C++中线程同步有几种方式，剥离开系统层面，只谈语言的话。常用的是mutex加锁同步和future捕捉同步。

本文简单谈一下自己对future的用法理解。
首先需要知道future是什么。future是C++中对于线程处理的库，可以进行线程异步处理和同步线程。

- 线程异步处理可以参照例子：

#include <iostream>       // std::cout
#include <future>         // std::async, std::future
#include <chrono>         // std::chrono::milliseconds

using namespace std;
// 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:
  future<bool> fut = async (is_prime,444444443); 

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

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

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

  return 0;
}

- 线程同步可以参照例子：

#include <iostream>
#include <string> // string
#include <thread> // thread
#include <future> // promise, future
#include <chrono> // seconds

using namespace std;

void recv(future<int> *future){
    try {
        cout << future->get() << endl;
    } catch (future_error &e){
        cerr << "err : " << e.code() << endl << e.what() << endl;
    }

}

void send(promise<int> *promise){
    int i = 0;
    while (i < 5){
        cout << "send while" << endl;
        ++i;
        this_thread::sleep_for(chrono::seconds(1));
    }
    promise->set_value("hello another thread");
}

int main(int argc, char **argv) {
    promise<string> promise;
    future<string> future = promise.get_future();

    thread thread_send(send, &promise);
    thread thread_recv(recv, &future);
    thread_send.join();
    thread_recv.join();
    return 0;
}