Странная ошибка C3867

Здравствуйте. Помогите исправить ошибку. Как не менял код, исправить не удалось.

1>------ Build started: Project: lab3, Configuration: Debug Win32 ------
1>Source.cpp
1>e:\програмування\lab3\lab3project\lab3\lab3\phonebook.h(100): error C3867: 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>::length': non-standard syntax; use '&' to create a pointer to member
1>e:\програмування\lab3\lab3project\lab3\lab3\phonebook.h(100): error C2446: '!=': no conversion from 'int' to 'unsigned int (__thiscall std::basic_string<char,std::char_traits<char>,std::allocator<char>>::* )(void) noexcept const'
1>e:\програмування\lab3\lab3project\lab3\lab3\phonebook.h(100): note: There are no conversions from integral values to pointer-to-member values
1>e:\програмування\lab3\lab3project\lab3\lab3\phonebook.h(104): error C3867: 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>::length': non-standard syntax; use '&' to create a pointer to member
1>e:\програмування\lab3\lab3project\lab3\lab3\phonebook.h(104): error C2446: '<': no conversion from 'unsigned int (__thiscall std::basic_string<char,std::char_traits<char>,std::allocator<char>>::* )(void) noexcept const' to 'int'
1>e:\програмування\lab3\lab3project\lab3\lab3\phonebook.h(104): note: There is no context in which this conversion is possible
1>Done building project "lab3.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Вот код:

https://gist.github.com/AndreyRafalsky/e84d389470e657a4e01e94e518b4ac68

Заранее спасибо за помощь.