File Explorer

1// Copyright 2020 the V8 project authors. All rights reserved.2// Use of this source code is governed by a BSD-style license that can be3// found in the LICENSE file.4 5#ifndef INCLUDE_CPPGC_LIVENESS_BROKER_H_6#define INCLUDE_CPPGC_LIVENESS_BROKER_H_7 8#include "cppgc/heap.h"9#include "cppgc/member.h"10#include "cppgc/sentinel-pointer.h"11#include "cppgc/trace-trait.h"12#include "v8config.h"  // NOLINT(build/include_directory)13 14namespace cppgc {15 16namespace internal {17class LivenessBrokerFactory;18}  // namespace internal19 20/**21 * The broker is passed to weak callbacks to allow (temporarily) querying22 * the liveness state of an object. References to non-live objects must be23 * cleared when `IsHeapObjectAlive()` returns false.24 *25 * \code26 * class GCedWithCustomWeakCallback final27 *   : public GarbageCollected<GCedWithCustomWeakCallback> {28 *  public:29 *   UntracedMember<Bar> bar;30 *31 *   void CustomWeakCallbackMethod(const LivenessBroker& broker) {32 *     if (!broker.IsHeapObjectAlive(bar))33 *       bar = nullptr;34 *   }35 *36 *   void Trace(cppgc::Visitor* visitor) const {37 *     visitor->RegisterWeakCallbackMethod<38 *         GCedWithCustomWeakCallback,39 *         &GCedWithCustomWeakCallback::CustomWeakCallbackMethod>(this);40 *   }41 * };42 * \endcode43 */44class V8_EXPORT LivenessBroker final {45 public:46  template <typename T>47  bool IsHeapObjectAlive(const T* object) const {48    // - nullptr objects are considered alive to allow weakness to be used from49    // stack while running into a conservative GC. Treating nullptr as dead50    // would mean that e.g. custom collections could not be strongified on51    // stack.52    // - Sentinel pointers are also preserved in weakness and not cleared.53    return !object || object == kSentinelPointer ||54           IsHeapObjectAliveImpl(55               TraceTrait<T>::GetTraceDescriptor(object).base_object_payload);56  }57 58  template <typename T>59  bool IsHeapObjectAlive(const WeakMember<T>& weak_member) const {60    return IsHeapObjectAlive<T>(weak_member.Get());61  }62 63  template <typename T>64  bool IsHeapObjectAlive(const UntracedMember<T>& untraced_member) const {65    return IsHeapObjectAlive<T>(untraced_member.Get());66  }67 68 private:69  LivenessBroker() = default;70 71  bool IsHeapObjectAliveImpl(const void*) const;72 73  friend class internal::LivenessBrokerFactory;74};75 76}  // namespace cppgc77 78#endif  // INCLUDE_CPPGC_LIVENESS_BROKER_H_79