C++11之std::future对象的基本用法

1、

// 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;
}

2、

// promise example
#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();
  std::cout << "value: " << x << '\n';
}

int main ()
{
  std::promise<int> prom;                      // create promise
  std::future<int> fut = prom.get_future();    // engagement with future
  std::thread th1 (print_int, std::ref(fut));  // send future to new thread
  prom.set_value (10);                         // fulfill promise                                               // (synchronizes with getting the future)
  th1.join();
  return 0;
}

3、

// 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;
}