This example demonstrates how to publish a "Hello World" message in a multithreaded environment using coroutines (co_spawn).

#include <vector>
#include <string>
#include <thread>

#include <boost/asio/as_tuple.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/deferred.hpp>
#include <boost/asio/strand.hpp>
#include <boost/asio/thread_pool.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 = 1883;
	std::string client_id = "async_mqtt5_tester";
};

// 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>;

// Modified completion token that will prevent co_await from throwing exceptions.
constexpr auto use_nothrow_awaitable = boost::asio::as_tuple(boost::asio::deferred);

boost::asio::awaitable<void> publish_hello_world(
	const config& cfg, client_type& client,
	const boost::asio::strand<boost::asio::thread_pool::executor_type>& strand
) {
	// Confirmation that the coroutine running in the strand.
	assert(strand.running_in_this_thread());

	// All these function calls will be executed by the strand that is executing the coroutine.
	// All the completion handler's associated executors will be that same strand
	// because the Client was constructed with it as the default associated executor.
	client.brokers(cfg.brokers, cfg.port) // Set the Broker to connect to.
		.credentials(cfg.client_id) // Set the Client Identifier. (optional)
		.async_run(boost::asio::detached); // Start the Client.

	auto&& [ec, rc, puback_props] = co_await client.async_publish<async_mqtt5::qos_e::at_least_once>(
		"async-mqtt5/test" /* topic */, "Hello world!" /* payload*/, 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];
	}

	// Create a thread pool with 4 threads.
	boost::asio::thread_pool tp(4);

	// Create an explicit strand from io_context's executor.
	// The strand guarantees a serialised handler execution regardless of the 
	// number of threads running in the io_context.
	boost::asio::strand strand = boost::asio::make_strand(tp.get_executor());

	// Create the Client with the explicit strand as the default associated executor.
	client_type client(strand, {} /* tls_context */, async_mqtt5::logger(async_mqtt5::log_level::info));

	// Spawn the coroutine.
	// The executor that executes the coroutine must be the same executor
	// that is the Client's default associated executor.
	co_spawn(
		strand,
		publish_hello_world(cfg, client, strand),
		[](std::exception_ptr e) {
			if (e)
				std::rethrow_exception(e);
		}
	);

	tp.join();

	return 0;
}