The good news is that you could use both libraries. `sqlite3cpp::database` supports "attaching" mode that won't close a `sqlite3` instance after sqlite3cpp::database going out of scope. Here is an example for coexisting with SqliteModernCpp:

  // db inited by SqliteModernCpp
  sqlite::database db(":memory:");
  {
    sqlite3cpp::database attached_db(db.connection().get());
    // create the SQL function as you like
    attached_db.create_scalar("mutiply", [](int x, int y) {
      return x * y;
    });
  }  // attached_db is going out of scope

  // SQL scalar function `multiply` is still availble
  db << "CREATE TABLE numbers (n1 INTEGER, n2 INTEGER);";
  db << "SELECT multiply(n1, n2) from numbers;";

Note that SqliteModernCpp provides customization of SQL function as well as sqlite3cpp. Though it doesn't seem to have aggregate function supporting (i.e. sqlite3cpp::database::create_aggregate).