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https://github.com/LadybirdBrowser/ladybird.git
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145e246a5e
This changes Variant::visit() to forward the value returned by the selected visitor invocation. By perfectly forwarding the returned value, this allows for the visitor to return by value or reference. Note that all provided visitors must return the same type - the compiler will otherwise fail with the message: "inconsistent deduction for auto return type".
385 lines
12 KiB
C++
385 lines
12 KiB
C++
/*
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* Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Array.h>
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#include <AK/BitCast.h>
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#include <AK/StdLibExtras.h>
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#include <AK/TypeList.h>
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namespace AK::Detail {
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template<typename T, typename IndexType, IndexType InitialIndex, typename... InTypes>
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struct VariantIndexOf {
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static_assert(DependentFalse<T, IndexType, InTypes...>, "Invalid VariantIndex instantiated");
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};
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template<typename T, typename IndexType, IndexType InitialIndex, typename InType, typename... RestOfInTypes>
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struct VariantIndexOf<T, IndexType, InitialIndex, InType, RestOfInTypes...> {
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consteval IndexType operator()()
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{
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if constexpr (IsSame<T, InType>)
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return InitialIndex;
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else
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return VariantIndexOf<T, IndexType, InitialIndex + 1, RestOfInTypes...> {}();
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}
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};
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template<typename T, typename IndexType, IndexType InitialIndex>
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struct VariantIndexOf<T, IndexType, InitialIndex> {
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consteval IndexType operator()() { return InitialIndex; }
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};
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template<typename T, typename IndexType, typename... Ts>
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consteval IndexType index_of()
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{
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return VariantIndexOf<T, IndexType, 0, Ts...> {}();
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}
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template<typename IndexType, IndexType InitialIndex, typename... Ts>
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struct Variant;
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template<typename IndexType, IndexType InitialIndex, typename F, typename... Ts>
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struct Variant<IndexType, InitialIndex, F, Ts...> {
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static constexpr auto current_index = VariantIndexOf<F, IndexType, InitialIndex, F, Ts...> {}();
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static void delete_(IndexType id, void* data)
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{
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if (id == current_index)
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bit_cast<F*>(data)->~F();
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::delete_(id, data);
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}
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static void move_(IndexType old_id, void* old_data, void* new_data)
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{
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if (old_id == current_index)
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new (new_data) F(move(*bit_cast<F*>(old_data)));
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::move_(old_id, old_data, new_data);
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}
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static void copy_(IndexType old_id, const void* old_data, void* new_data)
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{
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if (old_id == current_index)
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new (new_data) F(*bit_cast<F*>(old_data));
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else
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Variant<IndexType, InitialIndex + 1, Ts...>::copy_(old_id, old_data, new_data);
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}
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};
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template<typename IndexType, IndexType InitialIndex>
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struct Variant<IndexType, InitialIndex> {
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static void delete_(IndexType, void*) { }
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static void move_(IndexType, void*, void*) { }
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static void copy_(IndexType, const void*, void*) { }
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};
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template<typename IndexType, typename... Ts>
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struct VisitImpl {
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template<typename Visitor, IndexType CurrentIndex = 0>
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static constexpr inline decltype(auto) visit(IndexType id, const void* data, Visitor&& visitor) requires(CurrentIndex < sizeof...(Ts))
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{
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using T = typename TypeList<Ts...>::template Type<CurrentIndex>;
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if (id == CurrentIndex)
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return visitor(*bit_cast<T*>(data));
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if constexpr ((CurrentIndex + 1) < sizeof...(Ts))
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return visit<Visitor, CurrentIndex + 1>(id, data, forward<Visitor>(visitor));
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else
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VERIFY_NOT_REACHED();
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}
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};
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struct VariantNoClearTag {
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explicit VariantNoClearTag() = default;
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};
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struct VariantConstructTag {
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explicit VariantConstructTag() = default;
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};
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template<typename T, typename Base>
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struct VariantConstructors {
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VariantConstructors(T&& t)
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{
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internal_cast().template set<T>(forward<T>(t), VariantNoClearTag {});
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}
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VariantConstructors() { }
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Base& operator=(const T& value)
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{
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Base variant { value };
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internal_cast() = move(variant);
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return internal_cast();
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}
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Base& operator=(T&& value)
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{
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Base variant { move(value) };
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internal_cast() = move(variant);
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return internal_cast();
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}
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private:
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[[nodiscard]] Base& internal_cast()
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{
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// Warning: Internal type shenanigans - VariantsConstrutors<T, Base> <- Base
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// Not the other way around, so be _really_ careful not to cause issues.
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return *reinterpret_cast<Base*>(this);
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}
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};
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// Type list deduplication
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// Since this is a big template mess, each template is commented with how and why it works.
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struct ParameterPackTag {
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};
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// Pack<Ts...> is just a way to pass around the type parameter pack Ts
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template<typename... Ts>
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struct ParameterPack : ParameterPackTag {
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};
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// Blank<T> is a unique replacement for T, if T is a duplicate type.
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template<typename T>
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struct Blank {
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};
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template<typename A, typename P>
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inline constexpr bool IsTypeInPack = false;
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// IsTypeInPack<T, Pack<Ts...>> will just return whether 'T' exists in 'Ts'.
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template<typename T, typename... Ts>
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inline constexpr bool IsTypeInPack<T, ParameterPack<Ts...>> = (IsSame<T, Ts> || ...);
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// Replaces T with Blank<T> if it exists in Qs.
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template<typename T, typename... Qs>
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using BlankIfDuplicate = Conditional<(IsTypeInPack<T, Qs> || ...), Blank<T>, T>;
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template<unsigned I, typename...>
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struct InheritFromUniqueEntries;
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// InheritFromUniqueEntries will inherit from both Qs and Ts, but only scan entries going *forwards*
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// that is to say, if it's scanning from index I in Qs, it won't scan for duplicates for entries before I
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// as that has already been checked before.
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// This makes sure that the search is linear in time (like the 'merge' step of merge sort).
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template<unsigned I, typename... Ts, unsigned... Js, typename... Qs>
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struct InheritFromUniqueEntries<I, ParameterPack<Ts...>, IndexSequence<Js...>, Qs...>
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: public BlankIfDuplicate<Ts, Conditional<Js <= I, ParameterPack<>, Qs>...>... {
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using BlankIfDuplicate<Ts, Conditional<Js <= I, ParameterPack<>, Qs>...>::BlankIfDuplicate...;
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};
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template<typename...>
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struct InheritFromPacks;
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// InheritFromPacks will attempt to 'merge' the pack 'Ps' with *itself*, but skip the duplicate entries
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// (via InheritFromUniqueEntries).
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template<unsigned... Is, typename... Ps>
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struct InheritFromPacks<IndexSequence<Is...>, Ps...>
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: public InheritFromUniqueEntries<Is, Ps, IndexSequence<Is...>, Ps...>... {
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using InheritFromUniqueEntries<Is, Ps, IndexSequence<Is...>, Ps...>::InheritFromUniqueEntries...;
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};
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// Just a nice wrapper around InheritFromPacks, which will wrap any parameter packs in ParameterPack (unless it already is one).
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template<typename... Ps>
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using MergeAndDeduplicatePacks = InheritFromPacks<MakeIndexSequence<sizeof...(Ps)>, Conditional<IsBaseOf<ParameterPackTag, Ps>, Ps, ParameterPack<Ps>>...>;
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}
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namespace AK {
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struct Empty {
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};
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template<typename... Ts>
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struct Variant
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: public Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...> {
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private:
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using IndexType = Conditional<sizeof...(Ts) < 255, u8, size_t>; // Note: size+1 reserved for internal value checks
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static constexpr IndexType invalid_index = sizeof...(Ts);
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template<typename T>
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static constexpr IndexType index_of() { return Detail::index_of<T, IndexType, Ts...>(); }
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public:
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template<typename... NewTs>
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friend struct Variant;
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Variant(const Variant& old)
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: Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>()
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, m_index(old.m_index)
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{
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Helper::copy_(old.m_index, old.m_data, m_data);
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}
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// Note: A moved-from variant emulates the state of the object it contains
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// so if a variant containing an int is moved from, it will still contain that int
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// and if a variant with a nontrivial move ctor is moved from, it may or may not be valid
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// but it will still contain the "moved-from" state of the object it previously contained.
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Variant(Variant&& old)
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: Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>()
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, m_index(old.m_index)
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{
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Helper::move_(old.m_index, old.m_data, m_data);
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}
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~Variant()
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{
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Helper::delete_(m_index, m_data);
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}
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Variant& operator=(const Variant& other)
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{
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m_index = other.m_index;
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Helper::copy_(other.m_index, other.m_data, m_data);
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return *this;
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}
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Variant& operator=(Variant&& other)
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{
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m_index = other.m_index;
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Helper::move_(other.m_index, other.m_data, m_data);
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return *this;
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}
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using Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>::MergeAndDeduplicatePacks;
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template<typename T>
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void set(T&& t) requires(index_of<T>() != invalid_index)
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{
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using StrippedT = RemoveCV<RemoveReference<T>>;
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constexpr auto new_index = index_of<StrippedT>();
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Helper::delete_(m_index, m_data);
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new (m_data) StrippedT(forward<T>(t));
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m_index = new_index;
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}
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template<typename T>
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void set(T&& t, Detail::VariantNoClearTag)
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{
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using StrippedT = RemoveCV<RemoveReference<T>>;
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constexpr auto new_index = index_of<StrippedT>();
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new (m_data) StrippedT(forward<T>(t));
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m_index = new_index;
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}
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template<typename T>
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T* get_pointer()
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{
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if (index_of<T>() == m_index)
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return bit_cast<T*>(&m_data);
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return nullptr;
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}
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template<typename T>
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[[gnu::noinline]] T& get()
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{
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VERIFY(has<T>());
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return *bit_cast<T*>(&m_data);
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}
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template<typename T>
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[[gnu::noinline]] const T* get_pointer() const
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{
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if (index_of<T>() == m_index)
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return bit_cast<const T*>(&m_data);
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return nullptr;
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}
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template<typename T>
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[[gnu::noinline]] const T& get() const
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{
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VERIFY(has<T>());
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return *bit_cast<const T*>(&m_data);
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}
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template<typename T>
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[[nodiscard]] bool has() const
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{
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return index_of<T>() == m_index;
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}
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template<typename... Fs>
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decltype(auto) visit(Fs&&... functions)
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{
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Visitor<Fs...> visitor { forward<Fs>(functions)... };
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return VisitHelper::visit(m_index, m_data, move(visitor));
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}
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template<typename... Fs>
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decltype(auto) visit(Fs&&... functions) const
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{
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Visitor<Fs...> visitor { forward<Fs>(functions)... };
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return VisitHelper::visit(m_index, m_data, move(visitor));
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}
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template<typename... NewTs>
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Variant<NewTs...> downcast() &&
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{
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Variant<NewTs...> instance { Variant<NewTs...>::invalid_index, Detail::VariantConstructTag {} };
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visit([&](auto& value) {
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if constexpr (Variant<NewTs...>::template can_contain<RemoveCV<RemoveReference<decltype(value)>>>())
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instance.set(move(value), Detail::VariantNoClearTag {});
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});
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VERIFY(instance.m_index != instance.invalid_index);
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return instance;
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}
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template<typename... NewTs>
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Variant<NewTs...> downcast() &
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{
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Variant<NewTs...> instance { Variant<NewTs...>::invalid_index, Detail::VariantConstructTag {} };
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visit([&](const auto& value) {
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if constexpr (Variant<NewTs...>::template can_contain<RemoveCV<RemoveReference<decltype(value)>>>())
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instance.set(value, Detail::VariantNoClearTag {});
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});
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VERIFY(instance.m_index != instance.invalid_index);
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return instance;
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}
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template<typename T>
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static constexpr bool can_contain()
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{
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return index_of<T>() != invalid_index;
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}
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private:
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static constexpr auto data_size = integer_sequence_generate_array<size_t>(0, IntegerSequence<size_t, sizeof(Ts)...>()).max();
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static constexpr auto data_alignment = integer_sequence_generate_array<size_t>(0, IntegerSequence<size_t, alignof(Ts)...>()).max();
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using Helper = Detail::Variant<IndexType, 0, Ts...>;
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using VisitHelper = Detail::VisitImpl<IndexType, Ts...>;
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explicit Variant(IndexType index, Detail::VariantConstructTag)
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: Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...>()
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, m_index(index)
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{
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}
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template<typename... Fs>
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struct Visitor : Fs... {
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Visitor(Fs&&... args)
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: Fs(forward<Fs>(args))...
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{
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}
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using Fs::operator()...;
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};
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alignas(data_alignment) u8 m_data[data_size];
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// Note: Make sure not to default-initialize!
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// VariantConstructors::VariantConstructors(T) will set this to the correct value
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// So default-constructing to anything will leave the first initialization with that value instead of the correct one.
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IndexType m_index;
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};
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}
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using AK::Empty;
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using AK::Variant;
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