aboutsummaryrefslogtreecommitdiff
path: root/absl/status/statusor.h
blob: 469d486fdd2edb24bde9c0a7df1d32c396f4199c (plain)
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
// Copyright 2020 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.
//
// -----------------------------------------------------------------------------
// File: statusor.h
// -----------------------------------------------------------------------------
//
// An `absl::StatusOr<T>` represents a union of an `absl::Status` object
// and an object of type `T`. The `absl::StatusOr<T>` will either contain an
// object of type `T` (indicating a successful operation), or an error (of type
// `absl::Status`) explaining why such a value is not present.
//
// In general, check the success of an operation returning an
// `absl::StatusOr<T>` like you would an `absl::Status` by using the `ok()`
// member function.
//
// Example:
//
//   StatusOr<Foo> result = Calculation();
//   if (result.ok()) {
//     result->DoSomethingCool();
//   } else {
//     LOG(ERROR) << result.status();
//   }
#ifndef ABSL_STATUS_STATUSOR_H_
#define ABSL_STATUS_STATUSOR_H_

#include <exception>
#include <initializer_list>
#include <new>
#include <string>
#include <type_traits>
#include <utility>

#include "absl/base/attributes.h"
#include "absl/meta/type_traits.h"
#include "absl/status/internal/statusor_internal.h"
#include "absl/status/status.h"
#include "absl/types/variant.h"
#include "absl/utility/utility.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

// BadStatusOrAccess
//
// This class defines the type of object to throw (if exceptions are enabled),
// when accessing the value of an `absl::StatusOr<T>` object that does not
// contain a value. This behavior is analogous to that of
// `std::bad_optional_access` in the case of accessing an invalid
// `std::optional` value.
//
// Example:
//
// try {
//   absl::StatusOr<int> v = FetchInt();
//   DoWork(v.value());  // Accessing value() when not "OK" may throw
// } catch (absl::BadStatusOrAccess& ex) {
//   LOG(ERROR) << ex.status();
// }
class BadStatusOrAccess : public std::exception {
 public:
  explicit BadStatusOrAccess(absl::Status status);
  ~BadStatusOrAccess() override;

  // BadStatusOrAccess::what()
  //
  // Returns the associated explanatory string of the `absl::StatusOr<T>`
  // object's error code. This function only returns the string literal "Bad
  // StatusOr Access" for cases when evaluating general exceptions.
  //
  // The pointer of this string is guaranteed to be valid until any non-const
  // function is invoked on the exception object.
  const char* what() const noexcept override;

  // BadStatusOrAccess::status()
  //
  // Returns the associated `absl::Status` of the `absl::StatusOr<T>` object's
  // error.
  const absl::Status& status() const;

 private:
  absl::Status status_;
};

// Returned StatusOr objects may not be ignored.
template <typename T>
class ABSL_MUST_USE_RESULT StatusOr;

// absl::StatusOr<T>
//
// The `absl::StatusOr<T>` class template is a union of an `absl::Status` object
// and an object of type `T`. The `absl::StatusOr<T>` models an object that is
// either a usable object, or an error (of type `absl::Status`) explaining why
// such an object is not present. An `absl::StatusOr<T>` is typically the return
// value of a function which may fail.
//
// An `absl::StatusOr<T>` can never hold an "OK" status (an
// `absl::StatusCode::kOk` value); instead, the presence of an object of type
// `T` indicates success. Instead of checking for a `kOk` value, use the
// `absl::StatusOr<T>::ok()` member function. (It is for this reason, and code
// readability, that using the `ok()` function is preferred for `absl::Status`
// as well.)
//
// Example:
//
//   StatusOr<Foo> result = DoBigCalculationThatCouldFail();
//   if (result.ok()) {
//     result->DoSomethingCool();
//   } else {
//     LOG(ERROR) << result.status();
//   }
//
// Accessing the object held by an `absl::StatusOr<T>` should be performed via
// `operator*` or `operator->`, after a call to `ok()` confirms that the
// `absl::StatusOr<T>` holds an object of type `T`:
//
// Example:
//
//   absl::StatusOr<int> i = GetCount();
//   if (i.ok()) {
//     updated_total += *i
//   }
//
// NOTE: using `absl::StatusOr<T>::value()` when no valid value is present will
// throw an exception if exceptions are enabled or terminate the process when
// execeptions are not enabled.
//
// Example:
//
//   StatusOr<Foo> result = DoBigCalculationThatCouldFail();
//   const Foo& foo = result.value();    // Crash/exception if no value present
//   foo.DoSomethingCool();
//
// A `absl::StatusOr<T*>` can be constructed from a null pointer like any other
// pointer value, and the result will be that `ok()` returns `true` and
// `value()` returns `nullptr`. Checking the value of pointer in an
// `absl::StatusOr<T>` generally requires a bit more care, to ensure both that a
// value is present and that value is not null:
//
//  StatusOr<std::unique_ptr<Foo>> result = FooFactory::MakeNewFoo(arg);
//  if (!result.ok()) {
//    LOG(ERROR) << result.status();
//  } else if (*result == nullptr) {
//    LOG(ERROR) << "Unexpected null pointer";
//  } else {
//    (*result)->DoSomethingCool();
//  }
//
// Example factory implementation returning StatusOr<T>:
//
//  StatusOr<Foo> FooFactory::MakeFoo(int arg) {
//    if (arg <= 0) {
//      return absl::Status(absl::StatusCode::kInvalidArgument,
//                          "Arg must be positive");
//    }
//    return Foo(arg);
//  }
template <typename T>
class StatusOr : private internal_statusor::StatusOrData<T>,
                 private internal_statusor::CopyCtorBase<T>,
                 private internal_statusor::MoveCtorBase<T>,
                 private internal_statusor::CopyAssignBase<T>,
                 private internal_statusor::MoveAssignBase<T> {
  template <typename U>
  friend class StatusOr;

  typedef internal_statusor::StatusOrData<T> Base;

 public:
  // StatusOr<T>::value_type
  //
  // This instance data provides a generic `value_type` member for use within
  // generic programming. This usage is analogous to that of
  // `optional::value_type` in the case of `std::optional`.
  typedef T value_type;

  // Constructors

  // Constructs a new `absl::StatusOr` with an `absl::StatusCode::kUnknown`
  // status. This constructor is marked 'explicit' to prevent usages in return
  // values such as 'return {};', under the misconception that
  // `absl::StatusOr<std::vector<int>>` will be initialized with an empty
  // vector, instead of an `absl::StatusCode::kUnknown` error code.
  explicit StatusOr();

  // `StatusOr<T>` is copy constructible if `T` is copy constructible.
  StatusOr(const StatusOr&) = default;
  // `StatusOr<T>` is copy assignable if `T` is copy constructible and copy
  // assignable.
  StatusOr& operator=(const StatusOr&) = default;

  // `StatusOr<T>` is move constructible if `T` is move constructible.
  StatusOr(StatusOr&&) = default;
  // `StatusOr<T>` is moveAssignable if `T` is move constructible and move
  // assignable.
  StatusOr& operator=(StatusOr&&) = default;

  // Converting Constructors

  // Constructs a new `absl::StatusOr<T>` from an `absl::StatusOr<U>`, when `T`
  // is constructible from `U`. To avoid ambiguity, these constructors are
  // disabled if `T` is also constructible from `StatusOr<U>.`. This constructor
  // is explicit if and only if the corresponding construction of `T` from `U`
  // is explicit. (This constructor inherits its explicitness from the
  // underlying constructor.)
  template <
      typename U,
      absl::enable_if_t<
          absl::conjunction<
              absl::negation<std::is_same<T, U>>,
              std::is_constructible<T, const U&>,
              std::is_convertible<const U&, T>,
              absl::negation<
                  internal_statusor::IsConstructibleOrConvertibleFromStatusOr<
                      T, U>>>::value,
          int> = 0>
  StatusOr(const StatusOr<U>& other)  // NOLINT
      : Base(static_cast<const typename StatusOr<U>::Base&>(other)) {}
  template <
      typename U,
      absl::enable_if_t<
          absl::conjunction<
              absl::negation<std::is_same<T, U>>,
              std::is_constructible<T, const U&>,
              absl::negation<std::is_convertible<const U&, T>>,
              absl::negation<
                  internal_statusor::IsConstructibleOrConvertibleFromStatusOr<
                      T, U>>>::value,
          int> = 0>
  explicit StatusOr(const StatusOr<U>& other)
      : Base(static_cast<const typename StatusOr<U>::Base&>(other)) {}

  template <
      typename U,
      absl::enable_if_t<
          absl::conjunction<
              absl::negation<std::is_same<T, U>>, std::is_constructible<T, U&&>,
              std::is_convertible<U&&, T>,
              absl::negation<
                  internal_statusor::IsConstructibleOrConvertibleFromStatusOr<
                      T, U>>>::value,
          int> = 0>
  StatusOr(StatusOr<U>&& other)  // NOLINT
      : Base(static_cast<typename StatusOr<U>::Base&&>(other)) {}
  template <
      typename U,
      absl::enable_if_t<
          absl::conjunction<
              absl::negation<std::is_same<T, U>>, std::is_constructible<T, U&&>,
              absl::negation<std::is_convertible<U&&, T>>,
              absl::negation<
                  internal_statusor::IsConstructibleOrConvertibleFromStatusOr<
                      T, U>>>::value,
          int> = 0>
  explicit StatusOr(StatusOr<U>&& other)
      : Base(static_cast<typename StatusOr<U>::Base&&>(other)) {}

  // Converting Assignment Operators

  // Creates an `absl::StatusOr<T>` through assignment from an
  // `absl::StatusOr<U>` when:
  //
  //   * Both `absl::StatusOr<T>` and `absl::StatusOr<U>` are OK by assigning
  //     `U` to `T` directly.
  //   * `absl::StatusOr<T>` is OK and `absl::StatusOr<U>` contains an error
  //      code by destroying `absl::StatusOr<T>`'s value and assigning from
  //      `absl::StatusOr<U>'
  //   * `absl::StatusOr<T>` contains an error code and `absl::StatusOr<U>` is
  //      OK by directly initializing `T` from `U`.
  //   * Both `absl::StatusOr<T>` and `absl::StatusOr<U>` contain an error
  //     code by assigning the `Status` in `absl::StatusOr<U>` to
  //     `absl::StatusOr<T>`
  //
  // These overloads only apply if `absl::StatusOr<T>` is constructible and
  // assignable from `absl::StatusOr<U>` and `StatusOr<T>` cannot be directly
  // assigned from `StatusOr<U>`.
  template <
      typename U,
      absl::enable_if_t<
          absl::conjunction<
              absl::negation<std::is_same<T, U>>,
              std::is_constructible<T, const U&>,
              std::is_assignable<T, const U&>,
              absl::negation<
                  internal_statusor::
                      IsConstructibleOrConvertibleOrAssignableFromStatusOr<
                          T, U>>>::value,
          int> = 0>
  StatusOr& operator=(const StatusOr<U>& other) {
    this->Assign(other);
    return *this;
  }
  template <
      typename U,
      absl::enable_if_t<
          absl::conjunction<
              absl::negation<std::is_same<T, U>>, std::is_constructible<T, U&&>,
              std::is_assignable<T, U&&>,
              absl::negation<
                  internal_statusor::
                      IsConstructibleOrConvertibleOrAssignableFromStatusOr<
                          T, U>>>::value,
          int> = 0>
  StatusOr& operator=(StatusOr<U>&& other) {
    this->Assign(std::move(other));
    return *this;
  }

  // Constructs a new `absl::StatusOr<T>` with a non-ok status. After calling
  // this constructor, `this->ok()` will be `false` and calls to `value()` will
  // crash, or produce an exception if exceptions are enabled.
  //
  // The constructor also takes any type `U` that is convertible to
  // `absl::Status`. This constructor is explicit if an only if `U` is not of
  // type `absl::Status` and the conversion from `U` to `Status` is explicit.
  //
  // REQUIRES: !Status(std::forward<U>(v)).ok(). This requirement is DCHECKed.
  // In optimized builds, passing absl::OkStatus() here will have the effect
  // of passing absl::StatusCode::kInternal as a fallback.
  template <
      typename U = absl::Status,
      absl::enable_if_t<
          absl::conjunction<
              std::is_convertible<U&&, absl::Status>,
              std::is_constructible<absl::Status, U&&>,
              absl::negation<std::is_same<absl::decay_t<U>, absl::StatusOr<T>>>,
              absl::negation<std::is_same<absl::decay_t<U>, T>>,
              absl::negation<std::is_same<absl::decay_t<U>, absl::in_place_t>>,
              absl::negation<internal_statusor::HasConversionOperatorToStatusOr<
                  T, U&&>>>::value,
          int> = 0>
  StatusOr(U&& v) : Base(std::forward<U>(v)) {}

  template <
      typename U = absl::Status,
      absl::enable_if_t<
          absl::conjunction<
              absl::negation<std::is_convertible<U&&, absl::Status>>,
              std::is_constructible<absl::Status, U&&>,
              absl::negation<std::is_same<absl::decay_t<U>, absl::StatusOr<T>>>,
              absl::negation<std::is_same<absl::decay_t<U>, T>>,
              absl::negation<std::is_same<absl::decay_t<U>, absl::in_place_t>>,
              absl::negation<internal_statusor::HasConversionOperatorToStatusOr<
                  T, U&&>>>::value,
          int> = 0>
  explicit StatusOr(U&& v) : Base(std::forward<U>(v)) {}

  template <
      typename U = absl::Status,
      absl::enable_if_t<
          absl::conjunction<
              std::is_convertible<U&&, absl::Status>,
              std::is_constructible<absl::Status, U&&>,
              absl::negation<std::is_same<absl::decay_t<U>, absl::StatusOr<T>>>,
              absl::negation<std::is_same<absl::decay_t<U>, T>>,
              absl::negation<std::is_same<absl::decay_t<U>, absl::in_place_t>>,
              absl::negation<internal_statusor::HasConversionOperatorToStatusOr<
                  T, U&&>>>::value,
          int> = 0>
  StatusOr& operator=(U&& v) {
    this->AssignStatus(std::forward<U>(v));
    return *this;
  }

  // Perfect-forwarding value assignment operator.

  // If `*this` contains a `T` value before the call, the contained value is
  // assigned from `std::forward<U>(v)`; Otherwise, it is directly-initialized
  // from `std::forward<U>(v)`.
  // This function does not participate in overload unless:
  // 1. `std::is_constructible_v<T, U>` is true,
  // 2. `std::is_assignable_v<T&, U>` is true.
  // 3. `std::is_same_v<StatusOr<T>, std::remove_cvref_t<U>>` is false.
  // 4. Assigning `U` to `T` is not ambiguous:
  //  If `U` is `StatusOr<V>` and `T` is constructible and assignable from
  //  both `StatusOr<V>` and `V`, the assignment is considered bug-prone and
  //  ambiguous thus will fail to compile. For example:
  //    StatusOr<bool> s1 = true;  // s1.ok() && *s1 == true
  //    StatusOr<bool> s2 = false;  // s2.ok() && *s2 == false
  //    s1 = s2;  // ambiguous, `s1 = *s2` or `s1 = bool(s2)`?
  template <
      typename U = T,
      typename = typename std::enable_if<absl::conjunction<
          std::is_constructible<T, U&&>, std::is_assignable<T&, U&&>,
          absl::disjunction<
              std::is_same<absl::remove_cv_t<absl::remove_reference_t<U>>, T>,
              absl::conjunction<
                  absl::negation<std::is_convertible<U&&, absl::Status>>,
                  absl::negation<internal_statusor::
                                     HasConversionOperatorToStatusOr<T, U&&>>>>,
          internal_statusor::IsForwardingAssignmentValid<T, U&&>>::value>::type>
  StatusOr& operator=(U&& v) {
    this->Assign(std::forward<U>(v));
    return *this;
  }

  // Constructs the inner value `T` in-place using the provided args, using the
  // `T(args...)` constructor.
  template <typename... Args>
  explicit StatusOr(absl::in_place_t, Args&&... args);
  template <typename U, typename... Args>
  explicit StatusOr(absl::in_place_t, std::initializer_list<U> ilist,
                    Args&&... args);

  // Constructs the inner value `T` in-place using the provided args, using the
  // `T(U)` (direct-initialization) constructor. This constructor is only valid
  // if `T` can be constructed from a `U`. Can accept move or copy constructors.
  //
  // This constructor is explicit if `U` is not convertible to `T`. To avoid
  // ambiguity, this constuctor is disabled if `U` is a `StatusOr<J>`, where `J`
  // is convertible to `T`.
  template <
      typename U = T,
      absl::enable_if_t<
          absl::conjunction<
              internal_statusor::IsDirectInitializationValid<T, U&&>,
              std::is_constructible<T, U&&>, std::is_convertible<U&&, T>,
              absl::disjunction<
                  std::is_same<absl::remove_cv_t<absl::remove_reference_t<U>>,
                               T>,
                  absl::conjunction<
                      absl::negation<std::is_convertible<U&&, absl::Status>>,
                      absl::negation<
                          internal_statusor::HasConversionOperatorToStatusOr<
                              T, U&&>>>>>::value,
          int> = 0>
  StatusOr(U&& u)  // NOLINT
      : StatusOr(absl::in_place, std::forward<U>(u)) {
  }

  template <
      typename U = T,
      absl::enable_if_t<
          absl::conjunction<
              internal_statusor::IsDirectInitializationValid<T, U&&>,
              absl::disjunction<
                  std::is_same<absl::remove_cv_t<absl::remove_reference_t<U>>,
                               T>,
                  absl::conjunction<
                      absl::negation<std::is_constructible<absl::Status, U&&>>,
                      absl::negation<
                          internal_statusor::HasConversionOperatorToStatusOr<
                              T, U&&>>>>,
              std::is_constructible<T, U&&>,
              absl::negation<std::is_convertible<U&&, T>>>::value,
          int> = 0>
  explicit StatusOr(U&& u)  // NOLINT
      : StatusOr(absl::in_place, std::forward<U>(u)) {
  }

  // StatusOr<T>::ok()
  //
  // Returns whether or not this `absl::StatusOr<T>` holds a `T` value. This
  // member function is analagous to `absl::Status::ok()` and should be used
  // similarly to check the status of return values.
  //
  // Example:
  //
  // StatusOr<Foo> result = DoBigCalculationThatCouldFail();
  // if (result.ok()) {
  //    // Handle result
  // else {
  //    // Handle error
  // }
  ABSL_MUST_USE_RESULT bool ok() const { return this->status_.ok(); }

  // StatusOr<T>::status()
  //
  // Returns a reference to the current `absl::Status` contained within the
  // `absl::StatusOr<T>`. If `absl::StatusOr<T>` contains a `T`, then this
  // function returns `absl::OkStatus()`.
  const Status& status() const &;
  Status status() &&;

  // StatusOr<T>::value()
  //
  // Returns a reference to the held value if `this->ok()`. Otherwise, throws
  // `absl::BadStatusOrAccess` if exceptions are enabled, or is guaranteed to
  // terminate the process if exceptions are disabled.
  //
  // If you have already checked the status using `this->ok()`, you probably
  // want to use `operator*()` or `operator->()` to access the value instead of
  // `value`.
  //
  // Note: for value types that are cheap to copy, prefer simple code:
  //
  //   T value = statusor.value();
  //
  // Otherwise, if the value type is expensive to copy, but can be left
  // in the StatusOr, simply assign to a reference:
  //
  //   T& value = statusor.value();  // or `const T&`
  //
  // Otherwise, if the value type supports an efficient move, it can be
  // used as follows:
  //
  //   T value = std::move(statusor).value();
  //
  // The `std::move` on statusor instead of on the whole expression enables
  // warnings about possible uses of the statusor object after the move.
  const T& value() const&;
  T& value() &;
  const T&& value() const&&;
  T&& value() &&;

  // StatusOr<T>:: operator*()
  //
  // Returns a reference to the current value.
  //
  // REQUIRES: `this->ok() == true`, otherwise the behavior is undefined.
  //
  // Use `this->ok()` to verify that there is a current value within the
  // `absl::StatusOr<T>`. Alternatively, see the `value()` member function for a
  // similar API that guarantees crashing or throwing an exception if there is
  // no current value.
  const T& operator*() const&;
  T& operator*() &;
  const T&& operator*() const&&;
  T&& operator*() &&;

  // StatusOr<T>::operator->()
  //
  // Returns a pointer to the current value.
  //
  // REQUIRES: `this->ok() == true`, otherwise the behavior is undefined.
  //
  // Use `this->ok()` to verify that there is a current value.
  const T* operator->() const;
  T* operator->();

  // StatusOr<T>::value_or()
  //
  // Returns the current value if `this->ok() == true`. Otherwise constructs a
  // value using the provided `default_value`.
  //
  // Unlike `value`, this function returns by value, copying the current value
  // if necessary. If the value type supports an efficient move, it can be used
  // as follows:
  //
  //   T value = std::move(statusor).value_or(def);
  //
  // Unlike with `value`, calling `std::move()` on the result of `value_or` will
  // still trigger a copy.
  template <typename U>
  T value_or(U&& default_value) const&;
  template <typename U>
  T value_or(U&& default_value) &&;

  // StatusOr<T>::IgnoreError()
  //
  // Ignores any errors. This method does nothing except potentially suppress
  // complaints from any tools that are checking that errors are not dropped on
  // the floor.
  void IgnoreError() const;

  // StatusOr<T>::emplace()
  //
  // Reconstructs the inner value T in-place using the provided args, using the
  // T(args...) constructor. Returns reference to the reconstructed `T`.
  template <typename... Args>
  T& emplace(Args&&... args) {
    if (ok()) {
      this->Clear();
      this->MakeValue(std::forward<Args>(args)...);
    } else {
      this->MakeValue(std::forward<Args>(args)...);
      this->status_ = absl::OkStatus();
    }
    return this->data_;
  }

  template <
      typename U, typename... Args,
      absl::enable_if_t<
          std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value,
          int> = 0>
  T& emplace(std::initializer_list<U> ilist, Args&&... args) {
    if (ok()) {
      this->Clear();
      this->MakeValue(ilist, std::forward<Args>(args)...);
    } else {
      this->MakeValue(ilist, std::forward<Args>(args)...);
      this->status_ = absl::OkStatus();
    }
    return this->data_;
  }

 private:
  using internal_statusor::StatusOrData<T>::Assign;
  template <typename U>
  void Assign(const absl::StatusOr<U>& other);
  template <typename U>
  void Assign(absl::StatusOr<U>&& other);
};

// operator==()
//
// This operator checks the equality of two `absl::StatusOr<T>` objects.
template <typename T>
bool operator==(const StatusOr<T>& lhs, const StatusOr<T>& rhs) {
  if (lhs.ok() && rhs.ok()) return *lhs == *rhs;
  return lhs.status() == rhs.status();
}

// operator!=()
//
// This operator checks the inequality of two `absl::StatusOr<T>` objects.
template <typename T>
bool operator!=(const StatusOr<T>& lhs, const StatusOr<T>& rhs) {
  return !(lhs == rhs);
}

//------------------------------------------------------------------------------
// Implementation details for StatusOr<T>
//------------------------------------------------------------------------------

// TODO(sbenza): avoid the string here completely.
template <typename T>
StatusOr<T>::StatusOr() : Base(Status(absl::StatusCode::kUnknown, "")) {}

template <typename T>
template <typename U>
inline void StatusOr<T>::Assign(const StatusOr<U>& other) {
  if (other.ok()) {
    this->Assign(*other);
  } else {
    this->AssignStatus(other.status());
  }
}

template <typename T>
template <typename U>
inline void StatusOr<T>::Assign(StatusOr<U>&& other) {
  if (other.ok()) {
    this->Assign(*std::move(other));
  } else {
    this->AssignStatus(std::move(other).status());
  }
}
template <typename T>
template <typename... Args>
StatusOr<T>::StatusOr(absl::in_place_t, Args&&... args)
    : Base(absl::in_place, std::forward<Args>(args)...) {}

template <typename T>
template <typename U, typename... Args>
StatusOr<T>::StatusOr(absl::in_place_t, std::initializer_list<U> ilist,
                      Args&&... args)
    : Base(absl::in_place, ilist, std::forward<Args>(args)...) {}

template <typename T>
const Status& StatusOr<T>::status() const & { return this->status_; }
template <typename T>
Status StatusOr<T>::status() && {
  return ok() ? OkStatus() : std::move(this->status_);
}

template <typename T>
const T& StatusOr<T>::value() const& {
  if (!this->ok()) internal_statusor::ThrowBadStatusOrAccess(this->status_);
  return this->data_;
}

template <typename T>
T& StatusOr<T>::value() & {
  if (!this->ok()) internal_statusor::ThrowBadStatusOrAccess(this->status_);
  return this->data_;
}

template <typename T>
const T&& StatusOr<T>::value() const&& {
  if (!this->ok()) {
    internal_statusor::ThrowBadStatusOrAccess(std::move(this->status_));
  }
  return std::move(this->data_);
}

template <typename T>
T&& StatusOr<T>::value() && {
  if (!this->ok()) {
    internal_statusor::ThrowBadStatusOrAccess(std::move(this->status_));
  }
  return std::move(this->data_);
}

template <typename T>
const T& StatusOr<T>::operator*() const& {
  this->EnsureOk();
  return this->data_;
}

template <typename T>
T& StatusOr<T>::operator*() & {
  this->EnsureOk();
  return this->data_;
}

template <typename T>
const T&& StatusOr<T>::operator*() const&& {
  this->EnsureOk();
  return std::move(this->data_);
}

template <typename T>
T&& StatusOr<T>::operator*() && {
  this->EnsureOk();
  return std::move(this->data_);
}

template <typename T>
const T* StatusOr<T>::operator->() const {
  this->EnsureOk();
  return &this->data_;
}

template <typename T>
T* StatusOr<T>::operator->() {
  this->EnsureOk();
  return &this->data_;
}

template <typename T>
template <typename U>
T StatusOr<T>::value_or(U&& default_value) const& {
  if (ok()) {
    return this->data_;
  }
  return std::forward<U>(default_value);
}

template <typename T>
template <typename U>
T StatusOr<T>::value_or(U&& default_value) && {
  if (ok()) {
    return std::move(this->data_);
  }
  return std::forward<U>(default_value);
}

template <typename T>
void StatusOr<T>::IgnoreError() const {
  // no-op
}

ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STATUS_STATUSOR_H_