std::promise 被创建为 promise/future 对的端点，std::future（使用 get_future() 方法从 std::promise 创建）是另一个端点。这是一种简单的一次性方法，它为两个线程提供了一种同步方式，因为一个线程通过消息向另一个线程提供数据。
您可以将其视为一个线程创建一个提供数据的承诺，而另一个线程在未来收集该承诺。此机制只能使用一次。
promise/future 机制只有一个方向，从使用std::promise 的set_value() 方法的线程到使用std::future 的get() 的线程接收数据。如果多次调用future 的get() 方法会产生异常。
如果带有std::promise 的线程没有使用set_value() 来实现它的promise，那么当第二个线程调用std::future 的get() 来收集promise 时，第二个线程将进入等待状态，直到当第一个线程使用set_value() 方法发送数据时，该承诺由std::promise 的第一个线程实现。
借助Technical Specification N4663 Programming Languages — C++ Extensions for Coroutines 的建议协程和co_await 的Visual Studio 2017 C++ 编译器支持，还可以使用std::future 和std::async 编写协程功能。请参阅https://stackoverflow.com/a/50753040/1466970 中的讨论和示例，其中一节讨论了std::future 与co_await 的使用。
以下示例代码是一个简单的 Visual Studio 2013 Windows 控制台应用程序，展示了如何使用一些 C++11 并发类/模板和其他功能。它说明了一个运行良好的 promise/future 的使用，将执行一些任务并停止的自治线程，以及一个需要更多同步行为并且由于需要多个通知而 promise/future 对不起作用的用途。
关于此示例的一个注意事项是在各个地方添加了延迟。添加这些延迟只是为了确保使用std::cout 打印到控制台的各种消息清晰，并且来自多个线程的文本不会混合。
main() 的第一部分是创建三个额外的线程并使用std::promise 和std::future 在线程之间发送数据。一个有趣的地方是主线程启动了一个线程，T2，它将等待来自主线程的数据，做一些事情，然后将数据发送到第三个线程，T3，然后它会做一些事情并将数据发送回主线程。
main() 的第二部分创建了两个线程和一组队列，以允许从主线程向两个创建的线程中的每一个发送多条消息。我们不能为此使用std::promise 和std::future，因为promise/future duo 是一次性的，不能重复使用。
Sync_queue 类的来源来自 Stroustrup 的 The C++ Programming Language: 4th Edition。
// cpp_threads.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include <thread> // std::thread is defined here
#include <future> // std::future and std::promise defined here
#include <list> // std::list which we use to build a message queue on.
static std::atomic<int> kount(1); // this variable is used to provide an identifier for each thread started.
//------------------------------------------------
// create a simple queue to let us send notifications to some of our threads.
// a future and promise are one shot type of notifications.
// we use Sync_queue<> to have a queue between a producer thread and a consumer thread.
// this code taken from chapter 42 section 42.3.4
// The C++ Programming Language, 4th Edition by Bjarne Stroustrup
// copyright 2014 by Pearson Education, Inc.
template<typename Ttype>
class Sync_queue {
public:
void put(const Ttype &val);
void get(Ttype &val);
private:
std::mutex mtx; // mutex used to synchronize queue access
std::condition_variable cond; // used for notifications when things are added to queue
std::list <Ttype> q; // list that is used as a message queue
};
template<typename Ttype>
void Sync_queue<Ttype>::put(const Ttype &val) {
std::lock_guard <std::mutex> lck(mtx);
q.push_back(val);
cond.notify_one();
}
template<typename Ttype>
void Sync_queue<Ttype>::get(Ttype &val) {
std::unique_lock<std::mutex> lck(mtx);
cond.wait(lck, [this]{return !q.empty(); });
val = q.front();
q.pop_front();
}
//------------------------------------------------
// thread function that starts up and gets its identifier and then
// waits for a promise to be filled by some other thread.
void func(std::promise<int> &jj) {
int myId = std::atomic_fetch_add(&kount, 1); // get my identifier
std::future<int> intFuture(jj.get_future());
auto ll = intFuture.get(); // wait for the promise attached to the future
std::cout << " func " << myId << " future " << ll << std::endl;
}
// function takes a promise from one thread and creates a value to provide as a promise to another thread.
void func2(std::promise<int> &jj, std::promise<int>&pp) {
int myId = std::atomic_fetch_add(&kount, 1); // get my identifier
std::future<int> intFuture(jj.get_future());
auto ll = intFuture.get(); // wait for the promise attached to the future
auto promiseValue = ll * 100; // create the value to provide as promised to the next thread in the chain
pp.set_value(promiseValue);
std::cout << " func2 " << myId << " promised " << promiseValue << " ll was " << ll << std::endl;
}
// thread function that starts up and waits for a series of notifications for work to do.
void func3(Sync_queue<int> &q, int iBegin, int iEnd, int *pInts) {
int myId = std::atomic_fetch_add(&kount, 1);
int ll;
q.get(ll); // wait on a notification and when we get it, processes it.
while (ll > 0) {
std::cout << " func3 " << myId << " start loop base " << ll << " " << iBegin << " to " << iEnd << std::endl;
for (int i = iBegin; i < iEnd; i++) {
pInts[i] = ll + i;
}
q.get(ll); // we finished this job so now wait for the next one.
}
}
int _tmain(int argc, _TCHAR* argv[])
{
std::chrono::milliseconds myDur(1000);
// create our various promise and future objects which we are going to use to synchronise our threads
// create our three threads which are going to do some simple things.
std::cout << "MAIN #1 - create our threads." << std::endl;
// thread T1 is going to wait on a promised int
std::promise<int> intPromiseT1;
std::thread t1(func, std::ref(intPromiseT1));
// thread T2 is going to wait on a promised int and then provide a promised int to thread T3
std::promise<int> intPromiseT2;
std::promise<int> intPromiseT3;
std::thread t2(func2, std::ref(intPromiseT2), std::ref(intPromiseT3));
// thread T3 is going to wait on a promised int and then provide a promised int to thread Main
std::promise<int> intPromiseMain;
std::thread t3(func2, std::ref(intPromiseT3), std::ref(intPromiseMain));
std::this_thread::sleep_for(myDur);
std::cout << "MAIN #2 - provide the value for promise #1" << std::endl;
intPromiseT1.set_value(22);
std::this_thread::sleep_for(myDur);
std::cout << "MAIN #2.2 - provide the value for promise #2" << std::endl;
std::this_thread::sleep_for(myDur);
intPromiseT2.set_value(1001);
std::this_thread::sleep_for(myDur);
std::cout << "MAIN #2.4 - set_value 1001 completed." << std::endl;
std::future<int> intFutureMain(intPromiseMain.get_future());
auto t3Promised = intFutureMain.get();
std::cout << "MAIN #2.3 - intFutureMain.get() from T3. " << t3Promised << std::endl;
t1.join();
t2.join();
t3.join();
int iArray[100];
Sync_queue<int> q1; // notification queue for messages to thread t11
Sync_queue<int> q2; // notification queue for messages to thread t12
std::thread t11(func3, std::ref(q1), 0, 5, iArray); // start thread t11 with its queue and section of the array
std::this_thread::sleep_for(myDur);
std::thread t12(func3, std::ref(q2), 10, 15, iArray); // start thread t12 with its queue and section of the array
std::this_thread::sleep_for(myDur);
// send a series of jobs to our threads by sending notification to each thread's queue.
for (int i = 0; i < 5; i++) {
std::cout << "MAIN #11 Loop to do array " << i << std::endl;
std::this_thread::sleep_for(myDur); // sleep a moment for I/O to complete
q1.put(i + 100);
std::this_thread::sleep_for(myDur); // sleep a moment for I/O to complete
q2.put(i + 1000);
std::this_thread::sleep_for(myDur); // sleep a moment for I/O to complete
}
// close down the job threads so that we can quit.
q1.put(-1); // indicate we are done with agreed upon out of range data value
q2.put(-1); // indicate we are done with agreed upon out of range data value
t11.join();
t12.join();
return 0;
}
这个简单的应用程序创建以下输出。
MAIN #1 - create our threads.
MAIN #2 - provide the value for promise #1
func 1 future 22
MAIN #2.2 - provide the value for promise #2
func2 2 promised 100100 ll was 1001
func2 3 promised 10010000 ll was 100100
MAIN #2.4 - set_value 1001 completed.
MAIN #2.3 - intFutureMain.get() from T3. 10010000
MAIN #11 Loop to do array 0
func3 4 start loop base 100 0 to 5
func3 5 start loop base 1000 10 to 15
MAIN #11 Loop to do array 1
func3 4 start loop base 101 0 to 5
func3 5 start loop base 1001 10 to 15
MAIN #11 Loop to do array 2
func3 4 start loop base 102 0 to 5
func3 5 start loop base 1002 10 to 15
MAIN #11 Loop to do array 3
func3 4 start loop base 103 0 to 5
func3 5 start loop base 1003 10 to 15
MAIN #11 Loop to do array 4
func3 4 start loop base 104 0 to 5
func3 5 start loop base 1004 10 to 15