about summary refs log tree commit diff
path: root/absl/types/optional.h
blob: 1670022dde4ea2fcf5cf41c5fb7edb12b2be6c2a (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
// 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
//
//      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.
//
// -----------------------------------------------------------------------------
// 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"   // TODO(calabrese) IWYU removal?
#include "absl/utility/utility.h"

#ifdef ABSL_HAVE_STD_OPTIONAL

#include <optional>  // IWYU pragma: export

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

#else  // ABSL_HAVE_STD_OPTIONAL

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

#include "absl/base/attributes.h"
#include "absl/base/internal/inline_variable.h"
#include "absl/meta/type_traits.h"
#include "absl/types/bad_optional_access.h"
#include "absl/types/internal/optional.h"

namespace absl {

// nullopt_t
//
// Class type for `absl::nullopt` used to indicate an `absl::optional<T>` type
// that does not contain a value.
struct nullopt_t {
  // It must not be default-constructible to avoid ambiguity for opt = {}.
  explicit constexpr nullopt_t(optional_internal::init_t) noexcept {}
};

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

// -----------------------------------------------------------------------------
// 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`.
//    * `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 : private optional_internal::optional_data<T>,
                 private optional_internal::optional_ctor_base<
                     optional_internal::ctor_copy_traits<T>::traits>,
                 private optional_internal::optional_assign_base<
                     optional_internal::assign_copy_traits<T>::traits> {
  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.)
  template <typename InPlaceT, typename... Args,
            absl::enable_if_t<absl::conjunction<
                std::is_same<InPlaceT, in_place_t>,
                std::is_constructible<T, Args&&...> >::value>* = nullptr>
  constexpr explicit optional(InPlaceT, 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.
  ABSL_ATTRIBUTE_REINITIALIZES 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&&
          type_traits_internal::IsNothrowSwappable<T>::value) {
    if (*this) {
      if (rhs) {
        type_traits_internal::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.
  //
  // If you need myOpt->foo in constexpr, use (*myOpt).foo instead.
  const T* operator->() const {
    assert(this->engaged_);
    return std::addressof(this->data_);
  }
  T* operator->() {
    assert(this->engaged_);
    return std::addressof(this->data_);
  }

  // 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_; }

// Suppress bogus warning on MSVC: MSVC complains call to reference() after
// throw_bad_optional_access() is unreachable.
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable : 4702)
#endif  // _MSC_VER
  // 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()));
  }
#ifdef _MSC_VER
#pragma warning(pop)
#endif  // _MSC_VER

  // 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 move 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 reference to T.
  constexpr const T& reference() const { return this->data_; }
  T& reference() { return this->data_; }

  // 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.
template <typename T, typename std::enable_if<
                          std::is_move_constructible<T>::value &&
                              type_traits_internal::IsSwappable<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
                                             : static_cast<bool>(*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
                                             : static_cast<bool>(*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 : static_cast<bool>(*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 : static_cast<bool>(*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 : static_cast<bool>(*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 : static_cast<bool>(*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) ? static_cast<bool>(*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) ? static_cast<bool>(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) ? static_cast<bool>(*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) ? static_cast<bool>(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) ? static_cast<bool>(*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) ? static_cast<bool>(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) ? static_cast<bool>(*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) ? static_cast<bool>(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) ? static_cast<bool>(*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) ? static_cast<bool>(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) ? static_cast<bool>(*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) ? static_cast<bool>(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_MSVC_CONSTEXPR_BUG_IN_UNION_LIKE_CLASS

#endif  // ABSL_HAVE_STD_OPTIONAL

#endif  // ABSL_TYPES_OPTIONAL_H_