[C++知识库]C++语法（二十一）友元的三种实现

友元的目的是让类外函数或者其他类访问到类内的私有属性。友元实现的三种方式：全局函数做友元、类做友元、成员函数做友元。

1、全局函数做友元

#include <iostream>
#include <string.h>
using namespace std;

class Home{
    // 全局函数作为友元
    friend void goodfriend(Home &home);
public:
    Home(){
        bedroom = "bed room";
        livingroom = "living room";
    }
    string livingroom;
private:
    string bedroom;
};

void goodfriend(Home &home){
    cout << home.livingroom << endl;
    cout << home.bedroom << endl;
}

void test(){
    Home a;
    goodfriend(a);
}

int main() {
    test();
    return 0;
}

2、类做友元

#include <iostream>
#include <string.h>
using namespace std;

class Home{
    // 类作为友元
    friend class Person;
public:
    Home(){
        bedroom = "bed room";
        livingroom = "living room";
    }
    string livingroom;
private:
    string bedroom;
};

class Person{
public:
    Person(){
        home = new Home;
    }
    void visit(){
        cout << home->livingroom << endl;
        // 作为友元即可访问该类的私有属性
        cout << home->bedroom << endl;
    }
private:
    Home *home;
};

void test(){
    Person p;
    p.visit();
}

int main() {
    test();
    return 0;
}

3、成员函数做友元

#include <iostream>
#include <string.h>
using namespace std;

class Home;

class Person{
public:
    // 不能在此处实现函数，因为Home类只进行声明并没有实现
    Person();
    void visit1();
    void visit2();
private:
    Home *home;
};

class Home{
    // 成员函数作为友元
    friend void Person::visit1();
public:
    Home(){
        bedroom = "bed room";
        livingroom = "living room";
    }
    string livingroom;
private:
    string bedroom;
};

// Person类中函数的实现，放在Home类实现之后
Person::Person(){
    home = new Home;
}
void Person::visit1(){
    cout << "visit1 " << home->livingroom << endl;
    cout << "visit1 " << home->bedroom << endl;
}
void Person::visit2(){
    cout << "visit2 " << home->livingroom << endl;
    // 访问私有属性bedroom，不能进行，因为visit2不是Home类的友元
//  cout << "visit2 " << home->bedroom << endl;
}

void test(){
    Person p;
    p.visit1();
    p.visit2();
}

int main() {
    test();
    return 0;
}