DDS QoS: Is it possible to send data without specifying a key?

Question

I am currently using Eclipse Cyclone DDS C++ and I am wondering if it is possible to send data without any key specified with #pragma keylist in an IDL.

Is it possible to achive this through QoS-Settings? If so, how?

Answer 1

If you do #pragma keylist TYPE without listing any key fields, the IDL compiler will generate everything you need for creating a topic that doesn't have any key fields. So

struct S {
  long something;
  string orother;
};
#pragma keylist S

is perfectly valid, and then you can create a topic for S.

I am not entirely sure what you mean by

In this case writes to the same topic could not be distinguished by the readers.

The above will result in a single instance for this topic. You can still have a history and so, e.g., the reader can still ensure it sees every update by specifying a KEEP_ALL history setting. The reader can therefore still distinguish between the writes by looking at the content and/or the sample info, but for DDS these are all updates to this one instance.

Is it possible to achive this through the QoS-Settings?

No, you cannot (currently) override the key fields using the QoS settings.