左移和右移运算符的重载问题（里面的绝对盲点）在做了一遍，还是出错了

#include <iostream>
//实现左移和右移的重载。
using namespace std;

class A
{
public:
	friend ostream operator<<(ostream &t1,A &a0);
public:
	A(int i)
	{
		this->a=i;
	}
public:
	int a;
protected:
};

ostream operator<<(ostream &t1,A &a0)
{
	t1<<a0.a<<endl;
	return t1;
}
int main()
{
	A a1(10);
	cout<<a1;
	system("pause");
	return 0;
}

　　上面这种是错的，因为定义了类ostream的对象，其他人员是不能改变iostream的任何变量和对象的，所以这里要加一个引用才对呢

#include <iostream>
//实现左移和右移的重载。
using namespace std;

class A
{
public:
	friend ostream &operator<<(ostream &t1,A &a0);
public:
	A(int i)
	{
		this->a=i;
	}
public:
	int a;
protected:
};

ostream &operator<<(ostream &t1,A &a0)
{
	t1<<a0.a<<endl;
	return t1;
}
int main()
{
	A a1(10);
	cout<<a1;
	system("pause");
	return 0;
}

　　

#include <iostream>

using namespace std;

class sten_fri
{
public:
	sten_fri(int a)
	{
		this->a = a;
	}
	friend ostream &operator <<(ostream &out, sten_fri sf);

protected:
private:
	int a;
};
ostream &operator <<(ostream &out, sten_fri sf)
{
	out << sf.a << endl;
	return out;
}

int main()
{

	sten_fri a1(5);
	cout << a1;//这里的友元函数中ostream out中间必须加引用，要不然是错误的。
	system("pause");
	return 0;
}