mfbt/TypedEnumInternal.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 /* Internal stuff needed by TypedEnum.h and TypedEnumBits.h. */
   8
   9 // NOTE: When we can assume C++11 enum class support and TypedEnum.h goes away,
  10 // we should then consider folding TypedEnumInternal.h into TypedEnumBits.h.
  11
  12 #ifndef mozilla_TypedEnumInternal_h
  13 #define mozilla_TypedEnumInternal_h
  14
  15 #include "mozilla/Attributes.h"
  16
  17 #if defined(__cplusplus)
  18
  19 #if defined(__clang__)
  20    /*
  21     * Per Clang documentation, "Note that marketing version numbers should not
  22     * be used to check for language features, as different vendors use different
  23     * numbering schemes. Instead, use the feature checking macros."
  24     */
  25 #  ifndef __has_extension
  26 #    define __has_extension __has_feature /* compatibility, for older versions of clang */
  27 #  endif
  28 #  if __has_extension(cxx_strong_enums)
  29 #    define MOZ_HAVE_CXX11_ENUM_TYPE
  30 #    define MOZ_HAVE_CXX11_STRONG_ENUMS
  31 #  endif
  32 #elif defined(__GNUC__)
  33 #  if defined(__GXX_EXPERIMENTAL_CXX0X__) || __cplusplus >= 201103L
  34 #    if MOZ_GCC_VERSION_AT_LEAST(4, 6, 3)
  35 #      define MOZ_HAVE_CXX11_ENUM_TYPE
  36 #      define MOZ_HAVE_CXX11_STRONG_ENUMS
  37 #    endif
  38 #  endif
  39 #elif defined(_MSC_VER)
  40 #  define MOZ_HAVE_CXX11_ENUM_TYPE
  41 #  define MOZ_HAVE_CXX11_STRONG_ENUMS
  42 #endif
  43
  44 namespace mozilla {
  45
  46 /*
  47  * The problem that CastableTypedEnumResult aims to solve is that
  48  * typed enums are not convertible to bool, and there is no way to make them
  49  * be, yet user code wants to be able to write
  50  *
  51  *   if (myFlags & Flags::SOME_PARTICULAR_FLAG)              (1)
  52  *
  53  * There are different approaches to solving this. Most of them require
  54  * adapting user code. For example, we could implement operator! and have
  55  * the user write
  56  *
  57  *   if (!!(myFlags & Flags::SOME_PARTICULAR_FLAG))          (2)
  58  *
  59  * Or we could supply a IsNonZero() or Any() function returning whether
  60  * an enum value is nonzero, and have the user write
  61  *
  62  *   if (Any(Flags & Flags::SOME_PARTICULAR_FLAG))           (3)
  63  *
  64  * But instead, we choose to preserve the original user syntax (1) as it
  65  * is inherently more readable, and to ease porting existing code to typed
  66  * enums. We achieve this by having operator& and other binary bitwise
  67  * operators have as return type a class, CastableTypedEnumResult,
  68  * that wraps a typed enum but adds bool convertibility.
  69  */
  70 template<typename E>
  71 class CastableTypedEnumResult
  72 {
  73 private:
  74   const E mValue;
  75
  76 public:
  77   explicit MOZ_CONSTEXPR CastableTypedEnumResult(E aValue)
  78     : mValue(aValue)
  79   {}
  80
  81   MOZ_CONSTEXPR operator E() const { return mValue; }
  82
  83   template<typename DestinationType>
  84   MOZ_EXPLICIT_CONVERSION MOZ_CONSTEXPR
  85   operator DestinationType() const { return DestinationType(mValue); }
  86
  87   MOZ_CONSTEXPR bool operator !() const { return !bool(mValue); }
  88
  89 #ifndef MOZ_HAVE_CXX11_STRONG_ENUMS
  90   // This get() method is used to implement a constructor in the
  91   // non-c++11 fallback path for MOZ_BEGIN_ENUM_CLASS, taking a
  92   // CastableTypedEnumResult. If we try to implement it using the
  93   // above conversion operator E(), then at least clang 3.3
  94   // (when forced to take the non-c++11 fallback path) compiles
  95   // this constructor to an infinite recursion. So we introduce this
  96   // get() method, that does exactly the same as the conversion operator,
  97   // to work around this.
  98   MOZ_CONSTEXPR E get() const { return mValue; }
  99 #endif
 100 };
 101
 102 } // namespace mozilla
 103
 104 #endif // __cplusplus
 105
 106 #endif // mozilla_TypedEnumInternal_h