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DataWriter

A |DataWriter-api| is attached to exactly one :ref:`dds_layer_publisher_publisher` that acts as a factory for it. Additionally, each DataWriter is bound to a single :ref:`dds_layer_topic_topic` since its creation. This Topic must exist prior to the creation of the DataWriter, and must be bound to the data type that the DataWriter wants to publish.

The effect of creating a new DataWriter in a Publisher for a specific Topic is to initiate a new publication with the name and data type described by the Topic.

Once the DataWriter is created, the application can inform of changes in the data value using the |DataWriter::write-api| member function on the DataWriter. These changes will be transmitted to all subscriptions matched with this publication.

DataWriterQos

|DataWriterQos-api| controls the behavior of the DataWriter. Internally it contains the following |QosPolicy-api| objects:

QosPolicy class Accessor/Mutator Mutable
|DurabilityQosPolicy| |DataWriterQos::durability-api| No
|DurabilityServiceQosPolicy| |DataWriterQos::durability_service-api| No
|DeadlineQosPolicy| |DataWriterQos::deadline-api| Yes
|LatencyBudgetQosPolicy| |DataWriterQos::latency_budget-api| Yes
|LivelinessQosPolicy| |DataWriterQos::liveliness-api| No
|ReliabilityQosPolicy| |DataWriterQos::reliability-api| No (*)
|DestinationOrderQosPolicy| |DataWriterQos::destination_order-api| No
|HistoryQosPolicy| |DataWriterQos::history-api| No
|ResourceLimitsQosPolicy| |DataWriterQos::resource_limits-api| No
|TransportPriorityQosPolicy| |DataWriterQos::transport_priority-api| Yes
|LifespanQosPolicy| |DataWriterQos::lifespan-api| Yes
|UserDataQosPolicy| |DataWriterQos::user_data-api| Yes
|OwnershipQosPolicy| |DataWriterQos::ownership-api| No
|OwnershipStrengthQosPolicy| |DataWriterQos::ownership_strength-api| Yes
|WriterDataLifecycleQosPolicy| |DataWriterQos::writer_data_lifecycle-api| Yes
|PublishModeQosPolicy| |DataWriterQos::publish_mode-api| No
|DataRepresentationQosPolicy| |DataWriterQos::representation-api| No
|PropertyPolicyQos| |DataWriterQos::properties-api| No
|RTPSReliableWriterQos| |DataWriterQos::reliable_writer_qos-api| Yes (*)
|RTPSEndpointQos| |DataWriterQos::endpoint-api| No
|WriterResourceLimitsQos| |DataWriterQos::writer_resource_limits-api| No
|DataSharingQosPolicy| |DataWriterQos::data_sharing-api| No

The following non-consolidated property-assigned QoS apply to DataWriters:

Property name Non-consolidated QoS
fastdds.push_mode :ref:`push_mode_qos_policy`
partitions :ref:`property_policies_partitions`

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

Note

Reliability kind (whether the publication is reliable or best effort) is not mutable. However, the max_blocking_time data member of |ReliabilityQosPolicy| can be modified any time.

Note

Not all data members of RTPSReliableWriterQos are mutable, please refer to |RTPSReliableWriterQos| for more information.

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

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

Default DataWriterQos

The default :ref:`dds_layer_publisher_dataWriterQos` refers to the value returned by the |Publisher::get_default_datawriter_qos-api| member function on the Publisher instance. The special value :class:`DATAWRITER_QOS_DEFAULT` can be used as QoS argument on |Publisher::create_datawriter-api| or |DataWriter::set_qos-api| member functions to indicate that the current default DataWriterQos should be used.

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

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

|Publisher::set_default_datawriter_qos-api| member function also accepts the special value DATAWRITER_QOS_DEFAULT as input argument. This will reset the current default DataWriterQos to default constructed value |DataWriterQos::DataWriterQos-api|.

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

Note

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