Implementing a list iterator and operators: -- ++ * - c++

Question

Following on from this question: std::list implementation & pointer arithemetic.
I want to implement a list iterator that is interchangeable with other common containers types and their respective iterators, so I want to use operators such as: --, ++, * and be able to declare iterators as normal, so: list::iterator iter = list.begin();
The --, ++ operators now work as they should, but I ran up against the problem of de-referencing the iterator, as structs can't return a value: T iterator::operator*()
template <class T>
struct element {
element<T> *prev = NULL;
element<T> *next = NULL;
T data;
};
template <typename T>
class list {
public:
list::list();
element<T>* current;
struct iterator{
element<T>* iterator::operator++(){
this = *this->next; ..whatever it works
}
element<T>* iterator::operator--()
T iterator::operator*()
}
};

Answer 1

the iterators could look something like this:
(declared inside the list template)
struct iterator;
struct const_iterator : public std::iterator<std::bidirectional_iterator_tag, const T>
{
const_iterator() = default;
T operator*() { return itm->data; }
const T* operator->() { return &(itm->data); }
const_iterator operator++() { itm = itm->next; return *this; }
const_iterator operator--() { itm = itm->prev; return *this; }
const_iterator operator++(int) { const_iterator ret=*this; itm = itm->next; return ret; }
const_iterator operator--(int) { const_iterator ret=*this; itm = itm->prev; return ret; }
bool operator==(const_iterator oth) const { return itm==oth.itm; }
bool operator!=(const_iterator oth) const { return itm!=oth.itm; }
private:
element<T>* itm = nullptr;
const_iterator(element<T>* i) : itm(i) {}
friend
class list;
friend
struct iterator;
};
struct iterator : public std::iterator<std::bidirectional_iterator_tag, T>
{
iterator() = default;
T& operator*() { return itm->data; }
T* operator->() { return &(itm->data); }
iterator operator++() { itm = itm->next; return *this; }
iterator operator--() { itm = itm->prev; return *this; }
iterator operator++(int) { iterator ret=*this; itm = itm->next; return ret; }
iterator operator--(int) { iterator ret=*this; itm = itm->prev; return ret; }
bool operator==(iterator oth) const { return itm==oth.itm; }
bool operator!=(iterator oth) const { return itm!=oth.itm; }
operator const_iterator() { return {itm}; }
private:
element<T>* itm = nullptr;
iterator(element<T>* i) : itm(i) {}
friend
class list;
};
typedef std::reverse_iterator<iterator> reverse_iterator;
typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
this needs
#include <iterator>
to compile