一、说明

 std::promise 是C++11并发编程中常用的一个类，常配合std::future使用。其作用是在一个线程t1中保存一个类型typename T的值，可供相绑定的std::future对象在另一线程t2中获取

二、代码

#include <chrono>
#include <future>
#include <iostream>
#include <numeric>
#include <thread>
#include <vector>void accumulate(int task_cost_second, std::promise<int> accumulate_promise) {std::cout << "before set_value" << std::endl;std::this_thread::sleep_for(std::chrono::seconds(task_cost_second));accumulate_promise.set_value(task_cost_second);  // 提醒 futurestd::cout << "after set_value" << std::endl;
}void accumulate2(int task_cost_second, std::promise<int> accumulate_promise) {std::cout << "before set_value" << std::endl;std::this_thread::sleep_for(std::chrono::seconds(task_cost_second));accumulate_promise.set_value(task_cost_second);  // 提醒 futurestd::cout << "after set_value" << std::endl;
}int main() {// 用 promise<int> 在线程间传递结果。std::cout << "**********第一个测试**********" << std::endl;std::promise<int> accumulate_promise;std::future<int> accumulate_future = accumulate_promise.get_future();std::thread work_thread(accumulate, 3, std::move(accumulate_promise));// future::get() 将等待直至该 future 拥有合法结果并取得它// 无需在 get() 前调用 wait()// accumulate_future.wait();  // 等待结果std::cout << "before accumulate_future.get()" << std::endl;std::this_thread::sleep_for(std::chrono::seconds(5));std::cout << "result=" << accumulate_future.get() << '\n';std::cout << "ater accumulate_future.get()" << std::endl;work_thread.join();  // wait for thread completion// 演示用 promise<void> 在线程间对状态发信号std::cout << "**********第二个测试**********" << std::endl;std::promise<int> accumulate_promise1;std::future<int> accumulate_future1 = accumulate_promise1.get_future();std::thread work_thread1(accumulate2, 3, std::move(accumulate_promise1));// future::get() 将等待直至该 future 拥有合法结果并取得它// 无需在 get() 前调用 wait()// accumulate_future.wait();  // 等待结果std::cout << "before accumulate_future.get()" << std::endl;std::this_thread::sleep_for(std::chrono::seconds(1));std::cout << "result=" << accumulate_future1.get() << '\n';std::cout << "ater accumulate_future.get()" << std::endl;work_thread1.join();  // wait for thread completion
}

 三、结果

 不难发现两个测试用例线程的sleep时间不一样，导致的输出不一样。