Friend function. ERROR: modifiers not allowed on nonmember functions

[Rehan FASTian]

#include <iostream>
#include <fstream>
#include <cmath>

using namespace std;

class frac{
	friend void print(frac& o) const;
	double num;
	double denom;
public:
	void set(double,double);
	frac operator + (frac &) const;
	void print();
};

void frac::set(double n, double d){
	for (int i = 2; i<=n && i<=d; i++){
		if (fmod(n,i) == 0 && fmod(d,i) == 0){
			n/=i;
			d/=i;
			i = 2;
		}
	}
	num = n, denom = d;
}

frac frac::operator + (frac &o) const{
	frac a = *this;
	a.num = a.num*o.denom + o.num*a.denom;
	a.denom = a.denom*o.denom;
	return a;
}

void print(frac& o) const{
	cout << o.num << '/' << o.denom << endl;
}

int main(){
	frac o1, o2, o3;
	o1.set(10,5);
	o2.set(4,2);
	o3 = o1+o2;
	print(o3);//**
	return 0;
}

D:\VC 6.0\xnx\x.cpp(9) : error C2270: 'print' : modifiers not allowed on nonmember functions
D:\VC 6.0\xnx\x.cpp(36) : error C2270: 'print' : modifiers not allowed on nonmember functions

[firedraco]

print is not a member function so it cannot be const.

[Rehan FASTian]

Can a friend function never be const?

[cire]

The const after the declaration is applied to the implicit this parameter. Friend functions will have no implicit this parameter as they aren't member functions.

[Rehan FASTian]

Thanks a lot Cire and Firedraco. I love you all!