萌新求问，为啥这段代码会显示“使用了未定义类型”

#include<iostream>
using namespace std;
#include<string>
class building;
class bestfriend {
public:
void gotolivingroom1(building b) {
b.gotolivingroom();
}
void gotobedroom1(building b) {
b.gotobedroom();
}
};
class building {
friend class bestfriend;// 声明类友元
//friend void bestfriend::gotobedroom1(building b);//声明成员函数友元
public:
string room1 = "livingroom";
void gotolivingroom();
private:
string room2 = "bedroom";
void gotobedroom();
};
// 类外实现函数
void building::gotolivingroom() {
cout << "goto livingroom" << endl;
}
void building::gotobedroom() {
cout << "goto bedroom" << endl;
};
int main() {
building b = building();
bestfriend f = bestfriend();
f.gotolivingroom1(b);
f.gotobedroom1(b);
}