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

Question

我的头文件：

#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);

};

我的 CPP 文件：

#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;
}

我的testcpp.file:

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

using namespace std;

int main()
{
    Fraction f1;

    cout<<f1;

    return 0;
}

当我运行此代码时，我收到以下 error 消息：

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: 一旦我省略了复制构造函数并尝试了其余部分是否真的有效，但我得到了相同的 error 消息重载运算符(cin >>).

非常感谢。

Answer 1

你的拷贝构造函数是倒退的。您应该从参数化实例复制到 this 实例。

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

Answer 2

摆脱 const 并放置一个 ref。 &symbol 在分数旁边。为什么？对对象进行更改？

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;
}

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

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

c++

copy-constructor