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