atomic_ref_count.h (2788B)
1 // Copyright 2011 The Chromium Authors 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 13 namespace base { 14 15 namespace subtle { 16 class RefCountedOverflowTest; 17 } // namespace subtle 18 19 class AtomicRefCount { 20 public: 21 constexpr AtomicRefCount() : ref_count_(0) {} 22 explicit constexpr AtomicRefCount(int initial_value) 23 : ref_count_(initial_value) {} 24 25 // Increment a reference count. 26 // Returns the previous value of the count. 27 int Increment() { return Increment(1); } 28 29 // Increment a reference count by "increment", which must exceed 0. 30 // Returns the previous value of the count. 31 int Increment(int increment) { 32 return ref_count_.fetch_add(increment, std::memory_order_relaxed); 33 } 34 35 // Decrement a reference count, and return whether the result is non-zero. 36 // Insert barriers to ensure that state written before the reference count 37 // became zero will be visible to a thread that has just made the count zero. 38 bool Decrement() { 39 // TODO(jbroman): Technically this doesn't need to be an acquire operation 40 // unless the result is 1 (i.e., the ref count did indeed reach zero). 41 // However, there are toolchain issues that make that not work as well at 42 // present (notably TSAN doesn't like it). 43 return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1; 44 } 45 46 // Return whether the reference count is one. If the reference count is used 47 // in the conventional way, a refrerence count of 1 implies that the current 48 // thread owns the reference and no other thread shares it. This call 49 // performs the test for a reference count of one, and performs the memory 50 // barrier needed for the owning thread to act on the object, knowing that it 51 // has exclusive access to the object. 52 bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; } 53 54 // Return whether the reference count is zero. With conventional object 55 // referencing counting, the object will be destroyed, so the reference count 56 // should never be zero. Hence this is generally used for a debug check. 57 bool IsZero() const { 58 return ref_count_.load(std::memory_order_acquire) == 0; 59 } 60 61 // Returns the current reference count (with no barriers). This is subtle, and 62 // should be used only for debugging. 63 int SubtleRefCountForDebug() const { 64 return ref_count_.load(std::memory_order_relaxed); 65 } 66 67 private: 68 friend subtle::RefCountedOverflowTest; 69 70 std::atomic_int ref_count_; 71 }; 72 73 } // namespace base 74 75 #endif // BASE_ATOMIC_REF_COUNT_H_