Topics

18.1 Topics

Before you can create a Topic, you need a user data type (see Chapter 17 Data Types and DDS Data Samples) and a DomainParticipant (16.3 DomainParticipants). The user data type must be registered with the DomainParticipant (see 17.2.8 Type Codes for Built-in Types).

Once you have created a Topic, what do you do with it? Topics are primarily used as parameters in other Entities’ operations. For instance, a Topic is required when a Publisher or Subscriber creates a DataWriter or DataReader, respectively. Topics do have a few operations of their own, as listed in Table 18.1 Topic Operations. For details on using these operations, see the reference section or the API Reference HTML documentation.

Figure 18.1: Topic Module

Table 18.1 Topic Operations
Purpose	Operation	Description	Reference
Configuring the Topic	enable	Enables the Topic.	15.2 Enabling DDS Entities
	get_qos	Gets the Topic’s current QosPolicy settings. This is most often used in preparation for calling set_qos().	18.1.3 Setting Topic QosPolicies
	set_qos	Sets the Topic’s QoS. You can use this operation to change the values for the Topic’s QosPolicies. Note, however, that not all QosPolicies can be changed after the Topic has been created.	18.1.3 Setting Topic QosPolicies
	equals	Compares two Topic’s QoS structures for equality.	18.1.3.2 Comparing QoS Values
	set_qos_with_ profile	Sets the Topic’s QoS based on a specified QoS profile.
	get_listener	Gets the currently installed Listener.	18.1.5 Setting Up TopicListeners
	set_listener	Sets the Topic’s Listener. If you create the Topic without a Listener, you can use this operation to add one later. Setting the listener to NULL will remove the listener from the Topic.	18.1.5 Setting Up TopicListeners
	narrow	A type-safe way to cast a pointer. This takes a DDSTopicDescription pointer and ‘narrows’ it to a DDSTopic pointer.	31.7 Using a Type-Specific DataWriter (FooDataWriter)
Checking Status	get_inconsistent_ topic_status	Allows an application to retrieve a Topic’s INCONSISTENT_TOPIC_STATUS status.	18.2.1 INCONSISTENT_TOPIC Status
Checking Status	get_status_changes	Gets a list of statuses that have changed since the last time the application read the status or the listeners were called.	15.4 Getting Status and Status Changes
Navigating Relationships	get_name	Gets the topic_name string used to create the Topic.	18.1.1 Creating Topics
	get_type_name	Gets the type_name used to create the Topic.	18.1.1 Creating Topics
	get_participant	Gets the DomainParticipant to which this Topic belongs.	18.1.6.1 Finding a Topic’s DomainParticipant