gecko-dev/mfbt/TypedEnumInternal.h

113 lines
3.7 KiB
C++

/* -*- 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_ENUM_TYPE
# 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_ENUM_TYPE
# define MOZ_HAVE_CXX11_STRONG_ENUMS
# endif
# endif
#elif defined(_MSC_VER)
# if _MSC_VER >= 1400
# define MOZ_HAVE_CXX11_ENUM_TYPE
# endif
# if _MSC_VER >= 1700
# define MOZ_HAVE_CXX11_STRONG_ENUMS
# endif
#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 value)
: mValue(value)
{}
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