<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 0;
}