Documentation rework - add Configuring the Client chapter

Summary: related to T12804

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D28182

example/hello_world_over_tls.cpp

@@ -0,0 +1,85 @@
+//[hello_world_over_tls
+#include <iostream>
+
+#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.hpp>
+
+// External customization point.
+namespace async_mqtt5 {
+
+template <typename StreamBase>
+struct tls_handshake_type<boost::asio::ssl::stream<StreamBase>> {
+	static constexpr auto client = boost::asio::ssl::stream_base::client;
+	static constexpr auto server = boost::asio::ssl::stream_base::server;
+};
+
+// 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
+
+// The certificate file in the PEM format.
+constexpr char ca_cert[] =
+"-----BEGIN CERTIFICATE-----\n"
+"...........................\n"
+"-----END CERTIFICATE-----\n"
+;
+
+int main() {
+	boost::asio::io_context ioc;
+
+	// Context satisfying ``__TlsContext__`` requirements 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);
+
+	async_mqtt5::error_code ec;
+
+	// Add the trusted certificate authority for performing verification.
+	context.add_certificate_authority(boost::asio::buffer(ca_cert), ec);
+	if (ec)
+		std::cout << "Failed to add certificate authority!" << std::endl;
+	ec.clear();
+
+	// Set peer verification mode used by the context.
+	// This will verify that the server's certificate is valid and signed by a trusted certificate authority.
+	context.set_verify_mode(boost::asio::ssl::verify_peer, ec);
+	if (ec)
+		std::cout << "Failed to set peer verification mode!" << std::endl;
+	ec.clear();
+
+	// Construct the Client with ``__SSL_STREAM__`` as the underlying stream
+	// with ``__SSL_CONTEXT__`` as the ``__TlsContext__`` type.
+	async_mqtt5::mqtt_client<
+		boost::asio::ssl::stream<boost::asio::ip::tcp::socket>,
+		boost::asio::ssl::context
+	> client(ioc, std::move(context));
+
+	// 8883 is the default TLS MQTT port.
+	client.brokers("<your-mqtt-broker>", 8883)
+		.async_run(boost::asio::detached);
+
+	client.async_publish<async_mqtt5::qos_e::at_most_once>(
+		"<topic>", "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();
+}
+//]