71 lines
2.4 KiB
C++
71 lines
2.4 KiB
C++
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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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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/* Type traits for enums. */
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#ifndef mozilla_EnumTypeTraits_h
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#define mozilla_EnumTypeTraits_h
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#include <type_traits>
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namespace mozilla {
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namespace detail {
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template<size_t EnumSize, bool EnumSigned, size_t StorageSize, bool StorageSigned>
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struct EnumFitsWithinHelper;
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// Signed enum, signed storage.
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template<size_t EnumSize, size_t StorageSize>
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struct EnumFitsWithinHelper<EnumSize, true, StorageSize, true>
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: public std::integral_constant<bool, (EnumSize <= StorageSize)>
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{};
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// Signed enum, unsigned storage.
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template<size_t EnumSize, size_t StorageSize>
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struct EnumFitsWithinHelper<EnumSize, true, StorageSize, false>
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: public std::integral_constant<bool, false>
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{};
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// Unsigned enum, signed storage.
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template<size_t EnumSize, size_t StorageSize>
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struct EnumFitsWithinHelper<EnumSize, false, StorageSize, true>
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: public std::integral_constant<bool, (EnumSize * 2 <= StorageSize)>
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{};
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// Unsigned enum, unsigned storage.
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template<size_t EnumSize, size_t StorageSize>
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struct EnumFitsWithinHelper<EnumSize, false, StorageSize, false>
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: public std::integral_constant<bool, (EnumSize <= StorageSize)>
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{};
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} // namespace detail
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/*
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* Type trait that determines whether the enum type T can fit within the
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* integral type Storage without data loss. This trait should be used with
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* caution with an enum type whose underlying type has not been explicitly
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* specified: for such enums, the C++ implementation is free to choose a type
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* no smaller than int whose range encompasses all possible values of the enum.
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* So for an enum with only small non-negative values, the underlying type may
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* be either int or unsigned int, depending on the whims of the implementation.
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*/
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template<typename T, typename Storage>
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struct EnumTypeFitsWithin
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: public detail::EnumFitsWithinHelper<
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sizeof(T),
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std::is_signed<typename std::underlying_type<T>::type>::value,
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sizeof(Storage),
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std::is_signed<Storage>::value
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>
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{
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static_assert(std::is_enum<T>::value, "must provide an enum type");
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static_assert(std::is_integral<Storage>::value, "must provide an integral type");
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};
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} // namespace mozilla
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#endif /* mozilla_EnumTypeTraits_h */
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