290 lines
10 KiB
C++
290 lines
10 KiB
C++
// Copyright 2018 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// Helper class to perform the Empty Base Optimization.
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// Ts can contain classes and non-classes, empty or not. For the ones that
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// are empty classes, we perform the optimization. If all types in Ts are empty
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// classes, then CompressedTuple<Ts...> is itself an empty class.
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//
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// To access the members, use member get<N>() function.
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//
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// Eg:
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// absl::container_internal::CompressedTuple<int, T1, T2, T3> value(7, t1, t2,
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// t3);
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// assert(value.get<0>() == 7);
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// T1& t1 = value.get<1>();
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// const T2& t2 = value.get<2>();
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// ...
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//
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// https://en.cppreference.com/w/cpp/language/ebo
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#ifndef ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_
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#define ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_
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#include <initializer_list>
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#include <tuple>
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#include <type_traits>
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#include <utility>
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#include "absl/utility/utility.h"
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#if defined(_MSC_VER) && !defined(__NVCC__)
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// We need to mark these classes with this declspec to ensure that
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// CompressedTuple happens.
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#define ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC __declspec(empty_bases)
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#else
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#define ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
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#endif
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace container_internal {
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template <typename... Ts>
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class CompressedTuple;
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namespace internal_compressed_tuple {
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template <typename D, size_t I>
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struct Elem;
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template <typename... B, size_t I>
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struct Elem<CompressedTuple<B...>, I>
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: std::tuple_element<I, std::tuple<B...>> {};
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template <typename D, size_t I>
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using ElemT = typename Elem<D, I>::type;
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// Use the __is_final intrinsic if available. Where it's not available, classes
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// declared with the 'final' specifier cannot be used as CompressedTuple
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// elements.
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// TODO(sbenza): Replace this with std::is_final in C++14.
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template <typename T>
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constexpr bool IsFinal() {
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#if defined(__clang__) || defined(__GNUC__)
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return __is_final(T);
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#else
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return false;
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#endif
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}
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// We can't use EBCO on other CompressedTuples because that would mean that we
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// derive from multiple Storage<> instantiations with the same I parameter,
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// and potentially from multiple identical Storage<> instantiations. So anytime
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// we use type inheritance rather than encapsulation, we mark
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// CompressedTupleImpl, to make this easy to detect.
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struct uses_inheritance {};
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template <typename T>
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constexpr bool ShouldUseBase() {
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return std::is_class<T>::value && std::is_empty<T>::value && !IsFinal<T>() &&
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!std::is_base_of<uses_inheritance, T>::value;
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}
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// The storage class provides two specializations:
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// - For empty classes, it stores T as a base class.
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// - For everything else, it stores T as a member.
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template <typename T, size_t I,
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#if defined(_MSC_VER)
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bool UseBase =
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ShouldUseBase<typename std::enable_if<true, T>::type>()>
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#else
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bool UseBase = ShouldUseBase<T>()>
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#endif
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struct Storage {
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T value;
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constexpr Storage() = default;
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template <typename V>
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explicit constexpr Storage(absl::in_place_t, V&& v)
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: value(absl::forward<V>(v)) {}
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constexpr const T& get() const& { return value; }
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T& get() & { return value; }
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constexpr const T&& get() const&& { return absl::move(*this).value; }
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T&& get() && { return std::move(*this).value; }
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};
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template <typename T, size_t I>
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struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC Storage<T, I, true> : T {
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constexpr Storage() = default;
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template <typename V>
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explicit constexpr Storage(absl::in_place_t, V&& v)
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: T(absl::forward<V>(v)) {}
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constexpr const T& get() const& { return *this; }
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T& get() & { return *this; }
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constexpr const T&& get() const&& { return absl::move(*this); }
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T&& get() && { return std::move(*this); }
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};
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template <typename D, typename I, bool ShouldAnyUseBase>
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struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl;
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template <typename... Ts, size_t... I, bool ShouldAnyUseBase>
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struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
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CompressedTuple<Ts...>, absl::index_sequence<I...>, ShouldAnyUseBase>
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// We use the dummy identity function through std::integral_constant to
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// convince MSVC of accepting and expanding I in that context. Without it
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// you would get:
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// error C3548: 'I': parameter pack cannot be used in this context
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: uses_inheritance,
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Storage<Ts, std::integral_constant<size_t, I>::value>... {
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constexpr CompressedTupleImpl() = default;
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template <typename... Vs>
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explicit constexpr CompressedTupleImpl(absl::in_place_t, Vs&&... args)
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: Storage<Ts, I>(absl::in_place, absl::forward<Vs>(args))... {}
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friend CompressedTuple<Ts...>;
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};
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template <typename... Ts, size_t... I>
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struct ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTupleImpl<
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CompressedTuple<Ts...>, absl::index_sequence<I...>, false>
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// We use the dummy identity function as above...
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: Storage<Ts, std::integral_constant<size_t, I>::value, false>... {
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constexpr CompressedTupleImpl() = default;
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template <typename... Vs>
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explicit constexpr CompressedTupleImpl(absl::in_place_t, Vs&&... args)
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: Storage<Ts, I, false>(absl::in_place, absl::forward<Vs>(args))... {}
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friend CompressedTuple<Ts...>;
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};
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std::false_type Or(std::initializer_list<std::false_type>);
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std::true_type Or(std::initializer_list<bool>);
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// MSVC requires this to be done separately rather than within the declaration
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// of CompressedTuple below.
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template <typename... Ts>
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constexpr bool ShouldAnyUseBase() {
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return decltype(
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Or({std::integral_constant<bool, ShouldUseBase<Ts>()>()...})){};
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}
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template <typename T, typename V>
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using TupleElementMoveConstructible =
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typename std::conditional<std::is_reference<T>::value,
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std::is_convertible<V, T>,
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std::is_constructible<T, V&&>>::type;
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template <bool SizeMatches, class T, class... Vs>
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struct TupleMoveConstructible : std::false_type {};
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template <class... Ts, class... Vs>
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struct TupleMoveConstructible<true, CompressedTuple<Ts...>, Vs...>
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: std::integral_constant<
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bool, absl::conjunction<
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TupleElementMoveConstructible<Ts, Vs&&>...>::value> {};
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template <typename T>
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struct compressed_tuple_size;
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template <typename... Es>
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struct compressed_tuple_size<CompressedTuple<Es...>>
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: public std::integral_constant<std::size_t, sizeof...(Es)> {};
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template <class T, class... Vs>
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struct TupleItemsMoveConstructible
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: std::integral_constant<
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bool, TupleMoveConstructible<compressed_tuple_size<T>::value ==
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sizeof...(Vs),
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T, Vs...>::value> {};
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} // namespace internal_compressed_tuple
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// Helper class to perform the Empty Base Class Optimization.
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// Ts can contain classes and non-classes, empty or not. For the ones that
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// are empty classes, we perform the CompressedTuple. If all types in Ts are
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// empty classes, then CompressedTuple<Ts...> is itself an empty class. (This
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// does not apply when one or more of those empty classes is itself an empty
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// CompressedTuple.)
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//
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// To access the members, use member .get<N>() function.
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//
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// Eg:
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// absl::container_internal::CompressedTuple<int, T1, T2, T3> value(7, t1, t2,
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// t3);
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// assert(value.get<0>() == 7);
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// T1& t1 = value.get<1>();
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// const T2& t2 = value.get<2>();
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// ...
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//
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// https://en.cppreference.com/w/cpp/language/ebo
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template <typename... Ts>
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class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple
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: private internal_compressed_tuple::CompressedTupleImpl<
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CompressedTuple<Ts...>, absl::index_sequence_for<Ts...>,
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internal_compressed_tuple::ShouldAnyUseBase<Ts...>()> {
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private:
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template <int I>
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using ElemT = internal_compressed_tuple::ElemT<CompressedTuple, I>;
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template <int I>
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using StorageT = internal_compressed_tuple::Storage<ElemT<I>, I>;
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public:
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// There seems to be a bug in MSVC dealing in which using '=default' here will
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// cause the compiler to ignore the body of other constructors. The work-
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// around is to explicitly implement the default constructor.
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#if defined(_MSC_VER)
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constexpr CompressedTuple() : CompressedTuple::CompressedTupleImpl() {}
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#else
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constexpr CompressedTuple() = default;
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#endif
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explicit constexpr CompressedTuple(const Ts&... base)
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: CompressedTuple::CompressedTupleImpl(absl::in_place, base...) {}
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template <typename First, typename... Vs,
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absl::enable_if_t<
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absl::conjunction<
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// Ensure we are not hiding default copy/move constructors.
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absl::negation<std::is_same<void(CompressedTuple),
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void(absl::decay_t<First>)>>,
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internal_compressed_tuple::TupleItemsMoveConstructible<
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CompressedTuple<Ts...>, First, Vs...>>::value,
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bool> = true>
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explicit constexpr CompressedTuple(First&& first, Vs&&... base)
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: CompressedTuple::CompressedTupleImpl(absl::in_place,
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absl::forward<First>(first),
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absl::forward<Vs>(base)...) {}
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template <int I>
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ElemT<I>& get() & {
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return internal_compressed_tuple::Storage<ElemT<I>, I>::get();
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}
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template <int I>
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constexpr const ElemT<I>& get() const& {
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return StorageT<I>::get();
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}
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template <int I>
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ElemT<I>&& get() && {
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return std::move(*this).StorageT<I>::get();
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}
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template <int I>
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constexpr const ElemT<I>&& get() const&& {
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return absl::move(*this).StorageT<I>::get();
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}
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};
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// Explicit specialization for a zero-element tuple
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// (needed to avoid ambiguous overloads for the default constructor).
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template <>
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class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple<> {};
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} // namespace container_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#undef ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
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#endif // ABSL_CONTAINER_INTERNAL_COMPRESSED_TUPLE_H_
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