The Waite Group's C++ Primer Plus, Third Edition

String Access

There are four methods for accessing individual characters, two using the [] operator 0, and two using the at() method:

reference operator[](size_type pos);
const_reference operator[](size_type pos) const;
reference at(size_type n);
const_reference at(size_type n) const;

The first operator[]() method allows you to access an individual element of a string using array notation; it can be used to retrieve or alter the value. The second operator[]() method can be used with const objects, and it can be used only to retrieve the value:

string word("tack");
cout << word[0];    // display the t
word[3] = t';      // overwrite the k with a t
const ward("garlic");
cout << ward[2];    // display the r

The at() methods provide similar access, except ...