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#### boost::mqtt5::prop::subscription_identifiers

A class holding any number of Subscription Identifiers.

##### Synopsis

Defined in header `<boost/mqtt5/property_types.hpp>`

    class subscription_identifiers
        : public boost::container::small_vector< int32_t, 1 >;

##### Member Functions

Name | Description
--- | ---
emplace | Sets or replaces the (first) Subscription Identifier.
has_value | Checks whether there are any Subscription Identifiers.
operator bool | Checks whether there are any Subscription Identifiers.
operator* | Accesses the (first) Subscription Identifier.
reset | Clears the Subscription Identifiers.
subscription_identifiers [constructor] | Constructs Subscription Identifiers with given parameters.
value | Returns the first Subscription Identifier.
value_or | Returns the first Subscription Identifier if it exists, otherwise returns `default_val`.

##### Description

Subscription Identifier is an integer that can be set in `boost::mqtt5::subscribe_props` when subscribing to a Topic. Broker will store the mapping between that Subscription and the Subscription Identifier. When an incoming `PUBLISH` message matches one or more Subscriptions, respective Subscription Identifiers will be forwarded in the `boost::mqtt5::publish_props` of the received message.

Convenience header `<boost/mqtt5.hpp>`