ROSCpp - Using common Header files

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Should I create a new package / folder with all the needed headers?

If the headers contain ROS-related types, methods, classes, functions, etc, then: yes. That would be a good way to make them available to other ROS packages.

If they are not ROS-related, then treat them like any other system dependency and create a non-ROS, regular C/C++ project.

if so, how do I need them to the makefiles in the different packages?

Please see the Catkin documentation for this, and please search ROS Answers (use Google: site:answers.ros.org <your search terms>.

This has been asked and answered too many times already to answer again.

Note: Catkin is just about equivalent to CMake, so large parts of this will be CMake, not Catkin.