當基類中的函數被protected的時候，只有繼承的子類才能訪問，為了使得非繼承的類也可以使用，使用friend class (類名)來進行操作

#include <iostream>

using namespace std;

class Lover{
public:
    Lover(string theName);
    void kiss(Lover *lover);
    void ask(Lover *lover, string something);

protected:
    string name;
    friend class Others; //構造友元函數, 使得Others可以調用name屬性 
};
Lover::Lover(string theName) {
    name = theName;
}

void Lover::kiss(Lover *lover) {
    cout << name << "kiss" << lover->name << endl;
}

void Lover::ask(Lover *lover, string something){
    cout << name << "ask" << lover->name << "do" << something << endl;
}

class Boyfriend:public Lover {
public:
    Boyfriend(string theName);
};

Boyfriend::Boyfriend(string theName) : Lover(theName){ //進行有參數的繼承

}
class Girlfriend:public Lover {
public:
    Girlfriend(string theName);
};

Girlfriend::Girlfriend(string theName) : Lover(theName){}

class Others {
public:
    Others(string theName);
    void kiss(Lover *lover);
protected:
    string name;
};

Others::Others(string theName) {
    name = theName;
}
void Others::kiss(Lover *lover) {
    cout << name << "親一下" << lover->name << endl; //因為name是protected, 如果沒有友元就不能進行訪問 
}


int main() {

    Boyfriend boyfriend("A君");
    Girlfriend girlfriend("B妞");

    Others others("路人甲");

    girlfriend.kiss(&boyfriend);
    girlfriend.ask(&boyfriend, "洗衣服啦");

    cout << "
當當當:傳說的路人甲登場...
";
    others.kiss(&girlfriend);
}

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