尽管有朋友 class，但无法访问 class 的私人成员

Question

I am doing a homework assignment on overloading operators. This is my code:

#include<iostream>
#include<cmath>
using namespace std;

class ComplexNumber{
    double real;
    double imaginary;

  public:
    friend class Equation;
};

class Equation{
    ComplexNumber *broevi;
    char *znaci;
    int n;

  public:
    Equation(){
        broevi=new ComplexNumber[0];
        znaci=new char[0];
        n=0;
    }
    friend istream &operator >>(istream &in, Equation &e){
        int k=1;
        while(k){
            char *tmp; ComplexNumber *b;
            tmp=new char[e.n+1];
            for(int i=0; i<e.n; i++)
                tmp[i]=e.znaci[i];
            b=new ComplexNumber[e.n+1];
            for(int i=0; i<e.n; i++)
                b[i]=e.broevi[i];
            in>>e.broevi[e.n].real>>e.broevi[e.n].imaginary;
            in>>tmp[e.n];
            if(tmp[e.n]=='=')
                break;
            delete []e.znaci;
            e.znaci=tmp;
            delete[]e.broevi;
            e.broevi=b;
            ++e.n;
        }
        return in;
    }
    friend class ComplexNumber;
};

This is only a part of my code.

While compiling I get these two following errors:

error: 'double ComplexNumber::imaginary' is private

error: 'double ComplexNumber::real' is private

On this line:

in>>e.broevi[e.n].real>>e.broevi[e.n].imaginary;

Can you please point out where my mistake is?

Answer 1

Your operator>> is a friend of Equation , so it can access private fields of Equation . But, it is not a member of Equation , and it is not a friend of ComplexNumber , so it can't access private fields of ComplexNumber .

The simplest solution is to add a friend operator>> to ComplexNumber , and then use that in Equation' s operator>> :

class ComplexNumber {
    double real;
    double imaginary;

public:
    friend istream& operator >>(istream &in, ComplexNumber &cn) {
        in >> cn.real >> cn.imaginary;
        return in;
    }
};

class Equation {
    ...

public:
    ...

    friend istream& operator >>(istream &in, Equation &e) {
        ...
        //in >> e.broevi[e.n].real >> e.broevi[e.n].imaginary;
        in >> e.broevi[e.n];
        ...
    }
};

However, if that is not allowed for your assignment, then you can instead move your stream logic to a member method of Equation for its operator>> to call, and then that method will have access to the private fields of ComplexNumber :

class ComplexNumber {
    double real;
    double imaginary;

public:
    friend class Equation;
};

class Equation {
    ...

    void read(istream &in) {
        // your logic here...
    }

public:
    ...

    friend istream& operator >>(istream &in, Equation &e){
        e.read(in);
        return in;
    }
};