mfbt/EnumeratedArray.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* EnumeratedArray is like Array, but indexed by a typed enum. */
   8
   9 #ifndef mozilla_EnumeratedArray_h
  10 #define mozilla_EnumeratedArray_h
  11
  12 #include <utility>
  13
  14 #include "mozilla/Array.h"
  15 #include "EnumTypeTraits.h"
  16
  17 namespace mozilla {
  18
  19 /**
  20  * EnumeratedArray is a fixed-size array container for use when an
  21  * array is indexed by a specific enum class.
  22  *
  23  * This provides type safety by guarding at compile time against accidentally
  24  * indexing such arrays with unrelated values. This also removes the need
  25  * for manual casting when using a typed enum value to index arrays.
  26  *
  27  * Aside from the typing of indices, EnumeratedArray is similar to Array.
  28  *
  29  * Example:
  30  *
  31  *   enum class AnimalSpecies {
  32  *     Cow,
  33  *     Sheep,
  34  *     Count
  35  *   };
  36  *
  37  *   EnumeratedArray<AnimalSpecies, int, AnimalSpecies::Count> headCount;
  38  *
  39  *   headCount[AnimalSpecies::Cow] = 17;
  40  *   headCount[AnimalSpecies::Sheep] = 30;
  41  *
  42  * If the enum class has contiguous values and provides a specialization of
  43  * mozilla::MaxContiguousEnumValue then the size will be calculated as the max
  44  * value + 1.
  45  */
  46 template <typename Enum, typename ValueType,
  47           size_t Size = ContiguousEnumSize<Enum>::value>
  48 class EnumeratedArray {
  49  private:
  50   static_assert(UnderlyingValue(MinContiguousEnumValue<Enum>::value) == 0,
  51                 "All indexes would need to be corrected if min != 0");
  52
  53   using ArrayType = Array<ValueType, Size>;
  54
  55   ArrayType mArray;
  56
  57  public:
  58   EnumeratedArray() = default;
  59
  60   template <typename... Args>
  61   MOZ_IMPLICIT constexpr EnumeratedArray(Args&&... aArgs)
  62       : mArray{std::forward<Args>(aArgs)...} {}
  63
  64   ValueType& operator[](Enum aIndex) { return mArray[size_t(aIndex)]; }
  65
  66   const ValueType& operator[](Enum aIndex) const {
  67     return mArray[size_t(aIndex)];
  68   }
  69
  70   using iterator = typename ArrayType::iterator;
  71   using const_iterator = typename ArrayType::const_iterator;
  72   using reverse_iterator = typename ArrayType::reverse_iterator;
  73   using const_reverse_iterator = typename ArrayType::const_reverse_iterator;
  74
  75   // Methods for range-based for loops.
  76   iterator begin() { return mArray.begin(); }
  77   const_iterator begin() const { return mArray.begin(); }
  78   const_iterator cbegin() const { return mArray.cbegin(); }
  79   iterator end() { return mArray.end(); }
  80   const_iterator end() const { return mArray.end(); }
  81   const_iterator cend() const { return mArray.cend(); }
  82
  83   // Methods for reverse iterating.
  84   reverse_iterator rbegin() { return mArray.rbegin(); }
  85   const_reverse_iterator rbegin() const { return mArray.rbegin(); }
  86   const_reverse_iterator crbegin() const { return mArray.crbegin(); }
  87   reverse_iterator rend() { return mArray.rend(); }
  88   const_reverse_iterator rend() const { return mArray.rend(); }
  89   const_reverse_iterator crend() const { return mArray.crend(); }
  90 };
  91
  92 }  // namespace mozilla
  93
  94 #endif  // mozilla_EnumeratedArray_h