It's convention in C++ to pass-by-pointer if the function will mutate the argument. The other way, by (non-const) reference, is discouraged because the function invocation looks the same as the extremely common pass-by-value case.

  void ptr_add(int *f) {
    *f += 5;
  }

  void val_add(int f) {
    f += 5;   // only changes local copy of f
  }

  void ref_add(int &f) {
    f += 5;
  }

  int main() {
    int foo = 42;
    ptr_add(&foo);  // foo is now 47
    val_add(foo);   // foo is still 47
    ref_add(foo);   // foo is now 52
  }