diff -r 000000000000 -r 6474c204b198 dom/bindings/Nullable.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/bindings/Nullable.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,122 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-*/
+/* vim: set ts=2 sw=2 et tw=79: */
+/* 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/. */
+
+#ifndef mozilla_dom_Nullable_h
+#define mozilla_dom_Nullable_h
+
+#include "mozilla/Assertions.h"
+#include "nsTArrayForwardDeclare.h"
+#include "mozilla/Move.h"
+
+namespace mozilla {
+namespace dom {
+
+// Support for nullable types
+template <typename T>
+struct Nullable
+{
+private:
+  // mIsNull MUST COME FIRST because otherwise the casting in our array
+  // conversion operators would shift where it is found in the struct.
+  bool mIsNull;
+  T mValue;
+
+public:
+  Nullable()
+    : mIsNull(true)
+  {}
+
+  explicit Nullable(T aValue)
+    : mIsNull(false)
+    , mValue(aValue)
+  {}
+
+  explicit Nullable(Nullable<T>&& aOther)
+    : mIsNull(aOther.mIsNull)
+    , mValue(mozilla::Move(aOther.mValue))
+  {}
+
+  Nullable(const Nullable<T>& aOther)
+    : mIsNull(aOther.mIsNull)
+    , mValue(aOther.mValue)
+  {}
+
+  void operator=(const Nullable<T>& aOther)
+  {
+    mIsNull = aOther.mIsNull;
+    mValue = aOther.mValue;
+  }
+
+  void SetValue(T aValue) {
+    mValue = aValue;
+    mIsNull = false;
+  }
+
+  // For cases when |T| is some type with nontrivial copy behavior, we may want
+  // to get a reference to our internal copy of T and work with it directly
+  // instead of relying on the copying version of SetValue().
+  T& SetValue() {
+    mIsNull = false;
+    return mValue;
+  }
+
+  void SetNull() {
+    mIsNull = true;
+  }
+
+  const T& Value() const {
+    MOZ_ASSERT(!mIsNull);
+    return mValue;
+  }
+
+  T& Value() {
+    MOZ_ASSERT(!mIsNull);
+    return mValue;
+  }
+
+  bool IsNull() const {
+    return mIsNull;
+  }
+
+  bool Equals(const Nullable<T>& aOtherNullable) const
+  {
+    return (mIsNull && aOtherNullable.mIsNull) ||
+           (!mIsNull && !aOtherNullable.mIsNull &&
+            mValue == aOtherNullable.mValue);
+  }
+
+  bool operator==(const Nullable<T>& aOtherNullable) const
+  {
+    return Equals(aOtherNullable);
+  }
+
+  bool operator!=(const Nullable<T>& aOtherNullable) const
+  {
+    return !Equals(aOtherNullable);
+  }
+
+  // Make it possible to use a const Nullable of an array type with other
+  // array types.
+  template<typename U>
+  operator const Nullable< nsTArray<U> >&() const {
+    // Make sure that T is ok to reinterpret to nsTArray<U>
+    const nsTArray<U>& arr = mValue;
+    (void)arr;
+    return *reinterpret_cast<const Nullable< nsTArray<U> >*>(this);
+  }
+  template<typename U>
+  operator const Nullable< FallibleTArray<U> >&() const {
+    // Make sure that T is ok to reinterpret to FallibleTArray<U>
+    const FallibleTArray<U>& arr = mValue;
+    (void)arr;
+    return *reinterpret_cast<const Nullable< FallibleTArray<U> >*>(this);
+  }
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif /* mozilla_dom_Nullable_h */