error: overloaded 'operator<<' must be a binary operator (has 3 parameters)

Question

I know there are plenty of questions like these, but I couldn't find a solution that worked for me.

I am trying to make simple fraction calculator than can add or subtract any number of functions and write the answer as a reduced fraction.

Example: input= 3/2 + 4/ 8 , output = 2

I am trying overload operators in order to accomplish this.

So in the program I am trying to develop, the input consists of an expression made of fractions separated by the operators + or -.

The number of fractions in the expression is arbitrary.

Each of the following 6 lines is an example of a valid input expression:

1/2 + 3/4
1/2 -5/7+3/5
355/113
3    /9-21/    -7
4/7-5/-8
-2/-3+7/5

***The problem that I am having is that in when I run my program it has an overload operating error: *error: overloaded 'operator<<' must be a binary operator (has 3 parameters)****

  /Users/Spicycurryman/Desktop/ECS40/hw1/fraction.cpp:61:22: error: overloaded 'operator<<' must be a binary operator (has 3 parameters)
  ostream& Fraction::operator<<(ostream &os, Fraction& n)
                     ^
/Users/Spicycurryman/Desktop/ECS40/hw1/fraction.cpp:80:22: error: overloaded 'operator>>' must be a binary operator (has 3 parameters)
  istream& Fraction::operator>>(istream &os, Fraction& n)

I don't understand why that is an error.

My following code is below:

CPP FILE

#include "Fraction.h"

Fraction::Fraction(int a, int b)
{

}
int Fraction::find_gcd (int n1, int n2) 
{
  int gcd, remainder;

  remainder = n1 % n2; 
  while ( remainder != 0 )
  {
    n1 = n2;
    n2 = remainder; 
    remainder = n1 % n2; 
  } 
  gcd = n2; 

  return (gcd);
}

void Fraction::reduce_fraction(int nump,  int denomp) 
{
  this->nump = nump;
  this->denomp = denomp; 
  int gcd;   
  gcd = find_gcd(nump, denomp);
  nump = nump / gcd;
  denomp = denomp / gcd;

    if ((denomp<0 && nump < 0 ))
    {
        denomp*=-1;
        nump*=-1;
    }
    else if (denomp < 0 &&  nump >0){
        denomp*=-1;

    }
    if ( denomp ==0) {
        throw invalid_argument( "Error: zero denominator" );
    }   
}



Fraction& Fraction::operator+(const Fraction& n) {
    denom = denomp * n.denom;
    numera = (nump * n.numera) + (n.denom * n.nump);
    return (*this);

}

Fraction& Fraction::operator-(const Fraction& n) {
    denom = denomp * n.denom;
    numera = (nump * n.numera) - (n.denom* n.nump);
    return (*this);
}

  ostream& Fraction::operator<<(ostream &os, Fraction& n)
{
    if (n.numera == 0)
    {
        cout << 0 << endl;
        return os;
    }
    else if (n.numera == n.denom)
    {
        cout << 1 << endl;
        return os;
    }
    else
    {
        cout << n.numera << '/' << n.denom << endl;
        return os;
    }
}

  istream& Fraction::operator>>(istream &os, Fraction& n)
{
    char slash = 0;
    return os >> n.numera >> slash >> n.denom;

}

Header File

#ifndef FRACTION_H
#define FRACTION_H
#include <iostream>
#include <stdexcept>
using namespace std;


class Fraction{

    public: 
    Fraction(int a, int b);
    int fraction(int a,int b);
    int find_gcd(int n1, int n2); 
    void reduce_fraction(int nump,  int denomp);
    Fraction& operator+(const Fraction& n);
    Fraction& operator-(const Fraction& n);
    friend ostream& operator<<(ostream &os, const  Fraction& n);
    friend istream& operator>>(istream &is, const Fraction& n);
private:
    int denom;
    int numera;
    int denomp;
    int nump;



};

#endif

MAIN CPP FILE

#include "Fraction.h"
#include <iostream>
using namespace std;

int main()
{
  Fraction x(2,3);
  Fraction y(6,-2);

  cout << x << endl;
  cout << y << endl;

  cin >> y;
  cout << y << endl;
  Fraction z = x + y;
  cout << x << " + " << y << " = " << z << endl;
}

I know that the operators are member functions and a member function takes an implicit first parameter, meaning my operators now takes three parameters it may be fixed being a non-member function; however, that would not work in this program. How exactly in my case would I fix it so the program would work?

Thank you very much!

Solution

The problem is that you declared operator>> and operator<< as non-member functions, but defined as a member function.

This should fix that problem (but open another set of problems). So instead of

  ostream& Fraction::operator<<(ostream &os, Fraction& n)
  {
     ...


  istream& Fraction::operator>>(istream &os, Fraction& n)
  {
     ...

implement as :

  ostream& operator<<(ostream &os, Fraction& n)
{
...

  istream& operator>>(istream &os, Fraction& n)
{
...

---

Also, take a note that you declared functions as :

friend ostream& operator<<(ostream &os, const  Fraction& n);
friend istream& operator>>(istream &is, const Fraction& n);

but defined as (therefore you changed the signature) :

  ostream& Fraction::operator<<(ostream &os, Fraction& n)
  istream& Fraction::operator>>(istream &os, Fraction& n)

Proper way is to declare and define as :

  ostream& Fraction::operator<<(ostream &os, const Fraction& n)
  istream& Fraction::operator>>(istream &os, Fraction& n)

---

I am adding just changes. The rest is the same as in the question:

class Fraction{
    friend ostream& operator<<(ostream &os, const  Fraction& n);
    friend istream& operator>>(istream &is, Fraction& n);
  // the rest is the same
};

ostream& operator<<(ostream &os, const Fraction& n)
{
    if (n.numera == 0)
    {
        cout << 0 << endl;
        return os;
    }
    else if (n.numera == n.denom)
    {
        cout << 1 << endl;
        return os;
    }
    else
    {
        cout << n.numera << '/' << n.denom << endl;
        return os;
    }
}

  istream&  operator>>(istream &os, Fraction& n)
{
    char slash = 0;
    return os >> n.numera >> slash >> n.denom;

}

Comments

[Frédéric Hamidi]

You could use friend non-member functions.

[chris]

Friend functions aren't member functions. Member functions would have no need to be friends because they already have private access.

[clcto]

In your operator<<( ostream& os, ... ) you should output to os not to cout.

[Lundin]

The problem is only related to the overloading of <<. The rest of the post is irrelevant. Please try to minimize the question to only include the part you are having trouble with.

[Lundin]

possible duplicate of How to properly overload the << operator for an ostream?

[anonymous]

operator+ is a problem too. It should return new Fraction. Otherwise, x+y will cause one of the operands to change in value.

[Rohit Tigga]

When I ran the program fully with the makefile because they are different

[Ross]

This depends on where you defined the operator>>, if you implement the operator in the header, it must be declared inline. In Fraction.h inline istream& operator>>(istream &os, Fraction& n) If you declare the function in Fraction.h but implement it in Fraction.cpp, you do not need to make it an inline function. This may depend on the compiler.

[heretoinfinity]

@BЈовић, what is the other set of problems you refer to?