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

#include <iostream>
#include <string>

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

#include <async_mqtt5/logger.hpp>
#include <async_mqtt5/mqtt_client.hpp>
#include <async_mqtt5/types.hpp>

struct config {
	std::string brokers = "broker.hivemq.com";
	uint16_t port = 8883; // 8883 is the default TLS MQTT port.
	std::string client_id = "async_mqtt5_tester";
};

// External customization point.
namespace async_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 async_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 boost::asio::ssl::stream<boost::asio::ip::tcp::socket> as the underlying stream
	// with boost::asio::ssl::context as the TlsContext type
	// and logging enabled.
	async_mqtt5::mqtt_client<
		boost::asio::ssl::stream<boost::asio::ip::tcp::socket>,
		boost::asio::ssl::context,
		async_mqtt5::logger
	> client(ioc, std::move(context), async_mqtt5::logger(async_mqtt5::log_level::info));


	// If you want to use the Client without logging, initialise it with the following line instead.
	//async_mqtt5::mqtt_client<
	//	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<async_mqtt5::qos_e::at_most_once>(
		"async-mqtt5/test", "Hello world!",
		async_mqtt5::retain_e::no, async_mqtt5::publish_props{},
		[&client](async_mqtt5::error_code ec) {
			std::cout << ec.message() << std::endl;
			client.async_disconnect(boost::asio::detached);
		}
	);

	ioc.run();
}