diff -r 000000000000 -r 6474c204b198 mfbt/EnumeratedArray.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mfbt/EnumeratedArray.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,81 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+/* EnumeratedArray is like Array, but indexed by a typed enum. */
+
+#ifndef mozilla_EnumeratedArray_h
+#define mozilla_EnumeratedArray_h
+
+#include "mozilla/Array.h"
+#include "mozilla/TypedEnum.h"
+
+namespace mozilla {
+
+/**
+ * EnumeratedArray is a fixed-size array container for use when an
+ * array is indexed by a specific enum class, as currently implemented
+ * by MOZ_BEGIN_ENUM_CLASS.
+ *
+ * This provides type safety by guarding at compile time against accidentally
+ * indexing such arrays with unrelated values. This also removes the need
+ * for manual casting when using a typed enum value to index arrays.
+ *
+ * Aside from the typing of indices, EnumeratedArray is similar to Array.
+ *
+ * Example:
+ *
+ *   MOZ_BEGIN_ENUM_CLASS(AnimalSpecies)
+ *     Cow,
+ *     Sheep,
+ *     Count
+ *   MOZ_END_ENUM_CLASS(AnimalSpecies)
+ *
+ *   EnumeratedArray<AnimalSpecies, AnimalSpecies::Count, int> headCount;
+ *
+ *   headCount[AnimalSpecies::Cow] = 17;
+ *   headCount[AnimalSpecies::Sheep] = 30;
+ *
+ */
+template<typename IndexType,
+         MOZ_TEMPLATE_ENUM_CLASS_ENUM_TYPE(IndexType) SizeAsEnumValue,
+         typename ValueType>
+class EnumeratedArray
+{
+  public:
+    static const size_t Size = size_t(SizeAsEnumValue);
+
+  private:
+    Array<ValueType, Size> mArray;
+
+  public:
+    EnumeratedArray() {}
+
+    explicit EnumeratedArray(const EnumeratedArray& aOther)
+    {
+      for (size_t i = 0; i < Size; i++)
+        mArray[i] = aOther.mArray[i];
+    }
+
+    explicit EnumeratedArray(const ValueType (&aOther)[Size])
+    {
+      for (size_t i = 0; i < Size; i++)
+        mArray[i] = aOther[i];
+    }
+
+    ValueType& operator[](IndexType aIndex)
+    {
+      return mArray[size_t(aIndex)];
+    }
+
+    const ValueType& operator[](IndexType aIndex) const
+    {
+      return mArray[size_t(aIndex)];
+    }
+};
+
+} // namespace mozilla
+
+#endif // mozilla_EnumeratedArray_h