This example demonstrates how to use the mqtt_client with its support for per-operation cancellation to perform operations under a time constraint using awaitable operators. Specifically, in this example, a call to mqtt_client::async_publish and mqtt_client::async_disconnect must complete within 5 seconds. Otherwise, they will be cancelled.

#include <chrono>
#include <iostream>
#include <string>

#include <boost/asio/as_tuple.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/deferred.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/steady_timer.hpp>
#include <boost/asio/use_awaitable.hpp>
#include <boost/asio/experimental/awaitable_operators.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";
};

// 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 = async_mqtt5::mqtt_client<
	boost::asio::ip::tcp::socket, std::monostate /* TlsContext */, async_mqtt5::logger
>;

// client_type without logging
//using client_type = async_mqtt5::mqtt_client<boost::asio::ip::tcp::socket>;

boost::asio::awaitable<void> send_over_mqtt(
	const config& cfg, client_type& 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.

	auto&& [pec, prc, puback_props] = co_await client.async_publish<async_mqtt5::qos_e::at_least_once>(
		"async-mqtt5/test", "Hello World!",
		async_mqtt5::retain_e::no, async_mqtt5::publish_props {},
		use_nothrow_awaitable
	);

	co_await client.async_disconnect(use_nothrow_awaitable);
	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];
	}

	boost::asio::io_context ioc;

	co_spawn(
		ioc,
		[&ioc, &cfg]() -> boost::asio::awaitable<void> {
		// Initialise the Client to connect to the Broker over TCP.
		client_type client(ioc);

		// You can also initialise the Client and its logger with a specific log_level (default log_level::info).
		//client_type client(ioc, {} /* tls_context */, async_mqtt5::logger(async_mqtt5::log_level::debug));

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

		using namespace boost::asio::experimental::awaitable_operators;
		auto res = co_await (
			send_over_mqtt(cfg, client) ||
			timer.async_wait(boost::asio::as_tuple(boost::asio::use_awaitable))
		);

		// The timer expired first. The client is cancelled.
		if (res.index() == 1)
			std::cout << "Send over MQTT timed out!" << std::endl;
		// send_over_mqtt completed first. The timer is cancelled.
		else
			std::cout << "Send over MQTT completed!" << std::endl;
		
		},
		[](std::exception_ptr e) {
			if (e)
				std::rethrow_exception(e);
		}
	);

	ioc.run();
}