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Diffstat (limited to 'absl/container/internal/compressed_tuple.h')
| -rw-r--r-- | absl/container/internal/compressed_tuple.h | 175 |
1 files changed, 175 insertions, 0 deletions
diff --git a/absl/container/internal/compressed_tuple.h b/absl/container/internal/compressed_tuple.h new file mode 100644 index 000000000000..cc52614f5b37 --- /dev/null +++ b/absl/container/internal/compressed_tuple.h @@ -0,0 +1,175 @@ +// 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 +// +// 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. +// +// 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>(); +// ... +// +// http://en.cppreference.com/w/cpp/language/ebo + +#ifndef ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_ +#define ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_ + +#include <tuple> +#include <type_traits> +#include <utility> + +#include "absl/utility/utility.h" + +#ifdef _MSC_VER +// We need to mark these classes with this declspec to ensure that +// CompressedTuple happens. +#define ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC __declspec(empty_bases) +#else // _MSC_VER +#define ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC +#endif // _MSC_VER + +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 +} + +template <typename T> +constexpr bool ShouldUseBase() { + return std::is_class<T>::value && std::is_empty<T>::value && !IsFinal<T>(); +} + +// 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 D, size_t I, bool = ShouldUseBase<ElemT<D, I>>()> +struct Storage { + using T = ElemT<D, I>; + 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; } +}; + +template <typename D, size_t I> +struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC Storage<D, I, true> + : ElemT<D, I> { + using T = internal_compressed_tuple::ElemT<D, I>; + constexpr Storage() = default; + explicit constexpr Storage(T&& v) : T(absl::forward<T>(v)) {} + constexpr const T& get() const { return *this; } + T& get() { return *this; } +}; + +template <typename D, typename I> +struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl; + +template <typename... Ts, size_t... I> +struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC + CompressedTupleImpl<CompressedTuple<Ts...>, absl::index_sequence<I...>> + // 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 + : Storage<CompressedTuple<Ts...>, + std::integral_constant<size_t, I>::value>... { + constexpr CompressedTupleImpl() = default; + explicit constexpr CompressedTupleImpl(Ts&&... args) + : Storage<CompressedTuple<Ts...>, I>(absl::forward<Ts>(args))... {} +}; + +} // 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. +// +// 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>(); +// ... +// +// http://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...>> { + 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<CompressedTuple, I>::get(); + } + + template <int I> + constexpr const ElemT<I>& get() const { + return internal_compressed_tuple::Storage<CompressedTuple, 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_ |