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v8/include/cppgc/liveness-broker.h

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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_