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<utility>
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&);
}
Relational Operators
This namespace declares template functions for four relational operators (!=,>, <=, and >=), deriving their behavior from operator== (for !=) and from operator< (for >,<=, and >=):
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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); }
}
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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 operator>=, the type must be LessThanComparable.
A type is LessThanComparable when it supports operator< operations that define a valid strict weak ordering relation.
Example
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// 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;
}
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Output:
(a<b) is true
(a>b) is false
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Because we used rel_ops, all types for which operator< is defined (like vector2d) also have operator>.
See also
| utility | Utility components (header) |
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