I am just confused because we have to parse serial messages and I don’t know how to do that without json.
It would help if you were more specific in describing what issue you are experiencing.
As for sample C++ code, there are many different C++ scripts that come with the standard Jevois image, as well as documentation on how to create your own C++ and Python scripts using Jevois at their website.
If you have any experience using Python, I would recommend starting by creating a simple Python module using Jevois, as Jevois has tutorials that walk you through every step of the way on their master documentation page. Then, once you have a handle on how to do that, making your own C++ modules should be a little more straightforward, as the Python modules actually use C++ bindings to do a lot of the processing. 
Also have a look at this AWESOME white paper by billbo911 on team 2073 on how to use Jevois in FRC. Our team really learned a lot from this whitepaper when we were first getting our vision pipeline off the ground. Also keep in mind that there are going to be some differences in the processes described in this white paper, and the way the current Jevois image operates, as the Jevois image is updated quite frequently, but the main premise of the operation of Jevois always remains the same. One main difference between the methods Eagleforce employs in their white paper, and the current methods that are available, is the creation of the Jevois Inventor, which makes making live changes to some modules on Jevois possible (as it was pretty much impossible to make live changes to your code before) :yikes:
Have you looked at this Jevois example on decoding their standard messages to number values?