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gtest-linked_ptr.h
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1 // Copyright 2003 Google Inc.
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18 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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29 //
30 // Authors: Dan Egnor (egnor@google.com)
31 //
32 // A "smart" pointer type with reference tracking. Every pointer to a
33 // particular object is kept on a circular linked list. When the last pointer
34 // to an object is destroyed or reassigned, the object is deleted.
35 //
36 // Used properly, this deletes the object when the last reference goes away.
37 // There are several caveats:
38 // - Like all reference counting schemes, cycles lead to leaks.
39 // - Each smart pointer is actually two pointers (8 bytes instead of 4).
40 // - Every time a pointer is assigned, the entire list of pointers to that
41 // object is traversed. This class is therefore NOT SUITABLE when there
42 // will often be more than two or three pointers to a particular object.
43 // - References are only tracked as long as linked_ptr<> objects are copied.
44 // If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
45 // will happen (double deletion).
46 //
47 // A good use of this class is storing object references in STL containers.
48 // You can safely put linked_ptr<> in a vector<>.
49 // Other uses may not be as good.
50 //
51 // Note: If you use an incomplete type with linked_ptr<>, the class
52 // *containing* linked_ptr<> must have a constructor and destructor (even
53 // if they do nothing!).
54 //
55 // Bill Gibbons suggested we use something like this.
56 //
57 // Thread Safety:
58 // Unlike other linked_ptr implementations, in this implementation
59 // a linked_ptr object is thread-safe in the sense that:
60 // - it's safe to copy linked_ptr objects concurrently,
61 // - it's safe to copy *from* a linked_ptr and read its underlying
62 // raw pointer (e.g. via get()) concurrently, and
63 // - it's safe to write to two linked_ptrs that point to the same
64 // shared object concurrently.
65 // TODO(wan@google.com): rename this to safe_linked_ptr to avoid
66 // confusion with normal linked_ptr.
67 
68 #ifndef GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_
69 #define GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_
70 
71 #include <stdlib.h>
72 #include <assert.h>
73 
75 
76 namespace testing {
77 namespace internal {
78 
79 // Protects copying of all linked_ptr objects.
80 GTEST_API_ GTEST_DECLARE_STATIC_MUTEX_(g_linked_ptr_mutex);
81 
82 // This is used internally by all instances of linked_ptr<>. It needs to be
83 // a non-template class because different types of linked_ptr<> can refer to
84 // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
85 // So, it needs to be possible for different types of linked_ptr to participate
86 // in the same circular linked list, so we need a single class type here.
87 //
88 // DO NOT USE THIS CLASS DIRECTLY YOURSELF. Use linked_ptr<T>.
90  public:
91  // Create a new circle that includes only this instance.
92  void join_new() {
93  next_ = this;
94  }
95 
96  // Many linked_ptr operations may change p.link_ for some linked_ptr
97  // variable p in the same circle as this object. Therefore we need
98  // to prevent two such operations from occurring concurrently.
99  //
100  // Note that different types of linked_ptr objects can coexist in a
101  // circle (e.g. linked_ptr<Base>, linked_ptr<Derived1>, and
102  // linked_ptr<Derived2>). Therefore we must use a single mutex to
103  // protect all linked_ptr objects. This can create serious
104  // contention in production code, but is acceptable in a testing
105  // framework.
106 
107  // Join an existing circle.
108  void join(linked_ptr_internal const* ptr)
109  GTEST_LOCK_EXCLUDED_(g_linked_ptr_mutex) {
110  MutexLock lock(&g_linked_ptr_mutex);
111 
112  linked_ptr_internal const* p = ptr;
113  while (p->next_ != ptr) {
114  assert(p->next_ != this &&
115  "Trying to join() a linked ring we are already in. "
116  "Is GMock thread safety enabled?");
117  p = p->next_;
118  }
119  p->next_ = this;
120  next_ = ptr;
121  }
122 
123  // Leave whatever circle we're part of. Returns true if we were the
124  // last member of the circle. Once this is done, you can join() another.
125  bool depart()
126  GTEST_LOCK_EXCLUDED_(g_linked_ptr_mutex) {
127  MutexLock lock(&g_linked_ptr_mutex);
128 
129  if (next_ == this) return true;
130  linked_ptr_internal const* p = next_;
131  while (p->next_ != this) {
132  assert(p->next_ != next_ &&
133  "Trying to depart() a linked ring we are not in. "
134  "Is GMock thread safety enabled?");
135  p = p->next_;
136  }
137  p->next_ = next_;
138  return false;
139  }
140 
141  private:
142  mutable linked_ptr_internal const* next_;
143 };
144 
145 template <typename T>
146 class linked_ptr {
147  public:
148  typedef T element_type;
149 
150  // Take over ownership of a raw pointer. This should happen as soon as
151  // possible after the object is created.
152  explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
154 
155  // Copy an existing linked_ptr<>, adding ourselves to the list of references.
156  template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }
157  linked_ptr(linked_ptr const& ptr) { // NOLINT
158  assert(&ptr != this);
159  copy(&ptr);
160  }
161 
162  // Assignment releases the old value and acquires the new.
163  template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
164  depart();
165  copy(&ptr);
166  return *this;
167  }
168 
170  if (&ptr != this) {
171  depart();
172  copy(&ptr);
173  }
174  return *this;
175  }
176 
177  // Smart pointer members.
178  void reset(T* ptr = NULL) {
179  depart();
180  capture(ptr);
181  }
182  T* get() const { return value_; }
183  T* operator->() const { return value_; }
184  T& operator*() const { return *value_; }
185 
186  bool operator==(T* p) const { return value_ == p; }
187  bool operator!=(T* p) const { return value_ != p; }
188  template <typename U>
189  bool operator==(linked_ptr<U> const& ptr) const {
190  return value_ == ptr.get();
191  }
192  template <typename U>
193  bool operator!=(linked_ptr<U> const& ptr) const {
194  return value_ != ptr.get();
195  }
196 
197  private:
198  template <typename U>
199  friend class linked_ptr;
200 
203 
204  void depart() {
205  if (link_.depart()) delete value_;
206  }
207 
208  void capture(T* ptr) {
209  value_ = ptr;
210  link_.join_new();
211  }
212 
213  template <typename U> void copy(linked_ptr<U> const* ptr) {
214  value_ = ptr->get();
215  if (value_)
216  link_.join(&ptr->link_);
217  else
218  link_.join_new();
219  }
220 };
221 
222 template<typename T> inline
223 bool operator==(T* ptr, const linked_ptr<T>& x) {
224  return ptr == x.get();
225 }
226 
227 template<typename T> inline
228 bool operator!=(T* ptr, const linked_ptr<T>& x) {
229  return ptr != x.get();
230 }
231 
232 // A function to convert T* into linked_ptr<T>
233 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
234 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
235 template <typename T>
237  return linked_ptr<T>(ptr);
238 }
239 
240 } // namespace internal
241 } // namespace testing
242 
243 #endif // GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_