期货与承诺 [英] Futures vs. Promises
问题描述
显然,他们有不同的方法和东西,但是实际的用例是什么? / p>
是吗?:
- 当我管理一些异步任务时,未来使用未来获取值
- 当我是异步任务时,我使用promise作为返回类型,以允许用户从我的承诺中获得未来
Future和Promise是异步操作的两个独立方面。
std :: promise
由异步操作的生产者/作者使用。
std :: future
由异步操作的consumer / reader使用。
因为它被分成这两个单独的接口是隐藏写/设置功能从消费者/读者。
auto promise = std :: promise< std :: string>();
auto producer = std :: thread([&]
{
promise.set_value(Hello World);
});
auto future = promise.get_future();
auto consumer = std :: thread([&]
{
std :: cout<<<<<; future.get();
}
producer.join();
consumer.join();
使用std :: promise实现std :: async的一种(不完整)方法可能是:
template< typename F>
auto async(F&& func) - > std :: future< decltype(func())>
{
typedef decltype(func())result_type;
auto promise = std :: promise< result_type>();
auto future = promise.get_future();
std :: thread(std :: bind([=](std :: promise< result_type>& promise)
{
try
{
promise.set_value(func()); //注意:不能使用std :: promise< void>需要一些元模板编程超出了这个问题的范围
}
catch(...)
{
promise.set_exception(std :: current_exception());
}
},std :: move ();
return std :: move(future);
}
使用 std :: packaged_task
这是一个帮助(即它基本上做我们上面做的) std :: promise
你可以做以下更完整,可能更快: p>
template< typename F>
auto async(F&& func) - > std :: future< decltype(func())>
{
auto task = std :: packaged_task< decltype(func())()>(std :: forward< F>(func)
auto future = task.get_future();
std :: thread(std :: move(task))。
return std :: move(future);
}
请注意,这与 std :: async
其中返回的 std :: future
将在被销毁时实际阻塞,直到线程完成。
I'm confusing myself with difference between a future and a promise.
Obviously, they have different methods and stuff, but what is the actual use case?
Is it?:
- when I'm managing some async task, I use future to get the value "in future"
- when I'm the async task, I use promise as the return type to allow the user get a future from my promise
Future and Promise are the two separate sides of an asynchronous operation.
std::promise
is used by the "producer/writer" of the asynchronous operation.
std::future
is used by the "consumer/reader" of the asynchronous operation.
The reason it is separated into these two separate "interfaces" is to hide the "write/set" functionality from the "consumer/reader".
auto promise = std::promise<std::string>();
auto producer = std::thread([&]
{
promise.set_value("Hello World");
});
auto future = promise.get_future();
auto consumer = std::thread([&]
{
std::cout << future.get();
});
producer.join();
consumer.join();
One (incomplete) way to implement std::async using std::promise could be:
template<typename F>
auto async(F&& func) -> std::future<decltype(func())>
{
typedef decltype(func()) result_type;
auto promise = std::promise<result_type>();
auto future = promise.get_future();
std::thread(std::bind([=](std::promise<result_type>& promise)
{
try
{
promise.set_value(func()); // Note: Will not work with std::promise<void>. Needs some meta-template programming which is out of scope for this question.
}
catch(...)
{
promise.set_exception(std::current_exception());
}
}, std::move(promise))).detach();
return std::move(future);
}
Using std::packaged_task
which is a helper (i.e. it basically does what we were doing above) around std::promise
you could do the following which is more complete and possibly faster:
template<typename F>
auto async(F&& func) -> std::future<decltype(func())>
{
auto task = std::packaged_task<decltype(func())()>(std::forward<F>(func));
auto future = task.get_future();
std::thread(std::move(task)).detach();
return std::move(future);
}
Note that this is slightly different from std::async
where the returned std::future
will when destructed actually block until the thread is finished.
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