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// 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_