Why use string::iterator rather than index? [duplicate]

Answer 1

As stated in this question, size() method is not guaranteed to be O(1)

Answer 2

Both works.

The main reason would be consistency: you're iterating over a collection or the characters of a string the same way, by requesting an iterator and making it advance.

I would not say the implementation details of ++it resulting in a pointer increment compared to str[i] involving pointer arithmetics is worth mentioning. And range checking are implementation detail as well.

Answer 3

In cases where you don't know which class you're iterating over (because it's a template argument), you should use an iterator because not every class that provides an iterator also provides [] (and not every class that does provide [], provides one which works in O(1) time). So by using iterator you'll make sure that the function will work with as many classes as possible (though not with C arrays).

In this specific case, I see no reason to prefer one over the other except personal preference or maybe premature optimization.