3

我想创建一个异步函数，它的最后一个参数是 boost::asio::yield_context。例如：

int async_meaning_of_life(asio::yield_context yield);

我也想与 Asio 如何返回错误代码保持一致。也就是说，如果用户这样做：

int result = async_meaning_of_life(yield);

并且函数失败，然后它抛出system_error异常。但如果用户这样做：

boost::error_code ec;
int result = async_meaning_of_life(yield[ec]);

然后 - 而不是抛出 - 错误在ec.

问题是在实现该功能时，我似乎无法找到一种干净的方法来检查是否使用了 operator[] 并设置它。我们想出了这样的事情：

inline void set_error(asio::yield_context yield, sys::error_code ec)
{
    if (!yield.ec_) throw system_error(ec);
    *(yield.ec_) = ec;
}

但这很棘手，因为yield_context::ec_它被声明为私有（尽管仅在文档中）。

我能想到的另一种方法是将yield对象转换为asio::handler_type并执行它。但这个解决方案充其量似乎很尴尬。

还有其他方法吗？

4

1 回答 1

3

Asio 用于在其 API 接口async_result中透明地提供use_future或完成处理程序。¹yield_context

以下是模式的运行方式：

template <typename Token>
auto async_meaning_of_life(bool success, Token&& token)
{
    typename asio::handler_type<Token, void(error_code, int)>::type
                 handler (std::forward<Token> (token));

    asio::async_result<decltype (handler)> result (handler);

    if (success)
        handler(42);
    else
        handler(asio::error::operation_aborted, 0);

    return result.get ();
}

更新

从 Boost 1.66 开始，该模式遵循为标准化而提议的接口：

    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);

综合演示

展示如何使用 with

  • coro's 和产量[ec]
  • coro 和 yield + 异常
  • 标准::未来
  • 完成处理程序

Live On Coliru

#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();
}

印刷：

using_yield_ec: Result: Success
using_yield_ec: Answer: 42
using_yield_ec: Result: Operation canceled
using_yield_ec: Answer: 0
using_future: Answer: 42
using_yield_catch: Answer: 42
using_yield_catch: Caught: Operation canceled
using_future: Caught: Operation canceled
using_handler: Result: Success
using_handler: Answer: 42
using_handler: Result: Operation canceled
using_handler: Answer: 0

注意：为简单起见，我没有添加输出同步，因此输出可能会根据运行时执行顺序而混合

¹ 参见例如如何使用它来扩展库的出色演示，使用您自己的异步结果模式boost::asio 和 boost::unique_future

于 2017-10-21T01:00:08.120 回答