gecko-dev/mfbt/TypedEnumInternal.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* Internal stuff needed by TypedEnum.h and TypedEnumBits.h. */

// NOTE: When we can assume C++11 enum class support and TypedEnum.h goes away,
// we should then consider folding TypedEnumInternal.h into TypedEnumBits.h.

#ifndef mozilla_TypedEnumInternal_h
#define mozilla_TypedEnumInternal_h

#include "mozilla/Attributes.h"

#if defined(__cplusplus)

#if defined(__clang__)
   /*
    * Per Clang documentation, "Note that marketing version numbers should not
    * be used to check for language features, as different vendors use different
    * numbering schemes. Instead, use the feature checking macros."
    */
#  ifndef __has_extension
#    define __has_extension __has_feature /* compatibility, for older versions of clang */
#  endif
#  if __has_extension(cxx_strong_enums)
#    define MOZ_HAVE_CXX11_STRONG_ENUMS
#  endif
#elif defined(__GNUC__)
#  if defined(__GXX_EXPERIMENTAL_CXX0X__) || __cplusplus >= 201103L
#    if MOZ_GCC_VERSION_AT_LEAST(4, 6, 3)
#      define MOZ_HAVE_CXX11_STRONG_ENUMS
#    endif
#  endif
#elif defined(_MSC_VER)
#  define MOZ_HAVE_CXX11_STRONG_ENUMS
#endif

namespace mozilla {

/*
 * The problem that CastableTypedEnumResult aims to solve is that
 * typed enums are not convertible to bool, and there is no way to make them
 * be, yet user code wants to be able to write
 *
 *   if (myFlags & Flags::SOME_PARTICULAR_FLAG)              (1)
 *
 * There are different approaches to solving this. Most of them require
 * adapting user code. For example, we could implement operator! and have
 * the user write
 *
 *   if (!!(myFlags & Flags::SOME_PARTICULAR_FLAG))          (2)
 *
 * Or we could supply a IsNonZero() or Any() function returning whether
 * an enum value is nonzero, and have the user write
 *
 *   if (Any(Flags & Flags::SOME_PARTICULAR_FLAG))           (3)
 *
 * But instead, we choose to preserve the original user syntax (1) as it
 * is inherently more readable, and to ease porting existing code to typed
 * enums. We achieve this by having operator& and other binary bitwise
 * operators have as return type a class, CastableTypedEnumResult,
 * that wraps a typed enum but adds bool convertibility.
 */
template<typename E>
class CastableTypedEnumResult
{
private:
  const E mValue;

public:
  explicit MOZ_CONSTEXPR CastableTypedEnumResult(E aValue)
    : mValue(aValue)
  {}

  MOZ_CONSTEXPR operator E() const { return mValue; }

  template<typename DestinationType>
  MOZ_EXPLICIT_CONVERSION MOZ_CONSTEXPR
  operator DestinationType() const { return DestinationType(mValue); }

  MOZ_CONSTEXPR bool operator !() const { return !bool(mValue); }

#ifndef MOZ_HAVE_CXX11_STRONG_ENUMS
  // This get() method is used to implement a constructor in the
  // non-c++11 fallback path for MOZ_BEGIN_ENUM_CLASS, taking a
  // CastableTypedEnumResult. If we try to implement it using the
  // above conversion operator E(), then at least clang 3.3
  // (when forced to take the non-c++11 fallback path) compiles
  // this constructor to an infinite recursion. So we introduce this
  // get() method, that does exactly the same as the conversion operator,
  // to work around this.
  MOZ_CONSTEXPR E get() const { return mValue; }
#endif
};

} // namespace mozilla

#endif // __cplusplus

#endif // mozilla_TypedEnumInternal_h