Coding Guidelines
Every project should follow coding guidelines to ensure a consistent code base. Without a coding guideline a lot of the changes in the codebase over time will be noise.
Also knowledge how to write good code and lessons learned could be distilled into coding guidelines to avoid to do failures twice.
Good starting points to learn what should be inside a coding guideline are the LLVM coding standards, the Barr group coding standard, the MISRA rules or the Power of ten rules for safety critical code.
If possible these rules should be enforced automatically through code formatters like clang format and linter like clang tidy or CppCheck.
For new file creation it is also good to have templates which are based on the coding standards and examples to clarify how to use the rules during implementation.