const_reference operator[](size_type pos) const;
reference operator[](size_type pos);
Returns: *(begin() + pos) if pos < size(). Otherwise, returns a reference to an object of type charT with value charT(), where modifying the object to any value other than charT() leads to undefined behavior.
const_reference at(size_type pos) const;
reference at(size_type pos);
const charT& front() const;
charT& front();
const charT& back() const;
charT& back();