C++11 <future>中std::promise 介绍

#include <iostream>    // std::cout
#include <functional>   // std::ref
#include <thread>     // std::thread
#include <future>     // std::promise, std::future

void print_int(std::future<int>& fut) {
  int x = fut.get(); // 获取共享状态的值.
  std::cout << "value: " << x << '\n'; // 打印 value: 10.
}

int main ()
{
  std::promise<int> prom; // 生成一个 std::promise<int> 对象.
  std::future<int> fut = prom.get_future(); // 和 future 关联.
  std::thread t(print_int, std::ref(fut)); // 将 future 交给另外一个线程t.
  prom.set_value(10); // 设置共享状态的值, 此处和线程t保持同步.
  t.join();
  return 0;
}

#include <iostream>    // std::cout
#include <thread>     // std::thread
#include <future>     // std::promise, std::future

std::promise<int> prom;

void print_global_promise () {
  std::future<int> fut = prom.get_future();
  int x = fut.get();
  std::cout << "value: " << x << '\n';
}

int main ()
{
  std::thread th1(print_global_promise);
  prom.set_value(10);
  th1.join();

  prom = std::promise<int>();  // prom 被move赋值为一个新的 promise 对象.

  std::thread th2 (print_global_promise);
  prom.set_value (20);
  th2.join();

 return 0;
}

#include <iostream>    // std::cin, std::cout, std::ios
#include <functional>   // std::ref
#include <thread>     // std::thread
#include <future>     // std::promise, std::future
#include <exception>   // std::exception, std::current_exception

void get_int(std::promise<int>& prom) {
  int x;
  std::cout << "Please, enter an integer value: ";
  std::cin.exceptions (std::ios::failbit);  // throw on failbit
  try {
    std::cin >> x;             // sets failbit if input is not int
    prom.set_value(x);
  } catch (std::exception&) {
    prom.set_exception(std::current_exception());
  }
}

void print_int(std::future<int>& fut) {
  try {
    int x = fut.get();
    std::cout << "value: " << x << '\n';
  } catch (std::exception& e) {
    std::cout << "[exception caught: " << e.what() << "]\n";
  }
}

int main ()
{
  std::promise<int> prom;
  std::future<int> fut = prom.get_future();

  std::thread th1(get_int, std::ref(prom));
  std::thread th2(print_int, std::ref(fut));

  th1.join();
  th2.join();
  return 0;
}