std::promise, std::future

promise 객체에 의해 생성된 future객체를 통해 비동기 적으로 획득되는 값이나 예외를 저장할 수 있는 기능을 제공


#include <iostream>
#include <thread>
#include <future>

using namespace std;

void ThreadWork(std::promise<int> promise)
{
    cout << "thread begin\n";
   
    std::this_thread::sleep_for(std::chrono::seconds(1));
    promise.set_value(999); ///< 전달 값 셋팅
   
    cout << "thread end\n";
}


int main(int argc, char* argv[])
{
    // Demonstrate using promise<void> to signal state between threads.
    std::promise<int> promise;
    std::future<int> future = promise.get_future();
    std::thread new_work_thread(ThreadWork, std::move(promise));
   
    cout << "wait future\n";

    future.wait();
    cout << "future value : " << future.get() << "\n"; // 위의 "future.wait"이 없으면 future.get()에서 쓰레드가 끝날때까지 블록됨
   
    cout << "wait thread\n";
    new_work_thread.join();
   
    cout << "end main\n";
    return 0;
}


output
wait future
thread begin
thread end
future value : 999
wait thread
end main