SUBSCRIBE properties

The last field in the Variable header of SUBSCRIBE 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 SUBSCRIBE Properties and describes their usage:

Table 1.12. SUBSCRIBE properties

Identifier	Value type	Description
subscription_identifier	`int32_t`	Identifier of the Subscription in range of 1 to 268,435,455.
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. This property can be used to send subscription related properties from the Client to the Server. The meaning of these properties is not defined by the specification

Usage

After obtaining an instance of async_mqtt5::subscribe_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 `std::optional<int32_t>`.

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 std::optional<int32_t>.

Example

The following example shows how to set a Property value:

async_mqtt5::subscribe_props props;
props[async_mqtt5::prop::subscription_identifier] = 1234;
props[async_mqtt5::prop::user_property].emplace_back("name", "value");

The following example shows how to retrieve a Property value:

async_mqtt5::prop::subscription_identifiers sub_id = props[async_mqtt5::prop::subscription_identifier];
if (sub_id.has_value())
	// subscription identifier property was previously set
else
	// subscription identifier property was not set

std::vector<std::pair<std::string, std::string>>& user_props = props[async_mqtt5::prop::user_property];
if (!user_props.empty())
	// user property was previously set
else
	// user property was not set