error: cannot bind ‘std::ostream in implementation file - c++

Question

First: I have tried reading and modifying my code based off no less than 7 other similar questions. At best, the other options trigger an avalanche of errors. With my current code, I'm down to a single error.
Putting it inside the class and using "friend" doesn't work, and using ostream& operator<< (ostream &out, const Fraction &rhs) makes a whole lot more errors.
The frustrating thing is that in c9.io this code works, but not on Netbeans.
In my main.cpp:
#include <iostream>
#include "fraction.h"
using namespace std;
int main() {
Fraction f(3, 4);
cout << f;
return 0;
}
In fraction.h:
#ifndef FRACTION_H
#define FRACTION_H
class Fraction{
public:
//constructor defs
//accessors
//modifiers/mutators
void setNumer(int newNum);
void setDenom(int newDenom);
void reduce();
private:
//instance variables
int numer;
int denom;
//helper functions
int gcd(int a, int b);
};
#endif /* FRACTION_H */
And in fraction.cpp:
#include "fraction.h"
#include <cmath>
#include <iostream>
using namespace std;
//code for constructors
//accessors
int Fraction::getNumer(){
return numer;
}
int Fraction::getDenom(){
return denom;
}
//modifiers/mutators
//other operator definitions
ostream& operator<< (ostream &out, Fraction &rhs){
if(rhs.getNumer() == 0){
out << 0;
} else if(rhs.getNumer() == rhs.getDenom()){
out << 1;
} else {
rhs.reduce();
out << rhs.getNumer() << "/" << rhs.getDenom();
}
}
The output is:
g++ -c -g -std=c++11 -MMD -MP -MF "build/Debug/GNU-Linux/main.o.d" -o build/Debug/GNU-Linux/main.o main.cpp
main.cpp: In function ‘int main()’:
main.cpp:6:13: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
cout << f;
^
In file included from /usr/include/c++/4.8/iostream:39:0,
from main.cpp:1:
/usr/include/c++/4.8/ostream:602:5: error: initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = Fraction]’
operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
^

Answer 1

There is no knowledge of the operator<< function that is defined in Fraction.cpp in main.cpp Hence, the line
cout << f;
is a problem.
Add the declaration of the function in the .h file.
#ifndef FRACTION_H
#define FRACTION_H
#include <iostream>
class Fraction{
public:
//constructor defs
//accessors
//modifiers/mutators
void setNumer(int newNum);
void setDenom(int newDenom);
void reduce();
private:
//instance variables
int numer;
int denom;
//helper functions
int gcd(int a, int b);
};
std::ostream& operator<< (std::ostream &out, Fraction const& rhs);
// ^^^^ Using const&, not just Fraction&
#endif /* FRACTION

Answer 2

So the answer was partly what #r-sahu said:
I had to set getNumer() and getDenom() as const member functions BUT I could only get the errors to go away by NOT using const& in the function definition.
Good:
std::ostream& operator<< (std::ostream &out, Fraction rhs);
Not good:
std::ostream& operator<< (std::ostream &out, Fraction const& rhs);
It now compiles and runs. any insight on why making the member functions const but not passing the Fraction as const makes it work? I stumbled on this solution by accident and don't understand it.