通过指向 Base 的指针删除 Derived 对象时，一定会先调用 Derived 的析构函数，再调用 Base 的析构函数。
#include <iostream>
using namespace std;
class Base {
public:
~Base() {
cout << "Base destructor" << endl;
}
};
class Derived : public Base {
public:
~Derived() {
cout << "Derived destructor" << endl;
}
};
int main() {
Base* p = new Derived();
delete p;
return 0;
}