1// Copyright 2017 The Abseil Authors.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// https://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14//
15// This header file contains C++14 versions of standard <utility> header
16// abstractions available within C++17, and are designed to be drop-in
17// replacement for code compliant with C++14 and C++17.
18//
19// The following abstractions are defined:
20//
21// * apply<Functor, Tuple> == std::apply<Functor, Tuple>
22// * exchange<T> == std::exchange<T>
23// * make_from_tuple<T> == std::make_from_tuple<T>
24//
25// This header file also provides the tag types `in_place_t`, `in_place_type_t`,
26// and `in_place_index_t`, as well as the constant `in_place`, and
27// `constexpr` `std::move()` and `std::forward()` implementations in C++11.
28//
29// References:
30//
31// https://en.cppreference.com/w/cpp/utility/apply
32// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3658.html
33
34#ifndef ABSL_UTILITY_UTILITY_H_
35#define ABSL_UTILITY_UTILITY_H_
36
37#include <cstddef>
38#include <cstdlib>
39#include <tuple>
40#include <utility>
41
42#include "absl/base/config.h"
43#include "absl/base/internal/inline_variable.h"
44#include "absl/base/internal/invoke.h"
45#include "absl/meta/type_traits.h"
46
47namespace absl {
48ABSL_NAMESPACE_BEGIN
49
50// Historical note: Abseil once provided implementations of these
51// abstractions for platforms that had not yet provided them. Those
52// platforms are no longer supported. New code should simply use the
53// the ones from std directly.
54using std::exchange;
55using std::forward;
56using std::index_sequence;
57using std::index_sequence_for;
58using std::integer_sequence;
59using std::make_index_sequence;
60using std::make_integer_sequence;
61using std::move;
62
63namespace utility_internal {
64
65template <typename T>
66struct InPlaceTypeTag {
67 explicit InPlaceTypeTag() = delete;
68 InPlaceTypeTag(const InPlaceTypeTag&) = delete;
69 InPlaceTypeTag& operator=(const InPlaceTypeTag&) = delete;
70};
71
72template <size_t I>
73struct InPlaceIndexTag {
74 explicit InPlaceIndexTag() = delete;
75 InPlaceIndexTag(const InPlaceIndexTag&) = delete;
76 InPlaceIndexTag& operator=(const InPlaceIndexTag&) = delete;
77};
78
79} // namespace utility_internal
80
81// Tag types
82
83#ifdef ABSL_USES_STD_OPTIONAL
84
85using std::in_place_t;
86using std::in_place;
87
88#else // ABSL_USES_STD_OPTIONAL
89
90// in_place_t
91//
92// Tag type used to specify in-place construction, such as with
93// `absl::optional`, designed to be a drop-in replacement for C++17's
94// `std::in_place_t`.
95struct in_place_t {};
96
97ABSL_INTERNAL_INLINE_CONSTEXPR(in_place_t, in_place, {});
98
99#endif // ABSL_USES_STD_OPTIONAL
100
101#if defined(ABSL_USES_STD_ANY) || defined(ABSL_USES_STD_VARIANT)
102using std::in_place_type;
103using std::in_place_type_t;
104#else
105
106// in_place_type_t
107//
108// Tag type used for in-place construction when the type to construct needs to
109// be specified, such as with `absl::any`, designed to be a drop-in replacement
110// for C++17's `std::in_place_type_t`.
111template <typename T>
112using in_place_type_t = void (*)(utility_internal::InPlaceTypeTag<T>);
113
114template <typename T>
115void in_place_type(utility_internal::InPlaceTypeTag<T>) {}
116#endif // ABSL_USES_STD_ANY || ABSL_USES_STD_VARIANT
117
118#ifdef ABSL_USES_STD_VARIANT
119using std::in_place_index;
120using std::in_place_index_t;
121#else
122
123// in_place_index_t
124//
125// Tag type used for in-place construction when the type to construct needs to
126// be specified, such as with `absl::any`, designed to be a drop-in replacement
127// for C++17's `std::in_place_index_t`.
128template <size_t I>
129using in_place_index_t = void (*)(utility_internal::InPlaceIndexTag<I>);
130
131template <size_t I>
132void in_place_index(utility_internal::InPlaceIndexTag<I>) {}
133#endif // ABSL_USES_STD_VARIANT
134
135namespace utility_internal {
136// Helper method for expanding tuple into a called method.
137template <typename Functor, typename Tuple, std::size_t... Indexes>
138auto apply_helper(Functor&& functor, Tuple&& t, index_sequence<Indexes...>)
139 -> decltype(absl::base_internal::invoke(
140 absl::forward<Functor>(functor),
141 std::get<Indexes>(absl::forward<Tuple>(t))...)) {
142 return absl::base_internal::invoke(
143 absl::forward<Functor>(functor),
144 std::get<Indexes>(absl::forward<Tuple>(t))...);
145}
146
147} // namespace utility_internal
148
149// apply
150//
151// Invokes a Callable using elements of a tuple as its arguments.
152// Each element of the tuple corresponds to an argument of the call (in order).
153// Both the Callable argument and the tuple argument are perfect-forwarded.
154// For member-function Callables, the first tuple element acts as the `this`
155// pointer. `absl::apply` is designed to be a drop-in replacement for C++17's
156// `std::apply`. Unlike C++17's `std::apply`, this is not currently `constexpr`.
157//
158// Example:
159//
160// class Foo {
161// public:
162// void Bar(int);
163// };
164// void user_function1(int, std::string);
165// void user_function2(std::unique_ptr<Foo>);
166// auto user_lambda = [](int, int) {};
167//
168// int main()
169// {
170// std::tuple<int, std::string> tuple1(42, "bar");
171// // Invokes the first user function on int, std::string.
172// absl::apply(&user_function1, tuple1);
173//
174// std::tuple<std::unique_ptr<Foo>> tuple2(absl::make_unique<Foo>());
175// // Invokes the user function that takes ownership of the unique
176// // pointer.
177// absl::apply(&user_function2, std::move(tuple2));
178//
179// auto foo = absl::make_unique<Foo>();
180// std::tuple<Foo*, int> tuple3(foo.get(), 42);
181// // Invokes the method Bar on foo with one argument, 42.
182// absl::apply(&Foo::Bar, tuple3);
183//
184// std::tuple<int, int> tuple4(8, 9);
185// // Invokes a lambda.
186// absl::apply(user_lambda, tuple4);
187// }
188template <typename Functor, typename Tuple>
189auto apply(Functor&& functor, Tuple&& t)
190 -> decltype(utility_internal::apply_helper(
191 absl::forward<Functor>(functor), absl::forward<Tuple>(t),
192 absl::make_index_sequence<std::tuple_size<
193 typename std::remove_reference<Tuple>::type>::value>{})) {
194 return utility_internal::apply_helper(
195 absl::forward<Functor>(functor), absl::forward<Tuple>(t),
196 absl::make_index_sequence<std::tuple_size<
197 typename std::remove_reference<Tuple>::type>::value>{});
198}
199
200namespace utility_internal {
201template <typename T, typename Tuple, size_t... I>
202T make_from_tuple_impl(Tuple&& tup, absl::index_sequence<I...>) {
203 return T(std::get<I>(std::forward<Tuple>(tup))...);
204}
205} // namespace utility_internal
206
207// make_from_tuple
208//
209// Given the template parameter type `T` and a tuple of arguments
210// `std::tuple(arg0, arg1, ..., argN)` constructs an object of type `T` as if by
211// calling `T(arg0, arg1, ..., argN)`.
212//
213// Example:
214//
215// std::tuple<const char*, size_t> args("hello world", 5);
216// auto s = absl::make_from_tuple<std::string>(args);
217// assert(s == "hello");
218//
219template <typename T, typename Tuple>
220constexpr T make_from_tuple(Tuple&& tup) {
221 return utility_internal::make_from_tuple_impl<T>(
222 std::forward<Tuple>(tup),
223 absl::make_index_sequence<
224 std::tuple_size<absl::decay_t<Tuple>>::value>{});
225}
226
227ABSL_NAMESPACE_END
228} // namespace absl
229
230#endif // ABSL_UTILITY_UTILITY_H_
231