// 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 __declspec(empty_bases)
#else
#define ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
#endif
namespace absl {
namespace container_internal {
template <typename... Ts>
class CompressedTuple;
namespace internal_compressed_tuple {
template <typename D, size_t I>
struct Elem;
template <typename... B, size_t I>
struct Elem<CompressedTuple<B...>, I>
: std::tuple_element<I, std::tuple<B...>> {};
template <typename D, size_t I>
using ElemT = typename Elem<D, I>::type;
// Use the __is_final intrinsic if available. Where it's not available, classes
// declared with the 'final' specifier cannot be used as CompressedTuple
// elements.
// TODO(sbenza): Replace this with std::is_final in C++14.
template <typename T>
constexpr bool IsFinal() {
#if defined(__clang__) || defined(__GNUC__)
return __is_final(T);
#else
return false;
#endif
}
// 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() {
return std::is_class<T>::value && std::is_empty<T>::value && !IsFinal<T>() &&
!std::is_base_of<uses_inheritance, T>::value;
}
// 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,
#if defined(_MSC_VER)
bool UseBase =
ShouldUseBase<typename std::enable_if<true, T>::type>()>
#else
bool UseBase = ShouldUseBase<T>()>
#endif
struct Storage {
T value;
constexpr Storage() = default;
explicit constexpr Storage(T&& v) : value(absl::forward<T>(v)) {}
constexpr const T& get() const& { return value; }
T& get() & { return value; }
constexpr const T&& get() const&& { return absl::move(*this).value; }
T&& get() && { return std::move(*this).value; }
};
template <typename T, size_t I>
struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC Storage<T, I, true> : T {
constexpr Storage() = default;
explicit constexpr Storage(T&& v) : T(absl::forward<T>(v)) {}
constexpr const T& get() const& { return *this; }
T& get() & { return *this; }
constexpr const T&& get() const&& { return absl::move(*this); }
T&& get() && { return std::move(*this); }
};
template <typename D, typename I, bool ShouldAnyUseBase>
struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl;
template <typename... Ts, size_t... I, bool ShouldAnyUseBase>
struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
CompressedTuple<Ts...>, absl::index_sequence<I...>, ShouldAnyUseBase>
// We use the dummy identity function through std::integral_constant to
// convince MSVC of accepting and expanding I in that context. Without it
// you would get:
// error C3548: 'I': parameter pack cannot be used in this context
: uses_inheritance,
Storage<Ts, std::integral_constant<size_t, I>::value>... {
constexpr CompressedTupleImpl() = default;
explicit constexpr CompressedTupleImpl(Ts&&... args)
: Storage<Ts, I>(absl::forward<Ts>(args))... {}
friend CompressedTuple<Ts...>;
};
template <typename... Ts, size_t... I>
struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
CompressedTuple<Ts...>, absl::index_sequence<I...>, false>
// We use the dummy identity function as above...
: Storage<Ts, std::integral_constant<size_t, I>::value, false>... {
constexpr CompressedTupleImpl() = default;
explicit constexpr CompressedTupleImpl(Ts&&... args)
: Storage<Ts, I, false>(absl::forward<Ts>(args))... {}
friend CompressedTuple<Ts...>;
};
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() {
return decltype(
Or({std::integral_constant<bool, ShouldUseBase<Ts>()>()...})){};
}
} // 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...>()> {
private:
template <int I>
using ElemT = internal_compressed_tuple::ElemT<CompressedTuple, I>;
public:
constexpr CompressedTuple() = default;
explicit constexpr CompressedTuple(Ts... base)
: CompressedTuple::CompressedTupleImpl(absl::forward<Ts>(base)...) {}
template <int I>
ElemT<I>& get() & {
return internal_compressed_tuple::Storage<ElemT<I>, I>::get();
}
template <int I>
constexpr const ElemT<I>& get() const& {
return internal_compressed_tuple::Storage<ElemT<I>, I>::get();
}
template <int I>
ElemT<I>&& get() && {
return std::move(*this)
.internal_compressed_tuple::template Storage<ElemT<I>, I>::get();
}
template <int I>
constexpr const ElemT<I>&& get() const&& {
return absl::move(*this)
.internal_compressed_tuple::template Storage<ElemT<I>, I>::get();
}
};
// 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
} // namespace absl
#undef ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
#endif // ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_
