DDS11 — (R#120) Clarify use of DATAREADER_QOS_USE_TOPIC_QOS

Title: (R#120) Clarify use of DATAREADER_QOS_USE_TOPIC_QOS constant when creating DataReader on ContentFilteredTopic or MultiTopic

Summary:
The specification defines the constant DATAREADER_QOS_USE_TOPIC_QOS that may be used to specify the QoS of a DataReader. The meaning of such usage is unclear when the DataReader's TopicDescription is a ContentFilteredTopic or a MultiTopic since those types do not have QoS of their own.

Proposed Resolution:
A ContentFilteredTopic is based on a single Topic; therefore, the meaning of DATAREADER_QOS_USE_TOPIC_QOS is well-defined in that case: it refers to the QoS of the Topic accessible via the ContentFilteredTopic::get_related_topic operation.
The meaning of DATAREADER_QOS_USE_TOPIC_QOS is not well-defined in the case of a MultiTopic; using it to set the QoS of a DataReader of a MultiTopic is an error. Specifically, passing the constant to Subscriber::create_datareader when a MultiTopic is also passed to that operation will result in the operation returning nil.

Proposed Revised Text:
The last paragraph of section "2.1.2.5.2.5 create_datareader" (which begins "The special value…") should be rewritten as follows:
Provided that the TopicDescription passed to this method is a Topic or a ContentFilteredTopic, the special value DATAREADER_QOS_USE_TOPIC_QOS can be used to indicate that the DataReader should be created with a combination of the default DataReader QoS and the Topic QoS. (In the case of a ContentFilteredTopic, the Topic in question is the ContentFilteredTopic's "related Topic.") The use of this value is equivalent to the application obtaining the default DataReader QoS and the Topic QoS (by means of the operation Topic::get_qos) and then combining these two QoS using the operation copy_from_topic_qos whereby any policy that is set on the Topic QoS "overrides" the corresponding policy on the default QoS. The resulting QoS is then applied to the creation of the DataReader. It is an error to use DATAREADER_QOS_USE_TOPIC_QOS when creating a DataReader with a MultiTopic; this method will return a nil value in that case.

No Data Available