std::basic_string_view<CharT,Traits>::back
From cppreference.com
                    
                                        
                    < cpp | string | basic string view
                    
                                                            
                    |   constexpr const_reference back() const;  | 
(since C++17) | |
Returns reference to the last character in the view. The behavior is undefined if empty() == true.
Parameters
(none)
Return value
Reference to the last character, equivalent to operator[](size() - 1).
Complexity
Constant.
See also
|    accesses the first character   (public member function)  | |
|    checks whether the view is empty    (public member function)  |