Question

我想创建一个异步函数，它接受最后一个参数boost :: asio :: yield_context。 E.g：

int async_meaning_of_life(asio::yield_context yield);

我还想与Asio如何返回错误代码保持一致。也就是说，如果用户执行：

int result = async_meaning_of_life(yield);

并且函数失败，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 - 错误在ec中返回。

问题在于，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 []，如果是这样的话就设置它。我们想出了类似的东西：

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

但这很麻烦，因为yield_context::ec_是declared private（虽然仅在文档中）。

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只是尴尬。

还有其他办法吗？

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

这是模式的方式：

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

    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

显示如何与

一起使用

coro's and yield [ec]
coro's和yield + exceptions
的std ::将来
完成处理程序

<强> 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，因此输出可以根据运行时执行顺序混合

¹见例如这个优秀的演示如何使用它来扩展库，使用您自己的异步结果模式boost::asio with boost::unique_future

如何将error_code设置为asio :: yield_context

1 个答案:

更新

综合演示