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v8/include/cppgc/custom-space.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_CUSTOM_SPACE_H_
+#define INCLUDE_CPPGC_CUSTOM_SPACE_H_
+
+#include <stddef.h>
+
+namespace cppgc {
+
+/**
+ * Index identifying a custom space.
+ */
+struct CustomSpaceIndex {
+  constexpr CustomSpaceIndex(size_t value) : value(value) {}  // NOLINT
+  size_t value;
+};
+
+/**
+ * Top-level base class for custom spaces. Users must inherit from CustomSpace
+ * below.
+ */
+class CustomSpaceBase {
+ public:
+  virtual ~CustomSpaceBase() = default;
+  virtual CustomSpaceIndex GetCustomSpaceIndex() const = 0;
+  virtual bool IsCompactable() const = 0;
+};
+
+/**
+ * Base class custom spaces should directly inherit from. The class inheriting
+ * from `CustomSpace` must define `kSpaceIndex` as unique space index. These
+ * indices need for form a sequence starting at 0.
+ *
+ * Example:
+ * \code
+ * class CustomSpace1 : public CustomSpace<CustomSpace1> {
+ *  public:
+ *   static constexpr CustomSpaceIndex kSpaceIndex = 0;
+ * };
+ * class CustomSpace2 : public CustomSpace<CustomSpace2> {
+ *  public:
+ *   static constexpr CustomSpaceIndex kSpaceIndex = 1;
+ * };
+ * \endcode
+ */
+template <typename ConcreteCustomSpace>
+class CustomSpace : public CustomSpaceBase {
+ public:
+  /**
+   * Compaction is only supported on spaces that manually manage slots
+   * recording.
+   */
+  static constexpr bool kSupportsCompaction = false;
+
+  CustomSpaceIndex GetCustomSpaceIndex() const final {
+    return ConcreteCustomSpace::kSpaceIndex;
+  }
+  bool IsCompactable() const final {
+    return ConcreteCustomSpace::kSupportsCompaction;
+  }
+};
+
+/**
+ * User-overridable trait that allows pinning types to custom spaces.
+ */
+template <typename T, typename = void>
+struct SpaceTrait {
+  using Space = void;
+};
+
+namespace internal {
+
+template <typename CustomSpace>
+struct IsAllocatedOnCompactableSpaceImpl {
+  static constexpr bool value = CustomSpace::kSupportsCompaction;
+};
+
+template <>
+struct IsAllocatedOnCompactableSpaceImpl<void> {
+  // Non-custom spaces are by default not compactable.
+  static constexpr bool value = false;
+};
+
+template <typename T>
+struct IsAllocatedOnCompactableSpace {
+ public:
+  static constexpr bool value =
+      IsAllocatedOnCompactableSpaceImpl<typename SpaceTrait<T>::Space>::value;
+};
+
+}  // namespace internal
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_CUSTOM_SPACE_H_