Doubly linked list iterators implementaion approach - c++

Question

I have implemented a doubly linked list with iterators(regular and const) and it seems working ok, but I can't understand several things:
1) As I know one of the ways to implement iterators is inherit from std::iterator< ... > mentioning in template parameters all the necessary types, then typedef them in your class ... but why we need difference_type of two iterators ? bidirectional iterator must not overload operator - () inside so is it ok to not remove difference type ?
2) I know that it's not the best approach to duplicate logic of similar things (in my case regular iterator and const_iterator are two different classes) but that's what I did at the moment due to my basic knowledge about c++ templates. I made const_iterator as a friend for iterator and made implicit conversions from T (node value type) to const T using special constructor const_iterator(iterator &other) so when returning references and pointers from operator *() it and operator->() it automatically converts internal ptr->_node_value (list node data) to const version. Is this approach ok enough ??
template <typename T>
struct DLList_node
{
T _node_value;
DLList_node<T> *_next, *_prev;
DLList_node<T>(const T& = T(), DLList_node<T> * = NULL, DLList_node<T> * = NULL);
};
template <typename T>
class DLList;
//non-const regular iterator
template<typename T>
class DLList_iterator;
//const iterator
template<typename T>
class DLList_const_iterator;
template<typename T>
class DLList_iterator :
public std::iterator< std::bidirectional_iterator_tag, T, T*, T& >
{
public:
friend class DLList<T>;
//const iterator must have access to node pointer
friend class DLList_const_iterator<T>;
typedef T value_type;
typedef ptrdiff_t difference_type;
typedef T* pointer;
typedef T& reference;
typedef std::bidirectional_iterator_tag iterator_category;
typedef DLList_node<T>* node_ptr;
DLList_iterator()
: ptr(NULL)
{}
DLList_iterator(node_ptr _ptr): ptr(_ptr)
{}
reference operator*() const
{
assert(ptr != NULL && "list iterator not dereferencable!");
return ptr->_node_value;
}
pointer operator->() const
{
assert(ptr != NULL && "list iterator not dereferencable!");
return &(ptr->_node_value);
}
bool operator == (const DLList_iterator& other) const
{
return ptr == other.ptr;
}
bool operator != (const DLList_iterator& other) const
{
return ptr != other.ptr;
}
//pre incerement
DLList_iterator& operator++()
{
assert(ptr != NULL && "list iterator not incrementable!");
ptr = ptr->_next;
return *this;
}
//post increment
DLList_iterator operator++(int)
{
assert(ptr != NULL && "list iterator not incrementable!");
DLList_iterator tmp(*this);
ptr = ptr->_next;
return tmp;
}
DLList_iterator& operator--()
{
assert(ptr != NULL && "list iterator not decrementable!");
ptr = ptr->_prev;
return *this;
}
DLList_iterator operator--(int)
{
assert(ptr != NULL && "list iterator not decrementable!");
DLList_iterator tmp(*this);
ptr = ptr->_prev;
return tmp;
}
private:
node_ptr ptr;
};
//const iterator
template<typename T>
class DLList_const_iterator :
public std::iterator< std::bidirectional_iterator_tag, const T, const T*, const T& >
{
public:
friend class DLList<T>;
typedef const T value_type;
typedef ptrdiff_t difference_type;
typedef const T* const_pointer;
typedef const T& const_reference;
typedef std::bidirectional_iterator_tag iterator_category;
typedef DLList_node<T>* node_ptr;
DLList_const_iterator()
: cptr(NULL)
{}
DLList_const_iterator(DLList_iterator<T>& _iter): cptr(_iter.ptr)
{}
DLList_const_iterator(node_ptr _ptr): cptr(_ptr)
{}
const_reference operator*() const
{
assert(cptr != NULL && "list iterator not dereferencable!");
return cptr->_node_value;
}
const_pointer operator->() const
{
assert(cptr != NULL && "list iterator not dereferencable!");
return &(cptr->_node_value);
}
bool operator == (const DLList_const_iterator & other) const
{
return cptr == other.cptr;
}
bool operator != (const DLList_const_iterator & other) const
{
return cptr != other.cptr;
}
//pre incerement
DLList_const_iterator & operator++()
{
assert(cptr != NULL && "list iterator not incrementable!");
cptr = cptr->_next;
return *this;
}
//post increment
DLList_const_iterator operator++(int)
{
assert(cptr != NULL && "list iterator not incrementable!");
DLList_const_iterator tmp(*this);
cptr = cptr->_next;
return tmp;
}
DLList_const_iterator & operator--()
{
assert(cptr != NULL && "list iterator not decrementable!");
cptr = cptr->_prev;
return *this;
}
DLList_const_iterator operator--(int)
{
assert(cptr != NULL && "list iterator not decrementable!");
DLList_iterator tmp(*this);
cptr = cptr->_prev;
return tmp;
}
private:
node_ptr cptr;
};
template <typename T>
class DLList
{
private:
node_ptr _begin, _tail;
public:
typedef DLList_iterator<T> iterator;
typedef DLList_const_iterator<T> const_iterator;
typedef DLList_node<T>* node_ptr;
typedef DLList_node<T> node;
typedef typename DLList_iterator<T>::value_type value_type;
typedef typename DLList_iterator<T>::difference_type difference_type;
typedef typename DLList_iterator<T>::pointer pointer;
typedef typename DLList_iterator<T>::reference reference;
typedef typename DLList_const_iterator<T>::const_pointer const_pointer;
typedef typename DLList_const_iterator<T>::const_reference const_reference;
//member functions and constructors here
//...
};

Answer 1

1) As I know one of the ways to implement iterators is inherit from std::iterator< ... > mentioning in template parameters all the necessary types,
Yes.
then typedef them in your class ...
No. That's why you inherit from std::iterator so you don't have to bother doing that.
but why we need difference_type of two iterators ?
Note that std::iterator defaults most of the standard types. You only need to specify a difference type if you are doing something funky and none standard.
bidirectional iterator must not overload operator - () inside so is it ok to not remove difference type ?
You don't need to mention any of the types. std::iterator does all that work for you.
2) I know that it's not the best approach to duplicate logic of similar things (in my case regular iterator and const_iterator are two different classes) but that's what I did at the moment due to my basic knowledge about c++ templates.
Looking at your class's they are basically the same apart from some differences in retrieval. Why not move all the same code into a base class.
template<typename T>
class DLList_Base_iterator :
public std::iterator< std::bidirectional_iterator_tag, T, T*, T& >
{
// Common code
protected: // allow all concrete iterators to see pointer.
node_ptr ptr;
};
template<typename T>
class DLList_iterator : public DLList_Base_iterator<T>
{
DLList_iterator(): public DLList_Base_iterator<T>(NULL) {}
DLList_iterator(node_ptr _ptr): public DLList_Base_iterator<T>(_ptr) {}
reference operator*() const {return cptr->_node_value;}
pointer operator->() const {return &(cptr->_node_value);}
};
template<typename T>
class DLList_const_iterator : public DLList_Base_iterator<T>
{
DLList_const_iterator(): public DLList_Base_iterator<T>(NULL) {}
DLList_const_iterator(node_ptr _ptr): public DLList_Base_iterator<T>(_ptr) {}
const_reference operator*() const {return cptr->_node_value;}
const_pointer operator->() const {return &(cptr->_node_value);}
};
I made const_iterator as a friend for iterator and made implicit conversions from T (node value type) to const T using special constructor const_iterator(iterator &other) so when returning references and pointers from operator *() it and operator->() it automatically converts internal ptr->_node_value (list node data) to const version. Is this approach ok enough ??
I would just add a conversion operator to the non-const iterator that allows you to convert it to a const version.
template<typename T>
class DLList_iterator : public DLList_Base_iterator<T>
{
operator DLList_const_iterator<T> {
return DLList_const_iterator<T>(cptr);
}
};