mfbt/TypedEnum.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 /* Macros to emulate C++11 typed enums and enum classes. */
   8
   9 #ifndef mozilla_TypedEnum_h
  10 #define mozilla_TypedEnum_h
  11
  12 #include "mozilla/TypedEnumInternal.h"
  13 #include "mozilla/MacroArgs.h"
  14
  15 #if defined(__cplusplus)
  16
  17 /**
  18  * MOZ_ENUM_TYPE specifies the underlying numeric type for an enum.  It's
  19  * specified by placing MOZ_ENUM_TYPE(type) immediately after the enum name in
  20  * its declaration, and before the opening curly brace, like
  21  *
  22  *   enum MyEnum MOZ_ENUM_TYPE(uint16_t)
  23  *   {
  24  *     A,
  25  *     B = 7,
  26  *     C
  27  *   };
  28  *
  29  * In supporting compilers, the macro will expand to ": uint16_t".  The
  30  * compiler will allocate exactly two bytes for MyEnum and will require all
  31  * enumerators to have values between 0 and 65535.  (Thus specifying "B =
  32  * 100000" instead of "B = 7" would fail to compile.)  In old compilers the
  33  * macro expands to the empty string, and the underlying type is generally
  34  * undefined.
  35  */
  36 #ifdef MOZ_HAVE_CXX11_ENUM_TYPE
  37 #  define MOZ_ENUM_TYPE(type)   : type
  38 #else
  39 #  define MOZ_ENUM_TYPE(type)   /* no support */
  40 #endif
  41
  42 /**
  43  * MOZ_BEGIN_ENUM_CLASS and MOZ_END_ENUM_CLASS provide access to the
  44  * strongly-typed enumeration feature of C++11 ("enum class").  If supported
  45  * by the compiler, an enum defined using these macros will not be implicitly
  46  * converted to any other type, and its enumerators will be scoped using the
  47  * enumeration name.  Place MOZ_BEGIN_ENUM_CLASS(EnumName [, type]) in place of
  48  * "enum EnumName {", and MOZ_END_ENUM_CLASS(EnumName) in place of the closing
  49  * "};".  For example,
  50  *
  51  *   MOZ_BEGIN_ENUM_CLASS(Enum, int32_t)
  52  *     A,
  53  *     B = 6
  54  *   MOZ_END_ENUM_CLASS(Enum)
  55  *
  56  * This will make "Enum::A" and "Enum::B" appear in the global scope, but "A"
  57  * and "B" will not.  In compilers that support C++11 strongly-typed
  58  * enumerations, implicit conversions of Enum values to numeric types will
  59  * fail.  In other compilers, Enum itself will actually be defined as a class,
  60  * and some implicit conversions will fail while others will succeed.
  61  *
  62  * The optional type argument specifies the underlying type for the enum where
  63  * supported, as with MOZ_ENUM_TYPE().  As with MOZ_ENUM_TYPE(), it will do
  64  * nothing on compilers that do not support it.
  65  *
  66  * MOZ_{BEGIN,END}_ENUM_CLASS doesn't work for defining enum classes nested
  67  * inside classes.  To define an enum class nested inside another class, use
  68  * MOZ_{BEGIN,END}_NESTED_ENUM_CLASS, and place a MOZ_FINISH_NESTED_ENUM_CLASS
  69  * in namespace scope to handle bits that can only be implemented with
  70  * namespace-scoped code.  For example:
  71  *
  72  *   class FooBar
  73  *   {
  74  *     MOZ_BEGIN_NESTED_ENUM_CLASS(Enum, int32_t)
  75  *       A,
  76  *       B = 6
  77  *     MOZ_END_NESTED_ENUM_CLASS(Enum)
  78  *   };
  79  *
  80  *   MOZ_FINISH_NESTED_ENUM_CLASS(FooBar::Enum)
  81  */
  82 #if defined(MOZ_HAVE_CXX11_STRONG_ENUMS)
  83   /*
  84    * All compilers that support strong enums also support an explicit
  85    * underlying type, so no extra check is needed.
  86    */
  87
  88    /* Single-argument form. */
  89 #  define MOZ_BEGIN_NESTED_ENUM_CLASS_HELPER1(Name) \
  90      enum class Name {
  91    /* Two-argument form. */
  92 #  define MOZ_BEGIN_NESTED_ENUM_CLASS_HELPER2(Name, type) \
  93      enum class Name : type {
  94 #  define MOZ_END_NESTED_ENUM_CLASS(Name) \
  95      };
  96 #  define MOZ_FINISH_NESTED_ENUM_CLASS(Name) /* nothing */
  97
  98   /*
  99    * MOZ_ENUM_CLASS_ENUM_TYPE allows using enum classes
 100    * as template parameter types. For that, we need integer types.
 101    * In the present case where the compiler supports strong enums,
 102    * these are already integer types so there is nothing more to do.
 103    */
 104 #  define MOZ_ENUM_CLASS_ENUM_TYPE(Name) Name
 105   /*
 106    * See the comment below about MOZ_TEMPLATE_ENUM_CLASS_ENUM_TYPE.
 107    */
 108 #  define MOZ_TEMPLATE_ENUM_CLASS_ENUM_TYPE(Name) Name
 109 #else
 110    /**
 111     * We need Name to both name a type, and scope the provided enumerator
 112     * names.  Namespaces and classes both provide scoping, but namespaces
 113     * aren't types, so we need to use a class that wraps the enum values.  We
 114     * have an implicit conversion from the inner enum type to the class, so
 115     * statements like
 116     *
 117     *   Enum x = Enum::A;
 118     *
 119     * will still work.  We need to define an implicit conversion from the class
 120     * to the inner enum as well, so that (for instance) switch statements will
 121     * work.  This means that the class can be implicitly converted to a numeric
 122     * value as well via the enum type, since C++ allows an implicit
 123     * user-defined conversion followed by a standard conversion to still be
 124     * implicit.
 125     *
 126     * We have an explicit constructor from int defined, so that casts like
 127     * (Enum)7 will still work.  We also have a zero-argument constructor with
 128     * no arguments, so declaration without initialization (like "Enum foo;")
 129     * will work.
 130     *
 131     * Additionally, we'll delete as many operators as possible for the inner
 132     * enum type, so statements like this will still fail:
 133     *
 134     *   f(5 + Enum::B); // deleted operator+
 135     *
 136     * But we can't prevent things like this, because C++ doesn't allow
 137     * overriding conversions or assignment operators for enums:
 138     *
 139     *   int x = Enum::A;
 140     *   int f()
 141     *   {
 142     *     return Enum::A;
 143     *   }
 144     */
 145
 146    /* Single-argument form. */
 147 #  define MOZ_BEGIN_NESTED_ENUM_CLASS_HELPER1(Name) \
 148      class Name \
 149      { \
 150      public: \
 151        enum Enum \
 152        {
 153    /* Two-argument form. */
 154 #  define MOZ_BEGIN_NESTED_ENUM_CLASS_HELPER2(Name, type) \
 155      class Name \
 156      { \
 157      public: \
 158        enum Enum MOZ_ENUM_TYPE(type) \
 159        {
 160 #  define MOZ_END_NESTED_ENUM_CLASS(Name) \
 161        }; \
 162        Name() {} \
 163        MOZ_CONSTEXPR Name(Enum aEnum) : mEnum(aEnum) {} \
 164        template<typename Other> \
 165        explicit MOZ_CONSTEXPR Name(Other num) : mEnum((Enum)num) {} \
 166        MOZ_CONSTEXPR operator Enum() const { return mEnum; } \
 167        explicit MOZ_CONSTEXPR Name(const mozilla::CastableTypedEnumResult<Name>& aOther) \
 168          : mEnum(aOther.get()) \
 169        {} \
 170      private: \
 171        Enum mEnum; \
 172      };
 173 #  define MOZ_FINISH_NESTED_ENUM_CLASS(Name) \
 174      inline int operator+(const int&, const Name::Enum&) MOZ_DELETE; \
 175      inline int operator+(const Name::Enum&, const int&) MOZ_DELETE; \
 176      inline int operator-(const int&, const Name::Enum&) MOZ_DELETE; \
 177      inline int operator-(const Name::Enum&, const int&) MOZ_DELETE; \
 178      inline int operator*(const int&, const Name::Enum&) MOZ_DELETE; \
 179      inline int operator*(const Name::Enum&, const int&) MOZ_DELETE; \
 180      inline int operator/(const int&, const Name::Enum&) MOZ_DELETE; \
 181      inline int operator/(const Name::Enum&, const int&) MOZ_DELETE; \
 182      inline int operator%(const int&, const Name::Enum&) MOZ_DELETE; \
 183      inline int operator%(const Name::Enum&, const int&) MOZ_DELETE; \
 184      inline int operator+(const Name::Enum&) MOZ_DELETE; \
 185      inline int operator-(const Name::Enum&) MOZ_DELETE; \
 186      inline int& operator++(Name::Enum&) MOZ_DELETE; \
 187      inline int operator++(Name::Enum&, int) MOZ_DELETE; \
 188      inline int& operator--(Name::Enum&) MOZ_DELETE; \
 189      inline int operator--(Name::Enum&, int) MOZ_DELETE; \
 190      inline bool operator==(const int&, const Name::Enum&) MOZ_DELETE; \
 191      inline bool operator==(const Name::Enum&, const int&) MOZ_DELETE; \
 192      inline bool operator!=(const int&, const Name::Enum&) MOZ_DELETE; \
 193      inline bool operator!=(const Name::Enum&, const int&) MOZ_DELETE; \
 194      inline bool operator>(const int&, const Name::Enum&) MOZ_DELETE; \
 195      inline bool operator>(const Name::Enum&, const int&) MOZ_DELETE; \
 196      inline bool operator<(const int&, const Name::Enum&) MOZ_DELETE; \
 197      inline bool operator<(const Name::Enum&, const int&) MOZ_DELETE; \
 198      inline bool operator>=(const int&, const Name::Enum&) MOZ_DELETE; \
 199      inline bool operator>=(const Name::Enum&, const int&) MOZ_DELETE; \
 200      inline bool operator<=(const int&, const Name::Enum&) MOZ_DELETE; \
 201      inline bool operator<=(const Name::Enum&, const int&) MOZ_DELETE; \
 202      inline bool operator!(const Name::Enum&) MOZ_DELETE; \
 203      inline bool operator&&(const bool&, const Name::Enum&) MOZ_DELETE; \
 204      inline bool operator&&(const Name::Enum&, const bool&) MOZ_DELETE; \
 205      inline bool operator||(const bool&, const Name::Enum&) MOZ_DELETE; \
 206      inline bool operator||(const Name::Enum&, const bool&) MOZ_DELETE; \
 207      inline int operator&(const int&, const Name::Enum&) MOZ_DELETE; \
 208      inline int operator&(const Name::Enum&, const int&) MOZ_DELETE; \
 209      inline int operator|(const int&, const Name::Enum&) MOZ_DELETE; \
 210      inline int operator|(const Name::Enum&, const int&) MOZ_DELETE; \
 211      inline int operator^(const int&, const Name::Enum&) MOZ_DELETE; \
 212      inline int operator^(const Name::Enum&, const int&) MOZ_DELETE; \
 213      inline int operator<<(const int&, const Name::Enum&) MOZ_DELETE; \
 214      inline int operator<<(const Name::Enum&, const int&) MOZ_DELETE; \
 215      inline int operator>>(const int&, const Name::Enum&) MOZ_DELETE; \
 216      inline int operator>>(const Name::Enum&, const int&) MOZ_DELETE; \
 217      inline int& operator+=(int&, const Name::Enum&) MOZ_DELETE; \
 218      inline int& operator-=(int&, const Name::Enum&) MOZ_DELETE; \
 219      inline int& operator*=(int&, const Name::Enum&) MOZ_DELETE; \
 220      inline int& operator/=(int&, const Name::Enum&) MOZ_DELETE; \
 221      inline int& operator%=(int&, const Name::Enum&) MOZ_DELETE; \
 222      inline int& operator&=(int&, const Name::Enum&) MOZ_DELETE; \
 223      inline int& operator|=(int&, const Name::Enum&) MOZ_DELETE; \
 224      inline int& operator^=(int&, const Name::Enum&) MOZ_DELETE; \
 225      inline int& operator<<=(int&, const Name::Enum&) MOZ_DELETE; \
 226      inline int& operator>>=(int&, const Name::Enum&) MOZ_DELETE;
 227
 228   /*
 229    * MOZ_ENUM_CLASS_ENUM_TYPE allows using enum classes
 230    * as template parameter types. For that, we need integer types.
 231    * In the present case, the integer type is the Enum nested type.
 232    */
 233 #  define MOZ_ENUM_CLASS_ENUM_TYPE(Name) Name::Enum
 234   /*
 235    * MOZ_TEMPLATE_ENUM_CLASS_ENUM_TYPE is a variant of MOZ_ENUM_CLASS_ENUM_TYPE
 236    * to be used when the enum class at hand depends on template parameters.
 237    *
 238    * Indeed, if T depends on template parameters, in order to name a nested type
 239    * in T, C++ does not allow to just write "T::NestedType". Instead, we have
 240    * to write "typename T::NestedType". The role of this macro is to add
 241    * this "typename" keywords where needed.
 242    *
 243    * Example:
 244    *
 245    *    template<typename T, MOZ_TEMPLATE_ENUM_CLASS_ENUM_TYPE(T) Value>
 246    *    struct S {};
 247    *
 248    *    MOZ_BEGIN_ENUM_CLASS(E)
 249    *      Foo,
 250    *      Bar
 251    *    MOZ_END_ENUM_CLASS(E)
 252    *
 253    *    S<E, E::Bar> s;
 254    *
 255    * In this example, the second template parameter to S is meant to be of type
 256    * T, but on non-C++11 compilers, type T is a class type, not an integer
 257    * type, so it is not accepted as the type of a constant template parameter.
 258    * One would then want to use MOZ_ENUM_CLASS_ENUM_TYPE(T), but that doesn't
 259    * work either as T depends on template parameters (more specifically here, T
 260    * _is_ a template parameter) so as MOZ_ENUM_CLASS_ENUM_TYPE(T) expands to
 261    * T::Enum, we are missing the required "typename" keyword. So here,
 262    * MOZ_TEMPLATE_ENUM_CLASS_ENUM_TYPE is needed.
 263    */
 264 #  define MOZ_TEMPLATE_ENUM_CLASS_ENUM_TYPE(Name) typename Name::Enum
 265 #endif
 266
 267 #  define MOZ_BEGIN_NESTED_ENUM_CLASS_GLUE(a, b) a b
 268 #  define MOZ_BEGIN_NESTED_ENUM_CLASS(...) \
 269      MOZ_BEGIN_NESTED_ENUM_CLASS_GLUE( \
 270        MOZ_PASTE_PREFIX_AND_ARG_COUNT(MOZ_BEGIN_NESTED_ENUM_CLASS_HELPER, \
 271                                       __VA_ARGS__), \
 272        (__VA_ARGS__))
 273
 274 #  define MOZ_BEGIN_ENUM_CLASS(...) MOZ_BEGIN_NESTED_ENUM_CLASS(__VA_ARGS__)
 275 #  define MOZ_END_ENUM_CLASS(Name) \
 276      MOZ_END_NESTED_ENUM_CLASS(Name) \
 277      MOZ_FINISH_NESTED_ENUM_CLASS(Name)
 278
 279 #endif /* __cplusplus */
 280
 281 #endif /* mozilla_TypedEnum_h */