namespace rel_ops { template <class T> bool operator!= (const T& x, const T& y); template <class T> bool operator> (const T& x, const T& y); template <class T> bool operator<= (const T& x, const T& y); template <class T> bool operator>= (const T& x, const T& y);}!=,>, <=, and >=), deriving their behavior from operator== (for !=) and from operator< (for >,<=, and >=):1
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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); }
}operator== and operator<, and importing this namespace, all six operators will be defined for the type (they will not be selected by argument-dependent lookup when not importing it, though).operator!=, the type shall be EqualityComparable.operator== operations that follow the typical reflexive, symmetric and transitive properties of equalities.operator>, operator<=, and operator>=, the type shall be LessThanComparable.operator< operations that define a valid strict weak ordering relation.1
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// 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;
}(a<b) is true (a>b) is false