Implement iterator of a circular list

Question

I'm trying to implement a class of Circular List with a nested class of iterator and I wrote like this:

template <class T>
class CircularList {
    struct Item {
        T data;
        Item* next;
    };
    Item* head;
    int size;
public:
    CircularList() {
        head = new Item();
        head->next = head;
    }
    int sizeList() { return size; }
    void push(T data) {
        Item* i = new Item();
        i->data = data;
        i->next = head->next;
        head->next = i;
        size++;
    }
    class CircularListIterator {
        Item* p;
        CircularListIterator() {
            p = head->next;
        }
        bool hasNext() {
            if(p->next != head) {
                return true;
            }
            return false;
        }
        T next() {
            T data_temp = p->data;
            p = p->next;
            return data_temp;
        }
};
            CircularListIterator* iterator() {
                return new CircularListIterator();
            }
    }; 

int main() {
    CircularList<string>* letters = new CircularList<string>;
    letters->push("d");
    letters->push("c");
    letters->push("b");
    letters->push("a");

    Iterator<string>* it= new Iterator<string>;
    it = letters->iterator();
    while (it->hasNext()) {
        cout<< it->next() << "," << endl;
    }
    return 0;
}

But the Iterator is not working when I try to create an iterator in the main function, It said that it wasn't declared in the scope and has no member of it.

Answer 1

Assuming by "in the main class" you mean in the main function , the problem is quite straightforward: you're trying to construct a ::Iterator<string> , but there is no class in the global namespace (or anywhere else, in this code sample) called Iterator ! You could try constructing a CircularList<string>::CircularListIterator - that's at least a class that exists - but it wouldn't work because the iterator needs to be associated with a CircularList object for it to be able to access member variables like head .

The correct thing to do here is to promote the iterator function - the one that returns a CircularListIterator* - out of the CircularListIterator class and into the CircularList class. Then, in your main function, you can call letters->iterator() and it'll return a CircularListIterator* for the letters object.

Now, CircularListIterator doesn't inherit from any other iterator classes - neither the (nonexistent-in-this-code Iterator you've typed it as, nor the C++ std::iterator or any of its variants) - so you can't assign it to it or probably even compile the code that references Iterator . To make CircularListIterator a subclass of std::iterator , you'll need to extend std::iterator<Category, T> with the appropriate category. See https://www.cplusplus.com/reference/iterator/iterator/ for more information on the std::iterator class template, including an example of implementing it.