How to use .xacro file for spawning model by using spawn_model service in c++

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Xacro files need to be parsed by the xacro parser before you can load them into Gazebo, rViz, etc.

When you are running things through a launch file, you can do this easily at launch time, like the example below. But if you are using the file from a node, it's easier to run xacro first to produce an output that you save to a file and work with that processed file.

<param name="robot_description" command="$(find xacro)/xacro.py '$(find my_robot_description)/urdf/my_robot.urdf.xacro' "/>