--- /dev/null
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <cstddef>
+#include <exception>
+#include <type_traits>
+#include <utility>
+
+#include "arrow/util/macros.h"
+#include "arrow/util/type_traits.h"
+
+namespace arrow {
+namespace util {
+
+/// \brief a std::variant-like discriminated union
+///
+/// Simplifications from std::variant:
+///
+/// - Strictly defaultable. The first type of T... should be nothrow default constructible
+/// and it will be used for default Variants.
+///
+/// - Never valueless_by_exception. std::variant supports a state outside those specified
+/// by T... to which it can return in the event that a constructor throws. If a Variant
+/// would become valueless_by_exception it will instead return to its default state.
+///
+/// - Strictly nothrow move constructible and assignable
+///
+/// - Less sophisticated type deduction. std::variant<bool, std::string>("hello") will
+/// intelligently construct std::string while Variant<bool, std::string>("hello") will
+/// construct bool.
+///
+/// - Either both copy constructible and assignable or neither (std::variant independently
+/// enables copy construction and copy assignment). Variant is copy constructible if
+/// each of T... is copy constructible and assignable.
+///
+/// - Slimmer interface; several members of std::variant are omitted.
+///
+/// - Throws no exceptions; if a bad_variant_access would be thrown Variant will instead
+/// segfault (nullptr dereference).
+///
+/// - Mutable visit takes a pointer instead of mutable reference or rvalue reference,
+/// which is more conformant with our code style.
+template <typename... T>
+class Variant;
+
+namespace detail {
+
+template <typename T, typename = void>
+struct is_equality_comparable : std::false_type {};
+
+template <typename T>
+struct is_equality_comparable<
+ T, typename std::enable_if<std::is_convertible<
+ decltype(std::declval<T>() == std::declval<T>()), bool>::value>::type>
+ : std::true_type {};
+
+template <bool C, typename T, typename E>
+using conditional_t = typename std::conditional<C, T, E>::type;
+
+template <typename T>
+struct type_constant {
+ using type = T;
+};
+
+template <typename...>
+struct first;
+
+template <typename H, typename... T>
+struct first<H, T...> {
+ using type = H;
+};
+
+template <typename T>
+using decay_t = typename std::decay<T>::type;
+
+template <bool...>
+struct all : std::true_type {};
+
+template <bool H, bool... T>
+struct all<H, T...> : conditional_t<H, all<T...>, std::false_type> {};
+
+struct delete_copy_constructor {
+ template <typename>
+ struct type {
+ type() = default;
+ type(const type& other) = delete;
+ type& operator=(const type& other) = delete;
+ };
+};
+
+struct explicit_copy_constructor {
+ template <typename Copyable>
+ struct type {
+ type() = default;
+ type(const type& other) { static_cast<const Copyable&>(other).copy_to(this); }
+ type& operator=(const type& other) {
+ static_cast<Copyable*>(this)->destroy();
+ static_cast<const Copyable&>(other).copy_to(this);
+ return *this;
+ }
+ };
+};
+
+template <typename... T>
+struct VariantStorage {
+ VariantStorage() = default;
+ VariantStorage(const VariantStorage&) {}
+ VariantStorage& operator=(const VariantStorage&) { return *this; }
+ VariantStorage(VariantStorage&&) noexcept {}
+ VariantStorage& operator=(VariantStorage&&) noexcept { return *this; }
+ ~VariantStorage() {
+ static_assert(offsetof(VariantStorage, data_) == 0,
+ "(void*)&VariantStorage::data_ == (void*)this");
+ }
+
+ typename arrow::internal::aligned_union<0, T...>::type data_;
+ uint8_t index_ = 0;
+};
+
+template <typename V, typename...>
+struct VariantImpl;
+
+template <typename... T>
+struct VariantImpl<Variant<T...>> : VariantStorage<T...> {
+ static void index_of() noexcept {}
+ void destroy() noexcept {}
+ void move_to(...) noexcept {}
+ void copy_to(...) const {}
+
+ template <typename R, typename Visitor>
+ [[noreturn]] R visit_const(Visitor&& visitor) const {
+ std::terminate();
+ }
+ template <typename R, typename Visitor>
+ [[noreturn]] R visit_mutable(Visitor&& visitor) {
+ std::terminate();
+ }
+};
+
+template <typename... M, typename H, typename... T>
+struct VariantImpl<Variant<M...>, H, T...> : VariantImpl<Variant<M...>, T...> {
+ using VariantType = Variant<M...>;
+ using Impl = VariantImpl<VariantType, T...>;
+
+ static constexpr uint8_t kIndex = sizeof...(M) - sizeof...(T) - 1;
+
+ VariantImpl() = default;
+
+ using VariantImpl<VariantType, T...>::VariantImpl;
+ using Impl::operator=;
+ using Impl::index_of;
+
+ explicit VariantImpl(H value) {
+ new (this) H(std::move(value));
+ this->index_ = kIndex;
+ }
+
+ VariantImpl& operator=(H value) {
+ static_cast<VariantType*>(this)->destroy();
+ new (this) H(std::move(value));
+ this->index_ = kIndex;
+ return *this;
+ }
+
+ H& cast_this() { return *reinterpret_cast<H*>(this); }
+ const H& cast_this() const { return *reinterpret_cast<const H*>(this); }
+
+ void move_to(VariantType* target) noexcept {
+ if (this->index_ == kIndex) {
+ new (target) H(std::move(cast_this()));
+ target->index_ = kIndex;
+ } else {
+ Impl::move_to(target);
+ }
+ }
+
+ // Templated to avoid instantiation in case H is not copy constructible
+ template <typename Void>
+ void copy_to(Void* generic_target) const {
+ const auto target = static_cast<VariantType*>(generic_target);
+ try {
+ if (this->index_ == kIndex) {
+ new (target) H(cast_this());
+ target->index_ = kIndex;
+ } else {
+ Impl::copy_to(target);
+ }
+ } catch (...) {
+ target->construct_default();
+ throw;
+ }
+ }
+
+ void destroy() noexcept {
+ if (this->index_ == kIndex) {
+ if (!std::is_trivially_destructible<H>::value) {
+ cast_this().~H();
+ }
+ } else {
+ Impl::destroy();
+ }
+ }
+
+ static constexpr std::integral_constant<uint8_t, kIndex> index_of(
+ const type_constant<H>&) {
+ return {};
+ }
+
+ template <typename R, typename Visitor>
+ R visit_const(Visitor&& visitor) const {
+ if (this->index_ == kIndex) {
+ return std::forward<Visitor>(visitor)(cast_this());
+ }
+ return Impl::template visit_const<R>(std::forward<Visitor>(visitor));
+ }
+
+ template <typename R, typename Visitor>
+ R visit_mutable(Visitor&& visitor) {
+ if (this->index_ == kIndex) {
+ return std::forward<Visitor>(visitor)(&cast_this());
+ }
+ return Impl::template visit_mutable<R>(std::forward<Visitor>(visitor));
+ }
+};
+
+} // namespace detail
+
+template <typename... T>
+class Variant : detail::VariantImpl<Variant<T...>, T...>,
+ detail::conditional_t<
+ detail::all<(std::is_copy_constructible<T>::value &&
+ std::is_copy_assignable<T>::value)...>::value,
+ detail::explicit_copy_constructor,
+ detail::delete_copy_constructor>::template type<Variant<T...>> {
+ template <typename U>
+ static constexpr uint8_t index_of() {
+ return Impl::index_of(detail::type_constant<U>{});
+ }
+
+ using Impl = detail::VariantImpl<Variant<T...>, T...>;
+
+ public:
+ using default_type = typename util::detail::first<T...>::type;
+
+ Variant() noexcept { construct_default(); }
+
+ Variant(const Variant& other) = default;
+ Variant& operator=(const Variant& other) = default;
+ Variant& operator=(Variant&& other) noexcept {
+ this->destroy();
+ other.move_to(this);
+ return *this;
+ }
+
+ using Impl::Impl;
+ using Impl::operator=;
+
+ Variant(Variant&& other) noexcept { other.move_to(this); }
+
+ ~Variant() {
+ static_assert(offsetof(Variant, data_) == 0, "(void*)&Variant::data_ == (void*)this");
+ this->destroy();
+ }
+
+ /// \brief Return the zero-based type index of the value held by the variant
+ uint8_t index() const noexcept { return this->index_; }
+
+ /// \brief Get a const pointer to the value held by the variant
+ ///
+ /// If the type given as template argument doesn't match, a null pointer is returned.
+ template <typename U, uint8_t I = index_of<U>()>
+ const U* get() const noexcept {
+ return index() == I ? reinterpret_cast<const U*>(this) : NULLPTR;
+ }
+
+ /// \brief Get a pointer to the value held by the variant
+ ///
+ /// If the type given as template argument doesn't match, a null pointer is returned.
+ template <typename U, uint8_t I = index_of<U>()>
+ U* get() noexcept {
+ return index() == I ? reinterpret_cast<U*>(this) : NULLPTR;
+ }
+
+ /// \brief Replace the value held by the variant
+ ///
+ /// The intended type must be given as a template argument.
+ /// The value is constructed in-place using the given function arguments.
+ template <typename U, typename... A, uint8_t I = index_of<U>()>
+ void emplace(A&&... args) {
+ try {
+ this->destroy();
+ new (this) U(std::forward<A>(args)...);
+ this->index_ = I;
+ } catch (...) {
+ construct_default();
+ throw;
+ }
+ }
+
+ template <typename U, typename E, typename... A, uint8_t I = index_of<U>()>
+ void emplace(std::initializer_list<E> il, A&&... args) {
+ try {
+ this->destroy();
+ new (this) U(il, std::forward<A>(args)...);
+ this->index_ = I;
+ } catch (...) {
+ construct_default();
+ throw;
+ }
+ }
+
+ /// \brief Swap with another variant's contents
+ void swap(Variant& other) noexcept { // NOLINT google-runtime-references
+ Variant tmp = std::move(other);
+ other = std::move(*this);
+ *this = std::move(tmp);
+ }
+
+ using Impl::visit_const;
+ using Impl::visit_mutable;
+
+ private:
+ void construct_default() noexcept {
+ new (this) default_type();
+ this->index_ = 0;
+ }
+
+ template <typename V>
+ friend struct detail::explicit_copy_constructor::type;
+
+ template <typename V, typename...>
+ friend struct detail::VariantImpl;
+};
+
+/// \brief Call polymorphic visitor on a const variant's value
+///
+/// The visitor will receive a const reference to the value held by the variant.
+/// It must define overloads for each possible variant type.
+/// The overloads should all return the same type (no attempt
+/// is made to find a generalized return type).
+template <typename Visitor, typename... T,
+ typename R = decltype(std::declval<Visitor&&>()(
+ std::declval<const typename Variant<T...>::default_type&>()))>
+R visit(Visitor&& visitor, const util::Variant<T...>& v) {
+ return v.template visit_const<R>(std::forward<Visitor>(visitor));
+}
+
+/// \brief Call polymorphic visitor on a non-const variant's value
+///
+/// The visitor will receive a pointer to the value held by the variant.
+/// It must define overloads for each possible variant type.
+/// The overloads should all return the same type (no attempt
+/// is made to find a generalized return type).
+template <typename Visitor, typename... T,
+ typename R = decltype(std::declval<Visitor&&>()(
+ std::declval<typename Variant<T...>::default_type*>()))>
+R visit(Visitor&& visitor, util::Variant<T...>* v) {
+ return v->template visit_mutable<R>(std::forward<Visitor>(visitor));
+}
+
+/// \brief Get a const reference to the value held by the variant
+///
+/// If the type given as template argument doesn't match, behavior is undefined
+/// (a null pointer will be dereferenced).
+template <typename U, typename... T>
+const U& get(const Variant<T...>& v) {
+ return *v.template get<U>();
+}
+
+/// \brief Get a reference to the value held by the variant
+///
+/// If the type given as template argument doesn't match, behavior is undefined
+/// (a null pointer will be dereferenced).
+template <typename U, typename... T>
+U& get(Variant<T...>& v) {
+ return *v.template get<U>();
+}
+
+/// \brief Get a const pointer to the value held by the variant
+///
+/// If the type given as template argument doesn't match, a nullptr is returned.
+template <typename U, typename... T>
+const U* get_if(const Variant<T...>* v) {
+ return v->template get<U>();
+}
+
+/// \brief Get a pointer to the value held by the variant
+///
+/// If the type given as template argument doesn't match, a nullptr is returned.
+template <typename U, typename... T>
+U* get_if(Variant<T...>* v) {
+ return v->template get<U>();
+}
+
+namespace detail {
+
+template <typename... T>
+struct VariantsEqual {
+ template <typename U>
+ bool operator()(const U& r) const {
+ return get<U>(l_) == r;
+ }
+ const Variant<T...>& l_;
+};
+
+} // namespace detail
+
+template <typename... T, typename = typename std::enable_if<detail::all<
+ detail::is_equality_comparable<T>::value...>::value>>
+bool operator==(const Variant<T...>& l, const Variant<T...>& r) {
+ if (l.index() != r.index()) return false;
+ return visit(detail::VariantsEqual<T...>{l}, r);
+}
+
+template <typename... T>
+auto operator!=(const Variant<T...>& l, const Variant<T...>& r) -> decltype(l == r) {
+ return !(l == r);
+}
+
+/// \brief Return whether the variant holds a value of the given type
+template <typename U, typename... T>
+bool holds_alternative(const Variant<T...>& v) {
+ return v.template get<U>();
+}
+
+} // namespace util
+} // namespace arrow
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