observer pattern in C++

```
#include <iostream>
#include <vector>

class Subject;

class Observer
{
public:
    virtual ~Observer() {}

    virtual int getState() = 0;
    virtual void update( Subject *subject ) = 0;
};

class ConcreteObserver : public Observer
{
public:
    ConcreteObserver( const int state ) :
        observer_state( state ) {}

    ~ConcreteObserver() {}

    int getState()
    {
        return observer_state;
    }

    void update( Subject *subject );

private:
    int observer_state;
};

class Subject
{
public:
    virtual ~Subject() {}

    void attach( Observer *observer )
    {
        observers.push_back(observer);
    }

    void detach( const int index )
    {
        observers.erase( observers.begin() + index );
    }

    void notify()
    {
        for ( unsigned int i = 0; i < observers.size(); i++ )
        {
            observers.at( i )->update( this );
        }
    }

    virtual int getState() = 0;
    virtual void setState( const int s ) = 0;

private:
    std::vector<Observer*> observers;
};

class ConcreteSubject : public Subject
{
public:
    ~ConcreteSubject() {}

    int getState()
    {
        return subject_state;
    }

    void setState( const int s )
    {
        subject_state = s;
    }

private:
    int subject_state;
};

void ConcreteObserver::update( Subject *subject )
{
    observer_state = subject->getState();
    std::cout << "Observer state updated." << std::endl;
}


int main()
{
    ConcreteObserver observer1( 1 );
    ConcreteObserver observer2( 2 );

    std::cout << "Observer 1 state: " << observer1.getState() << std::endl;
    std::cout << "Observer 2 state: " << observer2.getState() << std::endl;

    Subject *subject = new ConcreteSubject();
    subject->attach( &observer1 );
    subject->attach( &observer2 );

    subject->setState( 10 );
    subject->notify();

    std::cout << "Observer 1 state: " << observer1.getState() << std::endl;
    std::cout << "Observer 2 state: " << observer2.getState() << std::endl;

    delete subject;
    return 0;
}

/*
Observer 1 state: 1
Observer 2 state: 2
Observer state updated.
Observer state updated.
Observer 1 state: 10
Observer 2 state: 10
*/
```
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