// 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 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_