This example demonstrates how to use the mqtt_client with its support for per-operation cancellation to perform operations under a time constraint using parallel group. Specifically, in this example, the mqtt_client will subscribe to a Topic and try to receive a message from the Topic within 5 seconds.

#include <array>
#include <chrono>
#include <iostream>
#include <string>
#include <vector>

#include <boost/asio/io_context.hpp>
#include <boost/asio/deferred.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/steady_timer.hpp>
#include <boost/asio/experimental/parallel_group.hpp>
#include <boost/asio/ip/tcp.hpp>

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

struct config {
	std::string brokers = "broker.hivemq.com";
	uint16_t port = 1883;
	std::string client_id = "async_mqtt5_tester";
};

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;

	// Construct the Client with boost::asio::ip::tcp::socket as the underlying stream and enabled logging.
	// Since we are not establishing a secure connection, set the TlsContext template parameter to std::monostate.
	async_mqtt5::mqtt_client<
		boost::asio::ip::tcp::socket, std::monostate /* TlsContext */, async_mqtt5::logger
	> client(ioc, {} /* tls_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::ip::tcp::socket> client(ioc);

	// Construct the timer.
	boost::asio::steady_timer timer(ioc, std::chrono::seconds(5));

	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.

	// Subscribe to a Topic.
	client.async_subscribe(
		{ "test" /* Topic */}, async_mqtt5::subscribe_props {},
		[](async_mqtt5::error_code ec, std::vector<async_mqtt5::reason_code> rcs, async_mqtt5::suback_props) {
			std::cout << "[subscribe ec]: " << ec.message() << std::endl;
			std::cout << "[subscribe rc]: " << rcs[0].message() << std::endl;
		}
	);

	// Create a parallel group to wait up to 5 seconds to receive a message
	// using client.async_receive(...).
	boost::asio::experimental::make_parallel_group(
		timer.async_wait(boost::asio::deferred),
		client.async_receive(boost::asio::deferred)
	).async_wait(
		boost::asio::experimental::wait_for_one(),
		[&client](
			std::array<std::size_t, 2> ord, // Completion order
			async_mqtt5::error_code /* timer_ec */, // timer.async_wait(...) handler signature
			// client.async_receive(...) handler signature
			async_mqtt5::error_code receive_ec,
			std::string topic, std::string payload, async_mqtt5::publish_props /* props */
		) {
			if (ord[0] == 1) {
				std::cout << "Received a message!" << std::endl;
				std::cout << "[receive ec]: " << receive_ec.message() << std::endl;
				std::cout << "[receive topic]: " << topic << std::endl;
				std::cout << "[receive payload]: " << payload << std::endl;
			}
			else
				std::cout << "Timed out! Did not receive a message within 5 seconds." << std::endl;

			client.cancel();
		}
	);

	ioc.run();
}