gecko-dev/mfbt/TypedEnumBits.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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/. */

/*
 * MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS allows using a typed enum as bit flags.
 */

#ifndef mozilla_TypedEnumBits_h
#define mozilla_TypedEnumBits_h

#include "mozilla/Attributes.h"
#include "mozilla/IntegerTypeTraits.h"

namespace mozilla {

/*
 * The problem that CastableTypedEnumResult aims to solve is that
 * typed enums are not convertible to bool, and there is no way to make them
 * be, yet user code wants to be able to write
 *
 *   if (myFlags & Flags::SOME_PARTICULAR_FLAG)              (1)
 *
 * There are different approaches to solving this. Most of them require
 * adapting user code. For example, we could implement operator! and have
 * the user write
 *
 *   if (!!(myFlags & Flags::SOME_PARTICULAR_FLAG))          (2)
 *
 * Or we could supply a IsNonZero() or Any() function returning whether
 * an enum value is nonzero, and have the user write
 *
 *   if (Any(Flags & Flags::SOME_PARTICULAR_FLAG))           (3)
 *
 * But instead, we choose to preserve the original user syntax (1) as it
 * is inherently more readable, and to ease porting existing code to typed
 * enums. We achieve this by having operator& and other binary bitwise
 * operators have as return type a class, CastableTypedEnumResult,
 * that wraps a typed enum but adds bool convertibility.
 */
template <typename E>
class CastableTypedEnumResult {
 private:
  const E mValue;

 public:
  explicit constexpr CastableTypedEnumResult(E aValue) : mValue(aValue) {}

  constexpr operator E() const { return mValue; }

  template <typename DestinationType>
  explicit constexpr operator DestinationType() const {
    return DestinationType(mValue);
  }

  constexpr bool operator!() const { return !bool(mValue); }
};

#define MOZ_CASTABLETYPEDENUMRESULT_BINOP(Op, OtherType, ReturnType)        \
  template <typename E>                                                     \
  constexpr ReturnType operator Op(const OtherType& aE,                     \
                                   const CastableTypedEnumResult<E>& aR) {  \
    return ReturnType(aE Op OtherType(aR));                                 \
  }                                                                         \
  template <typename E>                                                     \
  constexpr ReturnType operator Op(const CastableTypedEnumResult<E>& aR,    \
                                   const OtherType& aE) {                   \
    return ReturnType(OtherType(aR) Op aE);                                 \
  }                                                                         \
  template <typename E>                                                     \
  constexpr ReturnType operator Op(const CastableTypedEnumResult<E>& aR1,   \
                                   const CastableTypedEnumResult<E>& aR2) { \
    return ReturnType(OtherType(aR1) Op OtherType(aR2));                    \
  }

MOZ_CASTABLETYPEDENUMRESULT_BINOP(|, E, CastableTypedEnumResult<E>)
MOZ_CASTABLETYPEDENUMRESULT_BINOP(&, E, CastableTypedEnumResult<E>)
MOZ_CASTABLETYPEDENUMRESULT_BINOP(^, E, CastableTypedEnumResult<E>)
MOZ_CASTABLETYPEDENUMRESULT_BINOP(==, E, bool)
MOZ_CASTABLETYPEDENUMRESULT_BINOP(!=, E, bool)

template <typename E>
constexpr CastableTypedEnumResult<E> operator~(
    const CastableTypedEnumResult<E>& aR) {
  return CastableTypedEnumResult<E>(~(E(aR)));
}

#define MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(Op)        \
  template <typename E>                                           \
  E& operator Op(E& aR1, const CastableTypedEnumResult<E>& aR2) { \
    return aR1 Op E(aR2);                                         \
  }

MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(&=)
MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(|=)
MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP(^=)

#undef MOZ_CASTABLETYPEDENUMRESULT_COMPOUND_ASSIGN_OP

#undef MOZ_CASTABLETYPEDENUMRESULT_BINOP

namespace detail {
template <typename E>
struct UnsignedIntegerTypeForEnum : UnsignedStdintTypeForSize<sizeof(E)> {};
}  // namespace detail

}  // namespace mozilla

#define MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, Op)                       \
  inline constexpr mozilla::CastableTypedEnumResult<Name> operator Op( \
      Name a, Name b) {                                                \
    typedef mozilla::CastableTypedEnumResult<Name> Result;             \
    typedef mozilla::detail::UnsignedIntegerTypeForEnum<Name>::Type U; \
    return Result(Name(U(a) Op U(b)));                                 \
  }                                                                    \
                                                                       \
  inline Name& operator Op##=(Name& a, Name b) { return a = a Op b; }

/**
 * MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS generates standard bitwise operators
 * for the given enum type. Use this to enable using an enum type as bit-field.
 */
#define MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(Name)                           \
  MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, |)                                     \
  MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, &)                                     \
  MOZ_MAKE_ENUM_CLASS_BINOP_IMPL(Name, ^)                                     \
  inline constexpr mozilla::CastableTypedEnumResult<Name> operator~(Name a) { \
    typedef mozilla::CastableTypedEnumResult<Name> Result;                    \
    typedef mozilla::detail::UnsignedIntegerTypeForEnum<Name>::Type U;        \
    return Result(Name(~(U(a))));                                             \
  }

#endif  // mozilla_TypedEnumBits_h