mfbt/TypedEnumBits.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 /*
   8  * MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS allows using a typed enum as bit flags.
   9  */
  10
  11 #ifndef mozilla_TypedEnumBits_h
  12 #define mozilla_TypedEnumBits_h
  13
  14 #include "mozilla/Attributes.h"
  15 #include "mozilla/IntegerTypeTraits.h"
  16
  17 namespace mozilla {
  18
  19 /*
  20  * The problem that CastableTypedEnumResult aims to solve is that
  21  * typed enums are not convertible to bool, and there is no way to make them
  22  * be, yet user code wants to be able to write
  23  *
  24  *   if (myFlags & Flags::SOME_PARTICULAR_FLAG)              (1)
  25  *
  26  * There are different approaches to solving this. Most of them require
  27  * adapting user code. For example, we could implement operator! and have
  28  * the user write
  29  *
  30  *   if (!!(myFlags & Flags::SOME_PARTICULAR_FLAG))          (2)
  31  *
  32  * Or we could supply a IsNonZero() or Any() function returning whether
  33  * an enum value is nonzero, and have the user write
  34  *
  35  *   if (Any(Flags & Flags::SOME_PARTICULAR_FLAG))           (3)
  36  *
  37  * But instead, we choose to preserve the original user syntax (1) as it
  38  * is inherently more readable, and to ease porting existing code to typed
  39  * enums. We achieve this by having operator& and other binary bitwise
  40  * operators have as return type a class, CastableTypedEnumResult,
  41  * that wraps a typed enum but adds bool convertibility.
  42  */
  43 template<typename E>
  44 class CastableTypedEnumResult
  45 {
  46 private:
  47   const E mValue;
  48
  49 public:
  50   explicit constexpr CastableTypedEnumResult(E aValue)
  51     : mValue(aValue)
  52   {}
  53
  54   constexpr operator E() const { return mValue; }
  55
  56   template<typename DestinationType>
  57   explicit constexpr
  58   operator DestinationType() const { return DestinationType(mValue); }
  59
  60   constexpr bool operator !() const { return !bool(mValue); }
  61 };
  62
  63 #define MOZ_CASTABLETYPEDENUMRESULT_BINOP(Op, OtherType, ReturnType) \
  64 template<typename E> \
  65 constexpr ReturnType \
  66 operator Op(const OtherType& aE, const CastableTypedEnumResult<E>& aR) \
  67 { \
  68   return ReturnType(aE Op OtherType(aR)); \
  69 } \
  70 template<typename E> \
  71 constexpr ReturnType \
  72 operator Op(const CastableTypedEnumResult<E>& aR, const OtherType& aE) \
  73 { \
  74   return ReturnType(OtherType(aR) Op aE); \
  75 } \
  76 template<typename E> \
  77 constexpr ReturnType \
  78 operator Op(const CastableTypedEnumResult<E>& aR1, \
  79             const CastableTypedEnumResult<E>& aR2) \
  80 { \
  81   return ReturnType(OtherType(aR1) Op OtherType(aR2)); \
  82 }
  83
  84 MOZ_CASTABLETYPEDENUMRESULT_BINOP(|, E, CastableTypedEnumResult<E>)
  85 MOZ_CASTABLETYPEDENUMRESULT_BINOP(&, E, CastableTypedEnumResult<E>)
  86 MOZ_CASTABLETYPEDENUMRESULT_BINOP(^, E, CastableTypedEnumResult<E>)
  87 MOZ_CASTABLETYPEDENUMRESULT_BINOP(==, E, bool)
  88 MOZ_CASTABLETYPEDENUMRESULT_BINOP(!=, E, bool)
  89
  90 template <typename E>
  91 constexpr CastableTypedEnumResult<E>
  92 operator ~(const CastableTypedEnumResult<E>& aR)
  93 {
  94   return CastableTypedEnumResult<E>(~(E(aR)));
  95 }
  96
  97 #define MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(Op) \
  98 template<typename E> \
  99 E& \
 100 operator Op(E& aR1, \
 101             const CastableTypedEnumResult<E>& aR2) \
 102 { \
 103   return aR1 Op E(aR2); \
 104 }
 105
 106 MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(&=)
 107 MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(|=)
 108 MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(^=)
 109
 110 #undef MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP
 111
 112 #undef MOZ_CASTABLETYPEDENUMRESULT_BINOP
 113
 114 namespace detail {
 115 template<typename E>
 116 struct UnsignedIntegerTypeForEnum
 117   : UnsignedStdintTypeForSize<sizeof(E)>
 118 {};
 119 } // namespace detail
 120
 121 } // namespace mozilla
 122
 123 #define MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, Op) \
 124    inline constexpr mozilla::CastableTypedEnumResult<Name> \
 125    operator Op(Name a, Name b) \
 126    { \
 127      typedef mozilla::CastableTypedEnumResult<Name> Result; \
 128      typedef mozilla::detail::UnsignedIntegerTypeForEnum<Name>::Type U; \
 129      return Result(Name(U(a) Op U(b))); \
 130    } \
 131  \
 132    inline Name& \
 133    operator Op##=(Name& a, Name b) \
 134    { \
 135      return a = a Op b; \
 136    }
 137
 138 /**
 139  * MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS generates standard bitwise operators
 140  * for the given enum type. Use this to enable using an enum type as bit-field.
 141  */
 142 #define MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(Name) \
 143    MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, |) \
 144    MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, &) \
 145    MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, ^) \
 146    inline constexpr mozilla::CastableTypedEnumResult<Name> \
 147    operator~(Name a) \
 148    { \
 149      typedef mozilla::CastableTypedEnumResult<Name> Result; \
 150      typedef mozilla::detail::UnsignedIntegerTypeForEnum<Name>::Type U; \
 151      return Result(Name(~(U(a)))); \
 152    }
 153
 154 #endif // mozilla_TypedEnumBits_h