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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+// 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_