请问cpp中创建访问private的成员函数为什么会报错？

Question

请问cpp中创建访问private的成员函数为什么会报错？

# include<iostream>
# include<string>

using namespace std;


class GoodGay
{
private:
    /* data */
public:
    GoodGay();
    void visit();
    void visit2();
    Building *building;
    ~GoodGay(){
        delete building;
    }
};

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



//类外写构造函数(类内要声明)
Building::Building(){
    m_SittingRoom = "客厅";
    m_BedRoom = "卧室";
}
GoodGay::GoodGay(){
    //创建建筑物对象
    building = new Building;
}

void GoodGay::visit(){
    cout << "visit函数正在访问:" << building->m_BedRoom << endl;
}
void GoodGay::visit2(){
    cout << "visit2函数正在访问:" << building->m_SittingRoom << endl;
}


int main(){
    Building bd;
    GoodGay gg;
    gg.visit();
    gg.visit2();

}

Answer 1

要改两个地方：
1、在类GoodGay的前面加上类Building的前向声明
2、将类GoodGay析构函数的定义放在类外，且位于类Building定义的后面
改后代码：

# include<iostream>
# include<string>
using namespace std;
class Building;
class GoodGay
{
private:
    /* data */
public:
    GoodGay();
    void visit();
    void visit2();
    Building *building;
    ~GoodGay();/*{
        delete building;
    }*/
};
class Building
{
    //GoodGay成员函数做友元
    friend void GoodGay::visit();
private:
    string m_BedRoom;
public:
    Building();
    string m_SittingRoom;
};
//类外写构造函数(类内要声明)
Building::Building(){
    m_SittingRoom = "客厅";
    m_BedRoom = "卧室";
}
GoodGay::GoodGay(){
    //创建建筑物对象
    building = new Building;
}
GoodGay::~GoodGay(){
        delete building;
}
void GoodGay::visit(){
    cout << "visit函数正在访问:" << building->m_BedRoom << endl;
}
void GoodGay::visit2(){
    cout << "visit2函数正在访问:" << building->m_SittingRoom << endl;
}
int main(){
    //Building bd;
    GoodGay gg;
    gg.visit();
    gg.visit2();
    return 0;
}