Qos Use Cases

The different QoS classes are containers of QoS policies of a concrete dds::core::Entity. Each Entity has a qos() getter and setter.

• dds::domain::qos::DomainParticipantQos is the container of the dds::domain::DomainParticipant QoS policies.
• dds::topic::qos::TopicQos is the container of the dds::topic::Topic QoS policies.
• dds::pub::qos::PublisherQos is the container of the dds::pub::Publisher QoS policies.
• dds::sub::qos::SubscriberQos is the container of the dds::sub::Subscriber QoS policies.
• dds::pub::qos::DataWriterQos is the container of the dds::pub::DataWriter QoS policies.
• dds::sub::qos::DataReaderQos is the container of the dds::sub::DataReader QoS policies.

Setting Qos Values

There are a few different ways to set the values of a QoS policy within an entity's QoS object

• Setting a QoS policy with operator <<

      using namespace dds::core::policy;
  
      // Construct a Reliability QoS object with default values
      dds::core::policy::Reliability reliability; 
  
      // Construct a DataReaderQos object with default values
      dds::sub::qos::DataReaderQos reader_qos; 
  
      // Set the Reliability Qos policy field of the DataReaderQos using operator <<, 
      // setting the ReliabilityKind to RELIABLE inline
      reader_qos << reliability.kind(ReliabilityKind::RELIABLE);

  
  
• Many of the QoS policies have appropriately named constructors that will construct a QoS policy object with values for some of the fields in the policy. The following code example is equivalent to the above example.

      using namespace dds::core::policy;
  
      // Use a named constructor to set the ReliabilityKind
      reader_qos << Reliability::Reliable();

  
  
• You can also set a QoS policy on a QoS object using the QoS object's policy() setter which is templatized on the QoS policy that it is being used to set. The following code example is equivalent to the above two examples.

      using namespace dds::core::policy;
  
      // Set the reliability Qos on the DataReaderQos object
      reader_qos.policy<Reliability>(Reliability::Reliable());
  
      // Note that the above statement sets the whole policy, possibly overriding
      // other fields of Reliability (such as Reliability::acknowledgment_kind())
      // with their default values. If you only want to change a specific field
      // of the policy, get it by reference and modify it:
      reader_qos.policy<Reliability>().kind(ReliabilityKind::RELIABLE);

Getting Qos Values

There are a few different ways to get the values of a QoS policy from an entity's QoS object

• Getting a QoS policy with operator >>

      using namespace dds::core::policy;
  
      // Construct a Reliability QoS object with default values
      dds::core::policy::Reliability reliability; 
  
      // Construct a DataReaderQos object with default values
      dds::sub::qos::DataReaderQos reader_qos; 
  
      // Populate the Reliability Qos object with the values from the 
      // DataReaderQos object using operator >>
      reader_qos >> reliability;

  
  
• You can also get a QoS policy from a QoS object using the QoS object's policy() getter which is templatized on the QoS policy that it is being used to get. The following code example is equivalent to the above example.

      using namespace dds::core::policy;
  
      // Get the reliability Qos from the DataReaderQos object
      reliability = reader_qos.policy<Reliability>();