BSC33-A. Use valid references, pointers, and iterators to reference string objects

References, pointers, and iterators referring to the elements of a basic_string sequence may be invalidated
by the following uses of that {{basic_string object:

• As an argument to non-member functions swap(), operator>>(), and getline().
• As an argument to basic_string::swap().
• Calling data() and c_str() member functions.
• Calling non-const member functions, except operator[](), at(), begin(), rbegin(), end(), and rend().
• Subsequent to any of the above uses except the forms of insert() and erase() which return iterators, the first call to non-const member functions operator[](), at(), begin(), rbegin(), end(), or rend().