// Explicit use of a functor, as you might use it INSIDE a container.

#include <iostream>
#include <cmath>

using namespace std;

// Compare two doubles according to how close they are to 5.0.
// This does NOT sort anything!  This merely compares two elements,
// and returns true iff the first one should come before the second one.
// Sure, this could be a template functor, but we don’t need that generality.
//
// As written, 4.0 and 6.0 are “equal”, so either one might come first.
// It might be better to also have a secondary comparison based on value
// if the proximity to 5.0 is equal, so 4.0 would come before 6.0.
//
// Always declare operator() const, if you can.  Compilers are getting
// fussier about requiring such a comparison functor for set and map.

class Proximity {
  public:
    bool operator()(double a, double b) const {
	return fabs(a-5.0) < fabs(b-5.0);
    }
};

template<typename Compare>
void decide(double m, double n) {
    const Compare cmp;		    // An object of the functor type

    if (cmp(m, n))
	cout << m << " goes before " << n << "\n";
    else if (cmp(n, m))
	cout << n << " goes before " << m << "\n";
    else
	cout << m << " and " << n << " are equal\n";
}

int main() {
    decide<Proximity>(1,3);
    decide<Proximity>(9,9);
    decide<Proximity>(2,8);		// How on Earth are 2 and 8 “equal”?
}