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;
}