39.8 Finding a Subscriber’s Related Entities

These Subscriber operations are useful for obtaining a handle to related entities:

  • get_participant(): Gets the DomainParticipant with which a Subscriber was created.
  • lookup_datareader(): Finds a DataReader created by the Subscriber with a Topic of a particular name. Note that if multiple DataReaders were created by the same Subscriber with the same Topic, any one of them may be returned by this method.

    • You can use this operation on a built-in Subscriber to access the built-in DataReaders for the built-in topics. The built-in DataReader is created when this operation is called on a built-in topic for the first time.

    If you are going to modify the transport properties for the built-in DataReaders, do so before using this operation. Built-in transports are implicitly registered when the DomainParticipant is enabled or the first DataWriter/DataReader is created. To ensure that built-in DataReaders receive all the discovery traffic, you should lookup the DataReader before the DomainParticipant is enabled. Therefore the suggested sequence when looking up built-in DataReaders is:

    1. Create a disabled DomainParticipant (see 46.2 ENTITYFACTORY QosPolicy).
    2. If you want to use non-default values, modify the built-in transport properties (see 51.5 Setting Builtin Transport Properties of Default Transport Instance—get/set_builtin_transport_properties()).
    3. Call get_builtin_subscriber() (see 28.2 Built-in DataReaders).
    4. Call lookup_datareader().
    5. Call enable() on the DomainParticipant (see 15.2 Enabling DDS Entities).

  • DDS_Subscriber_as_Entity(): This method is provided for C applications and is necessary when invoking the parent class Entity methods on Subscribers. For example, to call the Entity method get_status_changes() on a Subscriber, my_sub, do the following:
DDS_Entity_get_status_changes(DDS_Subscriber_as_Entity(my_sub))
  • DDS_Subscriber_as_Entity() is not provided in the C++, C# and Java APIs because the object-oriented features of those languages make it unnecessary.