const_reference operator[](size_type pos) const;
reference operator[](size_type pos);
1 Returns: If pos < size(), returns data()pos. Otherwise, if pos == size(), the const
version returns charT(). Otherwise, the behavior is undefined.
const_reference at(size_type pos) const;
reference at(size_type pos);
2 Requires: pos < size()
3 Throws: out_of_range if pos >= size().
4 Returns: operator[](pos).
References
Section 21.3.4 basic_string element access