Search parameters in namespaces does not work

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I have the following code snippet:

  ros::init(argc, argv, "parameters");
  ros::NodeHandle n;
  double param_in_namespace = 7.0;
  std::string namespace;
  n.setParam("/namespace/param", param_in_namespace);
  n.searchParam("param", namespace);

I would expect that the "namespace" variable has the value "/namespace/param" but when I execute this example, I get an empty string for namespace.

Without knowing in which namespace you start your node, I believe this is by-design.

From the roscpp: Searching for Parameter Keys page (emphasis mine):

There are times where you want to get a parameter from the closest namespace. For example, if you have a "robot_name" parameter, you just want to search upwards from your private namespace until you find a matching parameter.

Note how it says "search upwards".

The code you show places a parameter in /namespace, but if your node is not started in namespace as well, it's likely param will not be found, as /namespace would be below the namespace in which your node is running (which is probably the global namespace).