c++之友元

客厅：客人可以随意走动；卧室：只有自己能进去；

现在想要自己的好朋友可以进去，就需要用到友元技术。即友元的目的就是让一个函数或者类访问另一个类中私有成员。

关键字：friend

三种实现方法：

• 全局函数做友元
• 类做友元
• 成员函数做友元

全局函数做友元：

#include<iostream>
using namespace std;
class Building{
    friend void goodGay(Building* building);
public:
    Building() {
        room = "客厅";
        myRoom = "卧室";
    }
public:
    string room;
private:
    string myRoom;
};
//全局函数
void goodGay(Building *building) {
    cout << "好朋友正在访问" <<building->room<< endl;
    //如果不将dooGay变成Building的友元函数，这里就会报错
    cout << "好朋友正在访问" << building->myRoom << endl;
}
void test() {
    Building building;
    goodGay(&building);
}
int main() {
    test();
    system("pause");
    return 0;
}

类做友元：

说明：在类外定义成员函数时，需要先在类中声明函数。利用类名::函数名()，可以定义类的成员函数。

#include<iostream>
using namespace std;

class Building{
    friend class GoodGay;
public:
    string room;
    Building();
private:
    string myRoom;
};
//类外写成员函数
Building::Building() {
    room = "客厅";
    myRoom = "卧室";
}

class GoodGay {
public:
    Building* building;
    GoodGay();
    void visit();
};
GoodGay::GoodGay() {
    building = new Building();
}
void GoodGay::visit() {
    cout << "好基友类正在访问：" << building->room << endl;
    cout << "好基友类正在访问：" << building->myRoom << endl;
}
void test() {
    GoodGay gg;
    gg.visit();
}
int main() {
    test();
    system("pause");
    return 0;
}

成员函数做友元：

#include<iostream>
using namespace std;
class Building;
class GoodGay;

//要先定义GoodGay类。。。
//我尝试在Building之后定义，就不行，之前也先声明了这两个类
class GoodGay {
public:
    GoodGay();
    void visit();//让该函数可以访问Building的私有成员
    void visit2();
private:
    Building* building;
};

class Building{
    friend void GoodGay::visit();
public:
    string room;
    Building();
private:
    string myRoom;
};
//类外写成员函数
Building::Building() {
    room = "客厅";
    myRoom = "卧室";
}

GoodGay::GoodGay() {
    building = new Building();
}
void GoodGay::visit() {
    cout << "好基友类正在访问：" << building->room << endl;
    cout << "好基友类正在访问：" << building->myRoom << endl;
}
void GoodGay::visit2() {
    cout << "好基友类正在访问：" << building->room << endl;
    //cout << "好基友类正在访问：" << building->myRoom << endl;
}
void test() {
    GoodGay gg;
    gg.visit();
}
void test2() {
    GoodGay gg;
    gg.visit2();
}
int main() {
    test();
    system("pause");
    return 0;
}