Communication¶

core/communication/ — FastDDS wrappers, topic specs, domain context.

TopicSpec¶

topic_spec.hpp

The contract type that connects the communication layer to the lifecycle layer. All topic configuration lives here.

template <typename T, const char* TopicName, size_t max_queue_size = 1>
struct TopicSpec {
  using type = T;                             // FastDDS PubSubType
  static constexpr const char* kName;         // topic name string
  static constexpr size_t kQueueSize;         // subscriber queue depth
};

T must derive from eprosima::fastdds::dds::TopicDataType. Enforced by static_assert.

is_topic_spec_v<T> — type trait for checking at instantiation sites.

In practice, TopicSpec instances are declared in generated headers (see Generators) — you don't write them by hand.

DDSContext¶

dds_context.hpp

Manages the FastDDS DomainParticipant and the topic registry. Two classes:

DDSContext — owns one DomainParticipant, creates and caches Topic objects:

DDSContext ctx("my_participant");
dds::Topic* topic = ctx.GetDDSTopic<MyTopicSpec>();

Topics are created on first request and cached by name. The participant and all topics are cleaned up via RAII deleters — no manual lifecycle calls.

DDSContextProvider — lazy singleton wrapper. Initialized by DDSAPPlication before any task starts:

DDSContextProvider::SetName("my_participant");  // call before any GetDDSTopic
DDSContext& ctx = DDSContextProvider::Get();    // creates instance on first call

SetName must be called before the first Get(). After that, the name is fixed.

Note: DDSContextProvider is not thread-safe on construction. The design assumption is single-threaded setup — tasks start only after the application is constructed.

DDSPublisher¶

dds_publisher.hpp

Thin wrapper over a FastDDS DataWriter. Called by DataEndpoint<Spec, DataDirection::Out> during Sync().

DDSPublisher<MyPubSubType> pub;
pub.Start("my_topic");
pub.Publish(my_message);

DDSSubscriber¶

dds_subscriber.hpp

Thin wrapper over a FastDDS DataReader. Accumulates incoming messages internally. Called by DataEndpoint<Spec, DataDirection::In> during FillInputs().

DDSSubscriber<MyPubSubType, kQueueSize> sub;
sub.Start("my_topic");
sub.DrainQueue(queue);  // moves buffered samples into a SizeConstrainedQueue

DrainQueue transfers everything accumulated since the last drain into the task-local SizeConstrainedQueue.

SizeConstrainedQueue (legacy)¶

core/communication/size_constrained_queue.hpp

Outdated. The canonical implementation is core/support/utils/size_constrained_queue.hpp. This file is kept for reference and will be removed. Do not use it in new code.

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