Question

so, I have been having issues in this header file overloading the stream insertion operator. I receive the error message in the title if I use the code as is. But when I put the declaration in the main file, it works fine.

Rational.h

#ifndef RATIONAL_H
#define RATIONAL_H

using namespace std;
class Rational{
private:
    int numerator;
    unsigned int denominator;
    bool isNegative;

public:
    Rational();
    Rational(int);
    Rational(int, int);

    bool operator==(const Rational&);
    Rational& operator++(int);          //Unused int
    Rational operator-(const Rational&);
    Rational operator+(const Rational&);
    Rational operator*(const Rational&);
    Rational operator/(const Rational&);
};

ostream& operator<<(ostream&, Rational&); //Erroneous code


#endif

The other two files, 1.c and Rational.c if needed:

#include "Rational.h"
#include <iostream>
#include <math.h>

using namespace std;

Rational::Rational(){
numerator   = 0;
denominator = 1;
}

Rational::Rational(int num){
numerator   = num;
denominator = 1;     
}

Rational::Rational(int num, int den){
//Determine negativity
if(num < 0 xor den < 0){    //If negative
    if(num > 0){
        num *= -1;
    }
}

numerator   = num;
denominator = abs(den);  
}

bool Rational::operator==(const Rational& rhs){
return (numerator/(double)denominator == rhs.numerator/(double)(rhs.denominator));
}

ostream& operator<<(ostream& os, Rational& input){
os << "Moo";
return os;
}
/*
private:
    int  numerator;
    unsigned int  denominator;
    bool isNegative;

public:
    Rational(int, int);

    bool operator==(const Rational&);
    Rational& operator++(int);          //Unused int
    Rational operator-(const Rational&);
    Rational operator+(const Rational&);
    Rational operator*(const Rational&);
    Rational operator/(const Rational&);
*/

1.c

#include <iostream>
#include "Rational.h"

using namespace std;

int main(){
Rational test = Rational(2);

cout << test << endl;
}

Answer 1

You need to include iostream in your Rational.h

//Rational.h

#include <iostream>

When you put the overload declaration in 1.c, iostream is included before Rational.h and hence compiler knows the type ostream and there is no error.

However, Rational.h does not include iostream so in that case compiler does not know the type ostream and hence the error.