#include <iostream>
#include <vector>
#include <cmath>
#include <string>

using namespace std;

class observer;

class subject
{
  public:
    subject()
    {
    };
    ~subject()
    {
    };
    void addObserver(observer *);
    void removeObserver(observer *);
    virtual void update() = 0;

  protected:
    vector < observer * >observerList;
};

class observer
{
  public:
    observer()
    {
    };
    ~observer()
    {
    };
    void subscribe(subject *);
    void unsubscribe(subject *);
    virtual void update(vector < int >) = 0;
};

void subject::addObserver(observer * o)
{
    observerList.push_back(o);
}

void subject::removeObserver(observer * o)
{
    for (vector < observer * >::iterator it =
         observerList.begin(); it != observerList.end(); ++it)
    {
        if (*it == o)
        {
            observerList.erase(it);
            break;
        }
    }
}

void observer::subscribe(subject * s)
{
    s->addObserver(this);
}

void observer::unsubscribe(subject * s)
{
    s->removeObserver(this);
}

class value:public subject
{
  public:
    value()
    {
        update();
    }

    void update()
    {
        int t;
        cout << "Enter value: ";
        cin >> t;
        values.push_back(t);

        for (vector < observer * >::iterator it =
             observerList.begin(); it != observerList.end(); ++it)
        {
            observer *o = *it;
            o->update(values);
        }
    }

  private:
    vector < int >values;
};

class avg3:public observer
{
  public:
    avg3(subject * s)
    {
        subscribe(s);
    }

    void update(vector < int >t)
    {
        int sum = 0;
        for (int i = 0; i < 3; i++)
        {
            int buf = t.back();
            sum += buf;
            t.pop_back();
        }
        cout << "The average for the last three is " << float (sum) /
            3 << endl;
    }
};

int main()
{
    subject *s = new value();
    observer *o;

    for (int j = 2; j <= 10; j++)
    {
        if (j == 3)
            o = new avg3(s);
        s->update();
    }

    delete s;
    delete o;
    return 0;
}