Implement iterator for std::uniqe_ptr Linked list - c++

Question

This is my code so far, it obviously does not work. I want this iterator to work on
both range and increment based for loops. How can I do it?
template<typename T>
class MyList {
public:
class Node {
public:
Node(const T& data): data(data) {
this->next = NULL;
}
std::unique_ptr<Node> next;
T data;
};
MyList() {
this->_size = 0;
}
int size() const;
void push_front(const T& data);
T pop_front();
T front() const;
void remove(T data);
typedef T* iterator;
typedef const Node* const_iterator;
iterator begin() {
return (&_head.get()->data);
}
iterator end() {
return (NULL);
}
private:
int _size;
std::unique_ptr<MyList<T>::Node> _head;
};

Answer 1

T* is not suitable as a linked-list iterator, as it has no way to get to the next node in the list when incremented. Also because &_head.get()->data is not valid when the list is empty.
And Node* will not work for either iterator or const_iterator, either, since it can't have a valid operator++ to iterator the list, or anoperator* to access the data. See the requirements for a ForwardIterator.
You are better off defining a separate type to act as a (const_)iterator and let it hold a Node* internally for iterating and dereferencing, eg:
template<typename T>
class MyList {
public:
class Node {
public:
Node(const T& data): data(data) {}
std::unique_ptr<Node> next;
T data;
};
template<typename D>
class my_iterator
{
public:
my_iterator(Node* node) : _current(node) {}
bool operator==(const my_iterator &rhs) const { return _current == rhs._current; }
D& operator*() { return _current->data; }
D* operator->() { return &(_current->data); }
my_iterator& operator++() { _current = _current->next.get(); return *this; }
my_iterator operator++(int) { my_iterator tmp(_current); ++(*this); return tmp; }
private:
Node* _current;
};
using iterator = my_iterator<T>;
using const_iterator = my_iterator<const T>;
...
iterator begin() {
return iterator(_head.get());
}
const_iterator cbegin() const {
return const_iterator(_head.get());
}
iterator end() {
return iterator(nullptr);
}
const_iterator cend() const {
return const_iterator(nullptr);
}
...
};