类的声明和实现分开遇到的坑

今天下午发现个坑，原理不太清楚，还是要记录一下
这份代码只是挪了一下位置，就编译不通过了
以下是编译不通过的

#include
#include
using namespace std;

class Building;
class goodGay
{
public:

	goodGay();
	void visit();

private:
	Building* building;
	Building m_b;
};

class Building
{
	//告诉编译器 goodGay类是Building类的好朋友，可以访问到Building类中私有内容
	friend class goodGay;

public:
	Building();

public:
	string m_SittingRoom; //客厅
private:
	string m_BedRoom;//卧室
};

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

goodGay::goodGay() :m_b()
{
	building = new Building;

}

void goodGay::visit()
{
	cout << "好基友正在访问" << building->m_SittingRoom << endl;
	cout << "好基友正在访问" << building->m_BedRoom << endl;
}

void test01()
{
	goodGay gg;
	gg.visit();

}

int main() {

	test01();

	system("pause");
	return 0;
}

一下是编译可以通过的代码

#include
#include
using namespace std;

class Building
{
	//告诉编译器 goodGay类是Building类的好朋友，可以访问到Building类中私有内容
	friend class goodGay;

public:
	Building();

public:
	string m_SittingRoom; //客厅
private:
	string m_BedRoom;//卧室
};
class goodGay
{
public:

	goodGay();
	void visit();

private:
	Building* building;
	Building m_b;
};

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

goodGay::goodGay() :m_b()
{
	building = new Building;

}

void goodGay::visit()
{
	cout << "好基友正在访问" << building->m_SittingRoom << endl;
	cout << "好基友正在访问" << building->m_BedRoom << endl;
}

void test01()
{
	goodGay gg;
	gg.visit();

}

int main() {

	test01();

	system("pause");
	return 0;
}

看报错说m_b调用的是不完整的类，估计是在给m_b初始化时类Building还没有实现