ROS NodeHandles : nh_ vs nh_priv_

asked 2021-06-28 05:07:37 -0500

electrophod
277 ●35 ●61 ●60

1) What exactly is a Nodehandle? Why are they only in C++ and not in Python?

2) In many C++ header files of a ROS package (for example this one), I have seen people declaring 2 NodeHandles namely, nh_ and nh_priv_.

Why are there 2 NodeHandles in the same code and what does the underscore _ at the end of both the identifiers mean?

Would like to know about these in more detail, resources would be amazing to get.

Help is appreciated, TIA

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Probably not what you came here to hear, but could you list the Q&As you've already found on this site which discuss this topic? Personally I always use Google to search and append site:answers.ros.org to my query.

There's a very good chance both of your questions have been discussed before, and we should try to a) avoid duplicates and b) avoid repeating discussions.

By listing existing Q&As in your own question, you can help us help you.

gvdhoorn ( 2021-06-28 05:38:42 -0500 )edit

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answered 2021-06-28 06:34:15 -0500

pvl
111 ●7 ●4

updated 2021-06-28 06:35:28 -0500

A nodehandle is a RAII (resource acquisition is initialization) type according to ros.org - NodeHandles. RAII is a C++ term meaning that resources are cleanup up afterwards when the object goes out of scope. In rospy the nodehandle-like class is the rospy module itself. Both solutions seems more idiomatic to their languages, but that is a guess on my part.
Private node handles are a way to access names private to the ROS node, which would create ambiguous names in some situations otherwise. They are a nodehandle to a different namespace. ros.org - AccessingPrivateNamesWithNodeHandle. I have not used them myself.
underscores at the end of a member variable are a C++ convention. As with all C++ things, this is not universal and will differ in different organisations.

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ROS NodeHandles : nh_ vs nh_priv_

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