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### The publisher

This example shows how to use ``mqtt_client`` as a publisher that publishes sensor readings every `5` seconds.

    #include <boost/mqtt5/logger.hpp>
    #include <boost/mqtt5/mqtt_client.hpp>
    #include <boost/mqtt5/reason_codes.hpp>
    #include <boost/mqtt5/types.hpp>

    #include <boost/asio/as_tuple.hpp>
    #include <boost/asio/co_spawn.hpp>
    #include <boost/asio/deferred.hpp>
    #include <boost/asio/detached.hpp>
    #include <boost/asio/io_context.hpp>
    #include <boost/asio/ip/tcp.hpp>
    #include <boost/asio/signal_set.hpp>
    #include <boost/asio/steady_timer.hpp>

    #include <chrono>
    #include <cstdlib>
    #include <iostream>
    #include <string>

    struct config {
        std::string brokers = "broker.hivemq.com";
        uint16_t port = 1883;
        std::string client_id = "boost_mqtt5_tester";
    };

    // Modified completion token that will prevent co_await from throwing exceptions.
    constexpr auto use_nothrow_awaitable = boost::asio::as_tuple(boost::asio::deferred);

    // client_type with logging enabled
    using client_type = boost::mqtt5::mqtt_client<
        boost::asio::ip::tcp::socket, std::monostate /* TlsContext */, boost::mqtt5::logger
    >;

    // client_type without logging
    //using client_type = boost::mqtt5::mqtt_client<boost::asio::ip::tcp::socket>;

    int next_sensor_reading() {
        srand(static_cast<unsigned int>(std::time(0)));
        return rand() % 100;
    }

    boost::asio::awaitable<void> publish_sensor_readings(
        const config& cfg, client_type& client, boost::asio::steady_timer& timer
    ) {
        // Configure the Client.
        // It is mandatory to call brokers() and async_run() to configure the Brokers to connect to and start the Client.
        client.brokers(cfg.brokers, cfg.port) // Broker that we want to connect to.
            .credentials(cfg.client_id) // Set the Client Identifier. (optional)
            .async_run(boost::asio::detached); // Start the Client.

        for (;;) {
            // Get the next sensor reading.
            auto reading = std::to_string(next_sensor_reading());

            // Publish the sensor reading with QoS 1.
            auto&& [ec, rc, props] = co_await client.async_publish<boost::mqtt5::qos_e::at_least_once>(
                "boost-mqtt5/test" /* topic */, reading /* payload */,
                boost::mqtt5::retain_e::no, boost::mqtt5::publish_props {}, use_nothrow_awaitable
            );
            // An error can occur as a result of:
            //  a) wrong publish parameters
            //  b) mqtt_client::cancel is called while the Client is publishing the message
            //     resulting in cancellation.
            if (ec) {
                std::cout << "Publish error occurred: " << ec.message() << std::endl;
                break;
            }

            // Reason code is the reply from the server presenting the result of the publish operation.
            std::cout << "Result of publish request: " << rc.message() << std::endl;
            if (!rc)
                std::cout << "Published sensor reading: " << reading << std::endl;

            // Wait 5 seconds before publishing the next reading.
            timer.expires_after(std::chrono::seconds(5));
            auto&& [tec] = co_await timer.async_wait(use_nothrow_awaitable);

            // An error occurred if we cancelled the timer.
            if (tec)
                break;
        }

        co_return;
    }

    int main(int argc, char** argv) {
        config cfg;

        if (argc == 4) {
            cfg.brokers = argv[1];
            cfg.port = uint16_t(std::stoi(argv[2]));
            cfg.client_id = argv[3];
        }

        // Initialise execution context.
        boost::asio::io_context ioc;

        // Initialise the Client to connect to the Broker over TCP.
        client_type client(ioc, {} /* tls_context */, boost::mqtt5::logger(boost::mqtt5::log_level::info));

        // Initialise the timer.
        boost::asio::steady_timer timer(ioc);

        // Set up signals to stop the program on demand.
        boost::asio::signal_set signals(ioc, SIGINT, SIGTERM);
        signals.async_wait([&client, &timer](boost::mqtt5::error_code /* ec */, int /* signal */) {
            // After we are done with publishing all the messages, cancel the timer and the Client.
            // Alternatively, use mqtt_client::async_disconnect.
            timer.cancel();
            client.cancel();
        });

        // Spawn the coroutine.
        co_spawn(
            ioc.get_executor(),
            publish_sensor_readings(cfg, client, timer),
            [](std::exception_ptr e) {
                if (e)
                    std::rethrow_exception(e);
            }
        );

        // Start the execution.
        ioc.run();
    }