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// 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.
#include <atomic>
namespace base {
class AtomicRefCount {
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);
std::atomic_int ref_count_;
} // namespace base