int main() {
   std::set<int> numbers;
   numbers.insert(3);
   numbers.insert(2);
   numbers.insert(99);
   numbers.insert(-21);
   numbers.insert(3);
   for (int i : numbers)
       std::cout << i << std::endl;
}

struct IntPair {
  int x;
  int y;
  bool operator<(const IntPair& other) const {
      return x < other.x && y < other.y;
  }
};
int main() {
   std::set<IntPair> numbers;
   numbers.insert({1,2});
   numbers.insert({2,3});
   numbers.insert({3,4});
   
   for (auto [x,y] : numbers)
       std::cout <<'( ' <<x << ' ' << y << ')' << std::endl;
}

class A{
public:
   A() = default;
   A(int x_, int y_) : x {x_}, y {y_} {}
   friend bool operator<(const A& left, const A& right);
private:
   int x {0}, y {0};
};

bool operator<(const A& left, const A& right) {
   return left.x < left.y || left.x == left.y  && right.x < right.y;
}

class B{
public:
   B() = default;
   B( double x_,  double y_) : x {x_}, y {y_} {}
   double get_x() const noexcept {
       return x;
   }
   double get_y() const noexcept {
       return y;
   }
private:
   double x {0}, y {0};
};


int main() {

std::set<A> s1; //operator<()

   auto comp {
       [](const B& left, const B& right) {
       return left.get_x() < left.get_y()
       || left.get_x() == left.get_y()  && right.get_x() < right.get_y();
   }};
   std::set<B, decltype(comp)> s2(comp);

return 0;
}

C++11/14/17 std::expected with functional-style extensions  - TartanLlama/expected