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150 lines
4.6 KiB
C++
150 lines
4.6 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 <stddef.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,
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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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// 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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// 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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// 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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} // 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), std::is_signed<Storage>::value> {
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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,
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"must provide an integral type");
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};
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/**
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* Get the underlying value of an enum, but typesafe.
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*
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* example:
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*
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* enum class Pet : int16_t {
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* Cat,
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* Dog,
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* Fish
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* };
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* enum class Plant {
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* Flower,
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* Tree,
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* Vine
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* };
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* UnderlyingValue(Pet::Fish) -> int16_t(2)
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* UnderlyingValue(Plant::Tree) -> int(1)
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*/
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template <typename T>
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inline constexpr auto UnderlyingValue(const T v) {
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static_assert(std::is_enum_v<T>);
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return static_cast<typename std::underlying_type<T>::type>(v);
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}
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/*
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* Specialize either MaxContiguousEnumValue or MaxEnumValue to provide the
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* highest enum member value for an enum class. Note that specializing
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* MaxContiguousEnumValue will make MaxEnumValue just take its value from the
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* MaxContiguousEnumValue specialization.
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*
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* Specialize MinContiguousEnumValue and MaxContiguousEnumValue to provide both
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* lowest and highest enum member values for an enum class with contiguous
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* values.
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*
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* Each specialization of these structs should define "static constexpr" member
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* variable named "value".
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*
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* example:
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*
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* enum ExampleEnum
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* {
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* CAT = 0,
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* DOG,
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* HAMSTER
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* };
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*
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* template <>
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* struct MaxEnumValue<ExampleEnum>
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* {
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* static constexpr ExampleEnumvalue = HAMSTER;
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* };
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*/
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template <typename T>
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struct MinContiguousEnumValue {
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static constexpr T value = static_cast<T>(0);
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};
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template <typename T>
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struct MaxContiguousEnumValue;
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template <typename T>
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struct MaxEnumValue {
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static constexpr auto value = MaxContiguousEnumValue<T>::value;
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};
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// Provides the min and max values for a contiguous enum (requires at least
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// MaxContiguousEnumValue to be defined).
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template <typename T>
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struct ContiguousEnumValues {
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static constexpr auto min = MinContiguousEnumValue<T>::value;
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static constexpr auto max = MaxContiguousEnumValue<T>::value;
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};
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// Provides the total number of values for a contiguous enum (requires at least
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// MaxContiguousEnumValue to be defined).
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template <typename T>
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struct ContiguousEnumSize {
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static constexpr size_t value =
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UnderlyingValue(ContiguousEnumValues<T>::max) + 1 -
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UnderlyingValue(ContiguousEnumValues<T>::min);
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};
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} // namespace mozilla
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#endif /* mozilla_EnumTypeTraits_h */
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