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Definition of data types

The definition of the data type exchanged in a :ref:`dds_layer_topic_topic` is divided in two classes: the |TypeSupport-api| and the |TopicDataType-api|.

TopicDataType describes the data type exchanged between a publication and a subscription, i.e., the data corresponding to a Topic. The user has to create a specialized class for each specific type that will be used by the application.

Any specialization of TopicDataType must be registered in the :ref:`dds_layer_domainParticipant` before it can be used to create Topic objects. A TypeSupport object encapsulates an instance of TopicDataType, providing the functions needed to register the type and interact with the publication and subscription. To register the data type, create a new TypeSupport with a TopicDataType instance and use the |TypeSupport::register_type-api| member function on the TypeSupport. Then the Topic can be created with the registered type name.

Note

Registering two different data types on the same DomainParticipant with identical names is not allowed and will issue an error. However, it is allowed to register the same data type within the same DomainParticipant, with the same or different names. If the same data type is registered twice on the same DomainParticipant with the same name, the second registering will have no effect, but will not issue any error.

.. literalinclude:: /../code/DDSCodeTester.cpp
   :language: c++
   :start-after: //DDS_TYPE_REGISTER
   :end-before: //!
   :dedent: 8

Dynamic data types

Instead of directly writing the specialized |TopicDataType-api| class, it is possible to dynamically define data types following the OMG Extensible and Dynamic Topic Types for DDS interface. Data types can also be described on an XML file that is dynamically loaded.

.. literalinclude:: /../code/DDSCodeTester.cpp
   :language: c++
   :start-after: //DDS_DYNAMIC_TYPES
   :end-before: //!
   :dedent: 8

A complete description of the dynamic definition of types can be found on the :ref:`dynamic-types` section.

Data types with a key

Data types that define a set of fields to form a unique key can distinguish different data sets within the same data type.

To define a keyed Topic, the |TopicDataType::getKey-api| member function on the |TopicDataType-api| has to be overridden to return the appropriate key value according to the data fields. Additionally, the |TopicDataType::m_isGetKeyDefined-api| data member needs to be set to true to let the entities know that this is a keyed Topic and that |TopicDataType::getKey-api| should be used. Types that do not define a key will have |TopicDataType::m_isGetKeyDefined-api| set to false.

There are three ways to implement keys on the TopicDataType:

Data types with key are used to define data sub flows on a single Topic. Data values with the same key on the same Topic represent data from the same sub-flow, while data values with different keys on the same Topic represent data from different sub-flows. The middleware keeps these sub-flows separated, but all will be restricted to the same QoS values of the Topic. If no key is provided, the data set associated with the Topic is restricted to a single flow.