c++ 迭代器

#include <iostream>
 #include <cstdlib>
using namespace  std;
template<class T,int ssize=100>
class MyList
{  T data[ssize];
   int length;
   public:
      MyList (){length=0;}
      void push_back(const T&i)
      {
           if(length>=ssize-1)exit(1);
           data[length++]=i;      
      }
      bool empty()
      {
      return(length==0);
      }
       class iterator;
       friend class iterator;
       class iterator
          {
            MyList&lst;
            int index;
          public:
          iterator(MyList &l):lst(l)
             {
              index=0;
             }
              iterator(MyList &l,bool):lst(l)
             {
              index=l.length;
             }
             T operator*() const
             {
             if(index>=0)
               return lst.data[index];
             else
                exit(1);
             }
              T operator++()
             {
             if(index>lst.length)
                        exit(2);
               return lst.data[++index];           
             }
           T operator++(int)
             {
             if(index>lst.length)
                        exit(3);
               return lst.data[index++];           
             }
             T operator--()
             {
             if(index<=0)
                        exit(4);
               return lst.data[--index];           
             }
             T operator--(int)
             {
             if(index<=0)
                        exit(5);
               return lst.data[index--];           
             }
                bool operator!=(const iterator &rv)const
              {
              return index!=rv.index;
              }
               bool operator==(const iterator &rv)const
              {
              return index==rv.index;
              }
               bool operator<=(const iterator &rv)const
              {
              return index<=rv.index;
              }
               bool operator<(const iterator &rv)const
              {
              return index<rv.index;
              }
               bool operator>=(const iterator &rv)const
              {
              return index>=rv.index;
              }
               bool operator>(const iterator &rv)const
              {
              return index>rv.index;
              }
          };
   iterator begin()
   {
   return iterator(*this);
   }
   iterator end()
   {
   return iterator(*this,true);
   }
};
int main()
{
MyList<int> lst;
lst.push_back(1);
lst.push_back(2);
lst.push_back(3);
lst.push_back(4);
MyList<int>::iterator iter=lst.begin();
    cout<<"從表頭到表尾：";
    for (; iter!=lst.end();iter++)  
       cout<<*iter<<" ";
       cout<<endl; 
       MyList<int>::iterator iterl=lst.end();
    cout<<"從表尾到表頭：";
    iterl--;
    for (; iterl>=lst.begin();iterl--)  
       cout<<*iterl<<" ";
       cout<<*iterl<<endl;  
}