C++ copy constructor error: Assignment of member 'Fraction::numerator' in read-only object

Question

My Header file:

#include <iostream>
using namespace std;

class Fraction
{
private:
    double numerator;
    double denominator;
public:
    Fraction();
    ~Fraction();
    Fraction(const Fraction& c);
    Fraction(double,double);
    //setter
    void setNumerator(double newnumerator);
    void setDenominator(double newdenominator);
    //getter
    double getNumerator();
    double getDenominator();

    //friend overlaoding operators

    friend ostream& operator<<(ostream& os, Fraction f);

};

My CPP file:

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

using namespace std;

Fraction::Fraction()
{
    cout<<"Empty constructor called"<<endl;
}

Fraction::~Fraction()
{
    cout<<"Deconstructor called"<<endl;
}

Fraction::Fraction(const Fraction& c)
{
    c.numerator = numerator;
    c.denominator = denominator;
}

Fraction::Fraction(double newnumerator, double newdenominator)
{
    numerator = newnumerator;
    denominator = newdenominator;
}

void Fraction::setNumerator(double newnumerator)
{
    numerator = newnumerator;
}

void Fraction::setDenominator(double newdenominator)
{
    denominator = newdenominator;
}

double Fraction::getNumerator()
{
    return numerator;
}

double Fraction::getDenominator()
{
    return denominator;
}

ostream& operator<<(ostream& os, Fraction f)
{
     os<<f.numerator<<"/"<<f.denominator<<endl;
     return os;
}

My testcpp.file:

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

using namespace std;

int main()
{
    Fraction f1;

    cout<<f1;

    return 0;
}

When I run this code I get bellow error message:

Fraction.cpp: In copy constructor ‘Fraction::Fraction(const Fraction&)’: Fraction.cpp:20:17: error: assignment of member ‘Fraction::numerator’ in read-only object c.numerator = numerator;

Fraction.cpp:21:19: error: assignment of member ‘Fraction::denominator’ in read-only object c.denominator = denominator;

PS: Once I left out the copy copy constructor and tried if the rest really works, but I get the same error message for the overloading operator(cin >>).

Thank you very much.

Your`>>` operator should not take a const parameter, as it changes it (which is what the compiler is telling you). — Karsten Koop, Jul 03 '16 at 08:37
I rolled this back, because your edits had completely changed the question. — juanchopanza, Jul 03 '16 at 11:18

score 2 · Answer 1 · answered Jul 03 '16 at 07:52

2

Your copy constructor is backwards. You're supposed to copy FROM the parameterized instance to the this instance.

Fraction::Fraction(const Fraction& c)
{
    numerator = c.numerator;
    denominator = c.denominator;
}

answered Jul 03 '16 at 07:52

xaxxon

19,189
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1

It'd be better to use the ctor initializer list (or even `= default` in this case) – M.M Jul 03 '16 at 07:55
And even better to let the compiler do this for you. – juanchopanza Jul 03 '16 at 07:56
I edited the question gents, could you check on the this one – mathquestion Jul 03 '16 at 08:24
@mathquestion you have a similar problem in another function... pay attention to the error messages, they tell you exactly where the problem is – M.M Jul 03 '16 at 09:03

score 0 · Accepted Answer · edited Jul 03 '16 at 11:20

0

get rid of the const and put a ref. &symbol next to Fraction. why? to make changes on the object ?

friend istream& operator>>(istream &is, Fraction &f);

istream& operator>>(istream &is, Fraction &)
{

    cout<<"Enter numerator"<<endl;
    is>>f.numerator;
    cout<<"Enter denominator"<<endl;
    is>>f.denominator;

    return is;
}

edited Jul 03 '16 at 11:20

BattleTested_закалённый в бою

4,037
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answered Jul 03 '16 at 08:46

C++ copy constructor error: Assignment of member 'Fraction::numerator' in read-only object

2 Answers2