boost::asio with boost::unique_future - c++

Question

According to http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/overview/cpp2011/futures.html, we can use boost::asio with std::future. But I couldn't find any information about working with boost::unique_future, which has more functions, such as then(). How can I use?

Answer 1

Boost.Asio only provides first-class support for asynchronous operations to return a C++11 std::future or an actual value in stackful coroutines. Nevertheless, the requirements on asynchronous operations documents how to customize the return type for other types, such as Boost.Thread's boost::unique_future. It requires:
A specialization of the handler_type template. This template is used to determine the actual handler to use based on the asynchronous operation's signature.
A specialization of the async_result template. This template is used both to determine the return type and to extract the return value from the handler.
Below is a minimal complete example demonstrating deadline_timer::async_wait() returning boost:unique_future with a basic calculation being performed over a series of continuations composed with .then(). To keep the example simple, I have opted to only specialize handler_type for the asynchronous operation signatures used in the example. For a complete reference, I highly suggest reviewing use_future.hpp and impl/use_future.hpp.
#include <exception> // current_exception, make_exception_ptr
#include <memory> // make_shared, shared_ptr
#include <thread> // thread
#include <utility> // move
#define BOOST_RESULT_OF_USE_DECLTYPE
#define BOOST_THREAD_PROVIDES_FUTURE_CONTINUATION
#include <boost/asio.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/thread/future.hpp>
/// #brief Class used to indicate an asynchronous operation should return
/// a boost::unique_future.
class use_unique_future_t {};
/// #brief A special value, similiar to std::nothrow.
constexpr use_unique_future_t use_unique_future;
namespace detail {
/// #brief Completion handler to adapt a boost::promise as a completion
/// handler.
template <typename T>
class unique_promise_handler;
/// #brief Completion handler to adapt a void boost::promise as a completion
/// handler.
template <>
class unique_promise_handler<void>
{
public:
/// #brief Construct from use_unique_future special value.
explicit unique_promise_handler(use_unique_future_t)
: promise_(std::make_shared<boost::promise<void> >())
{}
void operator()(const boost::system::error_code& error)
{
// On error, convert the error code into an exception and set it on
// the promise.
if (error)
promise_->set_exception(
std::make_exception_ptr(boost::system::system_error(error)));
// Otherwise, set the value.
else
promise_->set_value();
}
//private:
std::shared_ptr<boost::promise<void> > promise_;
};
// Ensure any exceptions thrown from the handler are propagated back to the
// caller via the future.
template <typename Function, typename T>
void asio_handler_invoke(
Function function,
unique_promise_handler<T>* handler)
{
// Guarantee the promise lives for the duration of the function call.
std::shared_ptr<boost::promise<T> > promise(handler->promise_);
try
{
function();
}
catch (...)
{
promise->set_exception(std::current_exception());
}
}
} // namespace detail
namespace boost {
namespace asio {
/// #brief Handler type specialization for use_unique_future.
template <typename ReturnType>
struct handler_type<
use_unique_future_t,
ReturnType(boost::system::error_code)>
{
typedef ::detail::unique_promise_handler<void> type;
};
/// #brief Handler traits specialization for unique_promise_handler.
template <typename T>
class async_result< ::detail::unique_promise_handler<T> >
{
public:
// The initiating function will return a boost::unique_future.
typedef boost::unique_future<T> type;
// Constructor creates a new promise for the async operation, and obtains the
// corresponding future.
explicit async_result(::detail::unique_promise_handler<T>& handler)
{
value_ = handler.promise_->get_future();
}
// Obtain the future to be returned from the initiating function.
type get() { return std::move(value_); }
private:
type value_;
};
} // namespace asio
} // namespace boost
int main()
{
boost::asio::io_service io_service;
boost::asio::io_service::work work(io_service);
// Run io_service in its own thread to demonstrate future usage.
std::thread thread([&io_service](){ io_service.run(); });
// Arm 3 second timer.
boost::asio::deadline_timer timer(
io_service, boost::posix_time::seconds(3));
// Asynchronously wait on the timer, then perform basic calculations
// within the future's continuations.
boost::unique_future<int> result =
timer.async_wait(use_unique_future)
.then([](boost::unique_future<void> future){
std::cout << "calculation 1" << std::endl;
return 21;
})
.then([](boost::unique_future<int> future){
std::cout << "calculation 2" << std::endl;
return 2 * future.get();
})
;
std::cout << "Waiting for result" << std::endl;
// Wait for the timer to trigger and for its continuations to calculate
// the result.
std::cout << result.get() << std::endl;
// Cleanup.
io_service.stop();
thread.join();
}
Output:
Waiting for result
calculation 1
calculation 2
42