Yes, you should put the Caesar:: bit in the place where you actually write the code for them (the second code snippet that you give).
Make sure you fix the other error noted as well.
BTW it would massively help if you put everything in one file for the purposes of getting help on the forum. Then it can be run as-is in cpp.sh without having to download multiple files or cut and paste several separate includes into cpp.sh.
As @salem c noted, you should give the whole error message, whether you understand it or not.