Extend std::list to cylic list - c++

Question

I want to implement a cyclic list based on std::list. I want to profit from the benfits of the list but add one specific feature: its iterator operators ++ and -- should hop over the edges and operations (insert/erase) must not invalidate existing iterators. My skills in handling templates are weak and to understand the std containers is an impossible act for me. Hence i need your help. By now im not that far :D. Sorry but even the numerous posts dont help me any further.
EDIT:
Well after a lot of work, a steeeep learnig curve, the failed approach to inherit from std::list::iterator, a short-term depression and a abasing return to your approaches (yes, you all were right) I finally made it. Inspired by all your contibutions, I can now post what I did the last ... about 12 hours :D Basicly what you suggested, but with nice little operators.
#pragma once
#include <list>
using std::list;
template<class T>
class cyclic_iterator;
template<class T>
class cyclicList : public list<T>
{
public:
typedef cyclic_iterator<T> cyclic_iterator;
cyclic_iterator cycbegin()
{// not the purpose, but needed for instanziation
return cyclic_iterator( *this, this->begin());
}
cyclic_iterator cycend()
{// not the purpose, but needed for instanziation
return cyclic_iterator( *this, this->end());
}
};
template<class T>
class cyclic_iterator
{
public:
// To hop over edges need to know the container
cyclic_iterator(){}
cyclic_iterator(typename list<T>::iterator i)
: mIter(i){}
cyclic_iterator(list<T> &c)
: mContainer(&c){}
cyclic_iterator(list<T> &c, typename list<T>::iterator i)
: mContainer(&c), mIter(i){}
cyclic_iterator<T>& operator=(typename list<T>::iterator i)
{// assign an interator
mIter = i;
return *this;
}
cyclic_iterator<T>& operator=(list<T> &c)
{// assign a container
mContainer = &c;
return *this;
}
bool operator==(const cyclic_iterator<T>& rVal) const
{// check for equality
return (this->mIter == rVal.mIter && this->mContainer == rVal.mContainer) ? true : false;
}
bool operator!=(const cyclic_iterator<T>& rVal) const
{// check for inequality
return !(this->operator==(rVal));
}
cyclic_iterator<T>& operator++()
{// preincrement
++mIter;
if (mIter == mContainer->end())
{ mIter = mContainer->begin(); }
return *this;
}
cyclic_iterator<T> operator++(int)
{ // postincrement
cyclic_iterator<T> tmp = *this;
++*this;
return tmp;
}
cyclic_iterator<T>& operator--()
{// predecrement
if (mIter == mContainer->begin())
mIter = --mContainer->end();
else --mIter;
return *this;
}
cyclic_iterator<T> operator--(int)
{// postdecrement
cyclic_iterator<T> tmp = *this;
--*this;
return tmp;
}
cyclic_iterator<T>& operator+=(int j)
{// hop j nodes forward
for (int i = 0; i < j; ++i)
++(*this);
return *this;
}
cyclic_iterator<T>& operator-=(int j)
{// hop j nodes backwards
for (int i = 0; i < j; ++i)
--(*this);
return *this;
}
T& operator*()
{
return *mIter;
}
typename list<T>::iterator & getStdIterator()
{
return mIter;
}
private:
list<T>* mContainer;
typename list<T>::iterator mIter;
};

Answer 1

Can't you just make a different iterator type?
#include <iterator>
#include <list>
template <typename T, typename Alloc>
struct cyclic_iterator
: std::iterator<typename std::list<T, Alloc>::iterator::iterator_category, T>
{
typedef std::list<T, Alloc> list_type;
cyclic_iterator & operator++()
{
++iter;
if (iter == container.end()) { iter = container.begin(); }
return *this;
}
T & operator*() { return *iter; }
cyclic_iterator(typename list_type::iterator it, list_type & l)
: iter(it)
, container(l)
{
if (it == container.end()) { it = container.begin(); }
}
// everything else
private:
typename list_type::iterator iter;
list_type & container;
};
With a helper:
template <typename List>
cyclic_iterator<typename List::value_type, typename List::allocator_type>
make_cyclic_iterator(typename List::iterator it, List & l)
{
return cyclic_iterator<typename List::value_type, typename List::allocator_type>(it, l);
}
Usage:
// goes round and round forever
for (auto ci = make_cyclic_iterator(mylist.begin(), mylist); ; ++ci)
{
std::cout << *ci << std::endl;
}
(With a few modifications, this code could be made to work on any container that exposes begin/end iterators.)

Answer 2

It's not possible. The iterators and the implementation of the end element is implementation specific and not customizable. Containers were not designed for that kind of thing and it would make them really hard. You will have to go through the pain of implementing this yourself. Keep in mind that this gets very tricky because a cyclic list doesn't have a real past-the-end iterator and iterators aren't really able to handle that kind of situation. Some libraries have a Circulator concept to deal with circular structures.
NB: Inheriting from a standard container is a bad idea.

Answer 3

Of course you can implement it using std::list, but first you should encapsulate list in your class and do not derive from it second you must implement your own iterator to accomplish this, but since circular lists are fixed in size I prefer a container with a fixed size and linear memory like std::array or std::vector.
template<
class T,
class Container = std::vector<T>
>
class circular_list {
public:
// Following typedef are required to make your class a container
typedef typename Container::size_type size_type;
typedef typename Container::difference_type difference_type;
typedef typename Container::pointer pointer;
typedef typename Container::const_pointer const_pointer;
typedef typename Container::reference reference;
typedef typename Container::const_reference const_reference;
typedef typename Container::value_type value_type;
public:
class iterator : std::iterator<std::bidirectional_iterator_tag, value_type> {
public:
iterator() : c_(nullptr) {}
iterator(circular_buffer& c, size_type index)
: c_( &c.c_ ), index_( index ) {}
reference operator* () const {
return (*c_)[index_];
}
iterator& operator++ () {
if( ++index_ >= c_->size() ) index_ = 0;
return *this;
}
iterator& operator-- () {
if( index_ == 0 ) index_ = c_->size() - 1; else --index_;
return *this;
}
private:
size_type index_;
Container* c_;
};
public:
void push( const_reference val ) {add item to the container}
reference current() {return current item from the container}
void pop() {remove item from the container}
size_type size() const {return c_.size();}
iterator begin() {return iterator( *this, 0 );}
iterator end() {return iterator( *this, size() );}
private:
friend iterator;
Container c_;
}