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When is private nodehandle useful

asked 2014-09-08 13:27:18 -0500

Andromeda gravatar image

updated 2021-04-21 17:31:35 -0500

130s gravatar image

Looking into many codes and programs I see not rarely some nodes which are called toghether with a private name. For istance:

ros::NodeHandle nh;
ros::NodeHandle nh_private("~");

finding out this tutorial I read that:

  // Use a private node handle so that multiple instances of the node can
  // be run simultaneously while using different parameters.
  // Parameters defined in the .cfg file do not need to be initialized here
  // as the dynamic_reconfigure::Server does this for you.

ya ok, but I would ask now: Why?!?

In which case should I need to run simultaneously many istances of the same node? Is the risiko, that some nodes publishing on the some topics create a lot of confusion running the main prog, too big?

Since im trying to write more complicated drivers for my robot and I look into the coe of other users, I see that private nodes are normal.


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answered 2014-09-11 00:59:25 -0500

Andromeda gravatar image

Many many Thanks! It helped me to understand much better how it works. The explanation of Sebastian was very clear about that. Here a link where I found some precious informations.


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answered 2014-09-08 18:50:04 -0500

One example is when you have multiple nodes that have the same parameter (e.g. "frequency"). How do you set each node's parameter to be different?

In a launch file, when you do

<node pkg="some_package" type="some_executable" name="some_name" >
    <param name="param_name" value="some_value" />

the parameter gets resolved to /some-name/param_name.

In order to access the param_name without worrying about the node name you need to use

ros::NodeHandle nh_private("~");
std::string param;
nh_private.getParam("param_name", param);

otherwise you won't be able to get the parameter.

Hope it helps.

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answered 2014-09-09 01:53:21 -0500

Sebastian Kasperski gravatar image

Also, you don't create many instances of the node, only different node handles. You can create node handles as you wish and your node will continue to exist until the last node handle gets out of scope.

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Asked: 2014-09-08 13:27:18 -0500

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Last updated: Apr 21 '21