A string allows you to have read or write access to the characters it contains. You can access a single
character via the subscript operator [ ] and the at() member function. Since C++11, front() and
back() are provided to also access the first or last character, respectively.
All these operations return a reference to the character at the position of the passed index, which
is a constant character if the string is constant. As usual, the first character has index 0, and the last
character has index length()-1. However, note the following differences: