poly.cpp

#include <cstdio>
#include <iostream>

using namespace std;

class A {
    public:
        virtual int a() {
            return 5;
        }
};

class B: public A {
    public:
        virtual int b() {
            return 17;
        }
};

void mymethod(A* a) {
    cout << a->a() << endl;
}

int main() {
    B* b = new B();

    mymethod(b);

    return 0;
}