name : liveness-broker.h // Copyright 2020 the V8 project authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. #ifndef INCLUDE_CPPGC_LIVENESS_BROKER_H_ #define INCLUDE_CPPGC_LIVENESS_BROKER_H_ #include "cppgc/heap.h" #include "cppgc/member.h" #include "cppgc/sentinel-pointer.h" #include "cppgc/trace-trait.h" #include "v8config.h" // NOLINT(build/include_directory) namespace cppgc { namespace internal { class LivenessBrokerFactory; } // namespace internal /** * The broker is passed to weak callbacks to allow (temporarily) querying * the liveness state of an object. References to non-live objects must be * cleared when `IsHeapObjectAlive()` returns false. * * \code * class GCedWithCustomWeakCallback final * : public GarbageCollected<GCedWithCustomWeakCallback> { * public: * UntracedMember<Bar> bar; * * void CustomWeakCallbackMethod(const LivenessBroker& broker) { * if (!broker.IsHeapObjectAlive(bar)) * bar = nullptr; * } * * void Trace(cppgc::Visitor* visitor) const { * visitor->RegisterWeakCallbackMethod< * GCedWithCustomWeakCallback, * &GCedWithCustomWeakCallback::CustomWeakCallbackMethod>(this); * } * }; * \endcode */ class V8_EXPORT LivenessBroker final { public: template <typename T> bool IsHeapObjectAlive(const T* object) const { // - nullptr objects are considered alive to allow weakness to be used from // stack while running into a conservative GC. Treating nullptr as dead // would mean that e.g. custom collections could not be strongified on // stack. // - Sentinel pointers are also preserved in weakness and not cleared. return !object || object == kSentinelPointer || IsHeapObjectAliveImpl( TraceTrait<T>::GetTraceDescriptor(object).base_object_payload); } template <typename T> bool IsHeapObjectAlive(const WeakMember<T>& weak_member) const { return IsHeapObjectAlive<T>(weak_member.Get()); } template <typename T> bool IsHeapObjectAlive(const UntracedMember<T>& untraced_member) const { return IsHeapObjectAlive<T>(untraced_member.Get()); } private: LivenessBroker() = default; bool IsHeapObjectAliveImpl(const void*) const; friend class internal::LivenessBrokerFactory; }; } // namespace cppgc #endif // INCLUDE_CPPGC_LIVENESS_BROKER_H_