详解C++中future和promise的使用

/** @file  20190815future.cpp
*  @note  
*  @brief
*  @author 
*  @date   2019-8-15
*  @note   
*  @history
*  @warning
*/
#include <iostream>
#include <functional>
#include <future>
#include <thread>
#include <chrono>
#include <cstdlib>

void thread_set_promise(std::promise<int>& promiseObj) {
    std::cout << "In a thread, making data...\n";
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    promiseObj.set_value(35);
    std::cout << "Finished\n";
}

int main() {
    std::promise<int> promiseObj;
    std::future<int> futureObj = promiseObj.get_future();
    std::thread t(&thread_set_promise, std::ref(promiseObj));
    std::cout << futureObj.get() << std::endl;
    t.join();

    system("pause");
    return 0;
}

/** @file  futureIsPrime.cpp
*  @note   
*  @brief
*  @author 
*  @date   2019-8-15
*  @note   
*  @history
*  @warning
*/
// future example
#include <iostream>       // std::cout
#include <future>         // std::async, std::future
#include <chrono>         // std::chrono::milliseconds

// a non-optimized way of checking for prime numbers:
bool is_prime (int x) {
  for (int i=2; i<x; ++i) if (x%i==0) return false;
  return true;
}

int main ()
{
  // call function asynchronously:
  std::future<bool> fut = std::async (is_prime,444444443);

  // do something while waiting for function to set future:
  std::cout << "checking, please wait";
  std::chrono::milliseconds span (100);
  while (fut.wait_for(span)==std::future_status::timeout)
    std::cout << '.' << std::flush;

  bool x = fut.get();     // retrieve return value

  std::cout << "\n444444443 " << (x?"is":"is not") << " prime.\n";

  return 0;
}

// packaged_task example
#include <iostream>     // std::cout
#include <future>       // std::packaged_task, std::future
#include <chrono>       // std::chrono::seconds
#include <thread>       // std::thread, std::this_thread::sleep_for

// count down taking a second for each value:
int countdown (int from, int to) {
  for (int i=from; i!=to; --i) {
    std::cout << i << '\n';
    std::this_thread::sleep_for(std::chrono::seconds(1));
  }
  std::cout << "Lift off!\n";
  return from-to;
}

int main ()
{
  std::packaged_task<int(int,int)> tsk (countdown);   // set up packaged_task
  std::future<int> ret = tsk.get_future();            // get future

  std::thread th (std::move(tsk),10,0);   // spawn thread to count down from 10 to 0

  // ...

  int value = ret.get();                  // wait for the task to finish and get result

  std::cout << "The countdown lasted for " << value << " seconds.\n";

  th.join();

  return 0;
}