include/arrow/util/aligned_storage.h

// 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 <cstring>
#include <type_traits>
#include <utility>

#include "arrow/util/launder.h"
#include "arrow/util/macros.h"

namespace arrow {
namespace internal {

template <typename T>
class AlignedStorage {
 public:
  static constexpr bool can_memcpy = std::is_trivial<T>::value;

  constexpr T* get() noexcept {
    return arrow::internal::launder(reinterpret_cast<T*>(&data_));
  }

  constexpr const T* get() const noexcept {
    // Use fully qualified name to avoid ambiguities with MSVC (ARROW-14800)
    return arrow::internal::launder(reinterpret_cast<const T*>(&data_));
  }

  void destroy() noexcept {
    if (!std::is_trivially_destructible<T>::value) {
      get()->~T();
    }
  }

  template <typename... A>
  void construct(A&&... args) noexcept {
    new (&data_) T(std::forward<A>(args)...);
  }

  template <typename V>
  void assign(V&& v) noexcept {
    *get() = std::forward<V>(v);
  }

  void move_construct(AlignedStorage* other) noexcept {
    new (&data_) T(std::move(*other->get()));
  }

  void move_assign(AlignedStorage* other) noexcept { *get() = std::move(*other->get()); }

  template <bool CanMemcpy = can_memcpy>
  static typename std::enable_if<CanMemcpy>::type move_construct_several(
      AlignedStorage* ARROW_RESTRICT src, AlignedStorage* ARROW_RESTRICT dest, size_t n,
      size_t memcpy_length) noexcept {
    memcpy(dest->get(), src->get(), memcpy_length * sizeof(T));
  }

  template <bool CanMemcpy = can_memcpy>
  static typename std::enable_if<CanMemcpy>::type
  move_construct_several_and_destroy_source(AlignedStorage* ARROW_RESTRICT src,
                                            AlignedStorage* ARROW_RESTRICT dest, size_t n,
                                            size_t memcpy_length) noexcept {
    memcpy(dest->get(), src->get(), memcpy_length * sizeof(T));
  }

  template <bool CanMemcpy = can_memcpy>
  static typename std::enable_if<!CanMemcpy>::type move_construct_several(
      AlignedStorage* ARROW_RESTRICT src, AlignedStorage* ARROW_RESTRICT dest, size_t n,
      size_t memcpy_length) noexcept {
    for (size_t i = 0; i < n; ++i) {
      new (dest[i].get()) T(std::move(*src[i].get()));
    }
  }

  template <bool CanMemcpy = can_memcpy>
  static typename std::enable_if<!CanMemcpy>::type
  move_construct_several_and_destroy_source(AlignedStorage* ARROW_RESTRICT src,
                                            AlignedStorage* ARROW_RESTRICT dest, size_t n,
                                            size_t memcpy_length) noexcept {
    for (size_t i = 0; i < n; ++i) {
      new (dest[i].get()) T(std::move(*src[i].get()));
      src[i].destroy();
    }
  }

  static void move_construct_several(AlignedStorage* ARROW_RESTRICT src,
                                     AlignedStorage* ARROW_RESTRICT dest,
                                     size_t n) noexcept {
    move_construct_several(src, dest, n, n);
  }

  static void move_construct_several_and_destroy_source(
      AlignedStorage* ARROW_RESTRICT src, AlignedStorage* ARROW_RESTRICT dest,
      size_t n) noexcept {
    move_construct_several_and_destroy_source(src, dest, n, n);
  }

  static void destroy_several(AlignedStorage* p, size_t n) noexcept {
    if (!std::is_trivially_destructible<T>::value) {
      for (size_t i = 0; i < n; ++i) {
        p[i].destroy();
      }
    }
  }

 private:
  alignas(T) std::byte data_[sizeof(T)];
};

}  // namespace internal
}  // namespace arrow