#include <iostream>
#include <cstdlib>

using namespace std;

struct Dude {
    virtual void howdy() = 0;
};
struct Bob : public Dude {
    virtual void howdy() { cout << "Hi, Bob!" << endl; }
};
struct Moe : public Dude {
    virtual void howdy() { cout << "Hi, Moe!" << endl; }
};
void say_hi(Dude& dude) {
    dude.howdy(); // <<== Here is the tricky line
}
int main(int argc, char* argv[]) {
    Bob b;
    Moe m;
    Dude *d = rand() & 1 ? (Dude*)&b : &m;
    say_hi(*d);
}