std::async比std::promise,std::packaged_task更高一层，它可以创建异步的task，异步任务返回的结果也保存在future中。当获取异步任务的结果时，调用future.get()；不关注异步任务的结果，只等待任务完成的话，调用future.wait()。

async的声明：

async(std::launch::async | std::launch::deferred, f, args...)

(1)第一个参数：线程的创建策略

std::launch::async：调用async时就开始创建线程。

std::launch::deferred：延迟加载方式创建线程。

(2)第二个参数：线程函数

(3)第三个参数：线程函数的参数

async的基本用法：

std::future<int> f1 = std::async(std::launch::async, [](){
    return 8;
});
cout << f1.get() << endl;

输出:
8

std::future<int> f2 = std::async(std::launch::async, [](){
    cout << 8 << endl;
});
f2.wait();

输出:
8

std::future<int> future = std::async(std::launch::async, [](){
    std::this_thread::sleep(std::chrono::seconds(3));
    return 8;
});
cout << "waiting..." << endl;
std::future_status status;
do {
    status = future.wait_for(std::chrono::seconds(1));
    if (status == std::future_status::deferred) {
        cout << "deferred" << endl;
    } else if (status == std::future_status::timeout) {
        cout << "timeout" << endl;
    } else if (status == std::future_status::ready) {
        cout << "ready" << endl;
    }
} while (status != std::future_status::ready);
cout << "result is " << futute.get << endl;

可能输出:
waiting...
timeout
timeout
ready
result is 8