Packio - An asynchronous msgpack-RPC and JSON-RPC library built on top of Boost.Asio.

Header-only | JSON-RPC | msgpack-RPC | asio | coroutines

This library requires C++17 and is designed as an extension to boost.asio. It will let you build asynchronous servers or client for JSON-RPC or msgpack-RPC.

The project is hosted on GitHub and available on Conan Center. Documentation is available on GitHub Pages.


#include <iostream>

#include <packio/packio.h>

using packio::arg;
using packio::nl_json_rpc::completion_handler;
using packio::nl_json_rpc::make_client;
using packio::nl_json_rpc::make_server;
using packio::nl_json_rpc::rpc;

int main(int, char**)
    using namespace packio::arg_literals;

    // Declare a server and a client, sharing the same io_context
    packio::net::io_context io;
    packio::net::ip::tcp::endpoint bind_ep{
        packio::net::ip::make_address(""), 0};
    auto server = make_server(packio::net::ip::tcp::acceptor{io, bind_ep});
    auto client = make_client(packio::net::ip::tcp::socket{io});

    // Declare a synchronous callback with named arguments
        "add", {"a", "b"}, [](int a, int b) { return a + b; });
    // Declare an asynchronous callback with named arguments
        "multiply", {"a", "b"}, [&io](completion_handler complete, int a, int b) {
            // Call the completion handler later
                io, [a, b, complete = std::move(complete)]() mutable {
                    complete(a * b);
    // Declare a coroutine with unnamed arguments
        "pow", io, [](int a, int b) -> packio::net::awaitable<int> {
            co_return std::pow(a, b);

    // Connect the client
    // Accept connections
    // Run the io_context
    std::thread thread{[&] {; }};

    // Make an asynchronous call with named arguments
    std::promise<int> add1_result, multiply_result;
        std::tuple{arg("a") = 42, arg("b") = 24},
        [&](packio::error_code, const rpc::response_type& r) {
    std::cout << "42 + 24 = " << add1_result.get_future().get() << std::endl;

    // Use packio::net::use_future with named arguments and literals
    auto add_future = client->async_call(
        std::tuple{"a"_arg = 12, "b"_arg = 23},
    std::cout << "12 * 23 = " << add_future.get().result.get<int>() << std::endl;

    // Spawn the coroutine and wait for its completion
    std::promise<int> pow_result;
        [&]() -> packio::net::awaitable<void> {
            // Call using an awaitable and positional arguments
            auto res = co_await client->async_call(
                "pow", std::tuple{2, 8}, packio::net::use_awaitable);
    std::cout << "2 ** 8 = " << pow_result.get_future().get() << std::endl;


    return 0;


  • C++17 or C++20
  • msgpack >= 3.2.1
  • nlohmann_json >= 3.9.1
  • boost.asio >= 1.70.0 or asio >= 1.13.0

Older versions of msgpack and nlohmann_json are probably compatible but they are not tested on the CI.


Standalone or Boost.Asio

By default, packio uses boost::asio. It is also compatible with standalone asio. To use the standalone version, the preprocessor macro PACKIO_STANDALONE_ASIO=1 must be defined.

If you are using the conan package, you can use the option standalone_asio=True.

Depending on your choice, the namespace packio::net will be an alias for either boost::asio or asio.

RPC components

You can define the following preprocessor macros to either 0 or 1 to force-disable or force-enable components of packio:


If you're using the conan package, use the associated options instead, conan will define these macros accordingly.

If you're not using the conan package, packio will try to auto-detect whether these components are available on your system. Define the macros to the appropriate value if you encounter any issue.

Boost before 1.75

If you're using the conan package with a boost version older than 1.75, you need to manually disable Boost.Json with the options boost_json=False.

Tested compilers

  • gcc-7
  • gcc-8
  • gcc-9
  • gcc-10
  • clang-6
  • clang-7
  • clang-8
  • clang-9
  • clang-10
  • clang-11
  • Apple clang-12
  • Visual Studio 2019 Version 16.8

Older compilers may be compatible but are not tested.

Install with conan

conan install packio/x.x.x


packio is compatible with C++20 coroutines:

  • calls can use the packio::asio::use_awaitable completion token
  • coroutines can be registered in the server

Coroutines are tested for the following compilers:

  • gcc-10 (with -fcoroutines)
  • clang-11 (with libc++)
  • Apple clang-12


You will find some samples in test_package/samples/ to help you get a hand on packio.


Let's compute fibonacci's numbers recursively over websockets with coroutines on a single thread ... in 65 lines of code.

#include <iostream>

#include <packio/extra/websocket.h>
#include <packio/packio.h>

using packio::msgpack_rpc::make_client;
using packio::msgpack_rpc::make_server;
using packio::net::ip::make_address;

using awaitable_tcp_stream = decltype(packio::net::use_awaitable_t<>::as_default_on(
using websocket = packio::extra::
    websocket_adapter<boost::beast::websocket::stream<awaitable_tcp_stream>, true>;
using ws_acceptor =
    packio::extra::websocket_acceptor_adapter<packio::net::ip::tcp::acceptor, websocket>;

int main(int argc, char** argv)
    if (argc < 2) {
        std::cerr << "I require one argument" << std::endl;
        return 1;
    const int n = std::atoi(argv[1]);

    packio::net::io_context io;
    packio::net::ip::tcp::endpoint bind_ep{make_address(""), 0};

    auto server = make_server(ws_acceptor{io, bind_ep});
    auto client = make_client(websocket{io});

        "fibonacci", io, [&](int n) -> packio::net::awaitable<int> {
            if (n <= 1) {
                co_return n;

            auto r1 = co_await client->async_call("fibonacci", std::tuple{n - 1});
            auto r2 = co_await client->async_call("fibonacci", std::tuple{n - 2});

            co_return<int>() +<int>();

    int result = 0;
        [&]() -> packio::net::awaitable<void> {
            auto ep = server->acceptor().local_endpoint();
            co_await client->socket().next_layer().async_connect(ep);
            co_await client->socket().async_handshake(
                "" + std::to_string(ep.port()), "/");
            auto ret = co_await client->async_call("fibonacci", std::tuple{n});
            result = ret.result.template as<int>();


    std::cout << "F{" << n << "} = " << result << std::endl;

    return 0;
  • Separated client and server

    Separated client and server

    Based on my previous request Closed Issue #65 i have a further question. I'm using the nl_json_rpc client and I tried to separate the client and server in two different programs (executable). From the client i can connect to the server socket (in the code below i get "The connection has been established!")

    packio::net::io_context io_context;
    packio::net::ip::tcp::socket socket{io_context};
    packio::net::ip::tcp::resolver resolver{io_context};
    auto endpoints = resolver.resolve("localhost", std::to_string(6543));
    packio::error_code error;
    auto client = make_client(std::move(socket));
    client->socket().connect(endpoints->endpoint(), error);
    if(not error)
        std::cout << "The connection has been established!";
        std::cerr << "Something went wrong :(";

    But for some reason the following RPC async_call does not work anymore, e.g. it seems it waits forever for the add_result.get_future().get()

    std::promise<int> add_result, multiply_result;
        std::tuple{arg("a") = 42, arg("b") = 24},
        [&](packio::error_code, const rpc::response_type& r) {
    std::cout << "42 + 24 = " << add_result.get_future().get() << std::endl;

    May i ask you if you know what the problem is or you can add an example with separated client & server?

  • packio in cmake project

    packio in cmake project

    I am trying to use packio in my cmake project but I get a few compiler errors:

    error 1

    .conan/data/packio/2.2.0/_/_/package/0c0791061c9ce0b41dcaec85e4a8aeeb1bfa1a31/include/packio/json_rpc/hash.h:66:8: error: redefinition of ‘struct std::hash<boost::json::value>’
       66 | struct hash<boost::json::value> {
    .conan/data/boost/1.80.0/_/_/package/cb420f7a9d4f5344407adad933bfd4a05ab58ddd/include/boost/json/value.hpp:4030:8: note: previous definition of ‘struct std::hash<boost::json::value>’
     4030 | struct hash< ::boost::json::value > {

    error 2

    error: ‘using element_type = class packio::dispatcher<packio::nl_json_rpc::rpc, packio::default_map, std::mutex>’ {aka ‘class packio::dispatcher<packio::nl_json_rpc::rpc, packio::default_map, std::mutex>’} has no member named ‘add_coro’
      108 |     server->dispatcher()->add_coro(

    error 3

    error: ‘awaitable’ in namespace ‘packio::net’ does not name a template type
      109 |             "pow", io, [](int a, int b) -> packio::net::awaitable<int> {

    It seems that boost asio is not correctly linked somehow?

    My cmake code for including packio:

    conan_cmake_configure(REQUIRES packio/2.2.0
            GENERATORS cmake_find_package)
    message("external: settings = ${settings}")
    conan_cmake_install(PATH_OR_REFERENCE .
            BUILD missing
            REMOTE conancenter
            SETTINGS ${settings})

    Any help would be very much appreciated.

    UPDATE: I am getting this error even when running test_package:

    packio$ conan test test_package/ packio/2.2.0
    .conan/data/packio/2.2.0/_/_/package/0c0791061c9ce0b41dcaec85e4a8aeeb1bfa1a31/include/packio/json_rpc/hash.h:66:8: error: redefinition of ‘struct std::hash<boost::json::value>’
       66 | struct hash<boost::json::value> {
    .conan/data/boost/1.80.0/_/_/package/cb420f7a9d4f5344407adad933bfd4a05ab58ddd/include/boost/json/value.hpp:4030:8: note: previous definition of ‘struct std::hash<boost::json::value>’
     4030 | struct hash< ::boost::json::value > {

    I have no experience with conan so please forgive me if I am missing something obvious.

  • RPC client as class member variable

    RPC client as class member variable

    How do i define the rpc client as a class member variable? I cannot use auto. I have to setup a rpc connection to serveral devices and i thought the best to do this is to instantiate a device object from a device class, each holding the rpc client as a member variable.

  • Accessing session from handler?

    Accessing session from handler?

    Hi! I'm working with your library, and I'm trying to figure out how to get the session (or socket) from within a handler. We'd like to send back notifications on the same websocket; the server may have several users connected.

    The only example code I saw related to this, was capturing a session_ptr when adding the handler (from basic_test_server_crash.cpp)

        std::shared_ptr<session_type> session_ptr;
        this->server_->async_serve([&](auto ec, auto session) {
            session_ptr = session;
            "close", [&]() { session_ptr->socket().close(); });

    But, I'm not sure something like that would work with more than one connection. I could capture the server itself, but I don't know how to go about finding the socket in the handler.

    Thank you for any hints here!


  • Progress support with WebSocket adapter

    Progress support with WebSocket adapter

    I have WebSocket server over JSON-RPC (MR, that can do long (from 1 to 30 minutes) the API call and can generate progress (%) with extra data (remaining time, sec). Can I do progress somehow?

  • Missing optional header

    Missing optional header

    The optional header isn't included in traits.h. It causes some errors with MSVC:

    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2065: 'optional': undeclared identifier
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(89): note: see reference to class template instantiation 'packio::traits::AsCallHandler<T,Result>' being compiled
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C3544: '_Args': parameter pack expects a type template argument
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2993: 'unknown-type': is not a valid type for non-type template parameter 'condition'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2955: 'packio::traits::Trait': use of class template requires template argument list
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(62): note: see declaration of 'packio::traits::Trait'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\traits.h(88): error C2143: syntax error: missing ',' before '>'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(166): error C2061: syntax error: identifier 'optional'
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(483): note: see reference to class template instantiation 'packio::client<Socket,Map>' being compiled
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(182): error C2061: syntax error: identifier 'optional'
    E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(414): error C2061: syntax error: identifier 'optional'
      E:\Users\Yoann\.conan\data\packio\1.3.0\_\_\package\5ab84d6acfe1f23c4fae0ab88f26e3a396351ac9\include\packio\client.h(472): note: see reference to class template instantiation 'packio::client<Socket,Map>::initiate_async_call<Buffer>' being compiled
  • How to deal with optional parameters

    How to deal with optional parameters

    When the dispatcher encounters a missing parameter (using the nlohmann::json implementation), it throws the request away as the request fails to parse in rpc::convert_named_args, due to the use of the json::at method which throws due to the parameter key not being present.

    We have a use-case where the other end omits optional parameters (instead of setting them to null), which we cannot change. Currently, I have hacked a workaround in, which is to attempt to return a default constructed object if the parameter is not found:

        template <typename T, typename NamesContainer, std::size_t... Idxs>
        static T convert_named_args(
            const nlohmann::json& args,
            const NamesContainer& names,
            return T{(args.template value<std::tuple_element_t<Idxs, T>>(, {}))...};

    It's not particularly elegant, and requires a suitably friendly type with a sensible default constructor to be passed into dispatcher::add (e.g. nlohmann::json, which can be explicitly tested for is_null() in the dispatcher callback).

    Do you think this change is appropriate for general inclusion in a PR? I'm not sure how well it would work for the Boost JSON or MessagePack variants. Alternatively, do you have a better solution?

  • Feature proposal - unsolicited notifications

    Feature proposal - unsolicited notifications

    I am currently using packio as a JSON RPC client in a commercial project. The server is a custom JSON RPC implementation that sends unsolicited notifications. This behaviour is outside the JSON RPC specification, but we have no control over the server. Other libraries have implemented this extension - see as an example.

    I have implemented the client-side support for this, extending the client class using the existing dispatcher class used by the server. We have been using this successfully over the last couple of months. What we have not done:

    • Server support for sending such notifications
    • Testing for Boost JSON and MessagePack variants (our project uses JSON for Modern C++)
    • Packio tests

    Before I do any further work - would you be interested in a PR for this feature?

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