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optional
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1 // <optional> -*- C++ -*-
2 
3 // Copyright (C) 2013-2023 Free Software Foundation, Inc.
4 // Copyright The GNU Toolchain Authors.
5 //
6 // This file is part of the GNU ISO C++ Library. This library is free
7 // software; you can redistribute it and/or modify it under the
8 // terms of the GNU General Public License as published by the
9 // Free Software Foundation; either version 3, or (at your option)
10 // any later version.
11 
12 // This library is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 // GNU General Public License for more details.
16 
17 // Under Section 7 of GPL version 3, you are granted additional
18 // permissions described in the GCC Runtime Library Exception, version
19 // 3.1, as published by the Free Software Foundation.
20 
21 // You should have received a copy of the GNU General Public License and
22 // a copy of the GCC Runtime Library Exception along with this program;
23 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
24 // <http://www.gnu.org/licenses/>.
25 
26 /** @file include/optional
27  * This is a Standard C++ Library header.
28  */
29 
30 #ifndef _GLIBCXX_OPTIONAL
31 #define _GLIBCXX_OPTIONAL 1
32 
33 #pragma GCC system_header
34 
35 #if __cplusplus >= 201703L
36 
37 #include <type_traits>
38 #include <exception>
39 #include <new>
40 #include <initializer_list>
41 #include <bits/enable_special_members.h>
42 #include <bits/exception_defines.h>
43 #include <bits/functional_hash.h>
44 #include <bits/stl_construct.h> // _Construct
45 #include <bits/utility.h> // in_place_t
46 #if __cplusplus > 201703L
47 # include <compare>
48 # include <bits/invoke.h> // std::__invoke
49 #endif
50 #if __cplusplus > 202002L
51 # include <concepts>
52 #endif
53 
54 namespace std _GLIBCXX_VISIBILITY(default)
55 {
56 _GLIBCXX_BEGIN_NAMESPACE_VERSION
57 
58  /**
59  * @addtogroup utilities
60  * @{
61  */
62 
63 #if __cplusplus > 202002L && __cpp_lib_concepts
64 # define __cpp_lib_optional 202110L
65 #elif __cplusplus >= 202002L
66 # define __cpp_lib_optional 202106L
67 #else
68 # define __cpp_lib_optional 201606L
69 #endif
70 
71  template<typename _Tp>
72  class optional;
73 
74  /// Tag type to disengage optional objects.
75  struct nullopt_t
76  {
77  // Do not user-declare default constructor at all for
78  // optional_value = {} syntax to work.
79  // nullopt_t() = delete;
80 
81  // Used for constructing nullopt.
82  enum class _Construct { _Token };
83 
84  // Must be constexpr for nullopt_t to be literal.
85  explicit constexpr nullopt_t(_Construct) noexcept { }
86  };
87 
88  /// Tag to disengage optional objects.
89  inline constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
90 
91  template<typename _Fn> struct _Optional_func { _Fn& _M_f; };
92 
93  /**
94  * @brief Exception class thrown when a disengaged optional object is
95  * dereferenced.
96  * @ingroup exceptions
97  */
98  class bad_optional_access : public exception
99  {
100  public:
101  bad_optional_access() = default;
102  virtual ~bad_optional_access() = default;
103 
104  const char* what() const noexcept override
105  { return "bad optional access"; }
106  };
107 
108  // XXX Does not belong here.
109  [[__noreturn__]] inline void
110  __throw_bad_optional_access()
111  { _GLIBCXX_THROW_OR_ABORT(bad_optional_access()); }
112 
113  // This class template manages construction/destruction of
114  // the contained value for a std::optional.
115  template <typename _Tp>
116  struct _Optional_payload_base
117  {
118  using _Stored_type = remove_const_t<_Tp>;
119 
120  _Optional_payload_base() = default;
121  ~_Optional_payload_base() = default;
122 
123  template<typename... _Args>
124  constexpr
125  _Optional_payload_base(in_place_t __tag, _Args&&... __args)
126  : _M_payload(__tag, std::forward<_Args>(__args)...),
127  _M_engaged(true)
128  { }
129 
130  template<typename _Up, typename... _Args>
131  constexpr
132  _Optional_payload_base(std::initializer_list<_Up> __il,
133  _Args&&... __args)
134  : _M_payload(__il, std::forward<_Args>(__args)...),
135  _M_engaged(true)
136  { }
137 
138  // Constructor used by _Optional_base copy constructor when the
139  // contained value is not trivially copy constructible.
140  constexpr
141  _Optional_payload_base(bool /* __engaged */,
142  const _Optional_payload_base& __other)
143  {
144  if (__other._M_engaged)
145  this->_M_construct(__other._M_get());
146  }
147 
148  // Constructor used by _Optional_base move constructor when the
149  // contained value is not trivially move constructible.
150  constexpr
151  _Optional_payload_base(bool /* __engaged */,
152  _Optional_payload_base&& __other)
153  {
154  if (__other._M_engaged)
155  this->_M_construct(std::move(__other._M_get()));
156  }
157 
158  // Copy constructor is only used to when the contained value is
159  // trivially copy constructible.
160  _Optional_payload_base(const _Optional_payload_base&) = default;
161 
162  // Move constructor is only used to when the contained value is
163  // trivially copy constructible.
164  _Optional_payload_base(_Optional_payload_base&&) = default;
165 
166  _Optional_payload_base&
167  operator=(const _Optional_payload_base&) = default;
168 
169  _Optional_payload_base&
170  operator=(_Optional_payload_base&&) = default;
171 
172  // used to perform non-trivial copy assignment.
173  constexpr void
174  _M_copy_assign(const _Optional_payload_base& __other)
175  {
176  if (this->_M_engaged && __other._M_engaged)
177  this->_M_get() = __other._M_get();
178  else
179  {
180  if (__other._M_engaged)
181  this->_M_construct(__other._M_get());
182  else
183  this->_M_reset();
184  }
185  }
186 
187  // used to perform non-trivial move assignment.
188  constexpr void
189  _M_move_assign(_Optional_payload_base&& __other)
190  noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
191  is_nothrow_move_assignable<_Tp>>)
192  {
193  if (this->_M_engaged && __other._M_engaged)
194  this->_M_get() = std::move(__other._M_get());
195  else
196  {
197  if (__other._M_engaged)
198  this->_M_construct(std::move(__other._M_get()));
199  else
200  this->_M_reset();
201  }
202  }
203 
204  struct _Empty_byte { };
205 
206  template<typename _Up, bool = is_trivially_destructible_v<_Up>>
207  union _Storage
208  {
209  constexpr _Storage() noexcept : _M_empty() { }
210 
211  template<typename... _Args>
212  constexpr
213  _Storage(in_place_t, _Args&&... __args)
214  : _M_value(std::forward<_Args>(__args)...)
215  { }
216 
217  template<typename _Vp, typename... _Args>
218  constexpr
219  _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
220  : _M_value(__il, std::forward<_Args>(__args)...)
221  { }
222 
223 #if __cplusplus >= 202002L
224  template<typename _Fn, typename _Arg>
225  constexpr
226  _Storage(_Optional_func<_Fn> __f, _Arg&& __arg)
227  : _M_value(std::__invoke(std::forward<_Fn>(__f._M_f),
228  std::forward<_Arg>(__arg)))
229  { }
230 #endif
231 
232  _Empty_byte _M_empty;
233  _Up _M_value;
234  };
235 
236  template<typename _Up>
237  union _Storage<_Up, false>
238  {
239  constexpr _Storage() noexcept : _M_empty() { }
240 
241  template<typename... _Args>
242  constexpr
243  _Storage(in_place_t, _Args&&... __args)
244  : _M_value(std::forward<_Args>(__args)...)
245  { }
246 
247  template<typename _Vp, typename... _Args>
248  constexpr
249  _Storage(std::initializer_list<_Vp> __il, _Args&&... __args)
250  : _M_value(__il, std::forward<_Args>(__args)...)
251  { }
252 
253 #if __cplusplus >= 202002L
254  template<typename _Fn, typename _Arg>
255  constexpr
256  _Storage(_Optional_func<_Fn> __f, _Arg&& __arg)
257  : _M_value(std::__invoke(std::forward<_Fn>(__f._M_f),
258  std::forward<_Arg>(__arg)))
259  { }
260 #endif
261 
262  // User-provided destructor is needed when _Up has non-trivial dtor.
263  _GLIBCXX20_CONSTEXPR ~_Storage() { }
264 
265  _Empty_byte _M_empty;
266  _Up _M_value;
267  };
268 
269  _Storage<_Stored_type> _M_payload;
270 
271  bool _M_engaged = false;
272 
273  template<typename... _Args>
274  constexpr void
275  _M_construct(_Args&&... __args)
276  noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
277  {
278  std::_Construct(std::__addressof(this->_M_payload._M_value),
279  std::forward<_Args>(__args)...);
280  this->_M_engaged = true;
281  }
282 
283  constexpr void
284  _M_destroy() noexcept
285  {
286  _M_engaged = false;
287  _M_payload._M_value.~_Stored_type();
288  }
289 
290 #if __cplusplus >= 202002L
291  template<typename _Fn, typename _Up>
292  constexpr void
293  _M_apply(_Optional_func<_Fn> __f, _Up&& __x)
294  {
295  std::construct_at(std::__addressof(this->_M_payload),
296  __f, std::forward<_Up>(__x));
297  _M_engaged = true;
298  }
299 #endif
300 
301  // The _M_get() operations have _M_engaged as a precondition.
302  // They exist to access the contained value with the appropriate
303  // const-qualification, because _M_payload has had the const removed.
304 
305  constexpr _Tp&
306  _M_get() noexcept
307  { return this->_M_payload._M_value; }
308 
309  constexpr const _Tp&
310  _M_get() const noexcept
311  { return this->_M_payload._M_value; }
312 
313  // _M_reset is a 'safe' operation with no precondition.
314  constexpr void
315  _M_reset() noexcept
316  {
317  if (this->_M_engaged)
318  _M_destroy();
319  else // This seems redundant but improves codegen, see PR 112480.
320  this->_M_engaged = false;
321  }
322  };
323 
324  // Class template that manages the payload for optionals.
325  template <typename _Tp,
326  bool /*_HasTrivialDestructor*/ =
327  is_trivially_destructible_v<_Tp>,
328  bool /*_HasTrivialCopy */ =
329  is_trivially_copy_assignable_v<_Tp>
330  && is_trivially_copy_constructible_v<_Tp>,
331  bool /*_HasTrivialMove */ =
332  is_trivially_move_assignable_v<_Tp>
333  && is_trivially_move_constructible_v<_Tp>>
334  struct _Optional_payload;
335 
336  // Payload for potentially-constexpr optionals (trivial copy/move/destroy).
337  template <typename _Tp>
338  struct _Optional_payload<_Tp, true, true, true>
339  : _Optional_payload_base<_Tp>
340  {
341  using _Optional_payload_base<_Tp>::_Optional_payload_base;
342 
343  _Optional_payload() = default;
344  };
345 
346  // Payload for optionals with non-trivial copy construction/assignment.
347  template <typename _Tp>
348  struct _Optional_payload<_Tp, true, false, true>
349  : _Optional_payload_base<_Tp>
350  {
351  using _Optional_payload_base<_Tp>::_Optional_payload_base;
352 
353  _Optional_payload() = default;
354  ~_Optional_payload() = default;
355  _Optional_payload(const _Optional_payload&) = default;
356  _Optional_payload(_Optional_payload&&) = default;
357  _Optional_payload& operator=(_Optional_payload&&) = default;
358 
359  // Non-trivial copy assignment.
360  constexpr
361  _Optional_payload&
362  operator=(const _Optional_payload& __other)
363  {
364  this->_M_copy_assign(__other);
365  return *this;
366  }
367  };
368 
369  // Payload for optionals with non-trivial move construction/assignment.
370  template <typename _Tp>
371  struct _Optional_payload<_Tp, true, true, false>
372  : _Optional_payload_base<_Tp>
373  {
374  using _Optional_payload_base<_Tp>::_Optional_payload_base;
375 
376  _Optional_payload() = default;
377  ~_Optional_payload() = default;
378  _Optional_payload(const _Optional_payload&) = default;
379  _Optional_payload(_Optional_payload&&) = default;
380  _Optional_payload& operator=(const _Optional_payload&) = default;
381 
382  // Non-trivial move assignment.
383  constexpr
384  _Optional_payload&
385  operator=(_Optional_payload&& __other)
386  noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
387  is_nothrow_move_assignable<_Tp>>)
388  {
389  this->_M_move_assign(std::move(__other));
390  return *this;
391  }
392  };
393 
394  // Payload for optionals with non-trivial copy and move assignment.
395  template <typename _Tp>
396  struct _Optional_payload<_Tp, true, false, false>
397  : _Optional_payload_base<_Tp>
398  {
399  using _Optional_payload_base<_Tp>::_Optional_payload_base;
400 
401  _Optional_payload() = default;
402  ~_Optional_payload() = default;
403  _Optional_payload(const _Optional_payload&) = default;
404  _Optional_payload(_Optional_payload&&) = default;
405 
406  // Non-trivial copy assignment.
407  constexpr
408  _Optional_payload&
409  operator=(const _Optional_payload& __other)
410  {
411  this->_M_copy_assign(__other);
412  return *this;
413  }
414 
415  // Non-trivial move assignment.
416  constexpr
417  _Optional_payload&
418  operator=(_Optional_payload&& __other)
419  noexcept(__and_v<is_nothrow_move_constructible<_Tp>,
420  is_nothrow_move_assignable<_Tp>>)
421  {
422  this->_M_move_assign(std::move(__other));
423  return *this;
424  }
425  };
426 
427  // Payload for optionals with non-trivial destructors.
428  template <typename _Tp, bool _Copy, bool _Move>
429  struct _Optional_payload<_Tp, false, _Copy, _Move>
430  : _Optional_payload<_Tp, true, false, false>
431  {
432  // Base class implements all the constructors and assignment operators:
433  using _Optional_payload<_Tp, true, false, false>::_Optional_payload;
434  _Optional_payload() = default;
435  _Optional_payload(const _Optional_payload&) = default;
436  _Optional_payload(_Optional_payload&&) = default;
437  _Optional_payload& operator=(const _Optional_payload&) = default;
438  _Optional_payload& operator=(_Optional_payload&&) = default;
439 
440  // Destructor needs to destroy the contained value:
441  _GLIBCXX20_CONSTEXPR ~_Optional_payload() { this->_M_reset(); }
442  };
443 
444  // Common base class for _Optional_base<T> to avoid repeating these
445  // member functions in each specialization.
446  template<typename _Tp, typename _Dp>
447  class _Optional_base_impl
448  {
449  protected:
450  using _Stored_type = remove_const_t<_Tp>;
451 
452  // The _M_construct operation has !_M_engaged as a precondition
453  // while _M_destruct has _M_engaged as a precondition.
454  template<typename... _Args>
455  constexpr void
456  _M_construct(_Args&&... __args)
457  noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)
458  {
459  static_cast<_Dp*>(this)->_M_payload._M_construct(
460  std::forward<_Args>(__args)...);
461  }
462 
463  constexpr void
464  _M_destruct() noexcept
465  { static_cast<_Dp*>(this)->_M_payload._M_destroy(); }
466 
467  // _M_reset is a 'safe' operation with no precondition.
468  constexpr void
469  _M_reset() noexcept
470  { static_cast<_Dp*>(this)->_M_payload._M_reset(); }
471 
472  constexpr bool _M_is_engaged() const noexcept
473  { return static_cast<const _Dp*>(this)->_M_payload._M_engaged; }
474 
475  // The _M_get operations have _M_engaged as a precondition.
476  constexpr _Tp&
477  _M_get() noexcept
478  {
479  __glibcxx_assert(this->_M_is_engaged());
480  return static_cast<_Dp*>(this)->_M_payload._M_get();
481  }
482 
483  constexpr const _Tp&
484  _M_get() const noexcept
485  {
486  __glibcxx_assert(this->_M_is_engaged());
487  return static_cast<const _Dp*>(this)->_M_payload._M_get();
488  }
489  };
490 
491  /**
492  * @brief Class template that provides copy/move constructors of optional.
493  *
494  * Such a separate base class template is necessary in order to
495  * conditionally make copy/move constructors trivial.
496  *
497  * When the contained value is trivially copy/move constructible,
498  * the copy/move constructors of _Optional_base will invoke the
499  * trivial copy/move constructor of _Optional_payload. Otherwise,
500  * they will invoke _Optional_payload(bool, const _Optional_payload&)
501  * or _Optional_payload(bool, _Optional_payload&&) to initialize
502  * the contained value, if copying/moving an engaged optional.
503  *
504  * Whether the other special members are trivial is determined by the
505  * _Optional_payload<_Tp> specialization used for the _M_payload member.
506  *
507  * @see optional, _Enable_special_members
508  */
509  template<typename _Tp,
510  bool = is_trivially_copy_constructible_v<_Tp>,
511  bool = is_trivially_move_constructible_v<_Tp>>
512  struct _Optional_base
513  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
514  {
515  // Constructors for disengaged optionals.
516  constexpr _Optional_base() = default;
517 
518  // Constructors for engaged optionals.
519  template<typename... _Args,
520  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
521  constexpr explicit
522  _Optional_base(in_place_t, _Args&&... __args)
523  : _M_payload(in_place, std::forward<_Args>(__args)...)
524  { }
525 
526  template<typename _Up, typename... _Args,
527  enable_if_t<is_constructible_v<_Tp,
528  initializer_list<_Up>&,
529  _Args...>, bool> = false>
530  constexpr explicit
531  _Optional_base(in_place_t,
532  initializer_list<_Up> __il,
533  _Args&&... __args)
534  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
535  { }
536 
537  // Copy and move constructors.
538  constexpr
539  _Optional_base(const _Optional_base& __other)
540  : _M_payload(__other._M_payload._M_engaged, __other._M_payload)
541  { }
542 
543  constexpr
544  _Optional_base(_Optional_base&& __other)
545  noexcept(is_nothrow_move_constructible_v<_Tp>)
546  : _M_payload(__other._M_payload._M_engaged,
547  std::move(__other._M_payload))
548  { }
549 
550  // Assignment operators.
551  _Optional_base& operator=(const _Optional_base&) = default;
552  _Optional_base& operator=(_Optional_base&&) = default;
553 
554  _Optional_payload<_Tp> _M_payload;
555  };
556 
557  template<typename _Tp>
558  struct _Optional_base<_Tp, false, true>
559  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
560  {
561  // Constructors for disengaged optionals.
562  constexpr _Optional_base() = default;
563 
564  // Constructors for engaged optionals.
565  template<typename... _Args,
566  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
567  constexpr explicit
568  _Optional_base(in_place_t, _Args&&... __args)
569  : _M_payload(in_place, std::forward<_Args>(__args)...)
570  { }
571 
572  template<typename _Up, typename... _Args,
573  enable_if_t<is_constructible_v<_Tp,
574  initializer_list<_Up>&,
575  _Args...>, bool> = false>
576  constexpr explicit
577  _Optional_base(in_place_t,
578  initializer_list<_Up> __il,
579  _Args... __args)
580  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
581  { }
582 
583  // Copy and move constructors.
584  constexpr _Optional_base(const _Optional_base& __other)
585  : _M_payload(__other._M_payload._M_engaged, __other._M_payload)
586  { }
587 
588  constexpr _Optional_base(_Optional_base&& __other) = default;
589 
590  // Assignment operators.
591  _Optional_base& operator=(const _Optional_base&) = default;
592  _Optional_base& operator=(_Optional_base&&) = default;
593 
594  _Optional_payload<_Tp> _M_payload;
595  };
596 
597  template<typename _Tp>
598  struct _Optional_base<_Tp, true, false>
599  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
600  {
601  // Constructors for disengaged optionals.
602  constexpr _Optional_base() = default;
603 
604  // Constructors for engaged optionals.
605  template<typename... _Args,
606  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
607  constexpr explicit
608  _Optional_base(in_place_t, _Args&&... __args)
609  : _M_payload(in_place, std::forward<_Args>(__args)...)
610  { }
611 
612  template<typename _Up, typename... _Args,
613  enable_if_t<is_constructible_v<_Tp,
614  initializer_list<_Up>&,
615  _Args...>, bool> = false>
616  constexpr explicit
617  _Optional_base(in_place_t,
618  initializer_list<_Up> __il,
619  _Args&&... __args)
620  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
621  { }
622 
623  // Copy and move constructors.
624  constexpr _Optional_base(const _Optional_base& __other) = default;
625 
626  constexpr
627  _Optional_base(_Optional_base&& __other)
628  noexcept(is_nothrow_move_constructible_v<_Tp>)
629  : _M_payload(__other._M_payload._M_engaged,
630  std::move(__other._M_payload))
631  { }
632 
633  // Assignment operators.
634  _Optional_base& operator=(const _Optional_base&) = default;
635  _Optional_base& operator=(_Optional_base&&) = default;
636 
637  _Optional_payload<_Tp> _M_payload;
638  };
639 
640  template<typename _Tp>
641  struct _Optional_base<_Tp, true, true>
642  : _Optional_base_impl<_Tp, _Optional_base<_Tp>>
643  {
644  // Constructors for disengaged optionals.
645  constexpr _Optional_base() = default;
646 
647  // Constructors for engaged optionals.
648  template<typename... _Args,
649  enable_if_t<is_constructible_v<_Tp, _Args...>, bool> = false>
650  constexpr explicit
651  _Optional_base(in_place_t, _Args&&... __args)
652  : _M_payload(in_place, std::forward<_Args>(__args)...)
653  { }
654 
655  template<typename _Up, typename... _Args,
656  enable_if_t<is_constructible_v<_Tp,
657  initializer_list<_Up>&,
658  _Args...>, bool> = false>
659  constexpr explicit
660  _Optional_base(in_place_t,
661  initializer_list<_Up> __il,
662  _Args&&... __args)
663  : _M_payload(in_place, __il, std::forward<_Args>(__args)...)
664  { }
665 
666  // Copy and move constructors.
667  constexpr _Optional_base(const _Optional_base& __other) = default;
668  constexpr _Optional_base(_Optional_base&& __other) = default;
669 
670  // Assignment operators.
671  _Optional_base& operator=(const _Optional_base&) = default;
672  _Optional_base& operator=(_Optional_base&&) = default;
673 
674  _Optional_payload<_Tp> _M_payload;
675  };
676 
677  template<typename _Tp>
678  class optional;
679 
680  template<typename _Tp>
681  inline constexpr bool __is_optional_v = false;
682  template<typename _Tp>
683  inline constexpr bool __is_optional_v<optional<_Tp>> = true;
684 
685  template<typename _Tp, typename _Up>
686  using __converts_from_optional =
687  __or_<is_constructible<_Tp, const optional<_Up>&>,
688  is_constructible<_Tp, optional<_Up>&>,
689  is_constructible<_Tp, const optional<_Up>&&>,
690  is_constructible<_Tp, optional<_Up>&&>,
691  is_convertible<const optional<_Up>&, _Tp>,
692  is_convertible<optional<_Up>&, _Tp>,
693  is_convertible<const optional<_Up>&&, _Tp>,
694  is_convertible<optional<_Up>&&, _Tp>>;
695 
696  template<typename _Tp, typename _Up>
697  using __assigns_from_optional =
698  __or_<is_assignable<_Tp&, const optional<_Up>&>,
699  is_assignable<_Tp&, optional<_Up>&>,
700  is_assignable<_Tp&, const optional<_Up>&&>,
701  is_assignable<_Tp&, optional<_Up>&&>>;
702 
703  /**
704  * @brief Class template for optional values.
705  */
706  template<typename _Tp>
707  class optional
708  : private _Optional_base<_Tp>,
709  private _Enable_copy_move<
710  // Copy constructor.
711  is_copy_constructible_v<_Tp>,
712  // Copy assignment.
713  __and_v<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>,
714  // Move constructor.
715  is_move_constructible_v<_Tp>,
716  // Move assignment.
717  __and_v<is_move_constructible<_Tp>, is_move_assignable<_Tp>>,
718  // Unique tag type.
719  optional<_Tp>>
720  {
721  static_assert(!is_same_v<remove_cv_t<_Tp>, nullopt_t>);
722  static_assert(!is_same_v<remove_cv_t<_Tp>, in_place_t>);
723  static_assert(is_object_v<_Tp> && !is_array_v<_Tp>);
724 
725  private:
726  using _Base = _Optional_base<_Tp>;
727 
728  // SFINAE helpers
729  template<typename _Up>
730  using __not_self = __not_<is_same<optional, __remove_cvref_t<_Up>>>;
731  template<typename _Up>
732  using __not_tag = __not_<is_same<in_place_t, __remove_cvref_t<_Up>>>;
733  template<typename... _Cond>
734  using _Requires = enable_if_t<__and_v<_Cond...>, bool>;
735 
736  public:
737  using value_type = _Tp;
738 
739  constexpr optional() noexcept { }
740 
741  constexpr optional(nullopt_t) noexcept { }
742 
743  // Converting constructors for engaged optionals.
744  template<typename _Up = _Tp,
745  _Requires<__not_self<_Up>, __not_tag<_Up>,
746  is_constructible<_Tp, _Up>,
747  is_convertible<_Up, _Tp>> = true>
748  constexpr
749  optional(_Up&& __t)
750  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
751  : _Base(std::in_place, std::forward<_Up>(__t)) { }
752 
753  template<typename _Up = _Tp,
754  _Requires<__not_self<_Up>, __not_tag<_Up>,
755  is_constructible<_Tp, _Up>,
756  __not_<is_convertible<_Up, _Tp>>> = false>
757  explicit constexpr
758  optional(_Up&& __t)
759  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
760  : _Base(std::in_place, std::forward<_Up>(__t)) { }
761 
762  template<typename _Up,
763  _Requires<__not_<is_same<_Tp, _Up>>,
764  is_constructible<_Tp, const _Up&>,
765  is_convertible<const _Up&, _Tp>,
766  __not_<__converts_from_optional<_Tp, _Up>>> = true>
767  constexpr
768  optional(const optional<_Up>& __t)
769  noexcept(is_nothrow_constructible_v<_Tp, const _Up&>)
770  {
771  if (__t)
772  emplace(*__t);
773  }
774 
775  template<typename _Up,
776  _Requires<__not_<is_same<_Tp, _Up>>,
777  is_constructible<_Tp, const _Up&>,
778  __not_<is_convertible<const _Up&, _Tp>>,
779  __not_<__converts_from_optional<_Tp, _Up>>> = false>
780  explicit constexpr
781  optional(const optional<_Up>& __t)
782  noexcept(is_nothrow_constructible_v<_Tp, const _Up&>)
783  {
784  if (__t)
785  emplace(*__t);
786  }
787 
788  template<typename _Up,
789  _Requires<__not_<is_same<_Tp, _Up>>,
790  is_constructible<_Tp, _Up>,
791  is_convertible<_Up, _Tp>,
792  __not_<__converts_from_optional<_Tp, _Up>>> = true>
793  constexpr
794  optional(optional<_Up>&& __t)
795  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
796  {
797  if (__t)
798  emplace(std::move(*__t));
799  }
800 
801  template<typename _Up,
802  _Requires<__not_<is_same<_Tp, _Up>>,
803  is_constructible<_Tp, _Up>,
804  __not_<is_convertible<_Up, _Tp>>,
805  __not_<__converts_from_optional<_Tp, _Up>>> = false>
806  explicit constexpr
807  optional(optional<_Up>&& __t)
808  noexcept(is_nothrow_constructible_v<_Tp, _Up>)
809  {
810  if (__t)
811  emplace(std::move(*__t));
812  }
813 
814  template<typename... _Args,
815  _Requires<is_constructible<_Tp, _Args...>> = false>
816  explicit constexpr
817  optional(in_place_t, _Args&&... __args)
818  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
819  : _Base(std::in_place, std::forward<_Args>(__args)...) { }
820 
821  template<typename _Up, typename... _Args,
822  _Requires<is_constructible<_Tp,
823  initializer_list<_Up>&,
824  _Args...>> = false>
825  explicit constexpr
826  optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
827  noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
828  _Args...>)
829  : _Base(std::in_place, __il, std::forward<_Args>(__args)...) { }
830 
831 
832  // Assignment operators.
833  _GLIBCXX20_CONSTEXPR optional&
834  operator=(nullopt_t) noexcept
835  {
836  this->_M_reset();
837  return *this;
838  }
839 
840  template<typename _Up = _Tp>
841  _GLIBCXX20_CONSTEXPR
842  enable_if_t<__and_v<__not_self<_Up>,
843  __not_<__and_<is_scalar<_Tp>,
844  is_same<_Tp, decay_t<_Up>>>>,
845  is_constructible<_Tp, _Up>,
846  is_assignable<_Tp&, _Up>>,
847  optional&>
848  operator=(_Up&& __u)
849  noexcept(__and_v<is_nothrow_constructible<_Tp, _Up>,
850  is_nothrow_assignable<_Tp&, _Up>>)
851  {
852  if (this->_M_is_engaged())
853  this->_M_get() = std::forward<_Up>(__u);
854  else
855  this->_M_construct(std::forward<_Up>(__u));
856 
857  return *this;
858  }
859 
860  template<typename _Up>
861  _GLIBCXX20_CONSTEXPR
862  enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
863  is_constructible<_Tp, const _Up&>,
864  is_assignable<_Tp&, const _Up&>,
865  __not_<__converts_from_optional<_Tp, _Up>>,
866  __not_<__assigns_from_optional<_Tp, _Up>>>,
867  optional&>
868  operator=(const optional<_Up>& __u)
869  noexcept(__and_v<is_nothrow_constructible<_Tp, const _Up&>,
870  is_nothrow_assignable<_Tp&, const _Up&>>)
871  {
872  if (__u)
873  {
874  if (this->_M_is_engaged())
875  this->_M_get() = *__u;
876  else
877  this->_M_construct(*__u);
878  }
879  else
880  {
881  this->_M_reset();
882  }
883  return *this;
884  }
885 
886  template<typename _Up>
887  _GLIBCXX20_CONSTEXPR
888  enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
889  is_constructible<_Tp, _Up>,
890  is_assignable<_Tp&, _Up>,
891  __not_<__converts_from_optional<_Tp, _Up>>,
892  __not_<__assigns_from_optional<_Tp, _Up>>>,
893  optional&>
894  operator=(optional<_Up>&& __u)
895  noexcept(__and_v<is_nothrow_constructible<_Tp, _Up>,
896  is_nothrow_assignable<_Tp&, _Up>>)
897  {
898  if (__u)
899  {
900  if (this->_M_is_engaged())
901  this->_M_get() = std::move(*__u);
902  else
903  this->_M_construct(std::move(*__u));
904  }
905  else
906  {
907  this->_M_reset();
908  }
909 
910  return *this;
911  }
912 
913  template<typename... _Args>
914  _GLIBCXX20_CONSTEXPR
915  enable_if_t<is_constructible_v<_Tp, _Args...>, _Tp&>
916  emplace(_Args&&... __args)
917  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
918  {
919  this->_M_reset();
920  this->_M_construct(std::forward<_Args>(__args)...);
921  return this->_M_get();
922  }
923 
924  template<typename _Up, typename... _Args>
925  _GLIBCXX20_CONSTEXPR
926  enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>,
927  _Tp&>
928  emplace(initializer_list<_Up> __il, _Args&&... __args)
929  noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&,
930  _Args...>)
931  {
932  this->_M_reset();
933  this->_M_construct(__il, std::forward<_Args>(__args)...);
934  return this->_M_get();
935  }
936 
937  // Destructor is implicit, implemented in _Optional_base.
938 
939  // Swap.
940  _GLIBCXX20_CONSTEXPR void
941  swap(optional& __other)
942  noexcept(is_nothrow_move_constructible_v<_Tp>
943  && is_nothrow_swappable_v<_Tp>)
944  {
945  using std::swap;
946 
947  if (this->_M_is_engaged() && __other._M_is_engaged())
948  swap(this->_M_get(), __other._M_get());
949  else if (this->_M_is_engaged())
950  {
951  __other._M_construct(std::move(this->_M_get()));
952  this->_M_destruct();
953  }
954  else if (__other._M_is_engaged())
955  {
956  this->_M_construct(std::move(__other._M_get()));
957  __other._M_destruct();
958  }
959  }
960 
961  // Observers.
962  constexpr const _Tp*
963  operator->() const noexcept
964  { return std::__addressof(this->_M_get()); }
965 
966  constexpr _Tp*
967  operator->() noexcept
968  { return std::__addressof(this->_M_get()); }
969 
970  constexpr const _Tp&
971  operator*() const& noexcept
972  { return this->_M_get(); }
973 
974  constexpr _Tp&
975  operator*()& noexcept
976  { return this->_M_get(); }
977 
978  constexpr _Tp&&
979  operator*()&& noexcept
980  { return std::move(this->_M_get()); }
981 
982  constexpr const _Tp&&
983  operator*() const&& noexcept
984  { return std::move(this->_M_get()); }
985 
986  constexpr explicit operator bool() const noexcept
987  { return this->_M_is_engaged(); }
988 
989  constexpr bool has_value() const noexcept
990  { return this->_M_is_engaged(); }
991 
992  constexpr const _Tp&
993  value() const&
994  {
995  if (this->_M_is_engaged())
996  return this->_M_get();
997  __throw_bad_optional_access();
998  }
999 
1000  constexpr _Tp&
1001  value()&
1002  {
1003  if (this->_M_is_engaged())
1004  return this->_M_get();
1005  __throw_bad_optional_access();
1006  }
1007 
1008  constexpr _Tp&&
1009  value()&&
1010  {
1011  if (this->_M_is_engaged())
1012  return std::move(this->_M_get());
1013  __throw_bad_optional_access();
1014  }
1015 
1016  constexpr const _Tp&&
1017  value() const&&
1018  {
1019  if (this->_M_is_engaged())
1020  return std::move(this->_M_get());
1021  __throw_bad_optional_access();
1022  }
1023 
1024  template<typename _Up>
1025  constexpr _Tp
1026  value_or(_Up&& __u) const&
1027  {
1028  static_assert(is_copy_constructible_v<_Tp>);
1029  static_assert(is_convertible_v<_Up&&, _Tp>);
1030 
1031  if (this->_M_is_engaged())
1032  return this->_M_get();
1033  else
1034  return static_cast<_Tp>(std::forward<_Up>(__u));
1035  }
1036 
1037  template<typename _Up>
1038  constexpr _Tp
1039  value_or(_Up&& __u) &&
1040  {
1041  static_assert(is_move_constructible_v<_Tp>);
1042  static_assert(is_convertible_v<_Up&&, _Tp>);
1043 
1044  if (this->_M_is_engaged())
1045  return std::move(this->_M_get());
1046  else
1047  return static_cast<_Tp>(std::forward<_Up>(__u));
1048  }
1049 
1050 #if __cpp_lib_optional >= 202110L
1051  // [optional.monadic]
1052 
1053  template<typename _Fn>
1054  constexpr auto
1055  and_then(_Fn&& __f) &
1056  {
1057  using _Up = remove_cvref_t<invoke_result_t<_Fn, _Tp&>>;
1058  static_assert(__is_optional_v<remove_cvref_t<_Up>>,
1059  "the function passed to std::optional<T>::and_then "
1060  "must return a std::optional");
1061  if (has_value())
1062  return std::__invoke(std::forward<_Fn>(__f), **this);
1063  else
1064  return _Up();
1065  }
1066 
1067  template<typename _Fn>
1068  constexpr auto
1069  and_then(_Fn&& __f) const &
1070  {
1071  using _Up = remove_cvref_t<invoke_result_t<_Fn, const _Tp&>>;
1072  static_assert(__is_optional_v<_Up>,
1073  "the function passed to std::optional<T>::and_then "
1074  "must return a std::optional");
1075  if (has_value())
1076  return std::__invoke(std::forward<_Fn>(__f), **this);
1077  else
1078  return _Up();
1079  }
1080 
1081  template<typename _Fn>
1082  constexpr auto
1083  and_then(_Fn&& __f) &&
1084  {
1085  using _Up = remove_cvref_t<invoke_result_t<_Fn, _Tp>>;
1086  static_assert(__is_optional_v<remove_cvref_t<_Up>>,
1087  "the function passed to std::optional<T>::and_then "
1088  "must return a std::optional");
1089  if (has_value())
1090  return std::__invoke(std::forward<_Fn>(__f), std::move(**this));
1091  else
1092  return _Up();
1093  }
1094 
1095  template<typename _Fn>
1096  constexpr auto
1097  and_then(_Fn&& __f) const &&
1098  {
1099  using _Up = remove_cvref_t<invoke_result_t<_Fn, const _Tp>>;
1100  static_assert(__is_optional_v<remove_cvref_t<_Up>>,
1101  "the function passed to std::optional<T>::and_then "
1102  "must return a std::optional");
1103  if (has_value())
1104  return std::__invoke(std::forward<_Fn>(__f), std::move(**this));
1105  else
1106  return _Up();
1107  }
1108 
1109  template<typename _Fn>
1110  constexpr auto
1111  transform(_Fn&& __f) &
1112  {
1113  using _Up = remove_cv_t<invoke_result_t<_Fn, _Tp&>>;
1114  if (has_value())
1115  return optional<_Up>(_Optional_func<_Fn>{__f}, **this);
1116  else
1117  return optional<_Up>();
1118  }
1119 
1120  template<typename _Fn>
1121  constexpr auto
1122  transform(_Fn&& __f) const &
1123  {
1124  using _Up = remove_cv_t<invoke_result_t<_Fn, const _Tp&>>;
1125  if (has_value())
1126  return optional<_Up>(_Optional_func<_Fn>{__f}, **this);
1127  else
1128  return optional<_Up>();
1129  }
1130 
1131  template<typename _Fn>
1132  constexpr auto
1133  transform(_Fn&& __f) &&
1134  {
1135  using _Up = remove_cv_t<invoke_result_t<_Fn, _Tp>>;
1136  if (has_value())
1137  return optional<_Up>(_Optional_func<_Fn>{__f}, std::move(**this));
1138  else
1139  return optional<_Up>();
1140  }
1141 
1142  template<typename _Fn>
1143  constexpr auto
1144  transform(_Fn&& __f) const &&
1145  {
1146  using _Up = remove_cv_t<invoke_result_t<_Fn, const _Tp>>;
1147  if (has_value())
1148  return optional<_Up>(_Optional_func<_Fn>{__f}, std::move(**this));
1149  else
1150  return optional<_Up>();
1151  }
1152 
1153  template<typename _Fn> requires invocable<_Fn> && copy_constructible<_Tp>
1154  constexpr optional
1155  or_else(_Fn&& __f) const&
1156  {
1157  using _Up = invoke_result_t<_Fn>;
1158  static_assert(is_same_v<remove_cvref_t<_Up>, optional>,
1159  "the function passed to std::optional<T>::or_else "
1160  "must return a std::optional<T>");
1161 
1162  if (has_value())
1163  return *this;
1164  else
1165  return std::forward<_Fn>(__f)();
1166  }
1167 
1168  template<typename _Fn> requires invocable<_Fn> && move_constructible<_Tp>
1169  constexpr optional
1170  or_else(_Fn&& __f) &&
1171  {
1172  using _Up = invoke_result_t<_Fn>;
1173  static_assert(is_same_v<remove_cvref_t<_Up>, optional>,
1174  "the function passed to std::optional<T>::or_else "
1175  "must return a std::optional<T>");
1176 
1177  if (has_value())
1178  return std::move(*this);
1179  else
1180  return std::forward<_Fn>(__f)();
1181  }
1182 #endif
1183 
1184  _GLIBCXX20_CONSTEXPR void reset() noexcept { this->_M_reset(); }
1185 
1186  private:
1187 #if __cplusplus >= 202002L
1188  template<typename _Up> friend class optional;
1189 
1190  template<typename _Fn, typename _Value>
1191  explicit constexpr
1192  optional(_Optional_func<_Fn> __f, _Value&& __v)
1193  {
1194  this->_M_payload._M_apply(__f, std::forward<_Value>(__v));
1195  }
1196 #endif
1197  };
1198 
1199  template<typename _Tp>
1200  using __optional_relop_t =
1201  enable_if_t<is_convertible<_Tp, bool>::value, bool>;
1202 
1203  template<typename _Tp, typename _Up>
1204  using __optional_eq_t = __optional_relop_t<
1205  decltype(std::declval<const _Tp&>() == std::declval<const _Up&>())
1206  >;
1207 
1208  template<typename _Tp, typename _Up>
1209  using __optional_ne_t = __optional_relop_t<
1210  decltype(std::declval<const _Tp&>() != std::declval<const _Up&>())
1211  >;
1212 
1213  template<typename _Tp, typename _Up>
1214  using __optional_lt_t = __optional_relop_t<
1215  decltype(std::declval<const _Tp&>() < std::declval<const _Up&>())
1216  >;
1217 
1218  template<typename _Tp, typename _Up>
1219  using __optional_gt_t = __optional_relop_t<
1220  decltype(std::declval<const _Tp&>() > std::declval<const _Up&>())
1221  >;
1222 
1223  template<typename _Tp, typename _Up>
1224  using __optional_le_t = __optional_relop_t<
1225  decltype(std::declval<const _Tp&>() <= std::declval<const _Up&>())
1226  >;
1227 
1228  template<typename _Tp, typename _Up>
1229  using __optional_ge_t = __optional_relop_t<
1230  decltype(std::declval<const _Tp&>() >= std::declval<const _Up&>())
1231  >;
1232 
1233  // Comparisons between optional values.
1234  template<typename _Tp, typename _Up>
1235  constexpr auto
1236  operator==(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1237  -> __optional_eq_t<_Tp, _Up>
1238  {
1239  return static_cast<bool>(__lhs) == static_cast<bool>(__rhs)
1240  && (!__lhs || *__lhs == *__rhs);
1241  }
1242 
1243  template<typename _Tp, typename _Up>
1244  constexpr auto
1245  operator!=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1246  -> __optional_ne_t<_Tp, _Up>
1247  {
1248  return static_cast<bool>(__lhs) != static_cast<bool>(__rhs)
1249  || (static_cast<bool>(__lhs) && *__lhs != *__rhs);
1250  }
1251 
1252  template<typename _Tp, typename _Up>
1253  constexpr auto
1254  operator<(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1255  -> __optional_lt_t<_Tp, _Up>
1256  {
1257  return static_cast<bool>(__rhs) && (!__lhs || *__lhs < *__rhs);
1258  }
1259 
1260  template<typename _Tp, typename _Up>
1261  constexpr auto
1262  operator>(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1263  -> __optional_gt_t<_Tp, _Up>
1264  {
1265  return static_cast<bool>(__lhs) && (!__rhs || *__lhs > *__rhs);
1266  }
1267 
1268  template<typename _Tp, typename _Up>
1269  constexpr auto
1270  operator<=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1271  -> __optional_le_t<_Tp, _Up>
1272  {
1273  return !__lhs || (static_cast<bool>(__rhs) && *__lhs <= *__rhs);
1274  }
1275 
1276  template<typename _Tp, typename _Up>
1277  constexpr auto
1278  operator>=(const optional<_Tp>& __lhs, const optional<_Up>& __rhs)
1279  -> __optional_ge_t<_Tp, _Up>
1280  {
1281  return !__rhs || (static_cast<bool>(__lhs) && *__lhs >= *__rhs);
1282  }
1283 
1284 #ifdef __cpp_lib_three_way_comparison
1285  template<typename _Tp, three_way_comparable_with<_Tp> _Up>
1286  constexpr compare_three_way_result_t<_Tp, _Up>
1287  operator<=>(const optional<_Tp>& __x, const optional<_Up>& __y)
1288  {
1289  return __x && __y ? *__x <=> *__y : bool(__x) <=> bool(__y);
1290  }
1291 #endif
1292 
1293  // Comparisons with nullopt.
1294  template<typename _Tp>
1295  constexpr bool
1296  operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept
1297  { return !__lhs; }
1298 
1299 #ifdef __cpp_lib_three_way_comparison
1300  template<typename _Tp>
1301  constexpr strong_ordering
1302  operator<=>(const optional<_Tp>& __x, nullopt_t) noexcept
1303  { return bool(__x) <=> false; }
1304 #else
1305  template<typename _Tp>
1306  constexpr bool
1307  operator==(nullopt_t, const optional<_Tp>& __rhs) noexcept
1308  { return !__rhs; }
1309 
1310  template<typename _Tp>
1311  constexpr bool
1312  operator!=(const optional<_Tp>& __lhs, nullopt_t) noexcept
1313  { return static_cast<bool>(__lhs); }
1314 
1315  template<typename _Tp>
1316  constexpr bool
1317  operator!=(nullopt_t, const optional<_Tp>& __rhs) noexcept
1318  { return static_cast<bool>(__rhs); }
1319 
1320  template<typename _Tp>
1321  constexpr bool
1322  operator<(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
1323  { return false; }
1324 
1325  template<typename _Tp>
1326  constexpr bool
1327  operator<(nullopt_t, const optional<_Tp>& __rhs) noexcept
1328  { return static_cast<bool>(__rhs); }
1329 
1330  template<typename _Tp>
1331  constexpr bool
1332  operator>(const optional<_Tp>& __lhs, nullopt_t) noexcept
1333  { return static_cast<bool>(__lhs); }
1334 
1335  template<typename _Tp>
1336  constexpr bool
1337  operator>(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
1338  { return false; }
1339 
1340  template<typename _Tp>
1341  constexpr bool
1342  operator<=(const optional<_Tp>& __lhs, nullopt_t) noexcept
1343  { return !__lhs; }
1344 
1345  template<typename _Tp>
1346  constexpr bool
1347  operator<=(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
1348  { return true; }
1349 
1350  template<typename _Tp>
1351  constexpr bool
1352  operator>=(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
1353  { return true; }
1354 
1355  template<typename _Tp>
1356  constexpr bool
1357  operator>=(nullopt_t, const optional<_Tp>& __rhs) noexcept
1358  { return !__rhs; }
1359 #endif // three-way-comparison
1360 
1361  // Comparisons with value type.
1362  template<typename _Tp, typename _Up>
1363  constexpr auto
1364  operator==(const optional<_Tp>& __lhs, const _Up& __rhs)
1365  -> __optional_eq_t<_Tp, _Up>
1366  { return __lhs && *__lhs == __rhs; }
1367 
1368  template<typename _Tp, typename _Up>
1369  constexpr auto
1370  operator==(const _Up& __lhs, const optional<_Tp>& __rhs)
1371  -> __optional_eq_t<_Up, _Tp>
1372  { return __rhs && __lhs == *__rhs; }
1373 
1374  template<typename _Tp, typename _Up>
1375  constexpr auto
1376  operator!=(const optional<_Tp>& __lhs, const _Up& __rhs)
1377  -> __optional_ne_t<_Tp, _Up>
1378  { return !__lhs || *__lhs != __rhs; }
1379 
1380  template<typename _Tp, typename _Up>
1381  constexpr auto
1382  operator!=(const _Up& __lhs, const optional<_Tp>& __rhs)
1383  -> __optional_ne_t<_Up, _Tp>
1384  { return !__rhs || __lhs != *__rhs; }
1385 
1386  template<typename _Tp, typename _Up>
1387  constexpr auto
1388  operator<(const optional<_Tp>& __lhs, const _Up& __rhs)
1389  -> __optional_lt_t<_Tp, _Up>
1390  { return !__lhs || *__lhs < __rhs; }
1391 
1392  template<typename _Tp, typename _Up>
1393  constexpr auto
1394  operator<(const _Up& __lhs, const optional<_Tp>& __rhs)
1395  -> __optional_lt_t<_Up, _Tp>
1396  { return __rhs && __lhs < *__rhs; }
1397 
1398  template<typename _Tp, typename _Up>
1399  constexpr auto
1400  operator>(const optional<_Tp>& __lhs, const _Up& __rhs)
1401  -> __optional_gt_t<_Tp, _Up>
1402  { return __lhs && *__lhs > __rhs; }
1403 
1404  template<typename _Tp, typename _Up>
1405  constexpr auto
1406  operator>(const _Up& __lhs, const optional<_Tp>& __rhs)
1407  -> __optional_gt_t<_Up, _Tp>
1408  { return !__rhs || __lhs > *__rhs; }
1409 
1410  template<typename _Tp, typename _Up>
1411  constexpr auto
1412  operator<=(const optional<_Tp>& __lhs, const _Up& __rhs)
1413  -> __optional_le_t<_Tp, _Up>
1414  { return !__lhs || *__lhs <= __rhs; }
1415 
1416  template<typename _Tp, typename _Up>
1417  constexpr auto
1418  operator<=(const _Up& __lhs, const optional<_Tp>& __rhs)
1419  -> __optional_le_t<_Up, _Tp>
1420  { return __rhs && __lhs <= *__rhs; }
1421 
1422  template<typename _Tp, typename _Up>
1423  constexpr auto
1424  operator>=(const optional<_Tp>& __lhs, const _Up& __rhs)
1425  -> __optional_ge_t<_Tp, _Up>
1426  { return __lhs && *__lhs >= __rhs; }
1427 
1428  template<typename _Tp, typename _Up>
1429  constexpr auto
1430  operator>=(const _Up& __lhs, const optional<_Tp>& __rhs)
1431  -> __optional_ge_t<_Up, _Tp>
1432  { return !__rhs || __lhs >= *__rhs; }
1433 
1434 #ifdef __cpp_lib_three_way_comparison
1435  template<typename _Tp, typename _Up>
1436  requires (!__is_optional_v<_Up>)
1437  && three_way_comparable_with<_Up, _Tp>
1438  constexpr compare_three_way_result_t<_Tp, _Up>
1439  operator<=>(const optional<_Tp>& __x, const _Up& __v)
1440  { return bool(__x) ? *__x <=> __v : strong_ordering::less; }
1441 #endif
1442 
1443  // Swap and creation functions.
1444 
1445  // _GLIBCXX_RESOLVE_LIB_DEFECTS
1446  // 2748. swappable traits for optionals
1447  template<typename _Tp>
1448  _GLIBCXX20_CONSTEXPR
1449  inline enable_if_t<is_move_constructible_v<_Tp> && is_swappable_v<_Tp>>
1450  swap(optional<_Tp>& __lhs, optional<_Tp>& __rhs)
1451  noexcept(noexcept(__lhs.swap(__rhs)))
1452  { __lhs.swap(__rhs); }
1453 
1454  template<typename _Tp>
1455  enable_if_t<!(is_move_constructible_v<_Tp> && is_swappable_v<_Tp>)>
1456  swap(optional<_Tp>&, optional<_Tp>&) = delete;
1457 
1458  template<typename _Tp>
1459  constexpr
1460  enable_if_t<is_constructible_v<decay_t<_Tp>, _Tp>,
1461  optional<decay_t<_Tp>>>
1462  make_optional(_Tp&& __t)
1463  noexcept(is_nothrow_constructible_v<optional<decay_t<_Tp>>, _Tp>)
1464  { return optional<decay_t<_Tp>>{ std::forward<_Tp>(__t) }; }
1465 
1466  template<typename _Tp, typename... _Args>
1467  constexpr
1468  enable_if_t<is_constructible_v<_Tp, _Args...>,
1469  optional<_Tp>>
1470  make_optional(_Args&&... __args)
1471  noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
1472  { return optional<_Tp>{ in_place, std::forward<_Args>(__args)... }; }
1473 
1474  template<typename _Tp, typename _Up, typename... _Args>
1475  constexpr
1476  enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>,
1477  optional<_Tp>>
1478  make_optional(initializer_list<_Up> __il, _Args&&... __args)
1479  noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>)
1480  { return optional<_Tp>{ in_place, __il, std::forward<_Args>(__args)... }; }
1481 
1482  // Hash.
1483 
1484  template<typename _Tp, typename _Up = remove_const_t<_Tp>,
1485  bool = __poison_hash<_Up>::__enable_hash_call>
1486  struct __optional_hash_call_base
1487  {
1488  size_t
1489  operator()(const optional<_Tp>& __t) const
1490  noexcept(noexcept(hash<_Up>{}(*__t)))
1491  {
1492  // We pick an arbitrary hash for disengaged optionals which hopefully
1493  // usual values of _Tp won't typically hash to.
1494  constexpr size_t __magic_disengaged_hash = static_cast<size_t>(-3333);
1495  return __t ? hash<_Up>{}(*__t) : __magic_disengaged_hash;
1496  }
1497  };
1498 
1499  template<typename _Tp, typename _Up>
1500  struct __optional_hash_call_base<_Tp, _Up, false> {};
1501 
1502  template<typename _Tp>
1503  struct hash<optional<_Tp>>
1504  : private __poison_hash<remove_const_t<_Tp>>,
1505  public __optional_hash_call_base<_Tp>
1506  {
1507  using result_type [[__deprecated__]] = size_t;
1508  using argument_type [[__deprecated__]] = optional<_Tp>;
1509  };
1510 
1511  template<typename _Tp>
1512  struct __is_fast_hash<hash<optional<_Tp>>> : __is_fast_hash<hash<_Tp>>
1513  { };
1514 
1515  /// @}
1516 
1517 #if __cpp_deduction_guides >= 201606
1518  template <typename _Tp> optional(_Tp) -> optional<_Tp>;
1519 #endif
1520 
1521 _GLIBCXX_END_NAMESPACE_VERSION
1522 } // namespace std
1523 
1524 #endif // C++17
1525 
1526 #endif // _GLIBCXX_OPTIONAL