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意图：提供一种方法顺序访问一个聚合对象中的各个元素，而又不需要暴露该对象的内部表示。

上下文：集合对象内部结构常常变化各异。对于这些集合对象，能否在不暴露其内部结构的同时，让外部Client透明地访问其中包含的元素，同时让这种“透明遍历”也为“同一种算法在多种集合对象上进行操作”提供可能？

UML

Iterator：定义访问和遍历元素的接口（.NET中定义了标准的IEnumrator接口）。ConcreteIterator：实现Iterator接口，同时在对Aggregate遍历时跟踪当前的位置。Aggregate：定义创建相应Iterator对象的接口（.NET中定义了标准的IEnumrable接口）。ConcreteAggregate：实现创建相应Iterator对象的接口，该操作返回一个适当的ConcreteIterator实例。

注意：.NET中的foreach关键字在编译时会自动创建迭代器对象，并使用该对象对集合进行遍历。.NET中的yield return关键字使得定义迭代器对象更加容易。

代码

#include <iostream>
#include <list>
#include <array>
#include <vector>
using namespace std;//Iterator迭代器抽象类
class Iterator
{
public:virtual void* First() = 0;virtual void* Next() = 0;virtual bool IsDone() = 0;virtual void* CurrentItem() = 0;
};//Aggregate聚集抽象类
class Aggregate
{
public:virtual Iterator *CreateIterator() = 0;
};//ConcreteAggregate具体聚集类　继承Aggregate
class ConcreteAggregate:public Aggregate
{
public:vector<void*> items;// Aggregate interface
public:Iterator *CreateIterator();int getCount() const;//既可以作为左值，也可以作为又值void** operator[](unsigned int index){if(items.size() <= index){items.resize(index+1);}return &items[index];}
};class ConcreteIterator:public Iterator
{
public:ConcreteAggregate *ca;int current;ConcreteIterator(ConcreteAggregate *c):ca(c){this->current = 0;}virtual void* First();virtual void* Next();virtual bool IsDone();virtual void* CurrentItem();
};Iterator *ConcreteAggregate::CreateIterator()
{return new ConcreteIterator(this);
}int ConcreteAggregate::getCount() const
{return items.size();
}void *ConcreteIterator::First()
{return *(*ca)[0];
}void *ConcreteIterator::Next()
{if(current < ca->getCount()){current++;}if(current < ca->getCount()){return *(*ca)[current];}return nullptr;
}bool ConcreteIterator::IsDone()
{return current >= ca->getCount()?true:false;
}void *ConcreteIterator::CurrentItem()
{return *(*ca)[current];
}class ConcreteIteratorDesc:public Iterator
{
public:ConcreteAggregate *ca;int current;ConcreteIteratorDesc(ConcreteAggregate *c):ca(c){this->current = c->getCount() - 1;}virtual void* First();virtual void* Next();virtual bool IsDone();virtual void* CurrentItem();
};
void *ConcreteIteratorDesc::First()
{if(ca->getCount() == 0){return nullptr;}return *(*ca)[ca->getCount() - 1];
}void *ConcreteIteratorDesc::Next()
{if(current >= 0){current--;}if(current >= 0){return *(*ca)[current];}return nullptr;
}bool ConcreteIteratorDesc::IsDone()
{return current < 0?true:false;
}
void *ConcreteIteratorDesc::CurrentItem()
{return *(*ca)[current];
}
int main()
{ConcreteAggregate ca;*(ca[0]) = (void*)5;*ca[1] = (void*)10;*ca[2] = (void*)15;*ca[3] = (void*)20;
//    cout << (int) *ca[0] << endl;
//    cout << (int) *ca[1] << endl;
//    cout << ca.getCount() << endl;Iterator *i = new ConcreteIterator(&ca);cout << "开始遍历" << endl;while(!i->IsDone()){cout << (int)i->CurrentItem() << endl;i->Next();}Iterator *i_desc = new ConcreteIteratorDesc(&ca);cout << "开始反向遍历" << endl;while(!i_desc->IsDone()){cout << (int)i_desc->CurrentItem() << endl;i_desc->Next();}cout << "--end--" << endl;return 0;
}

结果：

开始遍历
5
10
15
20
开始反向遍历
20
15
10
5
--end--