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// This is a low level implementation of atomic semantics for reference 6// counting. Please use base/memory/ref_counted.h directly instead. 7 8#ifndef BASE_ATOMIC_REF_COUNT_H_ 9#define BASE_ATOMIC_REF_COUNT_H_ 10 11#include <atomic> 12 13namespace base { 14 15class AtomicRefCount { 16 public: 17 constexpr AtomicRefCount() : ref_count_(0) {} 18 explicit constexpr AtomicRefCount(int initial_value) 19 : ref_count_(initial_value) {} 20 21 // Increment a reference count. 22 void Increment() { Increment(1); } 23 24 // Increment a reference count by "increment", which must exceed 0. 25 void Increment(int increment) { 26 ref_count_.fetch_add(increment, std::memory_order_relaxed); 27 } 28 29 // Decrement a reference count, and return whether the result is non-zero. 30 // Insert barriers to ensure that state written before the reference count 31 // became zero will be visible to a thread that has just made the count zero. 32 bool Decrement() { 33 // TODO(jbroman): Technically this doesn't need to be an acquire operation 34 // unless the result is 1 (i.e., the ref count did indeed reach zero). 35 // However, there are toolchain issues that make that not work as well at 36 // present (notably TSAN doesn't like it). 37 return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1; 38 } 39 40 // Return whether the reference count is one. If the reference count is used 41 // in the conventional way, a reference count of 1 implies that the current 42 // thread owns the reference and no other thread shares it. This call 43 // performs the test for a reference count of one, and performs the memory 44 // barrier needed for the owning thread to act on the object, knowing that it 45 // has exclusive access to the object. 46 bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; } 47 48 // Return whether the reference count is zero. With conventional object 49 // referencing counting, the object will be destroyed, so the reference count 50 // should never be zero. Hence this is generally used for a debug check. 51 bool IsZero() const { 52 return ref_count_.load(std::memory_order_acquire) == 0; 53 } 54 55 // Returns the current reference count (with no barriers). This is subtle, and 56 // should be used only for debugging. 57 int SubtleRefCountForDebug() const { 58 return ref_count_.load(std::memory_order_relaxed); 59 } 60 61 private: 62 std::atomic_int ref_count_; 63}; 64 65} // namespace base 66 67#endif // BASE_ATOMIC_REF_COUNT_H_ 68