mfbt/EnumTypeTraits.h

   1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* This Source Code Form is subject to the terms of the Mozilla Public
   3  * License, v. 2.0. If a copy of the MPL was not distributed with this
   4  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   5
   6 /* Type traits for enums. */
   7
   8 #ifndef mozilla_EnumTypeTraits_h
   9 #define mozilla_EnumTypeTraits_h
  10
  11 #include <type_traits>
  12
  13 namespace mozilla {
  14
  15 namespace detail {
  16
  17 template<size_t EnumSize, bool EnumSigned, size_t StorageSize, bool StorageSigned>
  18 struct EnumFitsWithinHelper;
  19
  20 // Signed enum, signed storage.
  21 template<size_t EnumSize, size_t StorageSize>
  22 struct EnumFitsWithinHelper<EnumSize, true, StorageSize, true>
  23   : public std::integral_constant<bool, (EnumSize <= StorageSize)>
  24 {};
  25
  26 // Signed enum, unsigned storage.
  27 template<size_t EnumSize, size_t StorageSize>
  28 struct EnumFitsWithinHelper<EnumSize, true, StorageSize, false>
  29   : public std::integral_constant<bool, false>
  30 {};
  31
  32 // Unsigned enum, signed storage.
  33 template<size_t EnumSize, size_t StorageSize>
  34 struct EnumFitsWithinHelper<EnumSize, false, StorageSize, true>
  35   : public std::integral_constant<bool, (EnumSize * 2 <= StorageSize)>
  36 {};
  37
  38 // Unsigned enum, unsigned storage.
  39 template<size_t EnumSize, size_t StorageSize>
  40 struct EnumFitsWithinHelper<EnumSize, false, StorageSize, false>
  41   : public std::integral_constant<bool, (EnumSize <= StorageSize)>
  42 {};
  43
  44 } // namespace detail
  45
  46 /*
  47  * Type trait that determines whether the enum type T can fit within the
  48  * integral type Storage without data loss. This trait should be used with
  49  * caution with an enum type whose underlying type has not been explicitly
  50  * specified: for such enums, the C++ implementation is free to choose a type
  51  * no smaller than int whose range encompasses all possible values of the enum.
  52  * So for an enum with only small non-negative values, the underlying type may
  53  * be either int or unsigned int, depending on the whims of the implementation.
  54  */
  55 template<typename T, typename Storage>
  56 struct EnumTypeFitsWithin
  57   : public detail::EnumFitsWithinHelper<
  58       sizeof(T),
  59       std::is_signed<typename std::underlying_type<T>::type>::value,
  60       sizeof(Storage),
  61       std::is_signed<Storage>::value
  62     >
  63 {
  64   static_assert(std::is_enum<T>::value, "must provide an enum type");
  65   static_assert(std::is_integral<Storage>::value, "must provide an integral type");
  66 };
  67
  68 /*
  69  * Provides information about highest enum member value.
  70  * Each specialization of struct MaxEnumValue should define
  71  * "static constexpr unsigned int value".
  72  *
  73  * example:
  74  *
  75  *   enum ExampleEnum
  76  *   {
  77  *     CAT = 0,
  78  *     DOG,
  79  *     HAMSTER
  80  *   };
  81  *
  82  *   template <>
  83  *   struct MaxEnumValue<ExampleEnum>
  84  *   {
  85  *     static constexpr unsigned int value = static_cast<unsigned int>(HAMSTER);
  86  *   };
  87  */
  88 template <typename T>
  89 struct MaxEnumValue;  // no need to define the primary template
  90
  91 } // namespace mozilla
  92
  93 #endif /* mozilla_EnumTypeTraits_h */