Solution: CRTP Add Class

#include <iostream>
#include <string>
#include <algorithm>
template <typename Derived>
struct Relational {
    friend bool operator > (Derived const& op1, Derived const& op2){
       return op2 < op1;
    }
    friend bool operator == (Derived const& op1, Derived const& op2){
        return !(op1 < op2) && !(op2 < op1);
    }
    friend bool operator != (Derived const& op1, Derived const& op2){
        return (op1 < op2) || (op2 < op1);
    }
    friend bool operator <= (Derived const& op1, Derived const& op2){ 
        return (op1 < op2) || (op1 == op2);
    }
    friend bool operator >= (Derived const& op1, Derived const& op2){
        return (op1 > op2) || (op1 == op2);
    }
};
// class Person
class Person: public Relational<Person>{
public:
    std::string first;
    std::string second;
    Person(std::string fst, std::string sec): first(fst), second(sec){}
    friend bool operator < (Person const& p1, Person const& p2){
        return std::make_pair(p1.first, p2.second) < std::make_pair(p2.first, p2.second);
    }
};
class Apple: public Relational<Apple>{
public:
    explicit Apple(int s): size{s}{};
    friend bool operator < (Apple const& a1, Apple const& a2){    
        return a1.size < a2.size;
    }
private:
    int size;
};
class Man: public Relational<Man>{                                 
public:
    explicit Man(const std::string& n): name{n}{}
    friend bool operator < (Man const& m1, Man const& m2){         
        return m1.name < m2.name;
    }
private:
    std::string name;
};
int main(){
  
  std::cout << std::boolalpha << '\n';
  
  Apple apple1{5};
  Apple apple2{10}; 
  std::cout << "apple1 < apple2: " << (apple1 < apple2) << '\n';
  std::cout << "apple1 > apple2: " << (apple1 > apple2) << '\n';
  std::cout << "apple1 == apple2: " << (apple1 == apple2) << '\n';
  std::cout << "apple1 != apple2: " << (apple1 != apple2) << '\n';
  std::cout << "apple1 <= apple2: " << (apple1 <= apple2) << '\n';
  std::cout << "apple1 >= apple2: " << (apple1 >= apple2) << '\n';
  
  std::cout << '\n';
    
  Man man1{"grimm"};
  Man man2{"jaud"};
  std::cout << "man1 < man2: " << (man1 < man2) << '\n'; 
  std::cout << "man1 > man2: " << (man1 > man2) << '\n'; 
  std::cout << "man1 == man2: " << (man1 == man2) << '\n'; 
  std::cout << "man1 != man2: " << (man1 != man2) << '\n';
  std::cout << "man1 <= man2: " << (man1 <= man2) << '\n';
  std::cout << "man1 >= man2: " << (man1 >= man2) << '\n';
  
  std::cout << '\n';
  Person rainer{"Rainer", "Grimm"};
  Person marius{"Marius", "Grimm"};
  Person jon{"Jon", "Joe"};
  std::cout << "Rainer Grimm < Marius Grimm: " << (rainer < marius) << '\n'; 
  std::cout << "Rainer Grimm > Marius Grimm: " << (rainer > marius) << '\n'; 
  std::cout << "Rainer Grimm == Marius Grimm: " << (rainer == marius) << '\n'; 
  std::cout << "Rainer Grimm != Marius Grimm: " << (rainer != marius) << '\n';
  std::cout << "Rainer Grimm <= Marius Grimm: " << (rainer <= marius) << '\n';
  std::cout << "Rainer Grimm >= Marius Grimm: " << (rainer >= marius) << '\n';
  Person people[] = {rainer, marius, jon};
  int n = 3;
  std::sort(people, people + n);
  for(int i = 0; i < n; i++){
    std::cout << people[i].first << " " << people[i].second << '\n';
  }
  std::cout << '\n';
    
}