How can I get a future from boost::asio::post? - c++

Question

I am using Boost 1.66.0, in which asio has built-in support for interoperating with futures (and for some time now). The examples I've seen online indicate how to achieve this cleanly when using networking functions such as async_read, async_read_some, etc. That is done by providing boost::asio::use_future in place of the completion handler, which causes the initiating function to return a future as expected.
What kind of object do I need to provide or wrap my function in to get the same behavior from boost::asio::post?
My purpose for posting the work is to execute it in the context of a strand but otherwise wait for the work to complete, so I can get the behavior I want doing:
std::packaged_task<void()> task( [] { std::cout << "Hello world\n"; } );
auto f = task.get_future();
boost::asio::post(
boost::asio::bind_executor(
strand_, std::move( task ) ) );
f.wait();
but according to the boost::asio documentation, the return type for boost::asio::post is deduced in the same way as for functions like boost::asio::async_read, so I feel like there has to be a nicer way that can avoid the intermediate packaged_task. Unlike async_read there is no "other work" to be done by post so providing just boost::asio::use_future doesn't makes sense, but we could define an async_result trait to get the same behavior for post.
Is there a wrapper or something that has the necessary traits defined to get the behavior I want or do I need to define it myself?

Answer 1

UPDATE: With more recent boost, use this much simpler answer
What kind of object do I need to provide or wrap my function in to get the same behavior from boost::asio::post?
You can't. post is a void operation. So the only option to achieve it with post is to use a packaged-task, really.
The Real Question
It was hidden in the part "how to get the same behaviour" (just not from post):
template <typename Token>
auto async_meaning_of_life(bool success, Token&& token)
{
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);
if (success)
handler(error_code{}, 42);
else
handler(asio::error::operation_aborted, 0);
return result.get ();
}
You can use it with a future:
std::future<int> f = async_meaning_of_life(true, asio::use_future);
std::cout << f.get() << "\n";
Or you can just use a handler:
async_meaning_of_life(true, [](error_code ec, int i) {
std::cout << i << " (" << ec.message() << ")\n";
});
Simple demo: Live On Coliru
Extended Demo
The same mechanism extends to supporting coroutines (with or without exceptions). There's a slightly different dance with async_result for Asio pre-boost 1.66.0.
See all the different forms together here:
How to set error_code to asio::yield_context
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_yield_catch: Answer: 42
using_future: Answer: 42
using_yield_catch: Caught: Operation canceled
using_future: Answer: using_future: Caught: Operation canceled
using_handler: Result: Success
using_handler: Answer: 42
using_handler: Result: Operation canceled
using_handler: Answer: 0

Answer 2

#MartiNitro's idea with packaged_task has become part of the library: now you can just post a packaged_task and it will magically return its future:
auto f = post(strand_, std::packaged_task<int()>(task));
Live Demo
#include <boost/asio.hpp>
#include <iostream>
#include <future>
using namespace std::chrono_literals;
int task() {
std::this_thread::sleep_for(1s);
std::cout << "Hello world\n";
return 42;
}
int main() {
boost::asio::thread_pool ioc;
auto strand_ = make_strand(ioc.get_executor());
auto f = post(strand_, std::packaged_task<int()>(task));
// optionally wait for future:
f.wait();
// otherwise .get() would block:
std::cout << "Answer: " << f.get() << "\n";
ioc.join();
}
Prints
Hello world
Answer: 42

Answer 3

Thas what I came up with, it essentially wrapts the asio::post and plugs in a promise/future pair. I think it can be adapted to your needs as well.
// outer scope setup
asio::io_context context;
asio::io_context::strand strand(context);
std::future<void> async_send(tcp::socket& socket, std::string message) {
auto buffered = std::make_shared<std::string>(message);
std::promise<void> promise;
auto future = promise.get_future();
// completion handler which only sets the promise.
auto handler = [buffered, promise{std::move(promise)}](asio::error_code, std::size_t) mutable {
promise.set_value();
};
// post async_write call to strand. Thas *should* protecte agains concurrent
// writes to the same socket from multiple threads
asio::post(strand, [buffered, &socket, handler{std::move(handler)}]() mutable {
asio::async_write(socket, asio::buffer(*buffered), asio::bind_executor(strand, std::move(handler)));
});
return future;
}
The promise can be moved without the future becoming invalidated.
Adapted to your scenario it could be somethign like this:
template<typename C>
std::future<void> post_with_future(C&& handler)
{
std::promise<void> promise;
auto future = promise.get_future();
auto wrapper = [promise{std::move(promise)}]{ // maybe mutable required?
handler();
promise.set_value();
};
// need to move in, cause the promise needs to be transferred. (i think)
asio::post(strand, std::move(wrapper));
return future;
}
I would be happy about some feedback to those lines, as I am myself just learning the whole thing :)
Hope to help,
Marti