UNSUBSCRIBE properties

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

Table 1.14. UNSUBSCRIBE properties

Identifier	Value type	Description
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::unsubscribe_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
	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`, where it will return an instance of `std::vector<std::pair<std::string, std::string>>`.

Example

The following example shows how to set a Property value:

async_mqtt5::unsubscribe_props props;
props[async_mqtt5::prop::user_property].emplace_back("name", "value");

The following example shows how to retrieve a Property value:

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