Skip to content

Latest commit

 

History

History
122 lines (96 loc) · 6.83 KB

topic.rst

File metadata and controls

122 lines (96 loc) · 6.83 KB
Raw

Topic

A |Topic-api| is a specialization of the broader concept of :ref:`dds_layer_topic_topicDescription`. A Topic represents a single data flow between :ref:`dds_layer_publisher_publisher` and :ref:`dds_layer_subscriber_subscriber`, providing:

  • The name to identify the data flow.
  • The data type that is transmitted on that flow.
  • The QoS values related to the data itself.

The behavior of the Topic can be modified with the QoS values specified on :ref:`dds_layer_topic_topicQos`. The QoS values can be set at the creation of the Topic, or modified later with the |Topic::set_qos-api| member function.

Like other Entities, Topic accepts a Listener that will be notified of status changes on the Topic.

Please refer to :ref:`dds_layer_topic_creation` for more information about how to create a |Topic-api|.

TopicQos

|TopicQos-api| controls the behavior of the Topic. Internally it contains the following |QosPolicy-api| objects:

QosPolicy class Accessor Mutable
|TopicDataQosPolicy| |TopicQos::topic_data-api| Yes
|DurabilityQosPolicy| |TopicQos::durability-api| Yes
|DurabilityServiceQosPolicy| |TopicQos::durability_service-api| Yes
|DeadlineQosPolicy| |TopicQos::deadline-api| Yes
|LatencyBudgetQosPolicy| |TopicQos::latency_budget-api| Yes
|LivelinessQosPolicy| |TopicQos::liveliness-api| Yes
|ReliabilityQosPolicy| |TopicQos::reliability-api| Yes
|DestinationOrderQosPolicy| |TopicQos::destination_order-api| Yes
|HistoryQosPolicy| |TopicQos::history-api| Yes
|ResourceLimitsQosPolicy| |TopicQos::resource_limits-api| Yes
|TransportPriorityQosPolicy| |TopicQos::transport_priority-api| Yes
|LifespanQosPolicy| |TopicQos::lifespan-api| Yes
|OwnershipQosPolicy| |TopicQos::ownership-api| Yes
|DataRepresentationQosPolicy| |TopicQos::representation-api| Yes

Refer to the detailed description of each QosPolicy-api class for more information about their usage and default values.

The QoS value of a previously created Topic can be modified using the |Topic::set_qos-api| member function.

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

Default TopicQos

The default :ref:`dds_layer_topic_topicQos` refers to the value returned by the |DomainParticipant::get_default_topic_qos-api| member function on the :ref:`dds_layer_domainParticipant` instance. The special value TOPIC_QOS_DEFAULT can be used as QoS argument on |DomainParticipant::create_topic-api| or |Topic::set_qos-api| member functions to indicate that the current default TopicQos should be used.

When the system starts, the default TopicQos is equivalent to the default constructed value |TopicQos::TopicQos-api|. The default TopicQos can be modified at any time using the |DomainParticipant::get_default_topic_qos-api| member function on the DomainParticipant instance. Modifying the default TopicQos will not affect already existing Topic instances.

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

|DomainParticipant::get_default_topic_qos-api| member function also accepts the value TOPIC_QOS_DEFAULT as input argument. This will reset the current default TopicQos to default constructed value |TopicQos::TopicQos-api|.

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

Note

The value TOPIC_QOS_DEFAULT has different meaning depending on where it is used: