const成员函数中对此的非const指针

Question

I'm trying to implement a last() function for a linked list, which, when called from any node in the list, will return the final node in the list:

template <typename T>
LinkedList::LinkedListElement<T>* LinkedList::LinkedListElement<T>::last () const {
    LinkedList::LinkedListElement<T>* p = this;

    while(p->next) {
        p = p->next;
    }

    return p;
}

Since only returning the last node shouldn't change the list at all, I thought it made sense to make it a const function, although once the end user has that node, he can certainly modify the list by calling other functions, such as insert.

The compiler complains on line 3, that I'm trying to convert a const pointer to a non-const one. Apparently, making my function const caused this to become a constant pointer. So I've tried making pa pointer to const data, but then the line return p; gives the same error, that I'm trying to convert from const to non-const. Doing a const_cast when I assign p does the trick, but since casting should be avoided, I was wondering if there's a better way to do this.

Answer 1

Just consider this:

Someone calls last() on a const node that is already the last node. It returns itself, as a pointer to non- const node, which then can be used to modify it. This violates const correctness.

Either make the method non- const and return the pointer to non- const :

LinkedList::LinkedListElement<T>*
    LinkedList::LinkedListElement<T>::last ();

or make the method const and return a pointer to const :

const LinkedList::LinkedListElement<T>*
    LinkedList::LinkedListElement<T>::last () const;

Answer 2

Sorry for misunderstanding you. Since the last() function may return this pointer and you want to modify it,it's better to define this function as non-const;

Answer 3

And what about make "const END element" maybe like stl? Element is END if (next==NULL) and we will not use END for store value.

[LIST]->(END)                                    // empty list
[LIST]->(value1)->(value2)->(value3)->...->(END) // list with values

After this change, the END will be const all the time.

_{And the iteration while (next!=NULL) will be less buggy.}