我正在使用 Boost 1.66.0,其中 asio 内置了与 future 互操作的支持(并且已经有一段时间了)。我在网上看到的示例表明了如何在使用
async_read
、async_read_some
等网络功能时干净地实现此目的。这是通过提供 boost::asio::use_future
代替完成处理程序来完成的,这会导致启动函数按预期返回 future
。
我需要提供或包装我的函数什么样的对象才能从
boost::asio::post
获得相同的行为?
我发布工作的目的是在链的上下文中执行它,但否则等待工作完成,这样我就可以获得我想要做的行为:
std::packaged_task<void()> task( [] { std::cout << "Hello world\n"; } );
auto f = task.get_future();
boost::asio::post(
boost::asio::bind_executor(
strand_, std::move( task ) ) );
f.wait();
但是根据
boost::asio
文档,boost::asio::post
的返回类型是以与 boost::asio::async_read
等函数相同的方式推导的,所以我觉得必须有一种更好的方法来避免中间packaged_task
。与 async_read
不同,post
不需要完成“其他工作”,因此仅提供 boost::asio::use_future
没有意义,但我们可以定义一个 async_result
特征来获得相同的 post 行为。
是否有包装器或具有定义的必要特征来获得我想要的行为的东西,或者我需要自己定义它?
我需要提供或包装我的函数什么样的对象才能从
获得相同的行为?boost::asio::post
你不能。
post
是无效操作。因此,通过 post
实现这一目标的唯一选择是使用打包任务,真的。
它隐藏在“如何获得相同行为”部分(只是不是来自
post
):
template <typename Token>
auto async_meaning_of_life(bool success, Token&& token)
{
using result_type = typename asio::async_result<std::decay_t<Token>, void(error_code, int)>;
typename result_type::completion_handler_type handler(std::forward<Token>(token));
result_type result(handler);
if (success)
handler(error_code{}, 42);
else
handler(asio::error::operation_aborted, 0);
return result.get ();
}
您可以在未来使用它:
std::future<int> f = async_meaning_of_life(true, asio::use_future);
std::cout << f.get() << "\n";
或者您可以只使用处理程序:
async_meaning_of_life(true, [](error_code ec, int i) {
std::cout << i << " (" << ec.message() << ")\n";
});
简单演示:在 Coliru 上直播
相同的机制扩展到支持协程(有或没有例外)。对于 Asio pre-boost 1.66.0,
async_result
的舞蹈略有不同。
在这里查看所有不同的形式:
#define BOOST_COROUTINES_NO_DEPRECATION_WARNING
#include <iostream>
#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/asio/use_future.hpp>
using boost::system::error_code;
namespace asio = boost::asio;
template <typename Token>
auto async_meaning_of_life(bool success, Token&& token)
{
#if BOOST_VERSION >= 106600
using result_type = typename asio::async_result<std::decay_t<Token>, void(error_code, int)>;
typename result_type::completion_handler_type handler(std::forward<Token>(token));
result_type result(handler);
#else
typename asio::handler_type<Token, void(error_code, int)>::type
handler(std::forward<Token>(token));
asio::async_result<decltype (handler)> result (handler);
#endif
if (success)
handler(error_code{}, 42);
else
handler(asio::error::operation_aborted, 0);
return result.get ();
}
void using_yield_ec(asio::yield_context yield) {
for (bool success : { true, false }) {
boost::system::error_code ec;
auto answer = async_meaning_of_life(success, yield[ec]);
std::cout << __FUNCTION__ << ": Result: " << ec.message() << "\n";
std::cout << __FUNCTION__ << ": Answer: " << answer << "\n";
}
}
void using_yield_catch(asio::yield_context yield) {
for (bool success : { true, false })
try {
auto answer = async_meaning_of_life(success, yield);
std::cout << __FUNCTION__ << ": Answer: " << answer << "\n";
} catch(boost::system::system_error const& e) {
std::cout << __FUNCTION__ << ": Caught: " << e.code().message() << "\n";
}
}
void using_future() {
for (bool success : { true, false })
try {
auto answer = async_meaning_of_life(success, asio::use_future);
std::cout << __FUNCTION__ << ": Answer: " << answer.get() << "\n";
} catch(boost::system::system_error const& e) {
std::cout << __FUNCTION__ << ": Caught: " << e.code().message() << "\n";
}
}
void using_handler() {
for (bool success : { true, false })
async_meaning_of_life(success, [](error_code ec, int answer) {
std::cout << "using_handler: Result: " << ec.message() << "\n";
std::cout << "using_handler: Answer: " << answer << "\n";
});
}
int main() {
asio::io_service svc;
spawn(svc, using_yield_ec);
spawn(svc, using_yield_catch);
std::thread work([] {
using_future();
using_handler();
});
svc.run();
work.join();
}
打印
using_yield_ec: Result: Success
using_yield_ec: Answer: 42
using_yield_ec: Result: Operation canceled
using_yield_ec: Answer: 0
using_yield_catch: Answer: 42
using_future: Answer: 42
using_yield_catch: Caught: Operation canceled
using_future: Answer: using_future: Caught: Operation canceled
using_handler: Result: Success
using_handler: Answer: 42
using_handler: Result: Operation canceled
using_handler: Answer: 0
packaged_task
的想法已成为图书馆的一部分:现在您只需发布 packaged_task
,它将神奇地返回其未来:
auto f = post(strand_, std::packaged_task<int()>(task));
感谢@niXman的评论,在
use_future
代币上发现了更方便的界面:
auto f = post(ex, boost::asio::use_future(task));
#include <boost/asio.hpp>
#include <iostream>
#include <future>
using namespace std::chrono_literals;
int task() {
std::this_thread::sleep_for(1s);
std::cout << "Hello world\n";
return 42;
}
int main() {
boost::asio::thread_pool ioc(1);
auto ex = ioc.get_executor();
auto f = post(ex, std::packaged_task<int()>(task));
// optionally wait for future:
f.wait();
// otherwise .get() would block:
std::cout << "Answer: " << f.get() << "\n";
f = post(ex, boost::asio::use_future(task));
f.wait();
std::cout << "Second answer: " << f.get() << "\n";
ioc.join();
}
打印
Hello world
Answer: 42
Hello world
Second answer: 42
这就是我的想法,它本质上包裹了
asio::post
并插入了一个 Promise/Future 对。我认为它也可以适应您的需求。
// outer scope setup
asio::io_context context;
asio::io_context::strand strand(context);
std::future<void> async_send(tcp::socket& socket, std::string message) {
auto buffered = std::make_shared<std::string>(message);
std::promise<void> promise;
auto future = promise.get_future();
// completion handler which only sets the promise.
auto handler = [buffered, promise{std::move(promise)}](asio::error_code, std::size_t) mutable {
promise.set_value();
};
// post async_write call to strand. Thas *should* protecte agains concurrent
// writes to the same socket from multiple threads
asio::post(strand, [buffered, &socket, handler{std::move(handler)}]() mutable {
asio::async_write(socket, asio::buffer(*buffered), asio::bind_executor(strand, std::move(handler)));
});
return future;
}
可以改变承诺,而未来不会失效。
适应您的场景,它可能是这样的:
template<typename C>
std::future<void> post_with_future(C&& handler)
{
std::promise<void> promise;
auto future = promise.get_future();
auto wrapper = [promise{std::move(promise)}]{ // maybe mutable required?
handler();
promise.set_value();
};
// need to move in, cause the promise needs to be transferred. (i think)
asio::post(strand, std::move(wrapper));
return future;
}
我很高兴收到对这些台词的一些反馈,因为我自己刚刚学习了整件事:)
希望能有所帮助, 马蒂