<C++>友元与虚函数的组合

为类重载<<与>>这两个运算符时，重载函数必须为该类的友元函数。

当友元不能被继承，故不能当作虚函数，无法使用多态。

可以用以下结构实现友元与虚函数的组合。

 class base {
 public:
     friend ostream & operator << (ostream &o, const base &b);
 private:
     virtual ostream & print(ostream & o) const {
         return o << "Hello, Base";
     }
 };

 class derived : public base {
 public:
 private:
     virtual ostream & print(ostream & o) const override {
         return o << "Hello, Derived";
     }
 };

 ostream & operator << (ostream &o, const base &b)
 {
     return b.print(o);
 }

 int main()
 {
     base base_obj;
     derived derived_obj;
     vector<base *> v = {&base_obj, &derived_obj};
     for(auto i: v) {
         cout << *i << endl;
     }
     return ;
 }