虚函数你真的懂？出一道可以疑惑你的面试题吧！

#include <iostream>

struct Base {
    virtual void display(int i = 5) { std::cout << "Base::" << i << "\n"; }
};

struct Derived : public Base {
    virtual void display(int i = 9) override { std::cout << "Derived::" << i << "\n"; }
};

int main() {
    Base* a = new Derived();
    a->display();

    Base* aa = new Base();
    aa->display();

    Derived* b = new Derived();
    b->display();
}