Endless loop with std::distance on custom list - c++

Question

For the fun of it, I implemented a std::list similar container by myself.
I will never use it in reality. Just practising . . .
During the comparison of my implemented functionality with the std::list, I found a severe problem.
Testing std::distance with invalid parameters, as part of a simple unit test, shows different results.
Of course I expect UB and indeterminate results, but std::distance on my implementation goes into an endless loop, while it does not with a std::list. Why?
Why is my implementation causing an endles loop with std::distance?
I followd link1 and link2 in CPP reference to the source code of std::distance and tried to reproduce it. But this works also for my implemenation.
Test/driver code:
int main() {
// std::list works. No endless loop.
std::list<int> list1{ 1, 2, 3, 4, 5 };
std::cout << std::distance(list1.end(), list1.begin()) << '\n';
// Custom list
List<int> list2{ 1, 2, 3, 4, 5 };
// Delta works
std::cout << list2.begin() - list2.end() << '\n';
std::cout << list2.end() - list2.begin() << '\n';
// Hopp Count works
List<int>::iterator i = list2.end();
int counter = 0;
while (i != list2.end()) {
++counter;
++i;
}
std::cout << counter << '\n';
// Distance will go into endless loop ****************************
std::cout << std::distance(list2.end(), list2.begin()) << '\n';
}
What is wrong with my implementation?
Please see the full source code below.
#include <iostream>
#include <iterator>
#include <vector>
#include <type_traits>
#include <initializer_list>
#include <algorithm>
#include <list>
// ------------------------------------------------------------------------------------------------
// This would be in a header file -----------------------------------------------------------------
// Type trait helper to identify iterators --------------------------------------------------------
template<typename T, typename = void>
struct is_iterator { static constexpr bool value = false; };
template<typename T>
struct is_iterator<T, typename std::enable_if<!std::is_same<typename std::iterator_traits<T>::value_type, void>::value>::type> {
static constexpr bool value = true;
};
// The List class ---------------------------------------------------------------------------------
template <typename T>
class List {
// Sub class for a Node -----------
struct Node {
Node* next{};
Node* previous{};
T data{};
Node(Node* const n, Node* const p, const T& d) : next(n), previous(p), data(d) {}
Node(Node* const n, Node* const p) : next(n), previous(p) {}
Node() {}
};
// Private list data and functions --------
size_t numberOfElements{};
Node* head{};
void init() { head = new Node(); head->next = head; head->previous = head; numberOfElements = 0; }
public:
struct iterator; // Forward declaration
// Constructor --------------------
List() { init(); }
explicit List(const size_t count, const T& value) { init(); insert(begin(), count, value); };
explicit List(const size_t count) { init(); insert(begin(), count); }
template <typename Iter>
List(const Iter& first, const Iter& last) { init(); insert(begin(),first, last); }
List(const List& other) { init(), insert(begin(), other.begin(), other.end()); };
List(List&& other) : head(other.head), numberOfElements(other.numberOfElements) { other.init(); }
List(const std::initializer_list<T>& il) { init(); insert(begin(), il.begin(), il.end()); }
template <int N> List(const T(&other)[N]) { init(); insert(begin(), std::begin(other), std::end(other)); }
template <int N> List(T(&other)[N]) { init(); insert(begin(), std::begin(other), std::end(other)); }
// Assignment ---------------------
List& operator =(const List& other) { clear(); insert(begin(), other.begin(), other.end()); return *this; }
List& operator =(List&& other) { clear(); head = other.head; numberOfElements = other.numberOfElements; other.init(); return *this; }
List& operator =(const std::initializer_list<T>& il) { clear(); insert(begin(),il.begin(),il.end()); return *this; }
template <int N> List& operator =(const T(&other)[N]) { clear(); insert(begin(), std::begin(other), std::end(other)); return *this; }
template <int N> List& operator =(T(&other)[N]) { clear(); insert(begin(), std::begin(other), std::end(other)); return *this;}
void assign(const size_t count, const T& value) { clear(); insert(begin(), count, value); }
void assign(const std::initializer_list<T>& il) { clear(); insert(begin(), il.begin(), il.end()); }
template <typename Iter> void assign(const Iter& first, const Iter& last) { clear(); insert(begin(), first, last);}
template <int N> void assign(const T(&other)[N]) { clear(); insert(begin(), std::begin(other), std::end(other)); return *this; }
template <int N> void assign(T(&other)[N]) { clear(); insert(begin(), std::begin(other), std::end(other)); return *this; }
// Destructor ---------------------
~List() { clear(); }
// Element Access -----------------
T& front() { return *begin(); }
T& back() { return *(--end()); }
// Iterators ----------------------
iterator begin() const { return iterator(head->next, head); }
iterator end() const { return iterator(head, head); }
// Capacity -----------------------
size_t size() const { return numberOfElements; }
bool empty() { return size() == 0; }
// Modifiers ----------------------
void clear();
iterator insert(const iterator& insertBeforePosition, const T& value);
iterator insert(const iterator& insertBeforePosition);
template <class Iter, std::enable_if_t<is_iterator<Iter>::value, bool> = true>
iterator insert(const iterator& insertBeforePosition, const Iter& first, const Iter& last);
iterator insert(const iterator& insertBeforePosition, const size_t& count, const T& value);
iterator insert(const iterator& insertBeforePosition, const std::initializer_list<T>& il);
iterator erase(const iterator& posToDelete);
iterator erase(const iterator& first, const iterator& last);
void push_back(const T& d) { insert(end(), d); }
void pop_back() { erase(--end()); };
void push_front(const T& d) { insert(begin(), d); }
void pop_front() { erase(begin()); };
void resize(size_t count);
void resize(size_t count, const T& value);
void swap(List& other) { std::swap(head, other.head); std::swap(numberOfElements, other.numberOfElements); }
// Operations --------------------
void reverse();
// Non standard inefficient functions --------------------------
T& operator[](const size_t index) const { return begin()[index]; }
// ------------------------------------------------------------------------
// Define iterator capability ---------------------------------------------
struct iterator {
// Definitions ----------------
using iterator_category = std::bidirectional_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = T;
using pointer = T*;
using reference = T&;
// Data -----------------------
Node* iter{};
Node* head{};
// Constructor ----------------
iterator(Node*const node, Node* const h) : iter(node), head(h) {};
iterator() {};
// Dereferencing --------------
reference operator*() const { return iter->data; }
reference operator->() const { return &**this; }
// Arithmetic operations ------
iterator operator++() { if (iter != head) iter = iter->next; return *this; }
iterator operator++(int) { iterator tmp = *this; ++* this; return tmp; }
iterator operator--() { if (iter != head->next) iter = iter->previous; return *this; }
iterator operator--(int) { iterator tmp = *this; --* this; return tmp; }
iterator operator +(const difference_type& n) const {
iterator temp{ *this }; difference_type k{ n }; if (k > 0) while (k--)++temp; else while (k++)--temp; return temp;
}
iterator operator +=(const difference_type& n) {
difference_type k{ n }; if (k > 0) while (k--)++* this; else while (k++)--* this; return *this;
};
iterator operator -(const difference_type& n) const {
iterator temp{ *this }; difference_type k{ n }; if (k > 0) while (k--)--temp; else while (k++)++temp; return temp;
}
iterator operator -=(const difference_type& n) {
difference_type k{ n }; if (k > 0) while (k--)--* this; else while (k++)++* this; return *this;
};
// Comparison -----------------
bool operator ==(const iterator& other) const { return iter == other.iter; };
bool operator !=(const iterator& other) const { return iter != other.iter; };
bool operator < (const iterator& other) const { return other.iter - iter < 0; };
bool operator <= (const iterator& other) const { return other.iter - iter <= 0; };
bool operator > (const iterator& other) const { return other.iter - iter > 0; };
bool operator >= (const iterator& other) const { return other.iter - iter >= 0; };
// Special non standard functions -----------------
difference_type operator-(const iterator& other) const;
reference operator[] (const size_t index);
};
};
// ------------------------------------------------------------------------------------------------
// Implementation of list functions. This would normally go into a TCC file -----------------------
// List class functions ---------------
template <typename T>
void List<T>::clear() {
for (Node* nextNode{}, * currentNode(head->next); currentNode != head; currentNode = nextNode) {
nextNode = currentNode->next;
delete currentNode;
}
init();
}
template <typename T>
typename List<T>::iterator List<T>::insert(const List<T>::iterator& insertBeforePosition, const T& value)
{
Node* nodeInsertBeforePosition = insertBeforePosition.iter;
Node* newNode = new Node(nodeInsertBeforePosition, nodeInsertBeforePosition->previous, value);
nodeInsertBeforePosition->previous = newNode;
(newNode->previous)->next = newNode;
++numberOfElements;
return iterator(newNode, head);
}
template <typename T>
typename List<T>::iterator List<T>::insert(const List<T>::iterator& insertBeforePosition)
{
Node* nodeInsertBeforePosition = insertBeforePosition.iter;
Node* newNode = new Node(nodeInsertBeforePosition, nodeInsertBeforePosition->previous);
nodeInsertBeforePosition->previous = newNode;
(newNode->previous)->next = newNode;
++numberOfElements;
return iterator(newNode, head);
}
template <typename T>
template <class Iter, std::enable_if_t<is_iterator<Iter>::value, bool>>
typename List<T>::iterator List<T>::insert(const List<T>::iterator& insertBeforePosition, const Iter& first, const Iter& last) {
iterator result(insertBeforePosition.iter, head);
if (first != last) {
result = insert(insertBeforePosition, *first);
Iter i(first);
for (++i; i != last; ++i)
insert(insertBeforePosition, *i);
}
return result;
}
template <typename T>
typename List<T>::iterator List<T>::insert(const List<T>::iterator& insertBeforePosition, const size_t& count, const T& value) {
iterator result(insertBeforePosition.iter, head);
if (count != 0u) {
result = insert(insertBeforePosition, value);
for (size_t i{ 1u }; i < count; ++i)
insert(insertBeforePosition, value);
}
return result;
}
template <typename T>
typename List<T>::iterator List<T>::insert(const List<T>::iterator& insertBeforePosition, const std::initializer_list<T>& il) {
return insert(insertBeforePosition, il.begin(), il.end());
}
template <typename T>
typename List<T>::iterator List<T>::erase(const List<T>::iterator& posToDelete) {
iterator result = posToDelete;
++result;
Node* nodeToDelete = posToDelete.iter;
if (nodeToDelete != head) {
nodeToDelete->previous->next = nodeToDelete->next;
nodeToDelete->next->previous = nodeToDelete->previous;
delete nodeToDelete;
--numberOfElements;
}
return result;
}
template <typename T>
typename List<T>::iterator List<T>::erase(const List<T>::iterator& first, const List<T>::iterator& last) {
iterator result{ end() };
if (first == begin() && last == end())
clear();
else {
while (first != last)
first = erase(first);
result = last;
}
return result;
}
template <typename T>
void List<T>::resize(size_t count) {
if (numberOfElements < count)
for (size_t i{ numberOfElements }; i < count; ++i)
insert(end());
else
while (count--)
pop_back();
}
template <typename T>
void List<T>::resize(size_t count, const T& value) {
if (numberOfElements < count)
for (size_t i{ numberOfElements }; i < count; ++i)
insert(end(),value);
else
while (count--)
pop_back();
}
template <typename T>
void List<T>::reverse() {
const Node* oldHead = head;
for (Node* nptr = head; ; nptr = nptr->previous) {
std::swap(nptr->next, nptr->previous);
if (nptr->previous == oldHead) // Previous was the original next
break;
}
}
// ------------------------------------
// Iterator functions -----------------
template <typename T>
typename List<T>::iterator::difference_type List<T>::iterator::operator-(const iterator& other) const {
difference_type result{};
Node* nptr = head;
int indexThis{ -1 }, indexOther{ -1 }, index{};
do {
nptr = nptr->next;
if (nptr == iter)
indexThis = index;
if (nptr == other.iter)
indexOther = index;
++index;
} while (nptr != head);
if (indexThis >= 0 and indexOther >= 0)
result = indexThis - indexOther;
return result;
}
template <typename T>
typename List<T>::iterator::reference List<T>::iterator::operator[] (const size_t index) {
Node* nptr = head->next;
for (size_t i{}; i < index and nptr != head; ++i, nptr = nptr->next)
;
return nptr->data;
}
// ------------------------------------------------------------------------------------------------
// This would be in a cpp file --------------------------------------------------------------------
int main() {
// std::list works
std::list<int> list1{ 1, 2, 3, 4, 5 };
std::cout << std::distance(list1.end(), list1.begin()) << '\n';
// Custom list
List<int> list2{ 1, 2, 3, 4, 5 };
// Delta works
std::cout << list2.begin() - list2.end() << '\n';
std::cout << list2.end() - list2.begin() << '\n';
// Hopp Count works
List<int>::iterator i = list2.end();
int counter = 0;
while (i != list2.end()) {
++counter;
++i;
}
std::cout << counter << '\n';
// Distance will go into endless loop ****************************
std::cout << std::distance(list2.end(), list2.begin()) << '\n';
}

Answer 1

Based on the good comment of AlanBirtles, I found the bug.
I build in (already in the first versions) a wrong additional sentinel for the pre increment and pre decrement operator of the iterator.
The correct version is:
iterator operator++() { iter = iter->next; return *this; }
iterator operator--() { iter = iter->previous; return *this; }
I originally planned to delete the question, but because of the superb comments, I will leave it. Thank you.

Answer 2

This:
// Distance will go into endless loop ****************************
std::cout << std::distance(list2.end(), list2.begin()) << '\n';
should be:
// Distance will go into endless loop ****************************
std::cout << std::distance(list2.begin(), list2.end()) << '\n';
What you do here is start from the end and increase the iterator until it reaches the beginning. This is an endless loop