mqtt_client::async_subscribe (2 of 2 overloads)

Send a SUBSCRIBE packet to Broker to create a subscription to one Topic of interest.

template <
    typename CompletionToken = typename asio::default_completion_token<executor_type>::type
>
decltype(auto) async_subscribe(
    const subscribe_topic & topic,
    const subscribe_props & props,
    CompletionToken && token = {}
);

Description

After the subscription has been established, the Broker will send PUBLISH packets to the Client to forward Application Messages that were published to Topics that the Client subscribed to. The Application Messages can be received with mqtt_client::async_receive function.

Parameters

Name	Description
topic	A `subscribe_topic` of interest.
props	An instance of `async_mqtt5::subscribe_props`.
token	Completion token that will be used to produce a completion handler. The handler will be invoked when the operation completes. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `boost::asio::async_immediate`.

Handler signature

The handler signature for this operation:

void (
  async_mqtt5::error_code, // Result of operation.
  std::vector<async_mqtt5::reason_code>, // Vector of Reason Codes containing the
                      // single subscription result for the Topic
                      // in the SUBSCRIBE packet.
  async_mqtt5::suback_props, // Properties received in the SUBACK packet.
)

Completion condition

The asynchronous operation will complete when one of the following conditions is true:

The Client has successfully sent a SUBSCRIBE packet and has received a SUBACK response from the Broker.
An error occurred. This is indicated by an associated async_mqtt5::error_code in the handler.

Error codes

The list of all possible error codes that this operation can finish with:

boost::system::errc::errc_t::success
boost::asio::error::no_recovery
boost::asio::error::operation_aborted
async_mqtt5::client::error::malformed_packet
async_mqtt5::client::error::packet_too_large
async_mqtt5::client::error::pid_overrun
async_mqtt5::client::error::invalid_topic
async_mqtt5::client::error::wildcard_subscription_not_available
async_mqtt5::client::error::subscription_identifier_not_available
async_mqtt5::client::error::shared_subscription_not_available

Refer to the section on Error handling to find the underlying causes for each error code.

Per-Operation Cancellation

This asynchronous operation supports cancellation for the following boost::asio::cancellation_type values:

cancellation_type::terminal - invokes mqtt_client::cancel
cancellation_type::partial & cancellation_type::total - prevents potential resending of the SUBSCRIBE packet