无法访问朋友 ostream 中的私人成员

I tried to make friend ostream function. The compiler say i cannot access private member of the class, even though i declared it as friend. I read a similar question and it say the problem is with the namespcaes.(the question: C++ friend function can't access private members)

我的代码如下:

标题:

#include <iostream>
#include <string>
//using namespace std;

namespace biumath
{
#ifndef BIUMATH_H
#define BIUMATH_H
class Assignment
{
private:
    int **m_varArray;
    int m_rowsOfVarArray;
public:
     Assignment(); //1
     Assignment(char symbol, double value); //2
     bool containsValueFor(char symbol) const; //3
     double valueOf(char symbol) const; //4
     void add(char symbol, double val); //5
     friend std::ostream& operator<< (std::ostream& out, 
     const Assignment& assignment);
};
}
#endif

cpp:

#include <iostream>
#include "biumath.h"
using namespace biumath;
using std::cout;
using std::endl;

ostream& operator<< (ostream& out, 
     const Assignment& assignment){
        out<<assignment.m_rowsOfVarArray<<std::endl;
        //return the stream. cout print the stream result.
        return out;
}

推荐答案

或者更好的是，通过遵循公共实用程序方法来避免所有不必要的友谊(当您的 Assignment 是多态基类时，这也有好处):

namespace biumath
{
    class Assignment
    {
    private:
        int **m_varArray;
        int m_rowsOfVarArray;
    public:
        Assignment(); //1
        Assignment(char symbol, double value); //2
        bool containsValueFor(char symbol) const; //3
        double valueOf(char symbol) const; //4
        void add(char symbol, double val); //5
        void write(std::ostream& os) const; // <=== public helper
    };

    // free function overload which defers to helper.
    // NOTE: it's in the biumath namespace so ADL will find it

    inline std::ostream& operator<< (std::ostream& out,
                                       const Assignment& assignment){
        assignment.write(out);
        return out;
    }
}

namespace biumath {

    void Assignment::write(std::ostream& os) const {
        os << m_rowsOfVarArray << std::endl;
    }
}