C++ 11 为什么要有友元函数

#include 
using namespace std;
class ABC
{
private:
    int a;
    int b;

public:
    ABC(int a,int b):a(a),b(b){}
    const ABC& operator << (ostream& out)
    {//方式三:成员函数重载运算符,一般返回自身引用,实现函数功能的一致性
        out << a << " " << b << endl;
        return *this;
    }
    
    friend ostream& operator << (ostream& out,const ABC& c);
    friend class Out;
};

ostream& operator << (ostream& out,const ABC& c)
{//方式二:重载运算符,友元函数
    out << c.a << " " << c.b << endl;
    return out;
}

class Out : public ostream
{//方式一:继承输出流,重载运算符,申请友元类
public:
    Out& operator << (const ABC& c){
        cout << c.a << " " << c.b << endl;
        return *this;
    }
};

int main()
{
    ABC a[2] = {{1,2},{3,4}};
    Out out;
    out << a[0] << a[1];//缺点,外形怪异
    cout << a[0] << a[1];//形式一致,直观
    a[0] << cout;//只能调用一次,想要接连输出,又要用到方式二
    return 0;
}