How to set error_code to asio::yield_context

I'd like to create an asynchronous function which takes as it's last argument boost::asio::yield_context. E.g.:

int async_meaning_of_life(asio::yield_context yield);

I'd also like to be consistent with how Asio returns error codes. That is, if the user does:

int result = async_meaning_of_life(yield);

and the function fails, then it throws the system_error exception. But if the user does:

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

Then - instead of throwing - the error is returned in ec.

The problem is that when implementing the function, I can't seem to find a clean way to check whether the operator[] was used or not and set it if so. We came up with something like this:

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

But that's hacky, because yield_context::ec_ is declared private (although only in the documentation).

One other way I can think of doing this is to convert the yield object into asio::handler_type and execute it. But this solution seems awkward at best.

Is there another way?

Asio uses async_result to transparently provide use_future, yield_context or completion handlers in its API interfaces.¹

Here's how the pattern goes:

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 ();
}

Update

Starting with Boost 1.66 the pattern adheres to the interface proposed for standardization:

    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);

Comprehensive Demo

Showing how to use it with with

coro's and yield[ec]
coro's and yield + exceptions
std::future
completion handlers

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();
}

Prints:

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

Note: for simplicity I have not added output synchronization, so the output can become intermingled depending on runtime execution order

¹ see e.g. this excellent demonstration of how to use it to extend the library with your own async result pattern boost::asio with boost::unique_future