C++ async

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
 
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
int result =0;
 
void do_work()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    result = 123;
    {
        std::lock_guard<std::mutex> lk(m);
        ready = true;
    }
    cv.notify_one();
}
 
int main()
{
    std::thread thread(do_work);
    thread.detach();
    
    std::unique_lock<std::mutex> lk(m);
    cv.wait(lk, []{return ready;});

    std::cout << "result=" << result << ‘\n‘;
}

#include <iostream>
#include <cstdlib>
#include <future>
#include <chrono>

void do_work(std::promise<int> promise){
    std::this_thread::sleep_for(std::chrono::seconds(1));
    promise.set_value(123);
}

int main()
{  
    std::promise<int> promise;
    std::future<int> future = promise.get_future();
    std::thread thread( do_work , std::move(promise));
    thread.detach();
    future.wait();
    std::cout << "result=" << future.get() << ‘\n‘;
}

#include <iostream>
#include <cstdlib>
#include <future>
#include <chrono>

int main()
{  
    std::future<int> future = std::async( [](){ std::this_thread::sleep_for(std::chrono::seconds(1));
                                                return 123; });
    future.wait();
    std::cout << "result is " << future.get() << ‘\n‘;
}