[GitHub] [arrow] pitrou commented on a change in pull request #8472: ARROW-8113: [C++][WIP] Lighter weight variant<>

[GitBox <gi...@apache.org>]

pitrou commented on a change in pull request #8472:
URL: https://github.com/apache/arrow/pull/8472#discussion_r505758011



##########
File path: cpp/src/arrow/util/variant.h
##########
@@ -17,17 +17,363 @@
 
 #pragma once
 
-#include "arrow/vendored/variant.hpp"  // IWYU pragma: export
+#include <array>
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+#include "arrow/util/macros.h"
 
 namespace arrow {
 namespace util {
 
-using ::mpark::bad_variant_access;
-using ::mpark::get;
-using ::mpark::get_if;
-using ::mpark::holds_alternative;
-using ::mpark::variant;
-using ::mpark::visit;
+/// \brief a std::variant-like discriminated union
+///
+/// Simplifications from std::variant:
+///
+/// - Strictly defaultable. The first type of T... must 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, which is also required of each
+///   of T...
+///
+/// - Less sophisticated type deduction. std::variant<bool, std::string>("hello") will
+///   intelligently construct std::string while Variant 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> {};
+
+template <typename T>
+void copy_construct(void* ptr, const void* other) {
+  new (ptr) T(*static_cast<const T*>(other));
+}
+
+template <typename T>
+void move_construct(void* ptr, void* other) {
+  new (ptr) T(std::move(*static_cast<T*>(other)));
+}
+
+template <typename T>
+void explicit_destroy(void* ptr) {
+  static_cast<T*>(ptr)->~T();
+}
+
+inline void trivial_destroy(void*) {}
+
+template <typename T>
+bool equal(const void* l, const void* r) {
+  return *static_cast<const T*>(l) == *static_cast<const T*>(r);
+}
+
+template <typename Visitor, typename T>
+decltype(std::declval<Visitor&&>()(std::declval<const T&>())) visit_const_ref(
+    Visitor&& visitor, const void* ptr) {
+  return std::forward<Visitor>(visitor)(*static_cast<const T*>(ptr));
+}
+
+template <typename Visitor, typename T>
+decltype(std::declval<Visitor&&>()(std::declval<T*>())) visit_mutable_ptr(
+    Visitor&& visitor, void* ptr) {
+  return std::forward<Visitor>(visitor)(static_cast<T*>(ptr));
+}
+
+template <typename... T>
+struct variant_storage {
+  variant_storage() = default;
+  variant_storage(const variant_storage&) {}
+  variant_storage& operator=(const variant_storage&) { return *this; }
+  variant_storage(variant_storage&&) {}
+  variant_storage& operator=(variant_storage&&) { return *this; }
+  ~variant_storage() {
+    static_assert(offsetof(variant_storage, data_) == 0,
+                  "(void*)&variant_storage::data_ == (void*)this");
+  }
+
+  typename std::aligned_union<0, T...>::type data_;
+  uint8_t index_ = 0;
+};
+
+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<Copyable*>(this)->copy(other, /*assignment=*/false);
+    }
+    type& operator=(const type& other) {
+      static_cast<Copyable*>(this)->copy(other, /*assignment=*/true);
+      return *this;
+    }
+  };
+};
+
+template <typename V, uint8_t I, typename...>
+struct member_constructor {
+  static void index_of() {}
+};
+
+template <typename V, uint8_t I, typename H, typename... T>
+struct member_constructor<V, I, H, T...> : member_constructor<V, I + 1, T...> {
+  member_constructor() = default;
+
+  using member_constructor<V, I + 1, T...>::member_constructor;
+  using member_constructor<V, I + 1, T...>::operator=;
+  using member_constructor<V, I + 1, T...>::index_of;
+
+  explicit member_constructor(H value) {
+    new (this) H(std::move(value));
+    static_cast<V*>(this)->index_ = I;
+  }
+
+  member_constructor& operator=(H value) {
+    static_cast<V*>(this)->destroy();
+    new (this) H(std::move(value));
+    static_cast<V*>(this)->index_ = I;
+    return *this;
+  }
+
+  static constexpr std::integral_constant<uint8_t, I> index_of(const type_constant<H>&) {
+    return {};
+  }
+};
+
+}  // namespace detail
+
+template <typename... T>
+class Variant : detail::variant_storage<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...>>,
+                detail::member_constructor<Variant<T...>, 0, T...> {
+  static_assert(detail::all<(std::is_nothrow_move_constructible<T>::value &&
+                             std::is_nothrow_move_assignable<T>::value)...>::value,
+                "valueless_by_exception is not supported");
+
+  template <typename U>
+  static constexpr uint8_t index_of() {
+    return detail::member_constructor<Variant<T...>, 0, T...>::index_of(
+        detail::type_constant<U>{});
+  }
+
+ public:
+  using default_type = typename util::detail::first<T...>::type;
+  static_assert(std::is_nothrow_default_constructible<default_type>::value,
+                "valueless_by_exception, non-default constructible are not supported");
+
+  Variant() noexcept { construct_default(); }
+
+  Variant(const Variant& other) = default;
+  Variant& operator=(const Variant& other) = default;
+
+  using detail::member_constructor<Variant<T...>, 0, T...>::member_constructor;
+  using detail::member_constructor<Variant<T...>, 0, T...>::operator=;
+
+  Variant(Variant&& other) noexcept { move(&other, /*assignment=*/false); }
+
+  Variant& operator=(Variant&& other) noexcept {
+    move(&other, /*assignment=*/true);
+    return *this;
+  }
+
+  ~Variant() {
+    static_assert(offsetof(Variant, data_) == 0, "(void*)&Variant::data_ == (void*)this");
+    this->destroy();
+  }
+
+  uint8_t index() const noexcept { return this->index_; }
+
+  template <typename U, uint8_t I = index_of<U>()>
+  const U* get() const noexcept {
+    return index() == I ? reinterpret_cast<const U*>(this) : NULLPTR;
+  }
+
+  template <typename U, uint8_t I = index_of<U>()>
+  U* get() noexcept {
+    return index() == I ? reinterpret_cast<U*>(this) : NULLPTR;
+  }
+
+  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;
+  }
+
+  void swap(Variant& other) noexcept {  // NOLINT google-runtime-references
+    Variant tmp = std::move(other);
+    other = std::move(*this);
+    *this = std::move(tmp);
+  }
+
+ private:
+  void move(Variant* other, bool assignment) noexcept {
+    using impl_t = void(void*, void*);
+    static std::array<impl_t*, sizeof...(T)> impl = {detail::move_construct<T>...};

Review comment:
       `constexpr`, no?




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