学习中遇到的问题：为声明的标识符

#include<iostream>
#include<string>
using namespace std;
**class Building;**
class goodgay {
public:
	goodgay() ;
	void visit();
	Building * building;
};
class Building {
public:
	Building();
	string sittingroom;
private:
	string bedroom;
};
Building::Building() {
	sittingroom = "客厅";
	bedroom = "卧室";
}
goodgay::goodgay() {
	building = new Building;
}
void goodgay::visit() {
	cout << "goodgay正在访问：" << building->sittingroom << endl;
}
void test01() {
	goodgay g;
	g.visit();
}
int main() {
	test01();
}

由于在类goodgay中用了类Building中的对象building，而class Building的定义在class goodgay之后，会报错：“building”未声明的标识符。

解决办法：在class good gay之前添加“class Building；”作为声明，或交换Building和goodgay的定义顺序