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#### PUBACK properties

The last field in the Variable header of `PUBACK` 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 `PUBACK` Properties, along with descriptions of their usage:

Table 1.8. PUBACK properties

Identifier | Value type | Description
--- | --- | ---
reason_string | `std::string` | A UTF-8 Encoded String representing the reason associated with this response.
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 may be used to provide additional diagnostic or other information.

##### Usage

After obtaining an instance of `boost::mqtt5::puback_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`, 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:

    boost::mqtt5::puback_props props;
    props[boost::mqtt5::prop::reason_string] = "Some reason...";
    props[boost::mqtt5::prop::user_property].emplace_back("name", "value");

The following example shows how to retrieve a Property value:

    std::optional<std::string> reason_string = props[boost::mqtt5::prop::reason_string];
    if (reason_string.has_value())
        // reason string property was previously set
    else
        // reason string 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