compare greater on age

on.cc

#include <iostream>
#include <ostream>
#include <algorithm>
#include <iterator>
#include <functional>
#include <type_traits>

struct Person {
  int age;

  friend std::ostream& operator<<(std::ostream& out, const Person& person) {
    return out << person.age;
  }
};

// https://hackage.haskell.org/package/base-4.8.1.0/docs/Data-Function.html#v:on
template <typename Cmp, typename MemPtr>
auto inline on(Cmp cmp, const MemPtr memPtr) {
  static_assert(std::is_member_pointer<MemPtr>(), "requires member pointer");
  return [=](const auto& lhs, const auto& rhs) {
    return cmp(lhs.*memPtr, rhs.*memPtr);
  };
}

int main() {
  std::vector<Person> persons{{0}, {1}, {2}};

  std::sort(begin(persons), end(persons), on(std::greater<>{}, &Person::age));

  for (auto&& each : persons)
    std::cout << each << std::endl;
}

Assignment for you dear reader: extend with variadic template to be able to say:

on(std::greater<>{}, &Person::age, &Person::firstName, &Person::lastName);

which would semantically be the same as:

operator<(const Person& lhs, const Person& rhs) {
  return std::tie(lhs.age, lhs.firstName, lhs.lastName) > std::tie(rhs.age, rhs.firstName, rhs.lastName);
}