PUBLISH properties

The last field in the Variable header of PUBLISH packet is a set of Properties. A set contains a Property Length followed by the Properties. A Property consists of an Identifier and a value.

This section lists all possible PUBLISH Properties and describes their usage:

Table 1.7. PUBLISH properties

Identifier	Value type	Description
payload_format_indicator	`uint8_t`	Value of 0 indicates that the Payload is in unspecified bytes. Value of 1 indicates that the Payload is UTF-8 Encoded Character Data.
message_expiry_interval	`uint32_t`	The lifetime of the Application Message in seconds.
topic_alias	`uint16_t`	Two Byte integer representing the Topic Alias, an integer value that is used to identify the Topic instead of using the Topic Name.
response_topic	`std::string`	A UTF-8 Encoded String which is used as the Topic Name for a response message.
correlation_data	`std::string`	Binary Data used by the sender of the Request Message to identify which request the Response Message is for when it is received.
user_property	`std::pair<std::string, std::string>`	Name, value pair (`UTF-8 String Pair`) defining User Property. There can be multiple pairs in one packet. The meaning of these properties is not defined by the specification.
subscription_identifier	`int32_t`	Identifier of the matching subscription. If there are multiple matching subscriptions, multiple identifiers may be included.
content_type	`std::string`	A UTF-8 Encoded String describing the content of the Application Message.

Usage

After obtaining an instance of async_mqtt5::publish_props, the subscript operator can be used to access a Property.

The Identifiers listed in the table above are available within the async_mqtt5::prop namespace for Property access.

Note

	Note
When accessing a property value, the subscript operator will return a `std::optional` of the value type for all properties, except for `async_mqtt5::prop::user_property` and `async_mqtt5::prop::subscription_identifier`, where it will return an instance of `std::vector<std::pair<std::string, std::string>>` and `async_mqtt5::prop::subscription_identifiers` respectively. `async_mqtt5::prop::subscription_identifiers` has the interface of `boost::container::small_vector`.

When accessing a property value, the subscript operator will return a std::optional of the value type for all properties, except for async_mqtt5::prop::user_property and async_mqtt5::prop::subscription_identifier, where it will return an instance of std::vector<std::pair<std::string, std::string>> and async_mqtt5::prop::subscription_identifiers respectively. async_mqtt5::prop::subscription_identifiers has the interface of boost::container::small_vector.

Example

The following example shows how to set a Property value:

async_mqtt5::publish_props props;
props[async_mqtt5::prop::payload_format_indicator] = uint8_t(1);
props[async_mqtt5::prop::topic_alias] = uint16_t(12);
props[async_mqtt5::prop::response_topic] = "response_topic";
props[async_mqtt5::prop::subscription_identifier].push_back(40);

The following example shows how to retrieve a Property value:

std::optional<uint16_t> topic_alias = props[async_mqtt5::prop::topic_alias];
if (topic_alias.has_value())
	// topic alias property was previously set
else
	// topic alias property was not set

async_mqtt5::prop::subscription_identifiers& sub_ids = props[async_mqtt5::prop::subscription_identifier];
if (!sub_ids.empty())
	// subscription identifier property was previously set
else
	// subscription identifier property was not set