运行以下C++代码，屏幕将输出derived class。
#include <iostream>
using namespace std;
class base {
public:
virtual void show() {
cout << "base class" << endl;
}
};
class derived : public base {
public:
void show() override {
cout << "derived class" << endl;
}
};
int main() {
base* b;
derived d;
b = &d;
b->show();
return 0;
}