The following list is a reference on how to use the mqtt_client with boost::asio::use_future as the completion token.

Each get() call on std::future will block the current thread and wait until the future has a valid result. That is why it is essential to ensure that the execution context is running in more than one thread.

Publish

// Just like the asio::use_awaitable completion token
// (see Async functions with C++20 coroutines), the boost::asio::use_future completion token
// will not return the error_code. Instead, it will throw an exception if an error has occurred.
std::future<void> pub_qos0_fut =
	client.async_publish<async_mqtt5::qos_e::at_most_once>(
		"test/mqtt-test", "Hello world!",
		async_mqtt5::retain_e::yes, async_mqtt5::publish_props {},
		asio::use_future
);
pub_qos0_fut.get(); // Blocking call!

using qos1_fut_type = std::tuple<async_mqtt5::reason_code, async_mqtt5::puback_props>;
std::future<qos1_fut_type> pub_qos1_fut =
	client.async_publish<async_mqtt5::qos_e::at_least_once>(
		"test/mqtt-test", "Hello world!",
		async_mqtt5::retain_e::yes, async_mqtt5::publish_props {},
		asio::use_future
);
auto [qos1_rc, puback_props] = pub_qos1_fut.get();
std::cout << "Publish QoS 1 Reason Code: " << qos1_rc.message() << std::endl;

using qos2_fut_type = std::tuple<async_mqtt5::reason_code, async_mqtt5::pubcomp_props>;
std::future<qos2_fut_type> pub_qos2_fut =
	client.async_publish<async_mqtt5::qos_e::exactly_once>(
		"test/mqtt-test", "Hello world!",
		async_mqtt5::retain_e::yes, async_mqtt5::publish_props {},
		asio::use_future
);
auto [qos2_rc, pubcomp_props] = pub_qos2_fut.get();
std::cout << "Publish QoS 2 Reason Code: " << qos2_rc.message() << std::endl;

Subscribe

using sub_fut_type = std::tuple<std::vector<async_mqtt5::reason_code>, async_mqtt5::suback_props>;
std::future<sub_fut_type> sub_fut = client.async_subscribe(
	{ "test/mqtt-test", { async_mqtt5::qos_e::exactly_once } },
	async_mqtt5::subscribe_props {}, asio::use_future
);
auto [sub_rcs, suback_props] = sub_fut.get();
std::cout << "Subscribe Reason Code: " << sub_rcs[0].message() << std::endl;

Receive

// Note: the get() call on async_receive future could block indefinitely if the mqtt_client
// failed to subscribe or there are no Application Messages to be received from the subscribed Topic!
using rec_fut_type = std::tuple<std::string, std::string, async_mqtt5::publish_props>;
std::future<rec_fut_type> rec_fut = client.async_receive(asio::use_future);
auto [topic, payload, publish_props] = rec_fut.get();
std::cout << "Received message from Topic: " << topic << ", " << payload << std::endl;

Unsubscribe

using unsub_fut_type = std::tuple<std::vector<async_mqtt5::reason_code>, async_mqtt5::unsuback_props>;
std::future<unsub_fut_type> unsub_fut = client.async_unsubscribe(
	"test/mqtt-test", async_mqtt5::unsubscribe_props {}, asio::use_future
);
auto [unsub_rcs, unsuback_props] = unsub_fut.get();
std::cout << "Unubscribe Reason Code: " << unsub_rcs[0].message() << std::endl;

Disconnect

std::future<void> dc_fut = client.async_disconnect(asio::use_future);
dc_fut.get();