set robot_description in program?

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Sure. It's just 'another parameter'.

Note: you will have to make sure that any other nodes that require that parameter will be started after your 'c++ program', or they'll most likely fail (most nodes (such as the robot_state_publisher) try to access parameters only once, at init). This can be tricky, as there is no guaranteed order in which nodes are launched.

Letting roslaunch set robot_description as a parameter takes care of this automatically, as "parameters will be stored on the Parameter Server before any nodes are launched" (from here).