1 files changed, 70 insertions, 0 deletions
diff --git a/mfbt/EnumTypeTraits.h b/mfbt/EnumTypeTraits.h
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+++ b/mfbt/EnumTypeTraits.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+/* Type traits for enums. */
+
+#ifndef mozilla_EnumTypeTraits_h
+#define mozilla_EnumTypeTraits_h
+
+#include <type_traits>
+
+namespace mozilla {
+
+namespace detail {
+
+template<size_t EnumSize, bool EnumSigned, size_t StorageSize, bool StorageSigned>
+struct EnumFitsWithinHelper;
+
+// Signed enum, signed storage.
+template<size_t EnumSize, size_t StorageSize>
+struct EnumFitsWithinHelper<EnumSize, true, StorageSize, true>
+  : public std::integral_constant<bool, (EnumSize <= StorageSize)>
+{};
+
+// Signed enum, unsigned storage.
+template<size_t EnumSize, size_t StorageSize>
+struct EnumFitsWithinHelper<EnumSize, true, StorageSize, false>
+  : public std::integral_constant<bool, false>
+{};
+
+// Unsigned enum, signed storage.
+template<size_t EnumSize, size_t StorageSize>
+struct EnumFitsWithinHelper<EnumSize, false, StorageSize, true>
+  : public std::integral_constant<bool, (EnumSize * 2 <= StorageSize)>
+{};
+
+// Unsigned enum, unsigned storage.
+template<size_t EnumSize, size_t StorageSize>
+struct EnumFitsWithinHelper<EnumSize, false, StorageSize, false>
+  : public std::integral_constant<bool, (EnumSize <= StorageSize)>
+{};
+
+} // namespace detail
+
+/*
+ * Type trait that determines whether the enum type T can fit within the
+ * integral type Storage without data loss. This trait should be used with
+ * caution with an enum type whose underlying type has not been explicitly
+ * specified: for such enums, the C++ implementation is free to choose a type
+ * no smaller than int whose range encompasses all possible values of the enum.
+ * So for an enum with only small non-negative values, the underlying type may
+ * be either int or unsigned int, depending on the whims of the implementation.
+ */
+template<typename T, typename Storage>
+struct EnumTypeFitsWithin
+  : public detail::EnumFitsWithinHelper<
+      sizeof(T),
+      std::is_signed<typename std::underlying_type<T>::type>::value,
+      sizeof(Storage),
+      std::is_signed<Storage>::value
+    >
+{
+  static_assert(std::is_enum<T>::value, "must provide an enum type");
+  static_assert(std::is_integral<Storage>::value, "must provide an integral type");
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_EnumTypeTraits_h */