base/memory/linked_ptr.h

   1 // Copyright (c) 2011 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 // A "smart" pointer type with reference tracking.  Every pointer to a
   6 // particular object is kept on a circular linked list.  When the last pointer
   7 // to an object is destroyed or reassigned, the object is deleted.
   8 //
   9 // Used properly, this deletes the object when the last reference goes away.
  10 // There are several caveats:
  11 // - Like all reference counting schemes, cycles lead to leaks.
  12 // - Each smart pointer is actually two pointers (8 bytes instead of 4).
  13 // - Every time a pointer is released, the entire list of pointers to that
  14 //   object is traversed.  This class is therefore NOT SUITABLE when there
  15 //   will often be more than two or three pointers to a particular object.
  16 // - References are only tracked as long as linked_ptr<> objects are copied.
  17 //   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
  18 //   will happen (double deletion).
  19 //
  20 // A good use of this class is storing object references in STL containers.
  21 // You can safely put linked_ptr<> in a vector<>.
  22 // Other uses may not be as good.
  23 //
  24 // Note: If you use an incomplete type with linked_ptr<>, the class
  25 // *containing* linked_ptr<> must have a constructor and destructor (even
  26 // if they do nothing!).
  27 //
  28 // Thread Safety:
  29 //   A linked_ptr is NOT thread safe. Copying a linked_ptr object is
  30 //   effectively a read-write operation.
  31 //
  32 // Alternative: to linked_ptr is shared_ptr, which
  33 //  - is also two pointers in size (8 bytes for 32 bit addresses)
  34 //  - is thread safe for copying and deletion
  35 //  - supports weak_ptrs
  36
  37 #ifndef BASE_MEMORY_LINKED_PTR_H_
  38 #define BASE_MEMORY_LINKED_PTR_H_
  39
  40 #include "base/logging.h"  // for CHECK macros
  41
  42 // This is used internally by all instances of linked_ptr<>.  It needs to be
  43 // a non-template class because different types of linked_ptr<> can refer to
  44 // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
  45 // So, it needs to be possible for different types of linked_ptr to participate
  46 // in the same circular linked list, so we need a single class type here.
  47 //
  48 // DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
  49 class linked_ptr_internal {
  50  public:
  51   // Create a new circle that includes only this instance.
  52   void join_new() {
  53     next_ = this;
  54   }
  55
  56   // Join an existing circle.
  57   void join(linked_ptr_internal const* ptr) {
  58     next_ = ptr->next_;
  59     ptr->next_ = this;
  60   }
  61
  62   // Leave whatever circle we're part of.  Returns true iff we were the
  63   // last member of the circle.  Once this is done, you can join() another.
  64   bool depart() {
  65     if (next_ == this) return true;
  66     linked_ptr_internal const* p = next_;
  67     while (p->next_ != this) p = p->next_;
  68     p->next_ = next_;
  69     return false;
  70   }
  71
  72  private:
  73   mutable linked_ptr_internal const* next_;
  74 };
  75
  76 template <typename T>
  77 class linked_ptr {
  78  public:
  79   typedef T element_type;
  80
  81   // Take over ownership of a raw pointer.  This should happen as soon as
  82   // possible after the object is created.
  83   explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
  84   ~linked_ptr() { depart(); }
  85
  86   // Copy an existing linked_ptr<>, adding ourselves to the list of references.
  87   template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }
  88
  89   linked_ptr(linked_ptr const& ptr) {
  90     DCHECK_NE(&ptr, this);
  91     copy(&ptr);
  92   }
  93
  94   // Assignment releases the old value and acquires the new.
  95   template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
  96     depart();
  97     copy(&ptr);
  98     return *this;
  99   }
 100
 101   linked_ptr& operator=(linked_ptr const& ptr) {
 102     if (&ptr != this) {
 103       depart();
 104       copy(&ptr);
 105     }
 106     return *this;
 107   }
 108
 109   // Smart pointer members.
 110   void reset(T* ptr = NULL) {
 111     depart();
 112     capture(ptr);
 113   }
 114   T* get() const { return value_; }
 115   T* operator->() const { return value_; }
 116   T& operator*() const { return *value_; }
 117   // Release ownership of the pointed object and returns it.
 118   // Sole ownership by this linked_ptr object is required.
 119   T* release() {
 120     bool last = link_.depart();
 121     CHECK(last);
 122     T* v = value_;
 123     value_ = NULL;
 124     return v;
 125   }
 126
 127   bool operator==(const T* p) const { return value_ == p; }
 128   bool operator!=(const T* p) const { return value_ != p; }
 129   template <typename U>
 130   bool operator==(linked_ptr<U> const& ptr) const {
 131     return value_ == ptr.get();
 132   }
 133   template <typename U>
 134   bool operator!=(linked_ptr<U> const& ptr) const {
 135     return value_ != ptr.get();
 136   }
 137
 138  private:
 139   template <typename U>
 140   friend class linked_ptr;
 141
 142   T* value_;
 143   linked_ptr_internal link_;
 144
 145   void depart() {
 146     if (link_.depart()) delete value_;
 147   }
 148
 149   void capture(T* ptr) {
 150     value_ = ptr;
 151     link_.join_new();
 152   }
 153
 154   template <typename U> void copy(linked_ptr<U> const* ptr) {
 155     value_ = ptr->get();
 156     if (value_)
 157       link_.join(&ptr->link_);
 158     else
 159       link_.join_new();
 160   }
 161 };
 162
 163 template<typename T> inline
 164 bool operator==(T* ptr, const linked_ptr<T>& x) {
 165   return ptr == x.get();
 166 }
 167
 168 template<typename T> inline
 169 bool operator!=(T* ptr, const linked_ptr<T>& x) {
 170   return ptr != x.get();
 171 }
 172
 173 // A function to convert T* into linked_ptr<T>
 174 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
 175 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
 176 template <typename T>
 177 linked_ptr<T> make_linked_ptr(T* ptr) {
 178   return linked_ptr<T>(ptr);
 179 }
 180
 181 #endif  // BASE_MEMORY_LINKED_PTR_H_