#include <iostream>
using namespace std;
#define MAX_ITEMS	20

typedef const char * String;

class Iterator;

class Aggregate{
public:
 	virtual Iterator *CreateIterator()=0;
};

class ConcreteAggregate:public Aggregate{
	String items[MAX_ITEMS];
  	int noi;
public:
	friend class ConcreteIterator;
	ConcreteAggregate():noi(0){
		for(int i=0;i<MAX_ITEMS;i++) items[i] = 0;
	}
	Iterator *CreateIterator();
	void Add(String item){
		if (noi==MAX_ITEMS) throw "Too many items!";
    	items[noi++] = item; 
	}
  	int Count(){
		return noi;
	}
	String operator[](int i){
		if ((i<0)||(i>=noi)) throw "Out of bound!";
		return items[i];
	}
};

class Iterator{
public:
 	virtual String First()=0;
  	virtual String Next()=0;
  	virtual bool IsDone()=0;
  	virtual String CurrentItem()=0;
};

class ConcreteIterator: public Iterator{
  ConcreteAggregate* aggregate;
  int current;
public:
 	ConcreteIterator(ConcreteAggregate* aggregate):
		aggregate(aggregate),current(0){ }
	String First(){
		if (current != 0) throw "Invalid First";
    	return (*aggregate)[current++];
	}  	
	String Next(){
		return (current<aggregate->Count())?(*aggregate)[current++]:0;
	}  	
	String CurrentItem(){
    	return (*aggregate)[current];
	}
	bool IsDone(){
		return (current==aggregate->Count());
	}
};

Iterator *ConcreteAggregate::CreateIterator(){
	return new ConcreteIterator(this);
}  	

main(){
    ConcreteAggregate a;
    a.Add("Item A");
    a.Add("Item B");
    a.Add("Item C");
    a.Add("Item D");

    Iterator *i = new ConcreteIterator(&a);
    cout<<"Iterating over collection:"<<endl;
    String item = i->First();
    cout<<"isDone="<<(i->IsDone()?"True":"False")<<endl;
    while (item) {
      cout<<item<<endl;
      item = i->Next();
    }
    cout<<"isDone="<<(i->IsDone()?"True":"False")<<endl;
    delete i;
    return 0;
}

/*=================== OUTPUT ===================
Iterating over collection:
isDone=False
Item A
Item B
Item C
Item D
isDone=True
=================================================*/