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 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.

 // Use of this source code is governed by a BSD-style license that can be

 // found in the LICENSE file.

 //

 // A "smart" pointer type with reference tracking.  Every pointer to a

 // particular object is kept on a circular linked list.  When the last pointer

 // to an object is destroyed or reassigned, the object is deleted.

 //

 // Used properly, this deletes the object when the last reference goes away.

 // There are several caveats:

 // - Like all reference counting schemes, cycles lead to leaks.

 // - Each smart pointer is actually two pointers (8 bytes instead of 4).

 // - Every time a pointer is released, the entire list of pointers to that

 //   object is traversed.  This class is therefore NOT SUITABLE when there

 //   will often be more than two or three pointers to a particular object.

 // - References are only tracked as long as linked_ptr<> objects are copied.

 //   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS

 //   will happen (double deletion).

 //

 // A good use of this class is storing object references in STL containers.

 // You can safely put linked_ptr<> in a vector<>.

 // Other uses may not be as good.

 //

 // Note: If you use an incomplete type with linked_ptr<>, the class

 // *containing* linked_ptr<> must have a constructor and destructor (even

 // if they do nothing!).

 //

 // Thread Safety:

 //   A linked_ptr is NOT thread safe. Copying a linked_ptr object is

 //   effectively a read-write operation.

 //

 // Alternative: to linked_ptr is shared_ptr, which

 //  - is also two pointers in size (8 bytes for 32 bit addresses)

 //  - is thread safe for copying and deletion

 //  - supports weak_ptrs

 #ifndef BASE_LINKED_PTR_H_

 #define BASE_LINKED_PTR_H_

 #include "base/logging.h"  // for CHECK macros

 // This is used internally by all instances of linked_ptr<>.  It needs to be

 // a non-template class because different types of linked_ptr<> can refer to

 // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).

 // So, it needs to be possible for different types of linked_ptr to participate

 // in the same circular linked list, so we need a single class type here.

 //

 // DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.

 class linked_ptr_internal {

  public:

   // Create a new circle that includes only this instance.

   void join_new() {

     next_ = this;

   }

   // Join an existing circle.

   void join(linked_ptr_internal const* ptr) {

     next_ = ptr->next_;

     ptr->next_ = this;

   }

   // Leave whatever circle we're part of.  Returns true iff we were the

   // last member of the circle.  Once this is done, you can join() another.

   bool depart() {

     if (next_ == this) return true;

     linked_ptr_internal const* p = next_;

     while (p->next_ != this) p = p->next_;

     p->next_ = next_;

     return false;

   }

  private:

   mutable linked_ptr_internal const* next_;

 };

 template <typename T>

 class linked_ptr {

  public:

   typedef T element_type;

   // Take over ownership of a raw pointer.  This should happen as soon as

   // possible after the object is created.

   explicit linked_ptr(T* ptr = NULL) { capture(ptr); }

   ~linked_ptr() { depart(); }

   // Copy an existing linked_ptr<>, adding ourselves to the list of references.

   template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }

   linked_ptr(linked_ptr const& ptr) { DCHECK_NE(&ptr, this); copy(&ptr); }

   // Assignment releases the old value and acquires the new.

   template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {

     depart();

     copy(&ptr);

     return *this;

   }

   linked_ptr& operator=(linked_ptr const& ptr) {

     if (&ptr != this) {

       depart();

       copy(&ptr);

     }

     return *this;

   }

   // Smart pointer members.

   void reset(T* ptr = NULL) { depart(); capture(ptr); }

   T* get() const { return value_; }

   T* operator->() const { return value_; }

   T& operator*() const { return *value_; }

   // Release ownership of the pointed object and returns it.

   // Sole ownership by this linked_ptr object is required.

   T* release() {

     bool last = link_.depart();

     CHECK(last);

     T* v = value_;

     value_ = NULL;

     return v;

   }

   bool operator==(const T* p) const { return value_ == p; }

   bool operator!=(const T* p) const { return value_ != p; }

   template <typename U>

   bool operator==(linked_ptr<U> const& ptr) const {

     return value_ == ptr.get();

   }

   template <typename U>

   bool operator!=(linked_ptr<U> const& ptr) const {

     return value_ != ptr.get();

   }

  private:

   template <typename U>

   friend class linked_ptr;

   T* value_;

   linked_ptr_internal link_;

   void depart() {

     if (link_.depart()) delete value_;

   }

   void capture(T* ptr) {

     value_ = ptr;

     link_.join_new();

   }

   template <typename U> void copy(linked_ptr<U> const* ptr) {

     value_ = ptr->get();

     if (value_)

       link_.join(&ptr->link_);

     else

       link_.join_new();

   }

 };

 template<typename T> inline

 bool operator==(T* ptr, const linked_ptr<T>& x) {

   return ptr == x.get();

 }

 template<typename T> inline

 bool operator!=(T* ptr, const linked_ptr<T>& x) {

   return ptr != x.get();

 }

 // A function to convert T* into linked_ptr<T>

 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation

 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))

 template <typename T>

 linked_ptr<T> make_linked_ptr(T* ptr) {

   return linked_ptr<T>(ptr);

 }

 #endif  // BASE_LINKED_PTR_H_