cpp/src/arrow/util/type_traits.h

// Licensed to the Apache Software Foundation (ASF) under one
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// 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
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// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

#pragma once

#include <cstdint>
#include <type_traits>

namespace arrow {
namespace internal {

/// \brief Metafunction to allow checking if a type matches any of another set of types
template <typename...>
struct IsOneOf : std::false_type {};  /// Base case: nothing has matched

template <typename T, typename U, typename... Args>
struct IsOneOf<T, U, Args...> {
  /// Recursive case: T == U or T matches any other types provided (not including U).
  static constexpr bool value = std::is_same<T, U>::value || IsOneOf<T, Args...>::value;
};

/// \brief Shorthand for using IsOneOf + std::enable_if
template <typename T, typename... Args>
using EnableIfIsOneOf = typename std::enable_if<IsOneOf<T, Args...>::value, T>::type;

/// \brief is_null_pointer from C++17
template <typename T>
struct is_null_pointer : std::is_same<std::nullptr_t, typename std::remove_cv<T>::type> {
};

#ifdef __GLIBCXX__

// A aligned_union backport, because old libstdc++ versions don't include it.

constexpr std::size_t max_size(std::size_t a, std::size_t b) { return (a > b) ? a : b; }

template <typename...>
struct max_size_traits;

template <typename H, typename... T>
struct max_size_traits<H, T...> {
  static constexpr std::size_t max_sizeof() {
    return max_size(sizeof(H), max_size_traits<T...>::max_sizeof());
  }
  static constexpr std::size_t max_alignof() {
    return max_size(alignof(H), max_size_traits<T...>::max_alignof());
  }
};

template <>
struct max_size_traits<> {
  static constexpr std::size_t max_sizeof() { return 0; }
  static constexpr std::size_t max_alignof() { return 0; }
};

template <std::size_t Len, typename... T>
struct aligned_union {
  static constexpr std::size_t alignment_value = max_size_traits<T...>::max_alignof();
  static constexpr std::size_t size_value =
      max_size(Len, max_size_traits<T...>::max_sizeof());
  using type = typename std::aligned_storage<size_value, alignment_value>::type;
};

#else

template <std::size_t Len, typename... T>
using aligned_union = std::aligned_union<Len, T...>;

#endif

}  // namespace internal
}  // namespace arrow