std::async获取异步执行结果和状态

2022-12-09

简单来讲std::async是对std::future、std::promise和std::packaged_task的一个包装，将异步操作用std::packaged_task包装起来，然后将异步操作的结果放到std::promise中,在外层通过std::future.get/wait来获取结果。

函数原型

  std::async(std::launch::async | std::launch::deferred, f, args...)
  第一个参数是线程的创建策略;
std::launch::async：在调用async就开始创建线程
std::launch::deferred：延迟加载方式创建线程。调用async时不创建线程，直到调用了future的get或者wait时才创建线程。
  第二个参数是线程函数;
  第三个参数是线程函数的参数;

示例

#include <iostream>
#include <thread>
#include <future>

int main(int argc, char* argv[])
{

    std::future<int> f1 = std::async(std::launch::async, [](){
        return 1; 
    });
 
    std::cout << f1.get() << std::endl; //output: 1
 
    std::future<int> f2 = std::async(std::launch::async, [](){
        std::cout << 2 << std::endl;
        return 0;
    });
 
    f2.wait(); //output: 2
 
    std::future<int> future = std::async(std::launch::async, [](){
        std::this_thread::sleep_for(std::chrono::seconds(3));
        return 3; 
    });
  
    std::cout << "waiting...\n";
    std::future_status status;
    do {
        status = future.wait_for(std::chrono::seconds(1));
        if (status == std::future_status::deferred) {
            std::cout << "deferred\n";
        } else if (status == std::future_status::timeout) {
            std::cout << "timeout\n";
        } else if (status == std::future_status::ready) {
            std::cout << "ready!\n";
        }
    } while (status != std::future_status::ready);
  
    std::cout << "result is " << future.get() << '\n';

    return 0;
}

输出结果

1
2
waiting...
timeout
timeout
ready!
result is 3