c++ - 有类(class)的 friend 但不能访问私有(private)成员

Question

友元函数应该可以访问一个类的私有(private)成员吧？那么我在这里做错了什么？我已经将我的 .h 文件包含在运算符<<我打算与类(class)交 friend 。

#include <iostream>

using namespace std;
class fun
{
private:
    int a;
    int b;
    int c;


public:
    fun(int a, int b);
    void my_swap();
    int a_func();
    void print();

    friend ostream& operator<<(ostream& out, const fun& fun);
};

ostream& operator<<(ostream& out, fun& fun)
{
    out << "a= " << fun.a << ", b= " << fun.b << std::endl;

    return out;
}

Answer 1

在这里...

ostream& operator<<(ostream& out, fun& fun)
{
    out << "a= " << fun.a << ", b= " << fun.b << std::endl;

    return out;
}

你需要

ostream& operator<<(ostream& out, const fun& fun)
{
    out << "a= " << fun.a << ", b= " << fun.b << std::endl;

    return out;
}

(我被这件事折磨了无数次；你的运算符重载的定义与声明不完全匹配，所以它被认为是一个不同的函数。)