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265e672179
# ignore-this-changeset --HG-- extra : amend_source : 4d301d3b0b8711c4692392aa76088ba7fd7d1022
156 lines
7.1 KiB
C++
156 lines
7.1 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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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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/* Poor man's reflection for enumerations. */
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#ifndef mozilla_DefineEnum_h
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#define mozilla_DefineEnum_h
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#include <stddef.h> // for size_t
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#include "mozilla/MacroArgs.h" // for MOZ_ARG_COUNT
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#include "mozilla/MacroForEach.h" // for MOZ_FOR_EACH
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/**
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* MOZ_UNWRAP_ARGS is a helper macro that unwraps a list of comma-separated
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* items enclosed in parentheses, to yield just the items.
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*
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* Usage: |MOZ_UNWRAP_ARGS foo| (note the absence of parentheses in the
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* invocation), where |foo| is a parenthesis-enclosed list.
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* For exampe if |foo| is |(3, 4, 5)|, then the expansion is just |3, 4, 5|.
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*/
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#define MOZ_UNWRAP_ARGS(...) __VA_ARGS__
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/**
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* MOZ_DEFINE_ENUM(aEnumName, aEnumerators) is a macro that allows
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* simultaneously defining an enumeration named |aEnumName|, and a constant
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* that stores the number of enumerators it has.
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*
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* The motivation is to allow the enumeration to evolve over time without
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* either having to manually keep such a constant up to date, or having to
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* add a special "sentinel" enumerator for this purpose. (While adding a
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* "sentinel" enumerator is trivial, it causes headaches with "switch"
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* statements. We often try to write "switch" statements whose cases exhaust
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* the enumerators and don't have a "default" case, so that if a new
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* enumerator is added and we forget to handle it in the "switch", the
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* compiler points it out. But this means we need to explicitly handle the
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* sentinel in every "switch".)
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*
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* |aEnumerators| is expected to be a comma-separated list of enumerators,
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* enclosed in parentheses. The enumerators may NOT have associated
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* initializers (an attempt to have one will result in a compiler error).
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* This ensures that the enumerator values are in the range [0, N), where N
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* is the number of enumerators.
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*
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* The list of enumerators cannot contain a trailing comma. This is a
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* limitation of MOZ_FOR_EACH, which we use in the implementation; if
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* MOZ_FOR_EACH supported trailing commas, we could too.
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*
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* The generated constant has the name "k" + |aEnumName| + "Count", and type
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* "size_t". The enumeration and the constant are both defined in the scope
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* in which the macro is invoked.
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*
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* For convenience, a constant of the enumeration type named
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* "kHighest" + |aEnumName| is also defined, whose value is the highest
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* valid enumerator, assuming the enumerators have contiguous values starting
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* from 0.
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*
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* Invocation of the macro may be followed by a semicolon, if one prefers a
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* more declaration-like syntax.
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*
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* Example invocation:
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* MOZ_DEFINE_ENUM(MyEnum, (Foo, Bar, Baz));
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*
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* This expands to:
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* enum MyEnum { Foo, Bar, Baz };
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* constexpr size_t kMyEnumCount = 3;
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* constexpr MyEnum kHighestMyEnum = MyEnum(kMyEnumCount - 1);
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* // some static_asserts to ensure the values are in the range [0, 3)
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*
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* The macro also has several variants:
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*
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* - A |_CLASS| variant, which generates an |enum class| instead of
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* a plain enum.
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*
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* - A |_WITH_BASE| variant which generates an enum with a specified
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* underlying ("base") type, which is provided as an additional
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* argument in second position.
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*
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* - An |_AT_CLASS_SCOPE| variant, designed for enumerations defined
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* at class scope. For these, the generated constants are static,
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* and have names prefixed with "s" instead of "k" as per
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* naming convention.
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*
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* (and combinations of these).
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*/
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/*
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* A helper macro for asserting that an enumerator does not have an initializer.
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*
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* The static_assert and the comparison to 0 are just scaffolding; the
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* important part is forming the expression |aEnumName::aEnumeratorDecl|.
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*
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* If |aEnumeratorDecl| is just the enumerator name without an identifier,
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* this expression compiles fine. However, if |aEnumeratorDecl| includes an
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* initializer, as in |eEnumerator = initializer|, then this will fail to
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* compile in expression context, since |eEnumerator| is not an lvalue.
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*
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* (The static_assert itself should always pass in the absence of the above
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* error, since you can't get a negative enumerator value without having
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* an initializer somewhere. It just provides a place to put the expression
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* we want to form.)
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*/
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#define MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER(aEnumName, aEnumeratorDecl) \
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static_assert( \
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(aEnumName::aEnumeratorDecl) >= aEnumName(0), \
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"MOZ_DEFINE_ENUM does not allow enumerators to have initializers");
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#define MOZ_DEFINE_ENUM_IMPL(aEnumName, aClassSpec, aBaseSpec, aEnumerators) \
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enum aClassSpec aEnumName aBaseSpec{MOZ_UNWRAP_ARGS aEnumerators}; \
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constexpr size_t k##aEnumName##Count = MOZ_ARG_COUNT aEnumerators; \
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constexpr aEnumName k##Highest##aEnumName = \
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aEnumName(k##aEnumName##Count - 1); \
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MOZ_FOR_EACH(MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER, (aEnumName, ), \
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aEnumerators)
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#define MOZ_DEFINE_ENUM(aEnumName, aEnumerators) \
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MOZ_DEFINE_ENUM_IMPL(aEnumName, , , aEnumerators)
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#define MOZ_DEFINE_ENUM_WITH_BASE(aEnumName, aBaseName, aEnumerators) \
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MOZ_DEFINE_ENUM_IMPL(aEnumName, , : aBaseName, aEnumerators)
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#define MOZ_DEFINE_ENUM_CLASS(aEnumName, aEnumerators) \
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MOZ_DEFINE_ENUM_IMPL(aEnumName, class, , aEnumerators)
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#define MOZ_DEFINE_ENUM_CLASS_WITH_BASE(aEnumName, aBaseName, aEnumerators) \
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MOZ_DEFINE_ENUM_IMPL(aEnumName, class, : aBaseName, aEnumerators)
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#define MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, aClassSpec, aBaseSpec, \
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aEnumerators) \
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enum aClassSpec aEnumName aBaseSpec{MOZ_UNWRAP_ARGS aEnumerators}; \
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constexpr static size_t s##aEnumName##Count = MOZ_ARG_COUNT aEnumerators; \
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constexpr static aEnumName s##Highest##aEnumName = \
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aEnumName(s##aEnumName##Count - 1); \
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MOZ_FOR_EACH(MOZ_ASSERT_ENUMERATOR_HAS_NO_INITIALIZER, (aEnumName, ), \
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aEnumerators)
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#define MOZ_DEFINE_ENUM_AT_CLASS_SCOPE(aEnumName, aEnumerators) \
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MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, , , aEnumerators)
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#define MOZ_DEFINE_ENUM_WITH_BASE_AT_CLASS_SCOPE(aEnumName, aBaseName, \
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aEnumerators) \
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MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, , : aBaseName, aEnumerators)
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#define MOZ_DEFINE_ENUM_CLASS_AT_CLASS_SCOPE(aEnumName, aEnumerators) \
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MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, class, , aEnumerators)
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#define MOZ_DEFINE_ENUM_CLASS_WITH_BASE_AT_CLASS_SCOPE(aEnumName, aBaseName, \
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aEnumerators) \
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MOZ_DEFINE_ENUM_AT_CLASS_SCOPE_IMPL(aEnumName, class, \
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: aBaseName, aEnumerators)
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#endif // mozilla_DefineEnum_h
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