q_async.cpp

#include <iostream>
#include <mutex>
#include <future>

using namespace std;

int main()
{
    mutex m;
    
    auto t = [](int i) {
        
        lock_guard<mutex> g(m);
        
        cout << " " 
             << i 
             << " " 
             << this_thread::get_id() 
             << std::endl;
        
        return i; 
    };
    
    array v{
        async(launch::async, t, 1),
        async(launch::async, t, 2),
        async(launch::async, t, 3),
        async(launch::async, t, 4),
        async(launch::async, t, 5)};
    
    for (auto &x:v) {
        cout << x.get() 
             << std::endl;
    }
    
}

https://coliru.stacked-crooked.com/a/2155f4a88d59869e