base/linked_list.h

   1 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_LINKED_LIST_H_
   6 #define BASE_LINKED_LIST_H_
   7 #pragma once
   8
   9 // Simple LinkedList type. (See the Q&A section to understand how this
  10 // differs from std::list).
  11 //
  12 // To use, start by declaring the class which will be contained in the linked
  13 // list, as extending LinkNode (this gives it next/previous pointers).
  14 //
  15 //   class MyNodeType : public LinkNode<MyNodeType> {
  16 //     ...
  17 //   };
  18 //
  19 // Next, to keep track of the list's head/tail, use a LinkedList instance:
  20 //
  21 //   LinkedList<MyNodeType> list;
  22 //
  23 // To add elements to the list, use any of LinkedList::Append,
  24 // LinkNode::InsertBefore, or LinkNode::InsertAfter:
  25 //
  26 //   LinkNode<MyNodeType>* n1 = ...;
  27 //   LinkNode<MyNodeType>* n2 = ...;
  28 //   LinkNode<MyNodeType>* n3 = ...;
  29 //
  30 //   list.Append(n1);
  31 //   list.Append(n3);
  32 //   n3->InsertBefore(n3);
  33 //
  34 // Lastly, to iterate through the linked list forwards:
  35 //
  36 //   for (LinkNode<MyNodeType>* node = list.head();
  37 //        node != list.end();
  38 //        node = node->next()) {
  39 //     MyNodeType* value = node->value();
  40 //     ...
  41 //   }
  42 //
  43 // Or to iterate the linked list backwards:
  44 //
  45 //   for (LinkNode<MyNodeType>* node = list.tail();
  46 //        node != list.end();
  47 //        node = node->previous()) {
  48 //     MyNodeType* value = node->value();
  49 //     ...
  50 //   }
  51 //
  52 // Questions and Answers:
  53 //
  54 // Q. Should I use std::list or base::LinkedList?
  55 //
  56 // A. The main reason to use base::LinkedList over std::list is
  57 //    performance. If you don't care about the performance differences
  58 //    then use an STL container, as it makes for better code readability.
  59 //
  60 //    Comparing the performance of base::LinkedList<T> to std::list<T*>:
  61 //
  62 //    * Erasing an element of type T* from base::LinkedList<T> is
  63 //      an O(1) operation. Whereas for std::list<T*> it is O(n).
  64 //      That is because with std::list<T*> you must obtain an
  65 //      iterator to the T* element before you can call erase(iterator).
  66 //
  67 //    * Insertion operations with base::LinkedList<T> never require
  68 //      heap allocations.
  69 //
  70 // Q. How does base::LinkedList implementation differ from std::list?
  71 //
  72 // A. Doubly-linked lists are made up of nodes that contain "next" and
  73 //    "previous" pointers that reference other nodes in the list.
  74 //
  75 //    With base::LinkedList<T>, the type being inserted already reserves
  76 //    space for the "next" and "previous" pointers (base::LinkNode<T>*).
  77 //    Whereas with std::list<T> the type can be anything, so the implementation
  78 //    needs to glue on the "next" and "previous" pointers using
  79 //    some internal node type.
  80
  81 namespace base {
  82
  83 template <typename T>
  84 class LinkNode {
  85  public:
  86   LinkNode() : previous_(0), next_(0) {}
  87   LinkNode(LinkNode<T>* previous, LinkNode<T>* next)
  88       : previous_(previous), next_(next) {}
  89
  90   // Insert |this| into the linked list, before |e|.
  91   void InsertBefore(LinkNode<T>* e) {
  92     this->next_ = e;
  93     this->previous_ = e->previous_;
  94     e->previous_->next_ = this;
  95     e->previous_ = this;
  96   }
  97
  98   // Insert |this| into the linked list, after |e|.
  99   void InsertAfter(LinkNode<T>* e) {
 100     this->next_ = e->next_;
 101     this->previous_ = e;
 102     e->next_->previous_ = this;
 103     e->next_ = this;
 104   }
 105
 106   // Remove |this| from the linked list.
 107   void RemoveFromList() {
 108     this->previous_->next_ = this->next_;
 109     this->next_->previous_ = this->previous_;
 110   }
 111
 112   LinkNode<T>* previous() const {
 113     return previous_;
 114   }
 115
 116   LinkNode<T>* next() const {
 117     return next_;
 118   }
 119
 120   // Cast from the node-type to the value type.
 121   const T* value() const {
 122     return static_cast<const T*>(this);
 123   }
 124
 125   T* value() {
 126     return static_cast<T*>(this);
 127   }
 128
 129   // Work around a Clang bug reported upstream:
 130   //   http://llvm.org/bugs/show_bug.cgi?id=7974
 131   // TODO(evanm): remove this and its sole caller.
 132   void set(LinkNode<T>* prev, LinkNode<T>* next) {
 133     previous_ = prev; next_ = next;
 134   }
 135
 136  private:
 137   LinkNode<T>* previous_;
 138   LinkNode<T>* next_;
 139 };
 140
 141 template <typename T>
 142 class LinkedList {
 143  public:
 144   // The "root" node is self-referential, and forms the basis of a circular
 145   // list (root_.next() will point back to the start of the list,
 146   // and root_->previous() wraps around to the end of the list).
 147   LinkedList() { root_.set(&root_, &root_); }
 148
 149   // Appends |e| to the end of the linked list.
 150   void Append(LinkNode<T>* e) {
 151     e->InsertBefore(&root_);
 152   }
 153
 154   LinkNode<T>* head() const {
 155     return root_.next();
 156   }
 157
 158   LinkNode<T>* tail() const {
 159     return root_.previous();
 160   }
 161
 162   const LinkNode<T>* end() const {
 163     return &root_;
 164   }
 165
 166  private:
 167   LinkNode<T> root_;
 168 };
 169
 170 }  // namespace base
 171
 172 #endif  // BASE_LINKED_LIST_H_