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      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_
    182