Can someone please explain me the logic behind the solution for this question. Like how does using the less than operator

Also what does the exclamation mark mean? Is it mean not? So here: !(b.last_name_

Please explain thoroughly and clearly

1. Given a program with this code: employee.hxx 6 struct employee 8 std::string first_name_; 9 std::string last_name_; 10 double salary-; 11 }; Prof. X would like to be able to use std::sort to sort employee structs by prioritizing last_name before first_name. For example, the following names listed first name followed by last name, and each name is separated by commas): John Doe, Abby Smith, Jane Doe, Fred Smith after sorting would be ordered as follows: Jane Doe, John Doe, Abby Smith, Fred Smith Given the container std::vector employee> names; write the single function Prof. X needs to be able to sort names using the call std::sort(begin(names), end(names));. For full marks your solution must only rely on strict weak ordering operations and semantics per key-based sorting rules in C++. Solution: employee-sort-soln.cxx bool operator names; write the single function Prof. X needs to be able to sort names using the call std::sort(begin(names), end(names));. For full marks your solution must only rely on strict weak ordering operations and semantics per key-based sorting rules in C++. Solution: employee-sort-soln.cxx bool operator