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### The receiver

This example shows how to use ``mqtt_client`` as a receiver. The ``mqtt_client`` subscribes and indefinitely receives Application Messages from the Broker.

    #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 <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>;

    boost::asio::awaitable<bool> subscribe(client_type& client) {
        // Configure the request to subscribe to a Topic.
        boost::mqtt5::subscribe_topic sub_topic = boost::mqtt5::subscribe_topic{
            "test" /* topic */,
            boost::mqtt5::subscribe_options {
                boost::mqtt5::qos_e::exactly_once, // All messages will arrive at QoS 2.
                boost::mqtt5::no_local_e::no, // Forward message from Clients with same ID.
                boost::mqtt5::retain_as_published_e::retain, // Keep the original RETAIN flag.
                boost::mqtt5::retain_handling_e::send // Send retained messages when the subscription is established.
            }
        };

        // Subscribe to a single Topic.
        auto&& [ec, sub_codes, sub_props] = co_await client.async_subscribe(
            sub_topic, boost::mqtt5::subscribe_props {}, use_nothrow_awaitable
        );
        // Note: you can subscribe to multiple Topics in one mqtt_client::async_subscribe call.

        // An error can occur as a result of:
            //  a) wrong subscribe parameters
            //  b) mqtt_client::cancel is called while the Client is in the process of subscribing
        if (ec)
            std::cout << "Subscribe error occurred: " << ec.message() << std::endl;
        else
            std::cout << "Result of subscribe request: " << sub_codes[0].message() << std::endl;

        co_return !ec && !sub_codes[0]; // True if the subscription was successfully established.
    }

    boost::asio::awaitable<void> subscribe_and_receive(
        const config& cfg, client_type& client
    ) {
        // 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.

        // Before attempting to receive an Application Message from the Topic we just subscribed to,
        // it is advisable to verify that the subscription succeeded.
        // It is not recommended to call mqtt_client::async_receive if you do not have any
        // subscription established as the corresponding handler will never be invoked.
        if (!(co_await subscribe(client)))
            co_return;

        for (;;) {
            // Receive an Appplication Message from the subscribed Topic(s).
            auto&& [ec, topic, payload, publish_props] = co_await client.async_receive(use_nothrow_awaitable);

            if (ec == boost::mqtt5::client::error::session_expired) {
                // The Client has reconnected, and the prior session has expired.
                // As a result, any previous subscriptions have been lost and must be reinstated.
                if (co_await subscribe(client))
                    continue;
                else
                    break;
            } else if (ec)
                break;

            std::cout << "Received message from the Broker" << std::endl;
            std::cout << "\t topic: " << topic << std::endl;
            std::cout << "\t payload: " << payload << std::endl;
        }

        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));

        // Set up signals to stop the program on demand.
        boost::asio::signal_set signals(ioc, SIGINT, SIGTERM);
        signals.async_wait([&client](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.
            client.cancel();
        });

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

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