友元类和友元函数

一、友元函数

class FriendFunc
{
public:
    FriendFunc();
    ~FriendFunc();
    friend void friend_function(FriendFunc f1);
private:
    int x;
    int y;
protected:
    int z;
};
void friend_function(FriendFunc f1)
{
    //可以直接访问类的私有和保护成员
    cout<<"x:"<<f1.x<<endl<<"y"<<f1.y<<endl<<"z:"<<f1.z<<endl;
}

二、友元类
类A可以声明类B是自己的友元类，那么在类A的成员函数中可以直接访问类B的私有成员和保护成员。

class Girl;
class Boy;
class Boy
{
public:
    void disp_girl_info(Girl gl);
private:
    int weight;
    int age;
    friend Girl;//友元类
};
class Girl
{
public:
    void disp_boy_info(Boy by);
private:
    int weight;
    int age;
    friend Boy;//友元类
};
void Boy::disp_girl_info(Girl gl)
{
     cout<<"age:"<<gl.age<<endl<<"weight"<<gl.weight<<endl;
}
void Girl::disp_boy_info(Boy gl)
{
     cout<<"age:"<<gl.age<<endl<<"weight"<<gl.weight<<endl;
}

也可以只声明类A的某个成员函数是类B的友元，这样做的好处是只有类A使用friend关键词声明的函数可以直接访问类B的保护成员和私有成员；

class Girl;
class Boy;

class Boy
{
public:
    void disp_girl_info(Girl gl);
private:
    int weight;
    int age;
};

class Girl
{
public:
    void disp_boy_info(Boy by);
private:
    int weight;
    int age;
    friend void Boy::disp_girl_info(Girl gl);
};
void Boy::disp_girl_info(Girl gl)
{
     cout<<"age:"<<gl.age<<endl<<"weight"<<gl.weight<<endl;
}