30 June 2018
Linked Lists - Implementing
Commented source code of what it would mean to implement the core of a linked list in C++.
Introduction
I have chosen to implement a circular double linked list with the dummy node in the header:
The dl_list class below is a template on the sequence type for:
- a double linked list
- circular
- with dummy node in the header
- non-intrusive (nodes are provided by the list class)
- nodes are allocated on the heap (no custom allocator) and list owns the nodes
- getting size is linear time with number of elements (but splice is constant time)
The short name is misleading. To be explicit about all these variations would have made for a long name.
Another approach could have been a policy-based design. E.g. decisions such as caching the size or not would have been a choice made via an additional template parameter. But then still we would have to specify a lot of template parameters, as there are many choices to be made, leading again to a long actual name in practice.
dl_list.h file
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#pragma once
#include <cstddef> // for size_t and ptrdiff_t
#include <iterator> // for iterator tag and std::reverse_iterator
#include <utility> // for std::move and std::forward
template<typename T>
class dl_list {
// Nodes consist of links and value
struct node;
struct links {
node * next_;
node * prev_;
};
struct node : public links {
T value_;
// Constructor: next, prev, then args for value construction
template<typename ... Args>
node(node * next, node * prev, Args && ... args) :
links{ next, prev }, value_(std::forward<Args>(args)...)
{}
};
// Dummy node only has links.
// Design decision: the dummy node also deals with ownership, and move to
// help implement the list.
struct dummy_node : public links {
// Helper to look at this as a node
node * as_node_ptr() {
return reinterpret_cast<node*>(this);
}
// or as a const node
const node * as_node_ptr() const {
return reinterpret_cast<const node*>(this);
}
// Default constructor, next_ and prev_ point at this.
dummy_node() noexcept : links{ as_node_ptr(), as_node_ptr() } {}
// Free nodes in the destructor
~dummy_node() {
free();
}
// No copy
dummy_node(const dummy_node &) = delete;
dummy_node & operator= (const dummy_node &) = delete;
// Implement move constructor
dummy_node(dummy_node && other) noexcept {
move_impl(other.as_node_ptr());
}
// and move assignment.
// It turns out they both use a helper function move_impl
dummy_node & operator= (dummy_node && other) noexcept {
if (this != &other) {
free();
move_impl(other.as_node_ptr());
}
return *this;
}
// Dummy node helper functions
private:
// To free nodes, walk the chain following prev_ and delete the nodes,
// except this (the dummy_node for this list is part of the header)
void free() noexcept {
node * crt = this->prev_;
while(crt != as_node_ptr()) {
node * tmp = crt;
crt = crt->prev_;
delete tmp;
}
}
// Move helper function. This takes the minimalistic view that the moved
// from object (other) is in a state that it can be destroyed or assigned
// to. That state is prev_ pointing to &other (which is also what free
// tests for ownership).
void move_impl(node * other) noexcept {
if (other->prev_ == other) {
this->next_ = as_node_ptr();
this->prev_ = as_node_ptr();
}
else {
this->next_ = other->next_;
this->prev_ = other->prev_;
this->next_->prev_ = as_node_ptr();
this->prev_->next_ = as_node_ptr();
other->prev_ = other;
}
}
};
// A dummy_node is all that the header is
dummy_node header_;
public:
// Iterator
class iterator {
public:
// Type aliases used by algorithms (e.g. iterator_category is used for
// algorithm selection)
using iterator_category = std::bidirectional_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = T;
using pointer = T *;
using reference = T &;
private:
// has a pointer to a node
node * ptr_;
// It has a constructor that the list uses to initialize the pointer to the
// node
explicit iterator(node * ptr) : ptr_{ ptr } {};
// friendship required for the list to access the above constructor
friend class dl_list;
public:
// Default constructor. Needs to initialize pointer to nullptr to ensure
// equality comparison with other iterator object gives consistent result
iterator() : ptr_{ nullptr } {};
// Copy and move are default implemented by the compiler
// Equality compares node pointer for equality
friend bool operator== (const iterator & x, const iterator & y) {
return x.ptr_ == y.ptr_;
}
// Difference is negation of equality
friend bool operator!= (const iterator & x, const iterator & y) {
return !(x == y);
}
// Dereferencing returns the value type
T & operator* () {
return ptr_->value_;
}
// Increment and decrement, pre and post
iterator & operator++ () {
ptr_ = ptr_->next_;
return *this;
}
iterator operator++ (int) {
node * tmp = ptr_;
ptr_ = ptr_->next_;
return iterator(tmp);
}
iterator & operator-- () {
ptr_ = ptr_->prev_;
return *this;
}
iterator operator-- (int) {
node * tmp = ptr_;
ptr_ = ptr_->prev_;
return iterator(tmp);
}
};
// Const iterator
class const_iterator {
public:
// NOTE: The value_type does not have a const. Reason is one can use it
// like:
// std::iterator_traits<I>::value_type x = *it;
using iterator_category = std::bidirectional_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = T;
using pointer = const T *;
using reference = const T &;
private:
// has a pointer to a const node
const node * ptr_;
// Same private constructor and friend
explicit const_iterator(const node * ptr) : ptr_{ ptr } {};
friend class dl_list;
public:
// Default constructor.
const_iterator() : ptr_{ nullptr } {};
// Copy and move are default implemented by the compiler
// Construct from an iterator (not explicit)
const_iterator(const iterator & x) noexcept : ptr_{ x.ptr_ } {}
// Equality and difference
friend bool operator== (const const_iterator & x, const const_iterator & y) {
return x.ptr_ == y.ptr_;
}
friend bool operator!= (const const_iterator & x, const const_iterator & y) {
return !(x == y);
}
// Dereferencing returns the value type
const T & operator* () {
return ptr_->value_;
}
// Increment and decrement, pre and post
const_iterator & operator++ () {
ptr_ = ptr_->next_;
return *this;
}
const_iterator operator++ (int) {
node * tmp = ptr_;
ptr_ = ptr_->next_;
return const_iterator(tmp);
}
const_iterator & operator-- () {
ptr_ = ptr_->prev_;
return *this;
}
const_iterator operator-- (int) {
node * tmp = ptr_;
ptr_ = ptr_->prev_;
return const_iterator(tmp);
}
};
// Type alises
using value_type = T;
using reference = T &;
using const_reference = const T &;
// using iterator alias not required, see struct iterator above
// same for const_iterator
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
using difference_type = std::ptrdiff_t;
using size_type = std::size_t;
// Default constructor, nothing to do the dummy_node default constructor does
// the job
dl_list() noexcept {
}
// Initialize from a sequence. Shoud push_back throw, the dummy_node
// destructor will free nodes already allocated
template<typename I, typename S>
// requires I is InputIterator and S is sentinel for It
dl_list(I first, S last) {
while (first != last) {
push_back(*first);
++first;
}
}
// Copy constructor falls back on initializing from sequence
dl_list(const dl_list & other) : dl_list(other.begin(), other.end()) {
}
// Copy assignment makes a copy and moves header
dl_list & operator= (const dl_list & other) {
dl_list tmp(other);
header_ = std::move(tmp.header_);
return *this;
}
// Move constructor and assignment delegate to header
// defaults do the job
dl_list(dl_list &&) noexcept = default;
dl_list & operator= (dl_list &&) noexcept = default;
// Begin, end, const and reverse variants
iterator begin() noexcept {
return iterator(sentinel()->next_);
}
const_iterator begin() const noexcept {
return const_iterator(sentinel()->next_);
}
const_iterator cbegin() const noexcept {
return const_iterator(sentinel()->next_);
}
iterator end() noexcept {
return iterator(sentinel());
}
const_iterator end() const noexcept {
return const_iterator(sentinel());
}
const_iterator cend() const noexcept {
return const_iterator(sentinel());
}
reverse_iterator rbegin() noexcept {
return reverse_iterator(end());
}
const_reverse_iterator rbegin() const noexcept {
return reverse_iterator(end());
}
const_reverse_iterator crbegin() const noexcept {
return reverse_iterator(cend());
}
reverse_iterator rend() noexcept {
return reverse_iterator(begin());
}
const_reverse_iterator rend() const noexcept {
return reverse_iterator(begin());
}
const_reverse_iterator crend() const noexcept {
return reverse_iterator(cbegin());
}
// Finally some operations
void push_back(const T & value) {
insert_impl(sentinel(), value);
}
void push_back(T && value) {
insert_impl(sentinel(), std::move(value));
}
private:
// helper functions
node * sentinel() {
return header_.as_node_ptr();
}
const node * sentinel() const {
return header_.as_node_ptr();
}
template<typename ... Args>
void insert_impl(node * where, Args && ... args) {
node * x = new node(where, where->prev_, std::forward<Args>(args)...);
where->prev_->next_ = x;
where->prev_ = x;
}
};
Sample usage
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#include "dl_list.h"
#include <iostream>
int main() {
dl_list<int> list;
list.push_back(42);
list.push_back(53);
dl_list<int> another(std::move(list));
for (const auto & e: another) {
std::cout << e << '\n';
}
for (auto it = another.rbegin(); it != another.rend(); ++it) {
std::cout << *it << '\n';
}
std::cout << "Done!\n";
}