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

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

@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));

Update

Thanks to @niXman's comment, discovered a more convenient interface on the use_future token:

    auto f = post(ex, boost::asio::use_future(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(1);
    auto                     ex = ioc.get_executor();

    auto f = post(ex, std::packaged_task<int()>(task));
    // optionally wait for future:
    f.wait();
    // otherwise .get() would block:
    std::cout << "Answer: " << f.get() << "\n";

    f = post(ex, boost::asio::use_future(task));
    f.wait();
    std::cout << "Second answer: " << f.get() << "\n";

    ioc.join();
}

Prints

Hello world
Answer: 42
Hello world
Second answer: 42

Answer 2

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 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