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Try to replace

nhp.searchParam("/r_arm/position/my_param", temp);

with

nhp.getParam("/r_arm/position/my_param", temp);

searchParam() searches the namespace tree upwards and returns the name of the first existing parameter, not its value. You still would have to call getParam() to retrieve the value. The XmlRpc::XmlRpcValue is implicitly casted into a string reference when passed to searchParam() and therefore has type TypeString afterwards.

See http://wiki.ros.org/roscpp_tutorials/Tutorials/Parameters#Searching_for_Parameters for further details.