i tried to build up my own linked list class and got a problem with the `= operator overloading.` as far as i know, we should use const parameter when overloading the assignment operator, say using `linked_list<T>& operator=( const linked_list<T>& a)`. however, the complier gave me errors unless i put `linked_list<T>&` a instead. the complier would stop at 
`if(this->head==a.front())`, giving me Error 

```
11  error C2662: 'linked_list<T>::front' : cannot convert 'this' pointer from 'const linked_list<T>' to 'linked_list<T> &'

```

below are the details.

```
#ifndef _LINKED_LIST_H_
#define _LINKED_LIST_H_
template <class T>
struct node
{
    T data;
    node<T>* next;
};
template <class T>
class linked_list
{
   private:
    node<T>* head;
   public:
    linked_list<T>();
    linked_list<T>(const linked_list<T>& a);
    ~linked_list<T>();
    linked_list<T>& operator=(const linked_list<T>& a);
    bool isEmpty();
    int size() const;
    void insert_front(T a);
    void insert_end(T a);
    void erase_end();
    void erase_front();
    void print() const;
    void erase(node<T>* a);
    node<T>*& front()
    {
        node<T>* ptr = new node<T>;
        ptr = head;
        return ptr;
    }
    void setFront(node<T>* a);
};
#endif
template <class T>
linked_list<T>& linked_list<T>::operator=(const linked_list<T>& a)
{
    if (this->head == a.front())  // the error mentioned happened here. however,
                                  // if no const in the parameter, it would be
                                  // no error
    {
        return *this;
    }
    while (head != nullptr) erase_front();
    node<T>* copy;
    copy = a.front();
    while (copy->next != nullptr)
    {
        insert_end(copy->data);
        copy = copy->next;
    }
    return *this;
}

```

anybody can help? thanks.