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v8/include/cppgc/garbage-collected.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_GARBAGE_COLLECTED_H_
+#define INCLUDE_CPPGC_GARBAGE_COLLECTED_H_
+
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/platform.h"
+#include "cppgc/trace-trait.h"
+#include "cppgc/type-traits.h"
+
+namespace cppgc {
+
+class Visitor;
+
+/**
+ * Base class for managed objects. Only descendent types of `GarbageCollected`
+ * can be constructed using `MakeGarbageCollected()`. Must be inherited from as
+ * left-most base class.
+ *
+ * Types inheriting from GarbageCollected must provide a method of
+ * signature `void Trace(cppgc::Visitor*) const` that dispatchs all managed
+ * pointers to the visitor and delegates to garbage-collected base classes.
+ * The method must be virtual if the type is not directly a child of
+ * GarbageCollected and marked as final.
+ *
+ * \code
+ * // Example using final class.
+ * class FinalType final : public GarbageCollected<FinalType> {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) const {
+ *     // Dispatch using visitor->Trace(...);
+ *   }
+ * };
+ *
+ * // Example using non-final base class.
+ * class NonFinalBase : public GarbageCollected<NonFinalBase> {
+ *  public:
+ *   virtual void Trace(cppgc::Visitor*) const {}
+ * };
+ *
+ * class FinalChild final : public NonFinalBase {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) const final {
+ *     // Dispatch using visitor->Trace(...);
+ *     NonFinalBase::Trace(visitor);
+ *   }
+ * };
+ * \endcode
+ */
+template <typename T>
+class GarbageCollected {
+ public:
+  using IsGarbageCollectedTypeMarker = void;
+  using ParentMostGarbageCollectedType = T;
+
+  // Must use MakeGarbageCollected.
+  void* operator new(size_t) = delete;
+  void* operator new[](size_t) = delete;
+  // The garbage collector is taking care of reclaiming the object. Also,
+  // virtual destructor requires an unambiguous, accessible 'operator delete'.
+  void operator delete(void*) {
+#ifdef V8_ENABLE_CHECKS
+    internal::Fatal(
+        "Manually deleting a garbage collected object is not allowed");
+#endif  // V8_ENABLE_CHECKS
+  }
+  void operator delete[](void*) = delete;
+
+ protected:
+  GarbageCollected() = default;
+};
+
+/**
+ * Base class for managed mixin objects. Such objects cannot be constructed
+ * directly but must be mixed into the inheritance hierarchy of a
+ * GarbageCollected object.
+ *
+ * Types inheriting from GarbageCollectedMixin must override a virtual method
+ * of signature `void Trace(cppgc::Visitor*) const` that dispatchs all managed
+ * pointers to the visitor and delegates to base classes.
+ *
+ * \code
+ * class Mixin : public GarbageCollectedMixin {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) const override {
+ *     // Dispatch using visitor->Trace(...);
+ *   }
+ * };
+ * \endcode
+ */
+class GarbageCollectedMixin {
+ public:
+  using IsGarbageCollectedMixinTypeMarker = void;
+
+  /**
+   * This Trace method must be overriden by objects inheriting from
+   * GarbageCollectedMixin.
+   */
+  virtual void Trace(cppgc::Visitor*) const {}
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_GARBAGE_COLLECTED_H_