操作员=无法识别

我试着用迭代器制作一个类列表：

#ifndef UTIL_H
#define UTIL_H
#include <iostream>
#include <exception>
namespace Util
{
class IndexOutOfBounds : public std::exception
{
    virtual const char* what()
    {
        return "Index out of bounds";
    }
};
template<class T>
class Iterator;
template <class T>
class ListNode
{
    template<class R>
    friend class Iterator;
    template<class R>
    friend class List;
    public:
    ListNode()
    {
        init();
    }
    ListNode(T info)
    {
        init();
        this->info=info;
    }
    static void link(ListNode<T> first, ListNode<T> second)
    {
        first.next=&second;
        second.prev=&first;
    }
    template<class R>
    friend std::ostream& operator<< (std::ostream& out, const ListNode<R>& node);
    template<class R>
    friend std::istream& operator>> (std::istream& in, const ListNode<R>& node);
    private:
    T info;
    ListNode<T>* next;
    ListNode<T>* prev;
    void init()
    {
        next=prev=this;
    }
};
template<class T>
std::ostream& operator<< (std::ostream& out , const ListNode<T>& node)
{
    out << node.info;
    return out;
}
template<class T>
std::istream& operator>> (std::istream& in , const ListNode<T>& node)
{
    in >> node.info;
    return in;
}
template <class T>
class List
{
    friend class ListNode<T>;
    template <class R>
    friend class Iterator;
    private:
    unsigned int length;
    ListNode<T>* node;
    public:
    List()
    {
        node=new ListNode<T>();
        length=0;
    }
    unsigned int size() const
    {
        return length;
    }
    void insert(int i,T info) throw()
    {
        ListNode<T>* ptr=node,*next_ptr;
        try
        {
            if(i>(int)length || i<0)
                throw IndexOutOfBounds();
            for(int j=0;j<i;j++)
                ptr=ptr->next;
            next_ptr=ptr->next;
            ptr->next=new ListNode<T>(info);
            ptr->next->prev=ptr;
            ListNode<T>::link(*(ptr->next),*next_ptr);
            length++;
        }
        catch(IndexOutOfBounds& e)
        {
            throw e;
        }
    }
    void push_back(T info) throw()
    {
        try
        {
            insert(length,info);
        }
        catch(IndexOutOfBounds& e)
        {
            throw e;
        }
    }
    void push_front(T info) throw()
    {
        try
        {
            insert(0,info);
        }
        catch(IndexOutOfBounds& e)
        {
            throw e;
        }
    }
    void remove(int i) throw()
    {
        ListNode<T>* ptr=node,*next_ptr;
        try
        {
            if(i>=length || i<0)
                throw IndexOutOfBounds();
            for(int j=0;j<i;j++)
                ptr=ptr->next;
            next_ptr=ptr->next->next;
            delete ptr->next;
            ListNode<T>::link(*ptr,*next_ptr);
            length--;
        }
        catch(IndexOutOfBounds& e)
        {
            throw e;
        }
    }
    void pop_back() throw()
    {
        try
        {
            remove(length-1);
        }
        catch(IndexOutOfBounds& e)
        {
            throw e;
        }
    }
    void pop_front() throw()
    {
        try
        {
            remove(0);
        }
        catch(IndexOutOfBounds& e)
        {
            throw e;
        }
    }
    Iterator<T> begin()
    {
        Iterator<T> result;
        result.ptr=node->next;
        return result;
    }
    Iterator<T> last()
    {
        Iterator<T> result;
        result.ptr=node->prev;
        return result;
    }
    Iterator<T> end()
    {
        Iterator<T> result;
        result.ptr=node;
        return result;
    }
    template <class R>
    friend std::ostream& operator<< (std::ostream& out , const List<R>& l);
    template <class R>
    friend std::istream& operator>> (std::istream& in , const List<R>& l);
    typedef Iterator<T> iterator;
};
template<class T>
std::ostream& operator<< (std::ostream& out , const List<T>& l)
{
    Iterator<T> i=l.begin();
    return out;
}
template<class T>
std::istream& operator>> (std::istream& in , const List<T>& l)
{
    for(Iterator<T> i=l.begin();i!=l.end();i++)
        in >> *i;
    return in;
}
template <class T>
class Iterator
{
    friend class List<T>;
    friend class ListNode<T>;
    private:
    ListNode<T>* ptr;
    public:
    Iterator()
    {
        ptr=NULL;
    }
    Iterator(const Iterator<T>& i)
    {
        ptr=i.ptr;
    }
    Iterator<T>& operator= (Iterator<T> i)
    {
        ptr=i.ptr;
        return *this;
    }
    Iterator<T>& operator++ ()
    {
        ptr=ptr->next;
        return *this;
    }
    Iterator<T> operator++ (int)
    {
        Iterator<T> i=*this;
        ++*this;
        return i;
    }
    Iterator<T>& operator-- ()
    {
        ptr=ptr->prev;
        return *this;
    }
    Iterator<T> operator-- (int)
    {
        Iterator<T> i=*this;
        --*this;
        return i;
    }
    T& operator* ()
    {
        return ptr->info;
    }
    template<class R>
    friend bool operator!= (Iterator<R>& i, Iterator<R>& j);
    template<class R>
    friend bool operator== (Iterator<R>& i, Iterator<R>& j);
};
template <class T>
bool operator!= (Iterator<T>& i, Iterator<T>& j)
{
    return i.ptr!=j.ptr;
}
template <class T>
bool operator== (Iterator<T>& i, Iterator<T>& j)
{
    return i.ptr==j.ptr;
}
}
#endif

但我得到了这些错误：

/home/ramy/Documents/C++/Prova/Util.h||In function ‘std::ostream&     Util::operator<<(std::ostream&, const Util::List<T>&) [with T = int]’:|
/home/ramy/Documents/C++/Prova/main.cpp|11|instantiated from here|
/home/ramy/Documents/C++/Prova/Util.h|218|error: passing ‘const Util::List<int>’ as   ‘this’ argument of ‘Util::Iterator<T> Util::List<T>::begin() [with T = int]’ discards   qualifiers|
||=== Build finished: 1 errors, 0 warnings ===|

有了这个主：

#include <iostream>
#include "Util.h"
using namespace Util;
int main(int argc, char** argv)
{
List<int> l;
for(int i=0;i<10;i++)
    l.push_back(i);
std::cout << l;
return 0;
}

我不知道为什么我会得到这个错误，似乎运算符=被认为是不对的。这听起来很荒谬，但当然会有一个解释。