The following list is a reference on how to use asynchrous functions in mqtt_client with callbacks.

Publish

// Publish an Application Message with QoS 0.
client.async_publish<async_mqtt5::qos_e::at_most_once>(
	"test/mqtt-test", "Hello world!",
	async_mqtt5::retain_e::no, async_mqtt5::publish_props {},
	// Callback with signature void (error_code)
	[](async_mqtt5::error_code ec) {
		std::cout << "error_code: " << ec.message() << std::endl;
	}
);

// Publish an Application Message with QoS 1.
client.async_publish<async_mqtt5::qos_e::at_least_once>(
	"test/mqtt-test", "Hello world!",
	async_mqtt5::retain_e::yes, async_mqtt5::publish_props {},
	// Callback with signature void (error_code, reason_code, puback_props)
	[](async_mqtt5::error_code ec, async_mqtt5::reason_code rc, async_mqtt5::puback_props) {
		std::cout << "error_code: " << ec.message() << std::endl;
		std::cout << "reason_code: " << rc.message() << std::endl;
	}
);

// Publish an Application Message with QoS 2.
client.async_publish<async_mqtt5::qos_e::exactly_once>(
	"test/mqtt-test", "Hello world!",
	async_mqtt5::retain_e::no, async_mqtt5::publish_props {},
	// Callback with signature (error_code, reason_code, pubcomp_props)
	[](async_mqtt5::error_code ec, async_mqtt5::reason_code rc, async_mqtt5::pubcomp_props) {
		std::cout << "error_code: " << ec.message() << std::endl;
		std::cout << "reason_code: " << rc.message() << std::endl;
	}
);

Subscribe

// Subscribe to a single Topic.
client.async_subscribe(
	{ "test/mqtt-test", { async_mqtt5::qos_e::exactly_once } }, async_mqtt5::subscribe_props {},
	// Callback with signature void (error_code, std::vector<reason_code>, suback_props)
	[](async_mqtt5::error_code ec,
		std::vector<async_mqtt5::reason_code> codes, async_mqtt5::suback_props
	) {
		std::cout << "subscribe error_code: " << ec.message() << std::endl;
		std::cout << "subscribe reason_code: " << codes[0].message() << std::endl;
	}
);

Receive

// Receive an Application Message.
client.async_receive(
	// Callback with signature void (error_code, std::string, std::string, publish_props)
	[] (
		async_mqtt5::error_code ec, std::string topic,
		std::string payload, async_mqtt5::publish_props
	) {
		std::cout << "topic: " << topic << std::endl;
		std::cout << "payload: " << payload << std::endl;
	}
);

Unsubscribe

// Unsubscribe from the Topic.
client.async_unsubscribe("test/mqtt-test", async_mqtt5::unsubscribe_props {},
	//Callback with signature void (error_code, std::vector<reason_code>, unsuback_props)
	[](async_mqtt5::error_code ec,
		std::vector<async_mqtt5::reason_code> codes, async_mqtt5::unsuback_props
	) {
		std::cout << "unsubscribe error_code: " << ec.message() << std::endl;
		std::cout << "unsubscribe reason_code: " << codes[0].message() << std::endl;
	}
);

Disconnect

// Disconnect the Client.
client.async_disconnect(
	async_mqtt5::disconnect_rc_e::disconnect_with_will_message,
	async_mqtt5::disconnect_props {},
	// Callback with signature void (error_code)
	[](async_mqtt5::error_code) {}
);