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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
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