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