118讲 友元-全局函数做友元、友元类、成员函数做友元

全局函数做友元

• 友元打穿了访问权限，让特殊的函数或者成员能访问到私有属性。

//goodGay 全局函数是 Building的好基友，能访问，。让我访问。（打穿私有成员权限）
friend void goodGay(Building* building);

#include<iostream>
#include"string"
using namespace std;
class Building
{
    //goodGay 全局函数是 Building的好基友，能访问，。让我访问。（打穿私有成员权限）
    friend void goodGay(Building* building);
public:
    Building() {
        m_SettingRoom = "客厅";
        m_BedRoom = "卧室";
    }
public:
    string m_SettingRoom; //定义客厅
private:
    string m_BedRoom;//定义卧室
};

void goodGay(Building *building) {
    cout << "好基友的全局函数正在访问:   " << building->m_SettingRoom << endl;
    cout << "好基友的全局函数正在访问:   " << building->m_BedRoom << endl;
}

void test() {
    Building building;
    goodGay(&building);
}

void main() {
    test();
}

类做友元（友元类）

作用：一个类访问另外一个类的私有成员。

tip：在写类的时候，可以先声明，让编译器知道你会有这个类，不让编译器报错。

#include<iostream>
#include"string"

using namespace std;

//类做成员
class Building;

class GoodGay {
public:
    GoodGay();
public:
    void visit();
    Building * buildingl;
};

class Building {
    friend void GoodGay::visit();
public:
    Building();  //可以在类外写成员函数。
public:
    string m_SettingRoom;
private:
    string m_BedRoom;
};

void GoodGay::visit() {
    cout << "好基友正在访问:  " << buildingl->m_SettingRoom << endl;
    cout << "好基友正在访问:  " << buildingl->m_BedRoom << endl;   //可以访问私有属性
}
GoodGay::GoodGay() {
    buildingl = new Building;
}
Building::Building() {  //在类外写Building 类中的成员函数。。。也有这个方法。
        m_SettingRoom = "客厅";
        m_BedRoom = "卧室";
}
void test() {
    GoodGay goodgay;
    goodgay.visit();
}

void main() {
    test();
}

成员函数做友元

#include<iostream>
#include"string"
using namespace std;

class Building;
class GoodGay {
public:
    GoodGay();
    Building * building;
    void visit();//让visit() 函数可以访问Building 的成员
    void visit2();
};
class Building {
    friend void GoodGay::visit2();
public:
    Building();
public:
    string m_SettingRoom;
private :
    string m_BeddingRoom;
};

GoodGay::GoodGay() {
    building = new Building;
}
Building::Building() {
    m_SettingRoom = "客厅";
    m_BeddingRoom = "卧室";

}
void GoodGay::visit() {
    cout << "visit()正在访问Building的公有属性：" << building->m_SettingRoom << endl;
    //cout << "visit()正在访问Building的私有属性:" << building->m_BeddingRoom << endl;//报错，不可访问
}

void GoodGay::visit2() {
    cout << "visit2()友元成员正在访问Building的私有属性：" << building->m_BeddingRoom << endl;
}
void test() {
    GoodGay goodGay;
    goodGay.visit();
    goodGay.visit2();
}

void main() {
    test();
}