summaryrefslogtreecommitdiffstats
path: root/src/google/protobuf/stubs/shared_ptr.h
blob: d250bf4d33850d0a48507d10d1bcfd181cc2d853 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
// Protocol Buffers - Google's data interchange format
// Copyright 2014 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

// from google3/util/gtl/shared_ptr.h

#ifndef GOOGLE_PROTOBUF_STUBS_SHARED_PTR_H__
#define GOOGLE_PROTOBUF_STUBS_SHARED_PTR_H__

#include <google/protobuf/stubs/atomicops.h>

#include <algorithm>  // for swap
#include <stddef.h>
#include <memory>

namespace google {
namespace protobuf {
namespace internal {

// Alias to std::shared_ptr for any C++11 platform,
// and for any supported MSVC compiler.
#if !defined(UTIL_GTL_USE_STD_SHARED_PTR) && \
    (defined(COMPILER_MSVC) || defined(LANG_CXX11))
#define UTIL_GTL_USE_STD_SHARED_PTR 1
#endif

#if defined(UTIL_GTL_USE_STD_SHARED_PTR) && UTIL_GTL_USE_STD_SHARED_PTR

// These are transitional.  They will be going away soon.
// Please just #include <memory> and just type std::shared_ptr yourself, instead
// of relying on this file.
//
// Migration doc: http://go/std-shared-ptr-lsc
using std::enable_shared_from_this;
using std::shared_ptr;
using std::static_pointer_cast;
using std::weak_ptr;

#else  // below, UTIL_GTL_USE_STD_SHARED_PTR not set or set to 0.

// For everything else there is the google3 implementation.
inline bool RefCountDec(volatile Atomic32 *ptr) {
  return Barrier_AtomicIncrement(ptr, -1) != 0;
}

inline void RefCountInc(volatile Atomic32 *ptr) {
  NoBarrier_AtomicIncrement(ptr, 1);
}

template <typename T> class shared_ptr;
template <typename T> class weak_ptr;

// This class is an internal implementation detail for shared_ptr. If two
// shared_ptrs point to the same object, they also share a control block.
// An "empty" shared_pointer refers to NULL and also has a NULL control block.
// It contains all of the state that's needed for reference counting or any
// other kind of resource management. In this implementation the control block
// happens to consist of two atomic words, the reference count (the number
// of shared_ptrs that share ownership of the object) and the weak count
// (the number of weak_ptrs that observe the object, plus 1 if the
// refcount is nonzero).
//
// The "plus 1" is to prevent a race condition in the shared_ptr and
// weak_ptr destructors. We need to make sure the control block is
// only deleted once, so we need to make sure that at most one
// object sees the weak count decremented from 1 to 0.
class SharedPtrControlBlock {
  template <typename T> friend class shared_ptr;
  template <typename T> friend class weak_ptr;
 private:
  SharedPtrControlBlock() : refcount_(1), weak_count_(1) { }
  Atomic32 refcount_;
  Atomic32 weak_count_;
};

// Forward declaration. The class is defined below.
template <typename T> class enable_shared_from_this;

template <typename T>
class shared_ptr {
  template <typename U> friend class weak_ptr;
 public:
  typedef T element_type;

  shared_ptr() : ptr_(NULL), control_block_(NULL) {}

  explicit shared_ptr(T* ptr)
      : ptr_(ptr),
        control_block_(ptr != NULL ? new SharedPtrControlBlock : NULL) {
    // If p is non-null and T inherits from enable_shared_from_this, we
    // set up the data that shared_from_this needs.
    MaybeSetupWeakThis(ptr);
  }

  // Copy constructor: makes this object a copy of ptr, and increments
  // the reference count.
  template <typename U>
  shared_ptr(const shared_ptr<U>& ptr)
      : ptr_(NULL),
        control_block_(NULL) {
    Initialize(ptr);
  }
  // Need non-templated version to prevent the compiler-generated default
  shared_ptr(const shared_ptr<T>& ptr)
      : ptr_(NULL),
        control_block_(NULL) {
    Initialize(ptr);
  }

  // Assignment operator. Replaces the existing shared_ptr with ptr.
  // Increment ptr's reference count and decrement the one being replaced.
  template <typename U>
  shared_ptr<T>& operator=(const shared_ptr<U>& ptr) {
    if (ptr_ != ptr.ptr_) {
      shared_ptr<T> me(ptr);   // will hold our previous state to be destroyed.
      swap(me);
    }
    return *this;
  }

  // Need non-templated version to prevent the compiler-generated default
  shared_ptr<T>& operator=(const shared_ptr<T>& ptr) {
    if (ptr_ != ptr.ptr_) {
      shared_ptr<T> me(ptr);   // will hold our previous state to be destroyed.
      swap(me);
    }
    return *this;
  }

  // TODO(austern): Consider providing this constructor. The draft C++ standard
  // (20.8.10.2.1) includes it. However, it says that this constructor throws
  // a bad_weak_ptr exception when ptr is expired. Is it better to provide this
  // constructor and make it do something else, like fail with a CHECK, or to
  // leave this constructor out entirely?
  //
  // template <typename U>
  // shared_ptr(const weak_ptr<U>& ptr);

  ~shared_ptr() {
    if (ptr_ != NULL) {
      if (!RefCountDec(&control_block_->refcount_)) {
        delete ptr_;

        // weak_count_ is defined as the number of weak_ptrs that observe
        // ptr_, plus 1 if refcount_ is nonzero.
        if (!RefCountDec(&control_block_->weak_count_)) {
          delete control_block_;
        }
      }
    }
  }

  // Replaces underlying raw pointer with the one passed in.  The reference
  // count is set to one (or zero if the pointer is NULL) for the pointer
  // being passed in and decremented for the one being replaced.
  //
  // If you have a compilation error with this code, make sure you aren't
  // passing NULL, nullptr, or 0 to this function.  Call reset without an
  // argument to reset to a null ptr.
  template <typename Y>
  void reset(Y* p) {
    if (p != ptr_) {
      shared_ptr<T> tmp(p);
      tmp.swap(*this);
    }
  }

  void reset() {
    reset(static_cast<T*>(NULL));
  }

  // Exchanges the contents of this with the contents of r.  This function
  // supports more efficient swapping since it eliminates the need for a
  // temporary shared_ptr object.
  void swap(shared_ptr<T>& r) {
    using std::swap;  // http://go/using-std-swap
    swap(ptr_, r.ptr_);
    swap(control_block_, r.control_block_);
  }

  // The following function is useful for gaining access to the underlying
  // pointer when a shared_ptr remains in scope so the reference-count is
  // known to be > 0 (e.g. for parameter passing).
  T* get() const {
    return ptr_;
  }

  T& operator*() const {
    return *ptr_;
  }

  T* operator->() const {
    return ptr_;
  }

  long use_count() const {
    return control_block_ ? control_block_->refcount_ : 1;
  }

  bool unique() const {
    return use_count() == 1;
  }

 private:
  // If r is non-empty, initialize *this to share ownership with r,
  // increasing the underlying reference count.
  // If r is empty, *this remains empty.
  // Requires: this is empty, namely this->ptr_ == NULL.
  template <typename U>
  void Initialize(const shared_ptr<U>& r) {
    // This performs a static_cast on r.ptr_ to U*, which is a no-op since it
    // is already a U*. So initialization here requires that r.ptr_ is
    // implicitly convertible to T*.
    InitializeWithStaticCast<U>(r);
  }

  // Initializes *this as described in Initialize, but additionally performs a
  // static_cast from r.ptr_ (V*) to U*.
  // NOTE(gfc): We'd need a more general form to support const_pointer_cast and
  // dynamic_pointer_cast, but those operations are sufficiently discouraged
  // that supporting static_pointer_cast is sufficient.
  template <typename U, typename V>
  void InitializeWithStaticCast(const shared_ptr<V>& r) {
    if (r.control_block_ != NULL) {
      RefCountInc(&r.control_block_->refcount_);

      ptr_ = static_cast<U*>(r.ptr_);
      control_block_ = r.control_block_;
    }
  }

  // Helper function for the constructor that takes a raw pointer. If T
  // doesn't inherit from enable_shared_from_this<T> then we have nothing to
  // do, so this function is trivial and inline. The other version is declared
  // out of line, after the class definition of enable_shared_from_this.
  void MaybeSetupWeakThis(enable_shared_from_this<T>* ptr);
  void MaybeSetupWeakThis(...) { }

  T* ptr_;
  SharedPtrControlBlock* control_block_;

#ifndef SWIG
  template <typename U>
  friend class shared_ptr;

  template <typename U, typename V>
  friend shared_ptr<U> static_pointer_cast(const shared_ptr<V>& rhs);
#endif
};

// Matches the interface of std::swap as an aid to generic programming.
template <typename T> void swap(shared_ptr<T>& r, shared_ptr<T>& s) {
  r.swap(s);
}

template <typename T, typename U>
shared_ptr<T> static_pointer_cast(const shared_ptr<U>& rhs) {
  shared_ptr<T> lhs;
  lhs.template InitializeWithStaticCast<T>(rhs);
  return lhs;
}

// See comments at the top of the file for a description of why this
// class exists, and the draft C++ standard (as of July 2009 the
// latest draft is N2914) for the detailed specification.
template <typename T>
class weak_ptr {
  template <typename U> friend class weak_ptr;
 public:
  typedef T element_type;

  // Create an empty (i.e. already expired) weak_ptr.
  weak_ptr() : ptr_(NULL), control_block_(NULL) { }

  // Create a weak_ptr that observes the same object that ptr points
  // to.  Note that there is no race condition here: we know that the
  // control block can't disappear while we're looking at it because
  // it is owned by at least one shared_ptr, ptr.
  template <typename U> weak_ptr(const shared_ptr<U>& ptr) {
    CopyFrom(ptr.ptr_, ptr.control_block_);
  }

  // Copy a weak_ptr. The object it points to might disappear, but we
  // don't care: we're only working with the control block, and it can't
  // disappear while we're looking at because it's owned by at least one
  // weak_ptr, ptr.
  template <typename U> weak_ptr(const weak_ptr<U>& ptr) {
    CopyFrom(ptr.ptr_, ptr.control_block_);
  }

  // Need non-templated version to prevent default copy constructor
  weak_ptr(const weak_ptr& ptr) {
    CopyFrom(ptr.ptr_, ptr.control_block_);
  }

  // Destroy the weak_ptr. If no shared_ptr owns the control block, and if
  // we are the last weak_ptr to own it, then it can be deleted. Note that
  // weak_count_ is defined as the number of weak_ptrs sharing this control
  // block, plus 1 if there are any shared_ptrs. We therefore know that it's
  // safe to delete the control block when weak_count_ reaches 0, without
  // having to perform any additional tests.
  ~weak_ptr() {
    if (control_block_ != NULL &&
        !RefCountDec(&control_block_->weak_count_)) {
      delete control_block_;
    }
  }

  weak_ptr& operator=(const weak_ptr& ptr) {
    if (&ptr != this) {
      weak_ptr tmp(ptr);
      tmp.swap(*this);
    }
    return *this;
  }
  template <typename U> weak_ptr& operator=(const weak_ptr<U>& ptr) {
    weak_ptr tmp(ptr);
    tmp.swap(*this);
    return *this;
  }
  template <typename U> weak_ptr& operator=(const shared_ptr<U>& ptr) {
    weak_ptr tmp(ptr);
    tmp.swap(*this);
    return *this;
  }

  void swap(weak_ptr& ptr) {
    using std::swap;  // http://go/using-std-swap
    swap(ptr_, ptr.ptr_);
    swap(control_block_, ptr.control_block_);
  }

  void reset() {
    weak_ptr tmp;
    tmp.swap(*this);
  }

  // Return the number of shared_ptrs that own the object we are observing.
  // Note that this number can be 0 (if this pointer has expired).
  long use_count() const {
    return control_block_ != NULL ? control_block_->refcount_ : 0;
  }

  bool expired() const { return use_count() == 0; }

  // Return a shared_ptr that owns the object we are observing. If we
  // have expired, the shared_ptr will be empty. We have to be careful
  // about concurrency, though, since some other thread might be
  // destroying the last owning shared_ptr while we're in this
  // function.  We want to increment the refcount only if it's nonzero
  // and get the new value, and we want that whole operation to be
  // atomic.
  shared_ptr<T> lock() const {
    shared_ptr<T> result;
    if (control_block_ != NULL) {
      Atomic32 old_refcount;
      do {
        old_refcount = control_block_->refcount_;
        if (old_refcount == 0)
          break;
      } while (old_refcount !=
               NoBarrier_CompareAndSwap(
                   &control_block_->refcount_, old_refcount,
                   old_refcount + 1));
      if (old_refcount > 0) {
        result.ptr_ = ptr_;
        result.control_block_ = control_block_;
      }
    }

    return result;
  }

 private:
  void CopyFrom(T* ptr, SharedPtrControlBlock* control_block) {
    ptr_ = ptr;
    control_block_ = control_block;
    if (control_block_ != NULL)
      RefCountInc(&control_block_->weak_count_);
  }

 private:
  element_type* ptr_;
  SharedPtrControlBlock* control_block_;
};

template <typename T> void swap(weak_ptr<T>& r, weak_ptr<T>& s) {
  r.swap(s);
}

// See comments at the top of the file for a description of why this class
// exists, and section 20.8.10.5 of the draft C++ standard (as of July 2009
// the latest draft is N2914) for the detailed specification.
template <typename T>
class enable_shared_from_this {
  friend class shared_ptr<T>;
 public:
  // Precondition: there must be a shared_ptr that owns *this and that was
  // created, directly or indirectly, from a raw pointer of type T*. (The
  // latter part of the condition is technical but not quite redundant; it
  // rules out some complicated uses involving inheritance hierarchies.)
  shared_ptr<T> shared_from_this() {
    // Behavior is undefined if the precondition isn't satisfied; we choose
    // to die with a CHECK failure.
    CHECK(!weak_this_.expired()) << "No shared_ptr owns this object";
    return weak_this_.lock();
  }
  shared_ptr<const T> shared_from_this() const {
    CHECK(!weak_this_.expired()) << "No shared_ptr owns this object";
    return weak_this_.lock();
  }

 protected:
  enable_shared_from_this() { }
  enable_shared_from_this(const enable_shared_from_this& other) { }
  enable_shared_from_this& operator=(const enable_shared_from_this& other) {
    return *this;
  }
  ~enable_shared_from_this() { }

 private:
  weak_ptr<T> weak_this_;
};

// This is a helper function called by shared_ptr's constructor from a raw
// pointer. If T inherits from enable_shared_from_this<T>, it sets up
// weak_this_ so that shared_from_this works correctly. If T does not inherit
// from weak_this we get a different overload, defined inline, which does
// nothing.
template<typename T>
void shared_ptr<T>::MaybeSetupWeakThis(enable_shared_from_this<T>* ptr) {
  if (ptr) {
    CHECK(ptr->weak_this_.expired()) << "Object already owned by a shared_ptr";
    ptr->weak_this_ = *this;
  }
}

#endif  // UTIL_GTL_USE_STD_SHARED_PTR

}  // internal
}  // namespace protobuf
}  // namespace google

#endif  // GOOGLE_PROTOBUF_STUBS_SHARED_PTR_H__