Interface which declares functions for iteration - c++

Question

I have an assignment in which I need to make template classes LinkedList and Traversible. Class Traversible needs to be a interface which declares functions for indexing and iteration of some collection class. I don't exactly know how to make an interface for iterator so LinkedList can use it. I was thinking something like
template <class T, class U>
class ITraversible {
public:
virtual U begin() noexcept = 0;
virtual U end() noexcept = 0;
virtual T& operator[](int) = 0;
};
and then in LinkedList header file I would do:
template <class T>
class LinkedList : public ITraversible<T,typename LinkedList<T>::iterator> {
struct node {
T data;
node* next, *prev;
explicit node(const T&);
void connect(node*);
};
node *head, *tail;
int n;
public:
/*************************ITERATOR************************/
class iterator : public std::iterator<std::bidirectional_iterator_tag, node*> {
typename LinkedList<T>::node* itr;
explicit iterator(node*) noexcept;
friend class LinkedList;
public:
iterator& operator++();
iterator operator++(int);
iterator& operator--();
iterator operator--(int);
bool operator==(const iterator&) const noexcept;
bool operator!=(const iterator&) const noexcept;
T& operator*() const noexcept;
T& operator->() const noexcept;
};
/**********************************************************/
LinkedList() noexcept;
LinkedList(std::initializer_list<T>);
LinkedList(const LinkedList&);
LinkedList(LinkedList&&) noexcept;
~LinkedList() noexcept;
LinkedList& operator=(LinkedList) noexcept;
template <class A>
friend void swap(LinkedList<A>&, LinkedList<A>&);
void add(const T&);
void removeAt(int);
int size() const noexcept;
bool operator==(const LinkedList&) const noexcept;
bool operator!=(const LinkedList&) const noexcept;
virtual T& operator[](int) override;
virtual iterator begin() noexcept override;
virtual iterator end() noexcept override;
};
But then Traversable template has two parameters and it should have only one.
Is this what I am supposed to do? Keep in mind I am new to templates and iterators.

Answer 1

When creating an interface you'll need to nail down the static types of what is being returned. These may behave dynamically different but you can't change the type other than using a subtype relation when returning pointers or references.
Personally, I think this exercise is ill-advised for a C++ context. It may make some sense when using Java or C#. However, similar behavior can be obtained. A rought sketch would be something like this (although this should work it will be rather slow):
template <typename T>
struct iterator_base {
virtual iterator_base() {}
virtual iterator_base<T>* do_clone() = 0;
virtual T& do_value() = 0;
virtual void do_next() = 0;
virtual bool do_equal() = 0;
// other operations to implement operator--, operator[], ...
};
template <typename It>
class iterator: iterator_base<typename std::iterator_traits<It>::value_type> {
typedef typename std::iterator_traits<It>::value_type> type;
It it;
iterator_base<type>* do_clone() { return new iterator<It>(*this); }
type& do_value() { return *this->it; }
void do_next() { ++this->it; }
bool do_equal(iterator_base<type>* other) {
return this->it == static_cast<iterator<It>>(other)->it;
}
};
template <typename T>
class poly_iterator {
std::unique_ptr<iterator_base<T>> ptr;
public:
poly_iterator(iterator_base<T>* ptr): ptr(ptr) {}
poly_iterator(poly_iterator const& other): ptr(other.ptr->clone()) {}
poly_iterator& operator= (poly_iterator other) {
other.swap(this);
return *this;
}
void swap(poly_iterator& other) { swap(this->ptr, other.ptr); }
T& operator*() { return this->ptr->value(); }
T* operator->() { return &this->operator*(); }
poly_iterator& operator++() { this->ptr->next(); return *this; }
poly_iterator operator++(int) {
poly_iterator rc(*this);
this->operator++();
return rc;
}
bool operator== (poly_iterator const& other) {
return this->ptr->equal(other.ptr.ptr());
}
bool operator!= (poly_iterator const& other) {
return !(*this == other);
}
// other operations
};
// define a suitable specialization of std::iterator_traits<poly_iterator<T>>
template <typename T>
class ITraversible {
virtual iterator_base<T>* do_begin() = 0;
virutal iterator_base<T>* do_end() = 0;
public:
poly_iterator<T> begin() { return this->do_begin(); }
poly_iterator<T> end() { return this->do_end(); }
// other operations
};
template <typename T>
class List: public ITraversible<T> {
std::list<T> list;
iterator_base<T>* do_begin() {
return iterator<std::list<T>::iterator>(list.begin());
}
iterator_base<T>* do_end() {
return iterator<std::list<T>::iterator>(list.end());
}
public:
// whatever is needed to fill the list
};