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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 class extends prop::properties, which provides common functionalities for all property classes. Below is a list of possible SUBSCRIBE Properties, along with descriptions of their usage:

Table 1.12. SUBSCRIBE properties

Identifier

Value type

Description

subscription_identifier

boost::mqtt5::prop::subscription_identifiers

Identifier of the Subscription in range of 1 to 268,435,455.

user_property

std::vector<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 boost::mqtt5::subscribe_props, the subscript operator can be used to access a Property.

The Identifiers listed in the table above are available within the boost::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 boost::mqtt5::prop::user_property and boost::mqtt5::prop::subscription_identifier, where it will return an instance of std::vector<std::pair<std::string, std::string>> and prop::subscription_identifiers respectively. prop::subscription_identifiers has the interface of std::optional<int32_t>.

Example

The following example shows how to set a Property value:

boost::mqtt5::subscribe_props props;
props[boost::mqtt5::prop::subscription_identifier] = 1234;
props[boost::mqtt5::prop::user_property].emplace_back("name", "value");

The following example shows how to retrieve a Property value:

boost::mqtt5::prop::subscription_identifiers sub_id = props[boost::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[boost::mqtt5::prop::user_property];
if (!user_props.empty())
    // user property was previously set
else
    // user property was not set

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