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### Hello World over Websocket/TLS

This example illustrates the process of setting up the Client to connect to the Broker via Websocket/TLS and publish a "Hello World!" message.

    #include <boost/mqtt5/logger.hpp>
    #include <boost/mqtt5/mqtt_client.hpp>
    #include <boost/mqtt5/types.hpp>
    #include <boost/mqtt5/websocket_ssl.hpp> // WebSocket and OpenSSL traits

    #include <boost/asio/detached.hpp>
    #include <boost/asio/io_context.hpp>
    #include <boost/asio/ip/tcp.hpp>
    #include <boost/asio/ssl.hpp>
    #include <boost/beast/websocket.hpp>

    #include <iostream>
    #include <string>

    struct config {
        std::string brokers = "broker.hivemq.com/mqtt"; // Path example: localhost/mqtt
        uint16_t port = 8884; // 8884 is the default Websocket/TLS MQTT port.
        std::string client_id = "boost_mqtt5_tester";
    };

    // External customization point.
    namespace boost::mqtt5 {

    // Specify that the TLS handshake will be performed as a client.
    template <typename StreamBase>
    struct tls_handshake_type<boost::asio::ssl::stream<StreamBase>> {
        static constexpr auto client = boost::asio::ssl::stream_base::client;
    };

    // This client uses this function to indicate which hostname it is
    // attempting to connect to at the start of the handshaking process.
    template <typename StreamBase>
    void assign_tls_sni(
        const authority_path& ap,
        boost::asio::ssl::context& /*ctx*/,
        boost::asio::ssl::stream<StreamBase>& stream
    ) {
        SSL_set_tlsext_host_name(stream.native_handle(), ap.host.c_str());
    }

    } // end namespace boost::mqtt5

    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];
        }

        boost::asio::io_context ioc;

        // TLS context that the underlying SSL stream will use.
        // The purpose of the context is to allow us to set up TLS/SSL-related options.
        // See SSL for more information and options.
        boost::asio::ssl::context context(boost::asio::ssl::context::tls_client);

        // Set up the TLS context.
        // This step is highly dependent on the specific requirements of the Broker you are connecting to.
        // Each broker may have its own standards and expectations for establishing a secure TLS/SSL connection.
        // This can include verifying certificates, setting up private keys, PSK authentication, and others.

        // Construct the Client with WebSocket/SSL as the underlying stream
        // with boost::asio::ssl::context as the TlsContext type and enabled logging.
        boost::mqtt5::mqtt_client<
            boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::ip::tcp::socket>>,
            boost::asio::ssl::context,
            boost::mqtt5::logger
        > client(ioc, std::move(context), boost::mqtt5::logger(boost::mqtt5::log_level::info));

        // If you want to use the Client without logging, initialise it with the following line instead.
        //boost::mqtt5::mqtt_client<
        //    boost::beast::websocket::stream<boost::asio::ssl::stream<boost::asio::ip::tcp::socket>>,
        //    boost::asio::ssl::context
        //> client(ioc, std::move(context));

        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.

        client.async_publish<boost::mqtt5::qos_e::at_most_once>(
            "boost-mqtt5/test", "Hello world!",
            boost::mqtt5::retain_e::no, boost::mqtt5::publish_props{},
            [&client](boost::mqtt5::error_code ec) {
                std::cout << ec.message() << std::endl;
                client.async_disconnect(boost::asio::detached);
            }
        );

        ioc.run();
    }