1 // Copyright 2013 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 // This is a copy of base/linked_ptr.h with CHECKS/DCHECKS replaced with 6 // PP_DCHECKs. 7 // 8 // A "smart" pointer type with reference tracking. Every pointer to a 9 // particular object is kept on a circular linked list. When the last pointer 10 // to an object is destroyed or reassigned, the object is deleted. 11 // 12 // Used properly, this deletes the object when the last reference goes away. 13 // There are several caveats: 14 // - Like all reference counting schemes, cycles lead to leaks. 15 // - Each smart pointer is actually two pointers (8 bytes instead of 4). 16 // - Every time a pointer is released, the entire list of pointers to that 17 // object is traversed. This class is therefore NOT SUITABLE when there 18 // will often be more than two or three pointers to a particular object. 19 // - References are only tracked as long as linked_ptr<> objects are copied. 20 // If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS 21 // will happen (double deletion). 22 // 23 // A good use of this class is storing object references in STL containers. 24 // You can safely put linked_ptr<> in a vector<>. 25 // Other uses may not be as good. 26 // 27 // Note: If you use an incomplete type with linked_ptr<>, the class 28 // *containing* linked_ptr<> must have a constructor and destructor (even 29 // if they do nothing!). 30 // 31 // Thread Safety: 32 // A linked_ptr is NOT thread safe. Copying a linked_ptr object is 33 // effectively a read-write operation. 34 // 35 // Alternative: to linked_ptr is shared_ptr, which 36 // - is also two pointers in size (8 bytes for 32 bit addresses) 37 // - is thread safe for copying and deletion 38 // - supports weak_ptrs 39 40 #ifndef MEDIA_CDM_PPAPI_LINKED_PTR_H_ 41 #define MEDIA_CDM_PPAPI_LINKED_PTR_H_ 42 43 #include "ppapi/cpp/logging.h" 44 45 // This is used internally by all instances of linked_ptr<>. It needs to be 46 // a non-template class because different types of linked_ptr<> can refer to 47 // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)). 48 // So, it needs to be possible for different types of linked_ptr to participate 49 // in the same circular linked list, so we need a single class type here. 50 // 51 // DO NOT USE THIS CLASS DIRECTLY YOURSELF. Use linked_ptr<T>. 52 class linked_ptr_internal { 53 public: 54 // Create a new circle that includes only this instance. 55 void join_new() { 56 next_ = this; 57 } 58 59 // Join an existing circle. 60 void join(linked_ptr_internal const* ptr) { 61 next_ = ptr->next_; 62 ptr->next_ = this; 63 } 64 65 // Leave whatever circle we're part of. Returns true iff we were the 66 // last member of the circle. Once this is done, you can join() another. 67 bool depart() { 68 if (next_ == this) return true; 69 linked_ptr_internal const* p = next_; 70 while (p->next_ != this) p = p->next_; 71 p->next_ = next_; 72 return false; 73 } 74 75 private: 76 mutable linked_ptr_internal const* next_; 77 }; 78 79 template <typename T> 80 class linked_ptr { 81 public: 82 typedef T element_type; 83 84 // Take over ownership of a raw pointer. This should happen as soon as 85 // possible after the object is created. 86 explicit linked_ptr(T* ptr = NULL) { capture(ptr); } 87 ~linked_ptr() { depart(); } 88 89 // Copy an existing linked_ptr<>, adding ourselves to the list of references. 90 template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); } 91 92 linked_ptr(linked_ptr const& ptr) { 93 PP_DCHECK(&ptr != this); 94 copy(&ptr); 95 } 96 97 // Assignment releases the old value and acquires the new. 98 template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) { 99 depart(); 100 copy(&ptr); 101 return *this; 102 } 103 104 linked_ptr& operator=(linked_ptr const& ptr) { 105 if (&ptr != this) { 106 depart(); 107 copy(&ptr); 108 } 109 return *this; 110 } 111 112 // Smart pointer members. 113 void reset(T* ptr = NULL) { 114 depart(); 115 capture(ptr); 116 } 117 T* get() const { return value_; } 118 T* operator->() const { return value_; } 119 T& operator*() const { return *value_; } 120 // Release ownership of the pointed object and returns it. 121 // Sole ownership by this linked_ptr object is required. 122 T* release() { 123 bool last = link_.depart(); 124 PP_DCHECK(last); 125 (void)last; 126 T* v = value_; 127 value_ = NULL; 128 return v; 129 } 130 131 bool operator==(const T* p) const { return value_ == p; } 132 bool operator!=(const T* p) const { return value_ != p; } 133 template <typename U> 134 bool operator==(linked_ptr<U> const& ptr) const { 135 return value_ == ptr.get(); 136 } 137 template <typename U> 138 bool operator!=(linked_ptr<U> const& ptr) const { 139 return value_ != ptr.get(); 140 } 141 142 private: 143 template <typename U> 144 friend class linked_ptr; 145 146 T* value_; 147 linked_ptr_internal link_; 148 149 void depart() { 150 if (link_.depart()) delete value_; 151 } 152 153 void capture(T* ptr) { 154 value_ = ptr; 155 link_.join_new(); 156 } 157 158 template <typename U> void copy(linked_ptr<U> const* ptr) { 159 value_ = ptr->get(); 160 if (value_) 161 link_.join(&ptr->link_); 162 else 163 link_.join_new(); 164 } 165 }; 166 167 template<typename T> inline 168 bool operator==(T* ptr, const linked_ptr<T>& x) { 169 return ptr == x.get(); 170 } 171 172 template<typename T> inline 173 bool operator!=(T* ptr, const linked_ptr<T>& x) { 174 return ptr != x.get(); 175 } 176 177 // A function to convert T* into linked_ptr<T> 178 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation 179 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg)) 180 template <typename T> 181 linked_ptr<T> make_linked_ptr(T* ptr) { 182 return linked_ptr<T>(ptr); 183 } 184 185 #endif // MEDIA_CDM_PPAPI_LINKED_PTR_H_ 186
