sortBy (greater `on` age)

on.cc

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

struct Person {
  int id;
  int age;

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

template <typename Persons>
void print(const Persons& persons) {
  for (auto&& each : persons)
    std::cout << each << std::endl;
  endl(std::cout);
}

template <typename Persons, typename Cmp>
void sort(Persons& persons, Cmp cmp) {
  std::stable_sort(begin(persons), end(persons), cmp);
}


// 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() {
  using Persons = std::vector<Person>;

  Persons persons{{0,1}, {1,3}, {1,2}, {2,0}};

  sort(persons, on(std::greater<>{}, &Person::id));
  print(persons);
}