| // Copyright (c) 2011 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| // This is a low level implementation of atomic semantics for reference |
| // counting. Please use base/memory/ref_counted.h directly instead. |
| |
| #ifndef BASE_ATOMIC_REF_COUNT_H_ |
| #define BASE_ATOMIC_REF_COUNT_H_ |
| |
| #include <atomic> |
| |
| namespace base { |
| |
| class AtomicRefCount { |
| public: |
| constexpr AtomicRefCount() : ref_count_(0) {} |
| explicit constexpr AtomicRefCount(int initial_value) |
| : ref_count_(initial_value) {} |
| |
| // Increment a reference count. |
| // Returns the previous value of the count. |
| int Increment() { return Increment(1); } |
| |
| // Increment a reference count by "increment", which must exceed 0. |
| // Returns the previous value of the count. |
| int Increment(int increment) { |
| return ref_count_.fetch_add(increment, std::memory_order_relaxed); |
| } |
| |
| // Decrement a reference count, and return whether the result is non-zero. |
| // Insert barriers to ensure that state written before the reference count |
| // became zero will be visible to a thread that has just made the count zero. |
| bool Decrement() { |
| // TODO(jbroman): Technically this doesn't need to be an acquire operation |
| // unless the result is 1 (i.e., the ref count did indeed reach zero). |
| // However, there are toolchain issues that make that not work as well at |
| // present (notably TSAN doesn't like it). |
| return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1; |
| } |
| |
| // Return whether the reference count is one. If the reference count is used |
| // in the conventional way, a refrerence count of 1 implies that the current |
| // thread owns the reference and no other thread shares it. This call |
| // performs the test for a reference count of one, and performs the memory |
| // barrier needed for the owning thread to act on the object, knowing that it |
| // has exclusive access to the object. |
| bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; } |
| |
| // Return whether the reference count is zero. With conventional object |
| // referencing counting, the object will be destroyed, so the reference count |
| // should never be zero. Hence this is generally used for a debug check. |
| bool IsZero() const { |
| return ref_count_.load(std::memory_order_acquire) == 0; |
| } |
| |
| // Returns the current reference count (with no barriers). This is subtle, and |
| // should be used only for debugging. |
| int SubtleRefCountForDebug() const { |
| return ref_count_.load(std::memory_order_relaxed); |
| } |
| |
| private: |
| std::atomic_int ref_count_; |
| }; |
| |
| } // namespace base |
| |
| #endif // BASE_ATOMIC_REF_COUNT_H_ |