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DefineEnum.h
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/* -*- 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/. */
/* Poor man's reflection for enumerations. */
#ifndef mozilla_DefineEnum_h
#define mozilla_DefineEnum_h
#include <stddef.h> // for size_t
#include "mozilla/MacroArgs.h" // for MOZ_ARG_COUNT
#include "mozilla/MacroForEach.h" // for MOZ_FOR_EACH
/**
* MOZ_UNWRAP_ARGS is a helper macro that unwraps a list of comma-separated
* items enclosed in parentheses, to yield just the items.
*
* Usage: |MOZ_UNWRAP_ARGS foo| (note the absence of parentheses in the
* invocation), where |foo| is a parenthesis-enclosed list.
* For exampe if |foo| is |(3, 4, 5)|, then the expansion is just |3, 4, 5|.
*/
#define MOZ_UNWRAP_ARGS(...) __VA_ARGS__
/**
* MOZ_DEFINE_ENUM(aEnumName, aEnumerators) is a macro that allows
* simultaneously defining an enumeration named |aEnumName|, and a constant
* that stores the number of enumerators it has.
*
* The motivation is to allow the enumeration to evolve over time without
* either having to manually keep such a constant up to date, or having to
* add a special "sentinel" enumerator for this purpose. (While adding a
* "sentinel" enumerator is trivial, it causes headaches with "switch"
* statements. We often try to write "switch" statements whose cases exhaust
* the enumerators and don't have a "default" case, so that if a new
* enumerator is added and we forget to handle it in the "switch", the
* compiler points it out. But this means we need to explicitly handle the
* sentinel in every "switch".)
*
* |aEnumerators| is expected to be a comma-separated list of enumerators,
* enclosed in parentheses. The enumerators may NOT have associated
* initializers (an attempt to have one will result in a compiler error).
* This ensures that the enumerator values are in the range [0, N), where N
* is the number of enumerators.
*
* The list of enumerators cannot contain a trailing comma. This is a
* limitation of MOZ_FOR_EACH, which we use in the implementation; if
* MOZ_FOR_EACH supported trailing commas, we could too.
*
* The generated constant has the name "k" + |aEnumName| + "Count", and type
* "size_t". The enumeration and the constant are both defined in the scope
* in which the macro is invoked.
*
* For convenience, a constant of the enumeration type named
* "kHighest" + |aEnumName| is also defined, whose value is the highest
* valid enumerator, assuming the enumerators have contiguous values starting
* from 0.
*
* Invocation of the macro may be followed by a semicolon, if one prefers a
* more declaration-like syntax.
*
* Example invocation:
* MOZ_DEFINE_ENUM(MyEnum, (Foo, Bar, Baz));
*
* This expands to:
* enum MyEnum { Foo, Bar, Baz };
* constexpr size_t kMyEnumCount = 3;
* constexpr MyEnum kHighestMyEnum = MyEnum(kMyEnumCount - 1);
* // some static_asserts to ensure the values are in the range [0, 3)
*
* The macro also has several variants:
*
* - A |_CLASS| variant, which generates an |enum class| instead of
* a plain enum.
*
* - A |_WITH_BASE| variant which generates an enum with a specified
* underlying ("base") type, which is provided as an additional
* argument in second position.
*
* - An |_AT_CLASS_SCOPE| variant, designed for enumerations defined
* at class scope. For these, the generated constants are static,
* and have names prefixed with "s" instead of "k" as per
* naming convention.
*
* (and combinations of these).
*/
/*
* A helper macro for asserting that an enumerator does not have an initializer.
*
* The static_assert and the comparison to 0 are just scaffolding; the
* important part is forming the expression |aEnumName::aEnumeratorDecl|.
*
* If |aEnumeratorDecl| is just the enumerator name without an identifier,
* this expression compiles fine. However, if |aEnumeratorDecl| includes an
* initializer, as in |eEnumerator = initializer|, then this will fail to
* compile in expression context, since |eEnumerator| is not an lvalue.
*
* (The static_assert itself should always pass in the absence of the above
* error, since you can't get a negative enumerator value without having
* an initializer somewhere. It just provides a place to put the expression
* we want to form.)
*/
#define MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER(aEnumName, aEnumeratorDecl) \
static_assert((aEnumName::aEnumeratorDecl) >= aEnumName(0), \
"MOZ_DEFINE_ENUM does not allow enumerators to have initializers");
#define MOZ_DEFINE_ENUM_IMPL(aEnumName, aClassSpec, aBaseSpec, aEnumerators) \
enum aClassSpec aEnumName aBaseSpec { MOZ_UNWRAP_ARGS aEnumerators }; \
constexpr size_t k##aEnumName##Count = MOZ_ARG_COUNT aEnumerators; \
constexpr aEnumName k##Highest##aEnumName = aEnumName(k##aEnumName##Count - 1); \
MOZ_FOR_EACH(MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER, (aEnumName,), aEnumerators)
#define MOZ_DEFINE_ENUM(aEnumName, aEnumerators) \
MOZ_DEFINE_ENUM_IMPL(aEnumName, , , aEnumerators)
#define MOZ_DEFINE_ENUM_WITH_BASE(aEnumName, aBaseName, aEnumerators) \
MOZ_DEFINE_ENUM_IMPL(aEnumName, , : aBaseName, aEnumerators)
#define MOZ_DEFINE_ENUM_CLASS(aEnumName, aEnumerators) \
MOZ_DEFINE_ENUM_IMPL(aEnumName, class, , aEnumerators)
#define MOZ_DEFINE_ENUM_CLASS_WITH_BASE(aEnumName, aBaseName, aEnumerators) \
MOZ_DEFINE_ENUM_IMPL(aEnumName, class, : aBaseName, aEnumerators)
#define MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, aClassSpec, aBaseSpec, aEnumerators) \
enum aClassSpec aEnumName aBaseSpec { MOZ_UNWRAP_ARGS aEnumerators }; \
constexpr static size_t s##aEnumName##Count = MOZ_ARG_COUNT aEnumerators; \
constexpr static aEnumName s##Highest##aEnumName = aEnumName(s##aEnumName##Count - 1); \
MOZ_FOR_EACH(MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER, (aEnumName,), aEnumerators)
#define MOZ_DEFINE_ENUM_AT_CLASS_SCOPE(aEnumName, aEnumerators) \
MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, , , aEnumerators)
#define MOZ_DEFINE_ENUM_WITH_BASE_AT_CLASS_SCOPE(aEnumName, aBaseName, aEnumerators) \
MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, , : aBaseName, aEnumerators)
#define MOZ_DEFINE_ENUM_CLASS_AT_CLASS_SCOPE(aEnumName, aEnumerators) \
MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, class, , aEnumerators)
#define MOZ_DEFINE_ENUM_CLASS_WITH_BASE_AT_CLASS_SCOPE(aEnumName, aBaseName, aEnumerators) \
MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, class, : aBaseName, aEnumerators)
#endif // mozilla_DefineEnum_h