1 files changed, 167 insertions, 0 deletions
diff --git a/layout/base/ArenaRefPtr.h b/layout/base/ArenaRefPtr.h
new file mode 100644
index 000000000..bc8ecaa70
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+++ b/layout/base/ArenaRefPtr.h
@@ -0,0 +1,167 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=2 sw=2 et tw=78:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+/* smart pointer for strong references to nsPresArena-allocated objects
+   that might be held onto until the arena's destruction */
+
+#include "mozilla/Assertions.h"
+#include "mozilla/RefPtr.h"
+
+#ifndef mozilla_ArenaRefPtr_h
+#define mozilla_ArenaRefPtr_h
+
+class nsPresArena;
+
+namespace mozilla {
+
+/**
+ * A class for holding strong references to nsPresArena-allocated
+ * objects.
+ *
+ * Since the arena's lifetime is not related to the refcounts
+ * of the objects allocated within it, it is possible to have a strong
+ * reference to an arena-allocated object that lives until the
+ * destruction of the arena.  An ArenaRefPtr acts like a weak reference
+ * in that it will clear its referent if the arena is about to go away.
+ *
+ * T must be a class that has these two methods:
+ *
+ *   static mozilla::ArenaObjectID ArenaObjectID();
+ *   U* Arena();
+ *
+ * where U is a class that has these two methods:
+ *
+ *   void RegisterArenaRefPtr(ArenaRefPtr<T>*);
+ *   void DeregisterArenaRefPtr(ArenaRefPtr<T>*);
+ *
+ * Currently, both nsPresArena and nsIPresShell can be used as U.
+ *
+ * The ArenaObjectID method must return the mozilla::ArenaObjectID that
+ * uniquely identifies T, and the Arena method must return the nsPresArena
+ * (or a proxy for it) in which the object was allocated.
+ */
+template<typename T>
+class ArenaRefPtr
+{
+  friend class ::nsPresArena;
+
+public:
+  ArenaRefPtr()
+  {
+    AssertValidType();
+  }
+
+  template<typename I>
+  MOZ_IMPLICIT ArenaRefPtr(already_AddRefed<I>& aRhs)
+  {
+    AssertValidType();
+    assign(aRhs);
+  }
+
+  template<typename I>
+  MOZ_IMPLICIT ArenaRefPtr(already_AddRefed<I>&& aRhs)
+  {
+    AssertValidType();
+    assign(aRhs);
+  }
+
+  MOZ_IMPLICIT ArenaRefPtr(T* aRhs)
+  {
+    AssertValidType();
+    assign(aRhs);
+  }
+
+  template<typename I>
+  ArenaRefPtr<T>& operator=(already_AddRefed<I>& aRhs)
+  {
+    assign(aRhs);
+    return *this;
+  }
+
+  template<typename I>
+  ArenaRefPtr<T>& operator=(already_AddRefed<I>&& aRhs)
+  {
+    assign(aRhs);
+    return *this;
+  }
+
+  ArenaRefPtr<T>& operator=(T* aRhs)
+  {
+    assign(aRhs);
+    return *this;
+  }
+
+  ~ArenaRefPtr() { assign(nullptr); }
+
+#ifdef MOZ_HAVE_REF_QUALIFIERS
+  operator T*() const & { return get(); }
+  operator T*() const && = delete;
+  explicit operator bool() const { return !!mPtr; }
+  bool operator!() const { return !mPtr; }
+#else
+  operator T*() const { return get(); }
+#endif
+
+  T* operator->() const { return mPtr.operator->(); }
+  T& operator*() const { return *get(); }
+
+  T* get() const { return mPtr; }
+
+private:
+  void AssertValidType();
+
+  /**
+   * Clears the pointer to the arena-allocated object but skips the usual
+   * step of deregistering the ArenaRefPtr from the nsPresArena.  This
+   * method is called by nsPresArena when clearing all registered ArenaRefPtrs
+   * so that it can deregister them all at once, avoiding hash table churn.
+   */
+  void ClearWithoutDeregistering()
+  {
+    mPtr = nullptr;
+  }
+
+  template<typename I>
+  void assign(already_AddRefed<I>& aSmartPtr)
+  {
+    RefPtr<T> newPtr(aSmartPtr);
+    assignFrom(newPtr);
+  }
+
+  template<typename I>
+  void assign(already_AddRefed<I>&& aSmartPtr)
+  {
+    RefPtr<T> newPtr(aSmartPtr);
+    assignFrom(newPtr);
+  }
+
+  void assign(T* aPtr) { assignFrom(aPtr); }
+
+  template<typename I>
+  void assignFrom(I& aPtr)
+  {
+    if (aPtr == mPtr) {
+      return;
+    }
+    bool sameArena = mPtr && aPtr && mPtr->Arena() == aPtr->Arena();
+    if (mPtr && !sameArena) {
+      MOZ_ASSERT(mPtr->Arena());
+      mPtr->Arena()->DeregisterArenaRefPtr(this);
+    }
+    mPtr = Move(aPtr);
+    if (mPtr && !sameArena) {
+      MOZ_ASSERT(mPtr->Arena());
+      mPtr->Arena()->RegisterArenaRefPtr(this);
+    }
+  }
+
+  RefPtr<T> mPtr;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_ArenaRefPtr_h