compare struct using std::tie()

cpp_cmp_struct_using_tie.cpp

struct Student {
  int stu_id;
  int age;
  float grade;
};

std::vector<Student> students;
// fill students vector
// ...

std::sort(std::begin(students), std::end(students),
    [](const Student &lhs, const Student &rhs) -> bool {
  return std::tie(lhs.grade, lhs.stu_id, lhs.age) <
      std::tie(rhs.grade, rhs.stu_id, rhs.age);
});