chromium/third_party/abseil-cpp/absl/container/internal/compressed_tuple.h

// Copyright 2018 The Abseil Authors.
//
// Licensed 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
//
//      https://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.
//
// Helper class to perform the Empty Base Optimization.
// Ts can contain classes and non-classes, empty or not. For the ones that
// are empty classes, we perform the optimization. If all types in Ts are empty
// classes, then CompressedTuple<Ts...> is itself an empty class.
//
// To access the members, use member get<N>() function.
//
// Eg:
//   absl::container_internal::CompressedTuple<int, T1, T2, T3> value(7, t1, t2,
//                                                                    t3);
//   assert(value.get<0>() == 7);
//   T1& t1 = value.get<1>();
//   const T2& t2 = value.get<2>();
//   ...
//
// https://en.cppreference.com/w/cpp/language/ebo

#ifndef ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_
#define ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_

#include <initializer_list>
#include <tuple>
#include <type_traits>
#include <utility>

#include "absl/utility/utility.h"

#if defined(_MSC_VER) && !defined(__NVCC__)
// We need to mark these classes with this declspec to ensure that
// CompressedTuple happens.
#define ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC …
#else
#define ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
#endif

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace container_internal {

template <typename... Ts>
class CompressedTuple;

namespace internal_compressed_tuple {

template <typename D, size_t I>
struct Elem;
Elem<CompressedTuple<B...>, I>;
ElemT;

// We can't use EBCO on other CompressedTuples because that would mean that we
// derive from multiple Storage<> instantiations with the same I parameter,
// and potentially from multiple identical Storage<> instantiations.  So anytime
// we use type inheritance rather than encapsulation, we mark
// CompressedTupleImpl, to make this easy to detect.
struct uses_inheritance { … };

template <typename T>
constexpr bool ShouldUseBase() { … }

// The storage class provides two specializations:
//  - For empty classes, it stores T as a base class.
//  - For everything else, it stores T as a member.
template <typename T, size_t I, bool UseBase = ShouldUseBase<T>()>
struct Storage { … };

Storage<T, I, true>;

template <typename D, typename I, bool ShouldAnyUseBase>
struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl;

CompressedTupleImpl<CompressedTuple<Ts...>, absl::index_sequence<I...>, ShouldAnyUseBase>;

CompressedTupleImpl<CompressedTuple<Ts...>, absl::index_sequence<I...>, false>;

std::false_type Or(std::initializer_list<std::false_type>);
std::true_type Or(std::initializer_list<bool>);

// MSVC requires this to be done separately rather than within the declaration
// of CompressedTuple below.
template <typename... Ts>
constexpr bool ShouldAnyUseBase() { … }

TupleElementMoveConstructible;

template <bool SizeMatches, class T, class... Vs>
struct TupleMoveConstructible : std::false_type { … };

TupleMoveConstructible<true, CompressedTuple<Ts...>, Vs...>;

template <typename T>
struct compressed_tuple_size;

compressed_tuple_size<CompressedTuple<Es...>>;

template <class T, class... Vs>
struct TupleItemsMoveConstructible
    : std::integral_constant<
          bool, TupleMoveConstructible<compressed_tuple_size<T>::value ==
                                           sizeof...(Vs),
                                       T, Vs...>::value> { … };

}  // namespace internal_compressed_tuple

// Helper class to perform the Empty Base Class Optimization.
// Ts can contain classes and non-classes, empty or not. For the ones that
// are empty classes, we perform the CompressedTuple. If all types in Ts are
// empty classes, then CompressedTuple<Ts...> is itself an empty class.  (This
// does not apply when one or more of those empty classes is itself an empty
// CompressedTuple.)
//
// To access the members, use member .get<N>() function.
//
// Eg:
//   absl::container_internal::CompressedTuple<int, T1, T2, T3> value(7, t1, t2,
//                                                                    t3);
//   assert(value.get<0>() == 7);
//   T1& t1 = value.get<1>();
//   const T2& t2 = value.get<2>();
//   ...
//
// https://en.cppreference.com/w/cpp/language/ebo
template <typename... Ts>
class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
    : private internal_compressed_tuple::CompressedTupleImpl<
          CompressedTuple<Ts...>, absl::index_sequence_for<Ts...>,
          internal_compressed_tuple::ShouldAnyUseBase<Ts...>()> { … };

// Explicit specialization for a zero-element tuple
// (needed to avoid ambiguous overloads for the default constructor).
template <>
class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple<> { … };

}  // namespace container_internal
ABSL_NAMESPACE_END
}  // namespace absl

#undef ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC

#endif  // ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_