Two examples of how to make C++ output from flex and bison interoperable.
https://learnmoderncpp.com/2020/12/16/generating-c-programs-with-flex-and-bison-1/
https://learnmoderncpp.com/2020/12/17/generating-c-programs-with-flex-and-bison-2/
https://learnmoderncpp.com/2020/12/18/generating-c-programs-with-flex-and-bison-3/