Example usage of boost::asio::async_result

async_result.cpp

// Example of how to use boost::asio::async_result

#include <iostream>
#include <boost/asio.hpp>

#if BOOST_VERSION >= 106600

template<typename CompletionToken>
typename boost::asio::async_result
  <CompletionToken, void(boost::system::error_code, std::string)>::return_type
my_fancy_async_operation( boost::asio::io_service& ios
                        , const std::string&       message
                        , CompletionToken&&        token)
{
  namespace asio   = boost::asio;
  namespace system = boost::system;

  using Sig     = void(system::error_code, std::string);
  using Result  = asio::async_result<CompletionToken, Sig>;
  using Handler = typename Result::completion_handler_type;

  Handler handler(std::forward<decltype(token)>(token));
  Result result(handler);

  ios.post([handler, message]() mutable {
      handler(system::error_code(), message);
      });

  return result.get();
}

#else

// Note: the handler signatures here must contain
// boost::system::error_code as first argument (not std::error_code)
// for asio to understand it is to indicate errors.
template<typename CompletionToken>
typename boost::asio::async_result
  < typename boost::asio::handler_type< CompletionToken
                                      , void(boost::system::error_code, std::string)
                                      >::type
  >::type
my_fancy_async_operation( boost::asio::io_service& ios
                        , const std::string&       message
                        , CompletionToken&&        token)
{
  namespace asio   = boost::asio;
  namespace system = boost::system;

  using handler_type = typename asio::handler_type
                         <CompletionToken, void(system::error_code, std::string)>::type;

  handler_type handler(std::forward<decltype(token)>(token));

  // The result must be defined here, otherwise we'll get segmentation
  // fault when using coroutines (don't understand why though).
  asio::async_result<handler_type> result(handler);

  ios.post([handler, message]() mutable {
      handler(system::error_code(), message);
      });

  return result.get();
}
#endif

// Link flags: -lboost_coroutine  // If using coroutines
//             -lboost_context    // If using coroutines
//             -lboost_system     // For asio
//             -lpthread          // If using futures

#define USE_CALLBACKS  1
#define USE_FUTURES    1
#define USE_COROUTINES 1

#if USE_FUTURES
#  include <thread>
#  include <boost/asio/use_future.hpp>
#endif

#if USE_COROUTINES
#  include <boost/asio/spawn.hpp>
#endif

int main() {

  using namespace std;
  using namespace boost;

  asio::io_service ios;

#if USE_CALLBACKS
  {
    my_fancy_async_operation( ios
                            , "handler"
                            , []( system::error_code, const string& msg) {
                                cout << "hello " << msg << endl;
                              });

    ios.run();
  }
#endif

#if USE_FUTURES
  ios.reset();

  {
    asio::io_service::work work(ios);

    thread io_thread([&ios]() { ios.run(); });

    try {
      auto msg = my_fancy_async_operation(ios
                                         , "future"
                                         , boost::asio::use_future).get();
      cout << "hello " << msg << endl;
    }
    catch (boost::system::system_error error) {
      // ...
    }
    
    ios.stop();
    io_thread.join();
  }
#endif

#if USE_COROUTINES
  ios.reset();

  {
    boost::asio::spawn(ios, [&ios](boost::asio::yield_context yield) {
        system::error_code error;
        auto msg = my_fancy_async_operation(ios, "coroutine", yield[error]);
        cout << "hello " << msg << endl;
        });

    ios.run();
  }
#endif

}

Excellent! Exactly what I was looking for.

Great! Have long been searching for a good example. Looking at Asio code directly is just too confusing.