友元

友元的目的：让一个函数或者类访问一个类中的私有成员

关键字： friend

1. 全局函数做友元

class Building
{
    //goodFriend函数可以访问私有成员
    friend void goodFriend(Building &build); //全局函数做友元
public:
    string m_SittingRoom;   //客厅
    Building();
private:
    string m_BedRoom;   //卧室

};

Building::Building()
{
    m_SittingRoom = "客厅";
    m_BedRoom = "卧室";
}

//全局函数
void goodFriend(Building &build)
{
    cout << "好朋友正在访问: " << build.m_SittingRoom << endl;
    cout << "好朋友正在访问: " << build.m_BedRoom << endl; 
}

2. 类做友元

class Building
{
    //GoodFriend类做友元
    friend class GoodFriend;
public:
    string m_SittingRoom;   //客厅
    Building();
private:
    string m_BedRoom;   //卧室

};
Building::Building()
{
    m_SittingRoom = "客厅";
    m_BedRoom = "卧室";
}
////////////////////////////////////////////////
class GoodFriend
{
public:
    GoodFriend();
    Building * building;
    void visit(); //访问building中的属性

private:

};
GoodFriend::GoodFriend()
{
    //创建建筑物对象
    building = new Building;
}
////////////////////////////////////////////////
void GoodFriend::visit()
{
    cout << "好朋友类正在访问: " << this->building->m_SittingRoom << endl;
    cout << "好朋友类正在访问: " << this->building->m_BedRoom << endl;
}

3. 成员函数做友元

class Building;

class GoodFriend
{
public:
    GoodFriend();
    void visit(); //访问building中的属性
    Building * building;
    
};

class Building
{
    //GoodFriend::visit()成员函数做友元
    friend void GoodFriend::visit();
public:
    string m_SittingRoom;   //客厅
    Building();
private:
    string m_BedRoom;   //卧室

};

GoodFriend::GoodFriend()//注意构造函数的位置，用到了Building类，再Building之后再实现
{
    //创建建筑物对象
    building = new Building;  
}
Building::Building()
{
    m_SittingRoom = "客厅";
    m_BedRoom = "卧室";
}
void GoodFriend::visit()
{
    cout << "好朋友类正在访问: " << this->building->m_SittingRoom << endl;
    cout << "好朋友类正在访问: " << this->building->m_BedRoom << endl;
}
void test()
{
    GoodFriend f1;
    f1.visit();
}