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
