namespace rel_ops {
  template <class T> bool operator!= (const T&, const T&);
  template <class T> bool operator>  (const T&, const T&);
  template <class T> bool operator<= (const T&, const T&);
  template <class T> bool operator>= (const T&, const T&);
}

<utility>

Relational Operators

This namespace declares template functions for four relational operators (!=,>, <=, and >=), deriving their behavior from operator== (for !=) and from opertator< (for >,<=, and >=):

namespace rel_ops {
  template <class T> bool operator!= (const T& x, const T& y) { return !(x==y); }
  template <class T> bool operator>  (const T& x, const T& y) { return y<x; }
  template <class T> bool operator<= (const T& x, const T& y) { return !(y<x); }
  template <class T> bool operator>= (const T& x, const T& y) { return !(x<y); }
}

This avoids the necessity to declare all six relational operators for every complete type; By defining just two: operator== and operator<, and importing this namespace, all six operators will be defined for the type.

Still, because non-template functions take precedence over template functions, any of these operators may be defined with a different behavior for a specific type, even with rel_ops being used.

Template parameters

T

For operator!=, the type shall be EqualityComparable.
A type is EqualityComparable when it supports operator== operations that follow the typical reflexive, symmetric and transitive properties of equalities.

For operator>, operator<=, and perator>=, the type must be LessThanComparable.
A type is LessThanComparable when it supports operator< operations that define a valid strict weak ordering relation.

Example

// rel_ops example:
#include <iostream>
#include <utility>
#include <cmath>
using namespace std;

class vector2d {
public:
  double x,y;
  vector2d (double px,double py): x(px), y(py) {}
  double length () const {return 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();}
};

using namespace rel_ops;

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

Output:

(a<b) is true
(a>b) is false

Because we used rel_ops, all types for which operator< is defined (like vector2d) also have operator>.

See also