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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 
74 #include "gtest/internal/gtest-port.h"
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>.
89 class linked_ptr_internal {
90  public:
91   // Create a new circle that includes only this instance.
join_new()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.
join(linked_ptr_internal const * ptr)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) p = p->next_;
114     p->next_ = this;
115     next_ = ptr;
116   }
117 
118   // Leave whatever circle we're part of.  Returns true if we were the
119   // last member of the circle.  Once this is done, you can join() another.
depart()120   bool depart()
121       GTEST_LOCK_EXCLUDED_(g_linked_ptr_mutex) {
122     MutexLock lock(&g_linked_ptr_mutex);
123 
124     if (next_ == this) return true;
125     linked_ptr_internal const* p = next_;
126     while (p->next_ != this) p = p->next_;
127     p->next_ = next_;
128     return false;
129   }
130 
131  private:
132   mutable linked_ptr_internal const* next_;
133 };
134 
135 template <typename T>
136 class linked_ptr {
137  public:
138   typedef T element_type;
139 
140   // Take over ownership of a raw pointer.  This should happen as soon as
141   // possible after the object is created.
142   explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
~linked_ptr()143   ~linked_ptr() { depart(); }
144 
145   // Copy an existing linked_ptr<>, adding ourselves to the list of references.
linked_ptr(linked_ptr<U> const & ptr)146   template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }
linked_ptr(linked_ptr const & ptr)147   linked_ptr(linked_ptr const& ptr) {  // NOLINT
148     assert(&ptr != this);
149     copy(&ptr);
150   }
151 
152   // Assignment releases the old value and acquires the new.
153   template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
154     depart();
155     copy(&ptr);
156     return *this;
157   }
158 
159   linked_ptr& operator=(linked_ptr const& ptr) {
160     if (&ptr != this) {
161       depart();
162       copy(&ptr);
163     }
164     return *this;
165   }
166 
167   // Smart pointer members.
168   void reset(T* ptr = NULL) {
169     depart();
170     capture(ptr);
171   }
get()172   T* get() const { return value_; }
173   T* operator->() const { return value_; }
174   T& operator*() const { return *value_; }
175 
176   bool operator==(T* p) const { return value_ == p; }
177   bool operator!=(T* p) const { return value_ != p; }
178   template <typename U>
179   bool operator==(linked_ptr<U> const& ptr) const {
180     return value_ == ptr.get();
181   }
182   template <typename U>
183   bool operator!=(linked_ptr<U> const& ptr) const {
184     return value_ != ptr.get();
185   }
186 
187  private:
188   template <typename U>
189   friend class linked_ptr;
190 
191   T* value_;
192   linked_ptr_internal link_;
193 
depart()194   void depart() {
195     if (link_.depart()) delete value_;
196   }
197 
capture(T * ptr)198   void capture(T* ptr) {
199     value_ = ptr;
200     link_.join_new();
201   }
202 
copy(linked_ptr<U> const * ptr)203   template <typename U> void copy(linked_ptr<U> const* ptr) {
204     value_ = ptr->get();
205     if (value_)
206       link_.join(&ptr->link_);
207     else
208       link_.join_new();
209   }
210 };
211 
212 template<typename T> inline
213 bool operator==(T* ptr, const linked_ptr<T>& x) {
214   return ptr == x.get();
215 }
216 
217 template<typename T> inline
218 bool operator!=(T* ptr, const linked_ptr<T>& x) {
219   return ptr != x.get();
220 }
221 
222 // A function to convert T* into linked_ptr<T>
223 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
224 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
225 template <typename T>
make_linked_ptr(T * ptr)226 linked_ptr<T> make_linked_ptr(T* ptr) {
227   return linked_ptr<T>(ptr);
228 }
229 
230 }  // namespace internal
231 }  // namespace testing
232 
233 #endif  // GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_
234