对类中函数添加友元属性时报错

在学习c++过程中发现对类中函数添加友元属性而让函数调用其私有属性时，一直报错。

原代码如下：

class Goodgay
{
public:Goodgay();
public:  
	  Building* building;
	  void visit();
	  void visit2();
};

class Building
{
	friend void Goodgay::visit();
public:string sittingroom;
	  Building();
private:string bedroom;

};
Building::Building()
{
	this->bedroom = "卧室";
	this->sittingroom = "客厅";
}


Goodgay::Goodgay()
{
	building = new Building;
	
	
}
void Goodgay::visit()
{

		cout << "你的好基友正在访问" << building->sittingroom << endl;
		cout << "你的好基友正在访问" << building->bedroom<< endl;
	
}
void Goodgay::visit2()
{

	cout << "你的好基友正在访问" << building->sittingroom << endl;
	//cout << "你的好基友正在访问" << building->bedroom << endl;

}
void test()
{
	Goodgay gay;
	gay.visit();
	gay.visit();
}
int main()
{
	test();
	system("pause");
	return 0;
}

 

报错也千奇百怪；

看到很多人说需要将Building放到Goodgay前面；尝试后也报错

 后来发现只需要将类声明，就可以解决该问题既能解决类的定义问题，也能解决友元属性定义问题，但类building还是在类Goodgay后面才行；原代码如下

class Goodgay;
class Building;
class Goodgay
{
public:Goodgay();
public:  
	  Building* building;
	  void visit();
	  void visit2();
};

class Building
{
	friend void Goodgay::visit();
public:string sittingroom;
	  Building();
private:string bedroom;

};
Building::Building()
{
	this->bedroom = "卧室";
	this->sittingroom = "客厅";
}


Goodgay::Goodgay()
{
	building = new Building;
	
	
}
void Goodgay::visit()
{

		cout << "你的好基友正在访问" << building->sittingroom << endl;
		cout << "你的好基友正在访问" << building->bedroom<< endl;
	
}
void Goodgay::visit2()
{

	cout << "你的好基友正在访问" << building->sittingroom << endl;
	//cout << "你的好基友正在访问" << building->bedroom << endl;

}
void test()
{
	Goodgay gay;
	gay.visit();
	gay.visit();
}
int main()
{
	test();
	system("pause");
	return 0;
}