


void calculate(S s, int& result) {
    int out = s.x * 2;

    {
        // gard access to the sheared resource, i.e. cout
        lock_guard<mutex> lock(process_mutex);
        cout << "threadid=" << this_thread::get_id()
             <<": x=" << s.x << " out=" << out << endl;
    }

    result = out;

}

int main() {


    size_t no_of_threads = 5;

    // this will store the values calculated by thread.
    vector<int> results(no_of_threads, 0);

    // vector of threads
    vector<thread> threads;

    // create threads
    for(size_t i = 0; i < no_of_threads; ++i) {
        threads.emplace_back(&calculate, S{i}, ref(results.at(i)));
    }

    // wait for their completion and display results.
    for(size_t i = 0; i < no_of_threads; ++i) {
        threads.at(i).join();
    }

    cout << endl << endl;

    // display results.
    for(size_t i = 0; i < no_of_threads; ++i) {
        cout << "x=" << i << " out=" << results.at(i) << endl;
    }

    return 0;
}