This example demonstrates how to publish a "Hello World" message in a multithreaded environment using callbacks (post/dispatch).

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

#include <boost/asio/bind_executor.hpp>
#include <boost/asio/detached.hpp>
#include <boost/asio/dispatch.hpp>
#include <boost/asio/thread_pool.hpp>
#include <boost/asio/strand.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];
	}

	// Create a thread pool with 4 threads.
	boost::asio::thread_pool tp(4);
	
	// Create an explicit strand from thread_pool's executor.
	// The strand guarantees a serialised handler execution regardless of the 
	// number of threads running in the thread_pool.
	boost::asio::strand strand = boost::asio::make_strand(tp.get_executor());

	// Create the Client with the explicit strand as the default associated executor.
	async_mqtt5::mqtt_client<
		boost::asio::ip::tcp::socket, std::monostate /* TlsContext */, async_mqtt5::logger
	> client(strand, {} /* tls_context */, async_mqtt5::logger(async_mqtt5::log_level::info));

	// Configure the client.
	client.brokers(cfg.brokers, cfg.port) // Broker that we want to connect to.
		.credentials(cfg.client_id); // Set the Client Identifier. (optional)

	// Start the Client.
	// The async_run function call must be posted/dispatched to the strand.
	boost::asio::dispatch(
		boost::asio::bind_executor(
			strand,
			[&client, &strand, &cfg] {
				// Considering that the default associated executor of all completion handlers is the strand,
				// it is not necessary to explicitly bind it to async_run or other async_xxx's handlers.
				client.async_run(boost::asio::detached); // Start the Client.
			}
		)
	);
	
	// The async_publish function call must be posted/dispatched to the strand.
	// The associated executor of async_publish's completion handler must be the same strand.
	boost::asio::dispatch(
		boost::asio::bind_executor(
			strand,
			[&client, &strand, &cfg] {
				assert(strand.running_in_this_thread());

				client.async_publish<async_mqtt5::qos_e::at_least_once>(
					"async-mqtt5/test", "Hello World!", async_mqtt5::retain_e::no,
					async_mqtt5::publish_props {},
					// You may bind the strand to this handler, but it is not necessary
					// as the strand is already the default associated handler.
					[&client, &strand](
						async_mqtt5::error_code ec, async_mqtt5::reason_code rc,
						async_mqtt5::puback_props props
					) {
						assert(strand.running_in_this_thread());

						std::cout << ec.message() << std::endl;
						std::cout << rc.message() << std::endl;

						// Stop the Client.
						client.cancel();
					}
				);
			}
		)
	);

	tp.join();

	return 0;
}