if you have an application that needs to maintain state in a non-critical section or if you discover that using SQL is actually a good idea for some tasks (even in critical sections), SQLite is not only a good choice but it will save you a lot of time coming up with a brittle custom solution.

maintain an in-memory SQLite db and work it with SQL commands, and if you also want to preserve state across application restarts you can routinely save to disk or load from it: <https://www.sqlite.org/backup.html#example_1_loading_and_sav...>

this also happens to be the most convenient file-format (aka. application-format) I ever worked with.