mfbt/IntegerTypeTraits.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* Helpers to manipulate integer types that don't fit in TypeTraits.h */
   8
   9 #ifndef mozilla_IntegerTypeTraits_h
  10 #define mozilla_IntegerTypeTraits_h
  11
  12 #include "mozilla/TypeTraits.h"
  13 #include <stdint.h>
  14
  15 namespace mozilla {
  16
  17 namespace detail {
  18
  19 /**
  20  * StdintTypeForSizeAndSignedness returns the stdint integer type
  21  * of given size (can be 1, 2, 4 or 8) and given signedness
  22  * (false means unsigned, true means signed).
  23  */
  24 template<size_t Size, bool Signedness>
  25 struct StdintTypeForSizeAndSignedness;
  26
  27 template<>
  28 struct StdintTypeForSizeAndSignedness<1, true>
  29 {
  30   typedef int8_t Type;
  31 };
  32
  33 template<>
  34 struct StdintTypeForSizeAndSignedness<1, false>
  35 {
  36   typedef uint8_t Type;
  37 };
  38
  39 template<>
  40 struct StdintTypeForSizeAndSignedness<2, true>
  41 {
  42   typedef int16_t Type;
  43 };
  44
  45 template<>
  46 struct StdintTypeForSizeAndSignedness<2, false>
  47 {
  48   typedef uint16_t Type;
  49 };
  50
  51 template<>
  52 struct StdintTypeForSizeAndSignedness<4, true>
  53 {
  54   typedef int32_t Type;
  55 };
  56
  57 template<>
  58 struct StdintTypeForSizeAndSignedness<4, false>
  59 {
  60   typedef uint32_t Type;
  61 };
  62
  63 template<>
  64 struct StdintTypeForSizeAndSignedness<8, true>
  65 {
  66   typedef int64_t Type;
  67 };
  68
  69 template<>
  70 struct StdintTypeForSizeAndSignedness<8, false>
  71 {
  72   typedef uint64_t Type;
  73 };
  74
  75 } // namespace detail
  76
  77 template<size_t Size>
  78 struct UnsignedStdintTypeForSize
  79   : detail::StdintTypeForSizeAndSignedness<Size, false>
  80 {};
  81
  82 template<size_t Size>
  83 struct SignedStdintTypeForSize
  84   : detail::StdintTypeForSizeAndSignedness<Size, true>
  85 {};
  86
  87 template<typename IntegerType>
  88 struct PositionOfSignBit
  89 {
  90   static_assert(IsIntegral<IntegerType>::value,
  91                 "PositionOfSignBit is only for integral types");
  92   // 8 here should be CHAR_BIT from limits.h, but the world has moved on.
  93   static const size_t value = 8 * sizeof(IntegerType) - 1;
  94 };
  95
  96 /**
  97  * MinValue returns the minimum value of the given integer type as a
  98  * compile-time constant, which std::numeric_limits<IntegerType>::min()
  99  * cannot do in c++98.
 100  */
 101 template<typename IntegerType>
 102 struct MinValue
 103 {
 104 private:
 105   static_assert(IsIntegral<IntegerType>::value,
 106                 "MinValue is only for integral types");
 107
 108   typedef typename MakeUnsigned<IntegerType>::Type UnsignedIntegerType;
 109   static const size_t PosOfSignBit = PositionOfSignBit<IntegerType>::value;
 110
 111 public:
 112   // Bitwise ops may return a larger type, that's why we cast explicitly.
 113   // In C++, left bit shifts on signed values is undefined by the standard
 114   // unless the shifted value is representable.
 115   // Notice that signed-to-unsigned conversions are always well-defined in
 116   // the standard as the value congruent to 2**n, as expected. By contrast,
 117   // unsigned-to-signed is only well-defined if the value is representable.
 118   static const IntegerType value =
 119       IsSigned<IntegerType>::value
 120       ? IntegerType(UnsignedIntegerType(1) << PosOfSignBit)
 121       : IntegerType(0);
 122 };
 123
 124 /**
 125  * MaxValue returns the maximum value of the given integer type as a
 126  * compile-time constant, which std::numeric_limits<IntegerType>::max()
 127  * cannot do in c++98.
 128  */
 129 template<typename IntegerType>
 130 struct MaxValue
 131 {
 132   static_assert(IsIntegral<IntegerType>::value,
 133                 "MaxValue is only for integral types");
 134
 135   // Tricksy, but covered by the CheckedInt unit test.
 136   // Relies on the type of MinValue<IntegerType>::value
 137   // being IntegerType.
 138   static const IntegerType value = ~MinValue<IntegerType>::value;
 139 };
 140
 141 } // namespace mozilla
 142
 143 #endif // mozilla_IntegerTypeTraits_h