[C++知识库]C++友元类

友元类有以下两个特点：

（1）如果B声明成A的友元类，那么B可以访问A的protected、private成员。但是A不能访问B的protected、private成员。

（2）如果C继承B，C不能访问A的protected、private数据成员和函数，只能访问A的public成员。但是可以通过B的公共接口获取到A的protected、private信息。

现在按照以下代码深入分析：

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <assert.h>

class Father;
class Mother;


class Father
{
public:
    Father()
    {
        m_nAge = 1;
        m_nWeChat = 1;
        m_nSalary = 1;
    }

    ~Father() 
    {
        m_nAge = 0;
        m_nWeChat = 0;
        m_nSalary = 0;
    }

    friend class Mother;  //Mother是Father的友元类

public:
    void PrintfAge()
    {
        printf("Father's age is 30.\n");
    }


    //void FatherGetMotherInfo(Mother &tMother)
    //{
    //    printf("Father get mother info\n");
    //    //只能访问Mother的public成员
    //    printf("Mother public    m_nSex    :   [%d]\n", tMother.m_nSex);
    //    printf("------------------\n\n");
    //}

private:
    void PrintfSalay()
    {
        printf("Father's salary is secret.\n");
    }

public:
    int m_nAge; //年龄

protected:
    int m_nWeChat; //微信

private:
    int m_nSalary; //工资
};

class Mother
{
public:
    Mother()
    {
        m_nSex = 2;
        m_nQQ = 2;
        m_nPayCard = 2;
    }

    ~Mother()
    {
        m_nSex = 0;
        m_nQQ = 0;
        m_nPayCard = 0;
    }

public:
    void MotherGetFatherInfo(Father &tFather)
    {
        printf("Mother get father info\n");
        printf("------------------\n");
        //Mother是Father的友元类，可以访问Father的public、protected、private成员和函数

        printf("Father public    m_nAge    :   [%d]\n", tFather.m_nAge);
        printf("Father protected m_nWeChat :   [%d]\n", tFather.m_nWeChat);
        printf("Father private   m_nSalary :   [%d]\n", tFather.m_nSalary);

        tFather.PrintfAge();
        tFather.PrintfSalay();

        printf("------------------\n\n");
    }

public:
    int m_nSex; //性别

protected:
    int m_nQQ; //QQ

private:
    int m_nPayCard; //工资卡
};


class Son : public Mother
{
public:
    Son()
    {
        m_nGirlFriend = 3;
    }

    ~Son()
    {
        m_nGirlFriend = 0;
    }

public:
    void SunGetFatherInfo(Father &tFather)
    {
        printf("Son get father info\n");
        printf("------------------\n");
        
        printf("Father public    m_nAge    :   [%d]\n", tFather.m_nAge);

        //友元关系不能被继承，所以没法访问Father的m_nWeChat、m_nSalary、PrintfSalay等protected和private成员、函数
        //printf("Father protected m_nWeChat :   [%d]\n", tFather.m_nWeChat);  
        //printf("Father private   m_nSalary :   [%d]\n", tFather.m_nSalary);

        tFather.PrintfAge();
        //tFather.PrintfSalay();

        printf("------------------\n\n");
    }


    void SunGetFatherInfoByMother(Father& tFather)
    {
        printf("Son get father info by mother\n");
        MotherGetFatherInfo(tFather);//虽然自己获取不到Father私有信息，可以通过Mother获取Father私有信息
    }

private:
    int m_nGirlFriend;

};


int main()
{
    Father tFather;
    Mother tMother;
    Son tSon;
    
    //tFather.FatherGetMotherInfo(tMother);

    tMother.MotherGetFatherInfo(tFather);

    tSon.SunGetFatherInfo(tFather);
    tSon.SunGetFatherInfoByMother(tFather);
    return 0;
}

上述代码中，Mother是Father的友元函数，那么Father在Mother这没有任何隐私可言，即Mother可以访问Father的public年龄m_nAge、protected微信m_nWeChat、private工资m_nSalary，而Father只能访问Mother的public性别m_nSex。Son继承Mother，注意的是不能将友元关系继承下来，试想本身友元已经破坏了类的封装特性了，假如友元关系能继承下来，类与类的关系就非常乱了，难以维护这么错综复杂的关系，所以Son只能访问Father的public年龄m_nAge。但是Son可以通过Mother的public函数MotherGetFatherInfo获取到Father的私人信息。