#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

class A {
public:
    virtual void foo() = 0;
};

class B : public A {
public:
    void foo() {
        cout << "I'm B." << endl;
    }
};

class C : public A {
public:
    virtual void foo() = 0;
};

class D : public C {
public:
    void foo(){
        cout << "I'm D." << endl;
    }
};

class E : public C {
public:
    void foo() {
        cout << "I'm E." << endl;
    }
};

int main()
{
    A* b = new B();
    A* d = new D();
    A* e = new E();
    b->foo();
    d->foo();
    e->foo();
}