Subscriber node changes the topic to which it listens

[blaroche]

This defect is based on this question: http://answers.ros.org/question/48086/problem-with-subscribing-to-a-topic-that-contains-slash

I have a node that subscribes to /parent/child, which is an unadvertised topic. Then, a publisher comes in on /parent. The node subscriber automagically starts listening to /parent instead of /parent/child.

[dirk-thomas]

I can not reproduce the described behavior - I tried the following:

Running a simple subscriber on /foo/data (note: data is actually a field of the String message published on /foo later):
#!/usr/bin/env python import rospy from std_msgs.msg import String def callback(data): rospy.loginfo(rospy.get_caller_id() + 'Incoming message: %s',data.data) rospy.init_node('listener', anonymous=True) rospy.Subscriber('/foo/data', String, callback) rospy.spin()

Publish message on the parent topic:
rostopic pub -r 2 /foo std_msgs/String text

And the subscriber does not receive/output any messages.
rostopic info /foo and rostopic info /foo/data shows that both endpoints are clearly separated and publishing/listening on different topics:

Publishers: 
- /rostopic_7493_1353507454648 (http://localhost:60573/)
  Subscribers: None`
  and
  `Type: std_msgs/String
  Publishers: None
  Subscribers: 
- /listener_7483_1353507449360 (http://localhost:39999/)```

Can you please try to reproduce the issue with this command sequence?

For `rostopic echo` the behavior is intended that the subscriber tries to extract the fields from parent topics.

[blaroche]

I'm on a business trip so I won't be able to try it for a while. :-(

Have you tried the C++ example that I wrote in the original question? Do you see the behavior there or not?

[dirk-thomas]

You used rostopic for subscribing to the topic which has a different semantic as mentioned in the answers. rostopic "magically" subscribes to the parent topic to extract field from these messages. That is intended and documented behavior.

A "real" node does not behave like that - it will stick to the specific topic name. I verified that with a Python subscriber node and confirmed that it works in that scenario as expected.

So the subject of this issue would be more accurate with "rostopic changes the topic to which it listens" since that behavior does not appear for normal nodes.