<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。

 1 class base {
 2 public:
 3     friend ostream & operator << (ostream &o, const base &b);
 4 private:
 5     virtual ostream & print(ostream & o) const {
 6         return o << "Hello, Base";
 7     }
 8 };
 9
10 class derived : public base {
11 public:
12 private:
13     virtual ostream & print(ostream & o) const override {
14         return o << "Hello, Derived";
15     }
16 };
17
18 ostream & operator << (ostream &o, const base &b)
19 {
20     return b.print(o);
21 }
22
23 int main()
24 {
25     base base_obj;
26     derived derived_obj;
27     vector<base *> v = {&base_obj, &derived_obj};
28     for(auto i: v) {
29         cout << *i << endl;
30     }
31     return 0;
32 }