如何将 error_code 设置为 asio::yield_context

Question

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);

我还想与 Asio 返回错误代码的方式保持一致。也就是说，如果用户这样做：

int result = async_meaning_of_life(yield);

函数失败，然后抛出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 - 在 ec 中返回错误。

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。我们想出了这样的东西：

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

但这很老套，因为yield_context::ec_ 是declared private（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？

Answer 1

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

模式是这样的：

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，模式 adheres to the interface proposed 用于标准化：

    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

展示如何与 with 一起使用

• coro 和产量[ec]
• coro 和产量 + 异常
• std::future
• 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();
}

打印：

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

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¹ 参见例如这个很好的演示如何使用它来扩展你自己的异步结果模式库boost::asio with boost::unique_future