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111 lines
3.7 KiB
C++
111 lines
3.7 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* Internal stuff needed by TypedEnum.h and TypedEnumBits.h. */
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// NOTE: When we can assume C++11 enum class support and TypedEnum.h goes away,
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// we should then consider folding TypedEnumInternal.h into TypedEnumBits.h.
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#ifndef mozilla_TypedEnumInternal_h
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#define mozilla_TypedEnumInternal_h
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#include "mozilla/Attributes.h"
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#if defined(__cplusplus)
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#if defined(__clang__)
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/*
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* Per Clang documentation, "Note that marketing version numbers should not
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* be used to check for language features, as different vendors use different
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* numbering schemes. Instead, use the feature checking macros."
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*/
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# ifndef __has_extension
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# define __has_extension __has_feature /* compatibility, for older versions of clang */
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# endif
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# if __has_extension(cxx_strong_enums)
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# define MOZ_HAVE_CXX11_ENUM_TYPE
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# define MOZ_HAVE_CXX11_STRONG_ENUMS
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# endif
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#elif defined(__GNUC__)
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# if defined(__GXX_EXPERIMENTAL_CXX0X__) || __cplusplus >= 201103L
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# if MOZ_GCC_VERSION_AT_LEAST(4, 6, 3)
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# define MOZ_HAVE_CXX11_ENUM_TYPE
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# define MOZ_HAVE_CXX11_STRONG_ENUMS
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# endif
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# endif
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#elif defined(_MSC_VER)
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# if _MSC_VER >= 1400
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# define MOZ_HAVE_CXX11_ENUM_TYPE
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# endif
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# if _MSC_VER >= 1700
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# define MOZ_HAVE_CXX11_STRONG_ENUMS
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# endif
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#endif
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namespace mozilla {
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/*
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* The problem that CastableTypedEnumResult aims to solve is that
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* typed enums are not convertible to bool, and there is no way to make them
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* be, yet user code wants to be able to write
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*
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* if (myFlags & Flags::SOME_PARTICULAR_FLAG) (1)
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*
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* There are different approaches to solving this. Most of them require
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* adapting user code. For example, we could implement operator! and have
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* the user write
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*
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* if (!!(myFlags & Flags::SOME_PARTICULAR_FLAG)) (2)
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*
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* Or we could supply a IsNonZero() or Any() function returning whether
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* an enum value is nonzero, and have the user write
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*
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* if (Any(Flags & Flags::SOME_PARTICULAR_FLAG)) (3)
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*
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* But instead, we choose to preserve the original user syntax (1) as it
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* is inherently more readable, and to ease porting existing code to typed
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* enums. We achieve this by having operator& and other binary bitwise
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* operators have as return type a class, CastableTypedEnumResult,
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* that wraps a typed enum but adds bool convertibility.
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*/
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template<typename E>
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class CastableTypedEnumResult
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{
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private:
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const E mValue;
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public:
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explicit MOZ_CONSTEXPR CastableTypedEnumResult(E aValue)
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: mValue(aValue)
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{}
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MOZ_CONSTEXPR operator E() const { return mValue; }
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template<typename DestinationType>
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MOZ_EXPLICIT_CONVERSION MOZ_CONSTEXPR
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operator DestinationType() const { return DestinationType(mValue); }
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MOZ_CONSTEXPR bool operator !() const { return !bool(mValue); }
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#ifndef MOZ_HAVE_CXX11_STRONG_ENUMS
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// This get() method is used to implement a constructor in the
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// non-c++11 fallback path for MOZ_BEGIN_ENUM_CLASS, taking a
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// CastableTypedEnumResult. If we try to implement it using the
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// above conversion operator E(), then at least clang 3.3
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// (when forced to take the non-c++11 fallback path) compiles
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// this constructor to an infinite recursion. So we introduce this
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// get() method, that does exactly the same as the conversion operator,
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// to work around this.
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MOZ_CONSTEXPR E get() const { return mValue; }
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#endif
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};
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} // namespace mozilla
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#endif // __cplusplus
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#endif // mozilla_TypedEnumInternal_h
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