about summary refs log tree commit diff
path: root/absl/types/optional.h
blob: 581321dc37a11bdbcfab3340d5263a97ced87e62 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
// Copyright 2017 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.
//
// -----------------------------------------------------------------------------
// optional.h
// -----------------------------------------------------------------------------
//
// This header file defines the `absl::optional` type for holding a value which
// may or may not be present. This type is useful for providing value semantics
// for operations that may either wish to return or hold "something-or-nothing".
//
// Example:
//
//   // A common way to signal operation failure is to provide an output
//   // parameter and a bool return type:
//   bool AcquireResource(const Input&, Resource * out);
//
//   // Providing an absl::optional return type provides a cleaner API:
//   absl::optional<Resource> AcquireResource(const Input&);
//
// `absl::optional` is a C++11 compatible version of the C++17 `std::optional`
// abstraction and is designed to be a drop-in replacement for code compliant
// with C++17.
#ifndef ABSL_TYPES_OPTIONAL_H_
#define ABSL_TYPES_OPTIONAL_H_

#include "absl/base/config.h"
#include "absl/utility/utility.h"

#ifdef ABSL_HAVE_STD_OPTIONAL

#include <optional>

namespace absl {
using std::bad_optional_access;
using std::optional;
using std::make_optional;
using std::nullopt_t;
using std::nullopt;
}

#else  // ABSL_HAVE_STD_OPTIONAL

#include <cassert>
#include <functional>
#include <initializer_list>
#include <new>
#include <type_traits>
#include <utility>

#include "absl/memory/memory.h"
#include "absl/meta/type_traits.h"
#include "absl/types/bad_optional_access.h"

// ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
//
// Inheriting constructors is supported in GCC 4.8+, Clang 3.3+ and MSVC 2015.
// __cpp_inheriting_constructors is a predefined macro and a recommended way to
// check for this language feature, but GCC doesn't support it until 5.0 and
// Clang doesn't support it until 3.6.
// Also, MSVC 2015 has a bug: it doesn't inherit the constexpr template
// constructor. For example, the following code won't work on MSVC 2015 Update3:
// struct Base {
//   int t;
//   template <typename T>
//   constexpr Base(T t_) : t(t_) {}
// };
// struct Foo : Base {
//   using Base::Base;
// }
// constexpr Foo foo(0);  // doesn't work on MSVC 2015
#if defined(__clang__)
#if __has_feature(cxx_inheriting_constructors)
#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
#endif
#elif (defined(__GNUC__) &&                                       \
       (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 8)) || \
    (__cpp_inheriting_constructors >= 200802) ||                  \
    (defined(_MSC_VER) && _MSC_VER >= 1910)
#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
#endif

namespace absl {

// -----------------------------------------------------------------------------
// absl::optional
// -----------------------------------------------------------------------------
//
// A value of type `absl::optional<T>` holds either a value of `T` or an
// "empty" value.  When it holds a value of `T`, it stores it as a direct
// sub-object, so `sizeof(optional<T>)` is approximately
// `sizeof(T) + sizeof(bool)`.
//
// This implementation is based on the specification in the latest draft of the
// C++17 `std::optional` specification as of May 2017, section 20.6.
//
// Differences between `absl::optional<T>` and `std::optional<T>` include:
//
//    * `constexpr` is not used for non-const member functions.
//      (dependency on some differences between C++11 and C++14.)
//    * `absl::nullopt` and `absl::in_place` are not declared `constexpr`. We
//      need the inline variable support in C++17 for external linkage.
//    * Throws `absl::bad_optional_access` instead of
//      `std::bad_optional_access`.
//    * `optional::swap()` and `absl::swap()` relies on
//      `std::is_(nothrow_)swappable()`, which has been introduced in C++17.
//      As a workaround, we assume `is_swappable()` is always `true`
//      and `is_nothrow_swappable()` is the same as `std::is_trivial()`.
//    * `make_optional()` cannot be declared `constexpr` due to the absence of
//      guaranteed copy elision.
//    * The move constructor's `noexcept` specification is stronger, i.e. if the
//      default allocator is non-throwing (via setting
//      `ABSL_ALLOCATOR_NOTHROW`), it evaluates to `noexcept(true)`, because
//      we assume
//       a) move constructors should only throw due to allocation failure and
//       b) if T's move constructor allocates, it uses the same allocation
//          function as the default allocator.
template <typename T>
class optional;

// nullopt_t
//
// Class type for `absl::nullopt` used to indicate an `absl::optional<T>` type
// that does not contain a value.
struct nullopt_t {
  struct init_t {};
  static init_t init;

  // It must not be default-constructible to avoid ambiguity for opt = {}.
  // Note the non-const reference, which is to eliminate ambiguity for code
  // like:
  //
  // struct S { int value; };
  //
  // void Test() {
  //   optional<S> opt;
  //   opt = {{}};
  // }
  explicit constexpr nullopt_t(init_t& /*unused*/) {}
};

// nullopt
//
// A tag constant of type `absl::nullopt_t` used to indicate an empty
// `absl::optional` in certain functions, such as construction or assignment.
extern const nullopt_t nullopt;

namespace optional_internal {

struct empty_struct {};
// This class stores the data in optional<T>.
// It is specialized based on whether T is trivially destructible.
// This is the specialization for non trivially destructible type.
template <typename T, bool = std::is_trivially_destructible<T>::value>
class optional_data_dtor_base {
  struct dummy_type {
    static_assert(sizeof(T) % sizeof(empty_struct) == 0, "");
    // Use an array to avoid GCC 6 placement-new warning.
    empty_struct data[sizeof(T) / sizeof(empty_struct)];
  };

 protected:
  // Whether there is data or not.
  bool engaged_;
  // Data storage
  union {
    dummy_type dummy_;
    T data_;
  };

  void destruct() noexcept {
    if (engaged_) {
      data_.~T();
      engaged_ = false;
    }
  }

  // dummy_ must be initialized for constexpr constructor.
  constexpr optional_data_dtor_base() noexcept : engaged_(false), dummy_{{}} {}

  template <typename... Args>
  constexpr explicit optional_data_dtor_base(in_place_t, Args&&... args)
      : engaged_(true), data_(absl::forward<Args>(args)...) {}

  ~optional_data_dtor_base() { destruct(); }
};

// Specialization for trivially destructible type.
template <typename T>
class optional_data_dtor_base<T, true> {
  struct dummy_type {
    static_assert(sizeof(T) % sizeof(empty_struct) == 0, "");
    // Use array to avoid GCC 6 placement-new warning.
    empty_struct data[sizeof(T) / sizeof(empty_struct)];
  };

 protected:
  // Whether there is data or not.
  bool engaged_;
  // Data storage
  union {
    dummy_type dummy_;
    T data_;
  };
  void destruct() noexcept { engaged_ = false; }

  // dummy_ must be initialized for constexpr constructor.
  constexpr optional_data_dtor_base() noexcept : engaged_(false), dummy_{{}} {}

  template <typename... Args>
  constexpr explicit optional_data_dtor_base(in_place_t, Args&&... args)
      : engaged_(true), data_(absl::forward<Args>(args)...) {}
};

template <typename T>
class optional_data_base : public optional_data_dtor_base<T> {
 protected:
  using base = optional_data_dtor_base<T>;
#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
  using base::base;
#else
  optional_data_base() = default;

  template <typename... Args>
  constexpr explicit optional_data_base(in_place_t t, Args&&... args)
      : base(t, absl::forward<Args>(args)...) {}
#endif

  template <typename... Args>
  void construct(Args&&... args) {
    // Use dummy_'s address to work around casting cv-qualified T* to void*.
    ::new (static_cast<void*>(&this->dummy_)) T(std::forward<Args>(args)...);
    this->engaged_ = true;
  }

  template <typename U>
  void assign(U&& u) {
    if (this->engaged_) {
      this->data_ = std::forward<U>(u);
    } else {
      construct(std::forward<U>(u));
    }
  }
};

// TODO(absl-team): Add another class using
// std::is_trivially_move_constructible trait when available to match
// http://cplusplus.github.io/LWG/lwg-defects.html#2900, for types that
// have trivial move but nontrivial copy.
// Also, we should be checking is_trivially_copyable here, which is not
// supported now, so we use is_trivially_* traits instead.
template <typename T, bool = absl::is_trivially_copy_constructible<T>::value&&
                          absl::is_trivially_copy_assignable<
                              typename std::remove_cv<T>::type>::value&&
                              std::is_trivially_destructible<T>::value>
class optional_data;

// Trivially copyable types
template <typename T>
class optional_data<T, true> : public optional_data_base<T> {
 protected:
#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
  using optional_data_base<T>::optional_data_base;
#else
  optional_data() = default;

  template <typename... Args>
  constexpr explicit optional_data(in_place_t t, Args&&... args)
      : optional_data_base<T>(t, absl::forward<Args>(args)...) {}
#endif
};

template <typename T>
class optional_data<T, false> : public optional_data_base<T> {
 protected:
#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
  using optional_data_base<T>::optional_data_base;
#else
  template <typename... Args>
  constexpr explicit optional_data(in_place_t t, Args&&... args)
      : optional_data_base<T>(t, absl::forward<Args>(args)...) {}
#endif

  optional_data() = default;

  optional_data(const optional_data& rhs) {
    if (rhs.engaged_) {
      this->construct(rhs.data_);
    }
  }

  optional_data(optional_data&& rhs) noexcept(
      absl::default_allocator_is_nothrow::value ||
      std::is_nothrow_move_constructible<T>::value) {
    if (rhs.engaged_) {
      this->construct(std::move(rhs.data_));
    }
  }

  optional_data& operator=(const optional_data& rhs) {
    if (rhs.engaged_) {
      this->assign(rhs.data_);
    } else {
      this->destruct();
    }
    return *this;
  }

  optional_data& operator=(optional_data&& rhs) noexcept(
      std::is_nothrow_move_assignable<T>::value&&
          std::is_nothrow_move_constructible<T>::value) {
    if (rhs.engaged_) {
      this->assign(std::move(rhs.data_));
    } else {
      this->destruct();
    }
    return *this;
  }
};

// Ordered by level of restriction, from low to high.
// Copyable implies movable.
enum class copy_traits { copyable = 0, movable = 1, non_movable = 2 };

// Base class for enabling/disabling copy/move constructor.
template <copy_traits>
class optional_ctor_base;

template <>
class optional_ctor_base<copy_traits::copyable> {
 public:
  constexpr optional_ctor_base() = default;
  optional_ctor_base(const optional_ctor_base&) = default;
  optional_ctor_base(optional_ctor_base&&) = default;
  optional_ctor_base& operator=(const optional_ctor_base&) = default;
  optional_ctor_base& operator=(optional_ctor_base&&) = default;
};

template <>
class optional_ctor_base<copy_traits::movable> {
 public:
  constexpr optional_ctor_base() = default;
  optional_ctor_base(const optional_ctor_base&) = delete;
  optional_ctor_base(optional_ctor_base&&) = default;
  optional_ctor_base& operator=(const optional_ctor_base&) = default;
  optional_ctor_base& operator=(optional_ctor_base&&) = default;
};

template <>
class optional_ctor_base<copy_traits::non_movable> {
 public:
  constexpr optional_ctor_base() = default;
  optional_ctor_base(const optional_ctor_base&) = delete;
  optional_ctor_base(optional_ctor_base&&) = delete;
  optional_ctor_base& operator=(const optional_ctor_base&) = default;
  optional_ctor_base& operator=(optional_ctor_base&&) = default;
};

// Base class for enabling/disabling copy/move assignment.
template <copy_traits>
class optional_assign_base;

template <>
class optional_assign_base<copy_traits::copyable> {
 public:
  constexpr optional_assign_base() = default;
  optional_assign_base(const optional_assign_base&) = default;
  optional_assign_base(optional_assign_base&&) = default;
  optional_assign_base& operator=(const optional_assign_base&) = default;
  optional_assign_base& operator=(optional_assign_base&&) = default;
};

template <>
class optional_assign_base<copy_traits::movable> {
 public:
  constexpr optional_assign_base() = default;
  optional_assign_base(const optional_assign_base&) = default;
  optional_assign_base(optional_assign_base&&) = default;
  optional_assign_base& operator=(const optional_assign_base&) = delete;
  optional_assign_base& operator=(optional_assign_base&&) = default;
};

template <>
class optional_assign_base<copy_traits::non_movable> {
 public:
  constexpr optional_assign_base() = default;
  optional_assign_base(const optional_assign_base&) = default;
  optional_assign_base(optional_assign_base&&) = default;
  optional_assign_base& operator=(const optional_assign_base&) = delete;
  optional_assign_base& operator=(optional_assign_base&&) = delete;
};

template <typename T>
constexpr copy_traits get_ctor_copy_traits() {
  return std::is_copy_constructible<T>::value
             ? copy_traits::copyable
             : std::is_move_constructible<T>::value ? copy_traits::movable
                                                    : copy_traits::non_movable;
}

template <typename T>
constexpr copy_traits get_assign_copy_traits() {
  return std::is_copy_assignable<T>::value &&
                 std::is_copy_constructible<T>::value
             ? copy_traits::copyable
             : std::is_move_assignable<T>::value &&
                       std::is_move_constructible<T>::value
                   ? copy_traits::movable
                   : copy_traits::non_movable;
}

// Whether T is constructible or convertible from optional<U>.
template <typename T, typename U>
struct is_constructible_convertible_from_optional
    : std::integral_constant<
          bool, std::is_constructible<T, optional<U>&>::value ||
                    std::is_constructible<T, optional<U>&&>::value ||
                    std::is_constructible<T, const optional<U>&>::value ||
                    std::is_constructible<T, const optional<U>&&>::value ||
                    std::is_convertible<optional<U>&, T>::value ||
                    std::is_convertible<optional<U>&&, T>::value ||
                    std::is_convertible<const optional<U>&, T>::value ||
                    std::is_convertible<const optional<U>&&, T>::value> {};

// Whether T is constructible or convertible or assignable from optional<U>.
template <typename T, typename U>
struct is_constructible_convertible_assignable_from_optional
    : std::integral_constant<
          bool, is_constructible_convertible_from_optional<T, U>::value ||
                    std::is_assignable<T&, optional<U>&>::value ||
                    std::is_assignable<T&, optional<U>&&>::value ||
                    std::is_assignable<T&, const optional<U>&>::value ||
                    std::is_assignable<T&, const optional<U>&&>::value> {};

// Helper function used by [optional.relops], [optional.comp_with_t],
// for checking whether an expression is convertible to bool.
bool convertible_to_bool(bool);

// Base class for std::hash<absl::optional<T>>:
// If std::hash<std::remove_const_t<T>> is enabled, it provides operator() to
// compute the hash; Otherwise, it is disabled.
// Reference N4659 23.14.15 [unord.hash].
template <typename T, typename = size_t>
struct optional_hash_base {
  optional_hash_base() = delete;
  optional_hash_base(const optional_hash_base&) = delete;
  optional_hash_base(optional_hash_base&&) = delete;
  optional_hash_base& operator=(const optional_hash_base&) = delete;
  optional_hash_base& operator=(optional_hash_base&&) = delete;
};

template <typename T>
struct optional_hash_base<T, decltype(std::hash<absl::remove_const_t<T> >()(
                                 std::declval<absl::remove_const_t<T> >()))> {
  using argument_type = absl::optional<T>;
  using result_type = size_t;
  size_t operator()(const absl::optional<T>& opt) const {
    if (opt) {
      return std::hash<absl::remove_const_t<T> >()(*opt);
    } else {
      return static_cast<size_t>(0x297814aaad196e6dULL);
    }
  }
};

}  // namespace optional_internal

// -----------------------------------------------------------------------------
// absl::optional class definition
// -----------------------------------------------------------------------------

template <typename T>
class optional : private optional_internal::optional_data<T>,
                 private optional_internal::optional_ctor_base<
                     optional_internal::get_ctor_copy_traits<T>()>,
                 private optional_internal::optional_assign_base<
                     optional_internal::get_assign_copy_traits<T>()> {
  using data_base = optional_internal::optional_data<T>;

 public:
  typedef T value_type;

  // Constructors

  // Constructs an `optional` holding an empty value, NOT a default constructed
  // `T`.
  constexpr optional() noexcept {}

  // Constructs an `optional` initialized with `nullopt` to hold an empty value.
  constexpr optional(nullopt_t) noexcept {}  // NOLINT(runtime/explicit)

  // Copy constructor, standard semantics
  optional(const optional& src) = default;

  // Move constructor, standard semantics
  optional(optional&& src) = default;

  // Constructs a non-empty `optional` direct-initialized value of type `T` from
  // the arguments `std::forward<Args>(args)...`  within the `optional`.
  // (The `in_place_t` is a tag used to indicate that the contained object
  // should be constructed in-place.)
  //
  // TODO(absl-team): Add std::is_constructible<T, Args&&...> SFINAE.
  template <typename... Args>
  constexpr explicit optional(in_place_t, Args&&... args)
      : data_base(in_place_t(), absl::forward<Args>(args)...) {}

  // Constructs a non-empty `optional` direct-initialized value of type `T` from
  // the arguments of an initializer_list and `std::forward<Args>(args)...`.
  // (The `in_place_t` is a tag used to indicate that the contained object
  // should be constructed in-place.)
  template <typename U, typename... Args,
            typename = typename std::enable_if<std::is_constructible<
                T, std::initializer_list<U>&, Args&&...>::value>::type>
  constexpr explicit optional(in_place_t, std::initializer_list<U> il,
                              Args&&... args)
      : data_base(in_place_t(), il, absl::forward<Args>(args)...) {
  }

  // Value constructor (implicit)
  template <
      typename U = T,
      typename std::enable_if<
          absl::conjunction<absl::negation<std::is_same<
                                in_place_t, typename std::decay<U>::type> >,
                            absl::negation<std::is_same<
                                optional<T>, typename std::decay<U>::type> >,
                            std::is_convertible<U&&, T>,
                            std::is_constructible<T, U&&> >::value,
          bool>::type = false>
  constexpr optional(U&& v) : data_base(in_place_t(), absl::forward<U>(v)) {}

  // Value constructor (explicit)
  template <
      typename U = T,
      typename std::enable_if<
          absl::conjunction<absl::negation<std::is_same<
                                in_place_t, typename std::decay<U>::type>>,
                            absl::negation<std::is_same<
                                optional<T>, typename std::decay<U>::type>>,
                            absl::negation<std::is_convertible<U&&, T>>,
                            std::is_constructible<T, U&&>>::value,
          bool>::type = false>
  explicit constexpr optional(U&& v)
      : data_base(in_place_t(), absl::forward<U>(v)) {}

  // Converting copy constructor (implicit)
  template <typename U,
            typename std::enable_if<
                absl::conjunction<
                    absl::negation<std::is_same<T, U> >,
                    std::is_constructible<T, const U&>,
                    absl::negation<
                        optional_internal::
                            is_constructible_convertible_from_optional<T, U> >,
                    std::is_convertible<const U&, T> >::value,
                bool>::type = false>
  optional(const optional<U>& rhs) {
    if (rhs) {
      this->construct(*rhs);
    }
  }

  // Converting copy constructor (explicit)
  template <typename U,
            typename std::enable_if<
                absl::conjunction<
                    absl::negation<std::is_same<T, U>>,
                    std::is_constructible<T, const U&>,
                    absl::negation<
                        optional_internal::
                            is_constructible_convertible_from_optional<T, U>>,
                    absl::negation<std::is_convertible<const U&, T>>>::value,
                bool>::type = false>
  explicit optional(const optional<U>& rhs) {
    if (rhs) {
      this->construct(*rhs);
    }
  }

  // Converting move constructor (implicit)
  template <typename U,
            typename std::enable_if<
                absl::conjunction<
                    absl::negation<std::is_same<T, U> >,
                    std::is_constructible<T, U&&>,
                    absl::negation<
                        optional_internal::
                            is_constructible_convertible_from_optional<T, U> >,
                    std::is_convertible<U&&, T> >::value,
                bool>::type = false>
  optional(optional<U>&& rhs) {
    if (rhs) {
      this->construct(std::move(*rhs));
    }
  }

  // Converting move constructor (explicit)
  template <
      typename U,
      typename std::enable_if<
          absl::conjunction<
              absl::negation<std::is_same<T, U>>, std::is_constructible<T, U&&>,
              absl::negation<
                  optional_internal::is_constructible_convertible_from_optional<
                      T, U>>,
              absl::negation<std::is_convertible<U&&, T>>>::value,
          bool>::type = false>
  explicit optional(optional<U>&& rhs) {
    if (rhs) {
      this->construct(std::move(*rhs));
    }
  }

  // Destructor. Trivial if `T` is trivially destructible.
  ~optional() = default;

  // Assignment Operators

  // Assignment from `nullopt`
  //
  // Example:
  //
  //   struct S { int value; };
  //   optional<S> opt = absl::nullopt;  // Could also use opt = { };
  optional& operator=(nullopt_t) noexcept {
    this->destruct();
    return *this;
  }

  // Copy assignment operator, standard semantics
  optional& operator=(const optional& src) = default;

  // Move assignment operator, standard semantics
  optional& operator=(optional&& src) = default;

  // Value assignment operators
  template <
      typename U = T,
      typename = typename std::enable_if<absl::conjunction<
          absl::negation<
              std::is_same<optional<T>, typename std::decay<U>::type>>,
          absl::negation<
              absl::conjunction<std::is_scalar<T>,
                                std::is_same<T, typename std::decay<U>::type>>>,
          std::is_constructible<T, U>, std::is_assignable<T&, U>>::value>::type>
  optional& operator=(U&& v) {
    this->assign(std::forward<U>(v));
    return *this;
  }

  template <
      typename U,
      typename = typename std::enable_if<absl::conjunction<
          absl::negation<std::is_same<T, U>>,
          std::is_constructible<T, const U&>, std::is_assignable<T&, const U&>,
          absl::negation<
              optional_internal::
                  is_constructible_convertible_assignable_from_optional<
                      T, U>>>::value>::type>
  optional& operator=(const optional<U>& rhs) {
    if (rhs) {
      this->assign(*rhs);
    } else {
      this->destruct();
    }
    return *this;
  }

  template <typename U,
            typename = typename std::enable_if<absl::conjunction<
                absl::negation<std::is_same<T, U>>, std::is_constructible<T, U>,
                std::is_assignable<T&, U>,
                absl::negation<
                    optional_internal::
                        is_constructible_convertible_assignable_from_optional<
                            T, U>>>::value>::type>
  optional& operator=(optional<U>&& rhs) {
    if (rhs) {
      this->assign(std::move(*rhs));
    } else {
      this->destruct();
    }
    return *this;
  }

  // Modifiers

  // optional::reset()
  //
  // Destroys the inner `T` value of an `absl::optional` if one is present.
  void reset() noexcept { this->destruct(); }

  // optional::emplace()
  //
  // (Re)constructs the underlying `T` in-place with the given forwarded
  // arguments.
  //
  // Example:
  //
  //   optional<Foo> opt;
  //   opt.emplace(arg1,arg2,arg3);  // Constructs Foo(arg1,arg2,arg3)
  //
  // If the optional is non-empty, and the `args` refer to subobjects of the
  // current object, then behaviour is undefined, because the current object
  // will be destructed before the new object is constructed with `args`.
  template <typename... Args,
            typename = typename std::enable_if<
                std::is_constructible<T, Args&&...>::value>::type>
  T& emplace(Args&&... args) {
    this->destruct();
    this->construct(std::forward<Args>(args)...);
    return reference();
  }

  // Emplace reconstruction overload for an initializer list and the given
  // forwarded arguments.
  //
  // Example:
  //
  //   struct Foo {
  //     Foo(std::initializer_list<int>);
  //   };
  //
  //   optional<Foo> opt;
  //   opt.emplace({1,2,3});  // Constructs Foo({1,2,3})
  template <typename U, typename... Args,
            typename = typename std::enable_if<std::is_constructible<
                T, std::initializer_list<U>&, Args&&...>::value>::type>
  T& emplace(std::initializer_list<U> il, Args&&... args) {
    this->destruct();
    this->construct(il, std::forward<Args>(args)...);
    return reference();
  }

  // Swaps

  // Swap, standard semantics
  void swap(optional& rhs) noexcept(
      std::is_nothrow_move_constructible<T>::value&&
          std::is_trivial<T>::value) {
    if (*this) {
      if (rhs) {
        using std::swap;
        swap(**this, *rhs);
      } else {
        rhs.construct(std::move(**this));
        this->destruct();
      }
    } else {
      if (rhs) {
        this->construct(std::move(*rhs));
        rhs.destruct();
      } else {
        // No effect (swap(disengaged, disengaged)).
      }
    }
  }

  // Observers

  // optional::operator->()
  //
  // Accesses the underlying `T` value's member `m` of an `optional`. If the
  // `optional` is empty, behavior is undefined.
  constexpr const T* operator->() const { return this->pointer(); }
  T* operator->() {
    assert(this->engaged_);
    return this->pointer();
  }

  // optional::operator*()
  //
  // Accesses the underlying `T` value of an `optional`. If the `optional` is
  // empty, behavior is undefined.
  constexpr const T& operator*() const & { return reference(); }
  T& operator*() & {
    assert(this->engaged_);
    return reference();
  }
  constexpr const T&& operator*() const && {
    return absl::move(reference());
  }
  T&& operator*() && {
    assert(this->engaged_);
    return std::move(reference());
  }

  // optional::operator bool()
  //
  // Returns false if and only if the `optional` is empty.
  //
  //   if (opt) {
  //     // do something with opt.value();
  //   } else {
  //     // opt is empty.
  //   }
  //
  constexpr explicit operator bool() const noexcept { return this->engaged_; }

  // optional::has_value()
  //
  // Determines whether the `optional` contains a value. Returns `false` if and
  // only if `*this` is empty.
  constexpr bool has_value() const noexcept { return this->engaged_; }

  // optional::value()
  //
  // Returns a reference to an `optional`s underlying value. The constness
  // and lvalue/rvalue-ness of the `optional` is preserved to the view of
  // the `T` sub-object. Throws `absl::bad_optional_access` when the `optional`
  // is empty.
  constexpr const T& value() const & {
    return static_cast<bool>(*this)
               ? reference()
               : (optional_internal::throw_bad_optional_access(), reference());
  }
  T& value() & {
    return static_cast<bool>(*this)
               ? reference()
               : (optional_internal::throw_bad_optional_access(), reference());
  }
  T&& value() && {  // NOLINT(build/c++11)
    return std::move(
        static_cast<bool>(*this)
            ? reference()
            : (optional_internal::throw_bad_optional_access(), reference()));
  }
  constexpr const T&& value() const && {  // NOLINT(build/c++11)
    return absl::move(
        static_cast<bool>(*this)
            ? reference()
            : (optional_internal::throw_bad_optional_access(), reference()));
  }

  // optional::value_or()
  //
  // Returns either the value of `T` or a passed default `v` if the `optional`
  // is empty.
  template <typename U>
  constexpr T value_or(U&& v) const& {
    static_assert(std::is_copy_constructible<value_type>::value,
                  "optional<T>::value_or: T must by copy constructible");
    static_assert(std::is_convertible<U&&, value_type>::value,
                  "optional<T>::value_or: U must be convertible to T");
    return static_cast<bool>(*this)
               ? **this
               : static_cast<T>(absl::forward<U>(v));
  }
  template <typename U>
  T value_or(U&& v) && {  // NOLINT(build/c++11)
    static_assert(std::is_move_constructible<value_type>::value,
                  "optional<T>::value_or: T must by copy constructible");
    static_assert(std::is_convertible<U&&, value_type>::value,
                  "optional<T>::value_or: U must be convertible to T");
    return static_cast<bool>(*this) ? std::move(**this)
                                    : static_cast<T>(std::forward<U>(v));
  }

 private:
  // Private accessors for internal storage viewed as pointer to T.
  constexpr const T* pointer() const { return &this->data_; }
  T* pointer() { return &this->data_; }

  // Private accessors for internal storage viewed as reference to T.
  constexpr const T& reference() const { return *this->pointer(); }
  T& reference() { return *(this->pointer()); }

  // T constraint checks.  You can't have an optional of nullopt_t, in_place_t
  // or a reference.
  static_assert(
      !std::is_same<nullopt_t, typename std::remove_cv<T>::type>::value,
      "optional<nullopt_t> is not allowed.");
  static_assert(
      !std::is_same<in_place_t, typename std::remove_cv<T>::type>::value,
      "optional<in_place_t> is not allowed.");
  static_assert(!std::is_reference<T>::value,
                "optional<reference> is not allowed.");
};

// Non-member functions

// swap()
//
// Performs a swap between two `absl::optional` objects, using standard
// semantics.
//
// NOTE: we assume `is_swappable()` is always `true`. A compile error will
// result if this is not the case.
template <typename T,
          typename std::enable_if<std::is_move_constructible<T>::value,
                                  bool>::type = false>
void swap(optional<T>& a, optional<T>& b) noexcept(noexcept(a.swap(b))) {
  a.swap(b);
}

// make_optional()
//
// Creates a non-empty `optional<T>` where the type of `T` is deduced. An
// `absl::optional` can also be explicitly instantiated with
// `make_optional<T>(v)`.
//
// Note: `make_optional()` constructions may be declared `constexpr` for
// trivially copyable types `T`. Non-trivial types require copy elision
// support in C++17 for `make_optional` to support `constexpr` on such
// non-trivial types.
//
// Example:
//
//   constexpr absl::optional<int> opt = absl::make_optional(1);
//   static_assert(opt.value() == 1, "");
template <typename T>
constexpr optional<typename std::decay<T>::type> make_optional(T&& v) {
  return optional<typename std::decay<T>::type>(absl::forward<T>(v));
}

template <typename T, typename... Args>
constexpr optional<T> make_optional(Args&&... args) {
  return optional<T>(in_place_t(), absl::forward<Args>(args)...);
}

template <typename T, typename U, typename... Args>
constexpr optional<T> make_optional(std::initializer_list<U> il,
                                    Args&&... args) {
  return optional<T>(in_place_t(), il,
                     absl::forward<Args>(args)...);
}

// Relational operators [optional.relops]

// Empty optionals are considered equal to each other and less than non-empty
// optionals. Supports relations between optional<T> and optional<U>, between
// optional<T> and U, and between optional<T> and nullopt.
//
// Note: We're careful to support T having non-bool relationals.

// Requires: The expression, e.g. "*x == *y" shall be well-formed and its result
// shall be convertible to bool.
// The C++17 (N4606) "Returns:" statements are translated into
// code in an obvious way here, and the original text retained as function docs.
// Returns: If bool(x) != bool(y), false; otherwise if bool(x) == false, true;
// otherwise *x == *y.
template <typename T, typename U>
constexpr auto operator==(const optional<T>& x, const optional<U>& y)
    -> decltype(optional_internal::convertible_to_bool(*x == *y)) {
  return static_cast<bool>(x) != static_cast<bool>(y)
             ? false
             : static_cast<bool>(x) == false ? true : *x == *y;
}

// Returns: If bool(x) != bool(y), true; otherwise, if bool(x) == false, false;
// otherwise *x != *y.
template <typename T, typename U>
constexpr auto operator!=(const optional<T>& x, const optional<U>& y)
    -> decltype(optional_internal::convertible_to_bool(*x != *y)) {
  return static_cast<bool>(x) != static_cast<bool>(y)
             ? true
             : static_cast<bool>(x) == false ? false : *x != *y;
}
// Returns: If !y, false; otherwise, if !x, true; otherwise *x < *y.
template <typename T, typename U>
constexpr auto operator<(const optional<T>& x, const optional<U>& y)
    -> decltype(optional_internal::convertible_to_bool(*x < *y)) {
  return !y ? false : !x ? true : *x < *y;
}
// Returns: If !x, false; otherwise, if !y, true; otherwise *x > *y.
template <typename T, typename U>
constexpr auto operator>(const optional<T>& x, const optional<U>& y)
    -> decltype(optional_internal::convertible_to_bool(*x > *y)) {
  return !x ? false : !y ? true : *x > *y;
}
// Returns: If !x, true; otherwise, if !y, false; otherwise *x <= *y.
template <typename T, typename U>
constexpr auto operator<=(const optional<T>& x, const optional<U>& y)
    -> decltype(optional_internal::convertible_to_bool(*x <= *y)) {
  return !x ? true : !y ? false : *x <= *y;
}
// Returns: If !y, true; otherwise, if !x, false; otherwise *x >= *y.
template <typename T, typename U>
constexpr auto operator>=(const optional<T>& x, const optional<U>& y)
    -> decltype(optional_internal::convertible_to_bool(*x >= *y)) {
  return !y ? true : !x ? false : *x >= *y;
}

// Comparison with nullopt [optional.nullops]
// The C++17 (N4606) "Returns:" statements are used directly here.
template <typename T>
constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept {
  return !x;
}
template <typename T>
constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept {
  return !x;
}
template <typename T>
constexpr bool operator!=(const optional<T>& x, nullopt_t) noexcept {
  return static_cast<bool>(x);
}
template <typename T>
constexpr bool operator!=(nullopt_t, const optional<T>& x) noexcept {
  return static_cast<bool>(x);
}
template <typename T>
constexpr bool operator<(const optional<T>&, nullopt_t) noexcept {
  return false;
}
template <typename T>
constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept {
  return static_cast<bool>(x);
}
template <typename T>
constexpr bool operator<=(const optional<T>& x, nullopt_t) noexcept {
  return !x;
}
template <typename T>
constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept {
  return true;
}
template <typename T>
constexpr bool operator>(const optional<T>& x, nullopt_t) noexcept {
  return static_cast<bool>(x);
}
template <typename T>
constexpr bool operator>(nullopt_t, const optional<T>&) noexcept {
  return false;
}
template <typename T>
constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept {
  return true;
}
template <typename T>
constexpr bool operator>=(nullopt_t, const optional<T>& x) noexcept {
  return !x;
}

// Comparison with T [optional.comp_with_t]

// Requires: The expression, e.g. "*x == v" shall be well-formed and its result
// shall be convertible to bool.
// The C++17 (N4606) "Equivalent to:" statements are used directly here.
template <typename T, typename U>
constexpr auto operator==(const optional<T>& x, const U& v)
    -> decltype(optional_internal::convertible_to_bool(*x == v)) {
  return static_cast<bool>(x) ? *x == v : false;
}
template <typename T, typename U>
constexpr auto operator==(const U& v, const optional<T>& x)
    -> decltype(optional_internal::convertible_to_bool(v == *x)) {
  return static_cast<bool>(x) ? v == *x : false;
}
template <typename T, typename U>
constexpr auto operator!=(const optional<T>& x, const U& v)
    -> decltype(optional_internal::convertible_to_bool(*x != v)) {
  return static_cast<bool>(x) ? *x != v : true;
}
template <typename T, typename U>
constexpr auto operator!=(const U& v, const optional<T>& x)
    -> decltype(optional_internal::convertible_to_bool(v != *x)) {
  return static_cast<bool>(x) ? v != *x : true;
}
template <typename T, typename U>
constexpr auto operator<(const optional<T>& x, const U& v)
    -> decltype(optional_internal::convertible_to_bool(*x < v)) {
  return static_cast<bool>(x) ? *x < v : true;
}
template <typename T, typename U>
constexpr auto operator<(const U& v, const optional<T>& x)
    -> decltype(optional_internal::convertible_to_bool(v < *x)) {
  return static_cast<bool>(x) ? v < *x : false;
}
template <typename T, typename U>
constexpr auto operator<=(const optional<T>& x, const U& v)
    -> decltype(optional_internal::convertible_to_bool(*x <= v)) {
  return static_cast<bool>(x) ? *x <= v : true;
}
template <typename T, typename U>
constexpr auto operator<=(const U& v, const optional<T>& x)
    -> decltype(optional_internal::convertible_to_bool(v <= *x)) {
  return static_cast<bool>(x) ? v <= *x : false;
}
template <typename T, typename U>
constexpr auto operator>(const optional<T>& x, const U& v)
    -> decltype(optional_internal::convertible_to_bool(*x > v)) {
  return static_cast<bool>(x) ? *x > v : false;
}
template <typename T, typename U>
constexpr auto operator>(const U& v, const optional<T>& x)
    -> decltype(optional_internal::convertible_to_bool(v > *x)) {
  return static_cast<bool>(x) ? v > *x : true;
}
template <typename T, typename U>
constexpr auto operator>=(const optional<T>& x, const U& v)
    -> decltype(optional_internal::convertible_to_bool(*x >= v)) {
  return static_cast<bool>(x) ? *x >= v : false;
}
template <typename T, typename U>
constexpr auto operator>=(const U& v, const optional<T>& x)
    -> decltype(optional_internal::convertible_to_bool(v >= *x)) {
  return static_cast<bool>(x) ? v >= *x : true;
}

}  // namespace absl

namespace std {

// std::hash specialization for absl::optional.
template <typename T>
struct hash<absl::optional<T> >
    : absl::optional_internal::optional_hash_base<T> {};

}  // namespace std

#undef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
#undef ABSL_MSVC_CONSTEXPR_BUG_IN_UNION_LIKE_CLASS

#endif  // ABSL_HAVE_STD_OPTIONAL

#endif  // ABSL_TYPES_OPTIONAL_H_