eee.cpp

// rel_ops example:
#include <iostream>     // std::cout, std::boolalpha
#include <utility>      // std::rel_ops
#include <cmath>        // std::sqrt

class vector2d {
public:
  double x,y;
  vector2d (double px,double py): x(px), y(py) {}
  double length() const {return std::sqrt(x*x+y*y);}
  bool operator==(const vector2d& rhs) const {return length()==rhs.length();}
  bool operator< (const vector2d& rhs) const {return length()< rhs.length();}
};

int main () {
  using namespace std::rel_ops;
  vector2d a (10,10);	// length=14.14
  vector2d b (15,5);	// length=15.81
  std::cout << std::boolalpha;
  std::cout << "(a<b) is " << (a<b) << '\n';
  std::cout << "(a>b) is " << (a>b) << '\n';
  return 0;
}