Boost C++ Libraries

Things get a bit more interesting when the async operation’s callback passes multiple data items of interest. One approach would be to use std::pair<> to capture both:

std::pair< AsyncAPI::errorcode, std::string > read_ec( AsyncAPI & api) {
    typedef std::pair< AsyncAPI::errorcode, std::string > result_pair;
    boost::fibers::promise< result_pair > promise;
    boost::fibers::future< result_pair > future( promise.get_future() );
    // We promise that both 'promise' and 'future' will survive until our
    // lambda has been called.
#if ! defined(BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES)
    api.init_read([promise=std::move( promise)]( AsyncAPI::errorcode ec, std::string const& data) mutable {
                            promise.set_value( result_pair( ec, data) );
                  });
#else // defined(BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES)
    api.init_read(
            std::bind([]( boost::fibers::promise< result_pair > & promise,
                          AsyncAPI::errorcode ec, std::string const& data) mutable {
                            promise.set_value( result_pair( ec, data) );
                  },
                  std::move( promise),
                  std::placeholders::_1,
                  std::placeholders::_2) );
#endif // BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES
    return future.get();
}

Once you bundle the interesting data in std::pair<>, the code is effectively identical to write_ec(). You can call it like this:

std::tie( ec, data) = read_ec( api);