libstdc++
variant
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1 // <variant> -*- C++ -*-
2 
3 // Copyright (C) 2016-2025 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24 
25 /** @file variant
26  * This is the `<variant>` C++ Library header.
27  */
28 
29 #ifndef _GLIBCXX_VARIANT
30 #define _GLIBCXX_VARIANT 1
31 
32 #ifdef _GLIBCXX_SYSHDR
33 #pragma GCC system_header
34 #endif
35 
36 #define __glibcxx_want_freestanding_variant
37 #define __glibcxx_want_variant
38 #define __glibcxx_want_constrained_equality
39 #include <bits/version.h>
40 
41 #ifdef __cpp_lib_variant // C++ >= 17
42 #include <initializer_list>
43 #include <type_traits>
44 #include <bits/enable_special_members.h>
45 #include <bits/exception_defines.h>
46 #include <bits/functional_hash.h>
47 #include <bits/invoke.h>
48 #include <bits/monostate.h>
49 #include <bits/parse_numbers.h> // _Select_int
50 #include <bits/stl_iterator_base_funcs.h>
51 #include <bits/stl_construct.h>
52 #include <bits/utility.h> // in_place_index_t
53 #if __cplusplus >= 202002L
54 # include <concepts>
55 # include <compare>
56 #endif
57 
58 // C++ < 20 || __cpp_concepts < 202002L || __cpp_constexpr < 201811L
59 #if __cpp_lib_variant < 202106L
60 # include <ext/aligned_buffer.h> // Use __aligned_membuf for storage.
61 #endif
62 
63 namespace std _GLIBCXX_VISIBILITY(default)
64 {
65 _GLIBCXX_BEGIN_NAMESPACE_VERSION
66 
67  template<typename... _Types> class tuple;
68  template<typename... _Types> class variant;
69 
70  template<typename _Variant>
71  struct variant_size;
72 
73  template<typename _Variant>
74  struct variant_size<const _Variant> : variant_size<_Variant> {};
75 
76  template<typename _Variant>
77  struct variant_size<volatile _Variant> : variant_size<_Variant> {};
78 
79  template<typename _Variant>
80  struct variant_size<const volatile _Variant> : variant_size<_Variant> {};
81 
82  template<typename... _Types>
83  struct variant_size<variant<_Types...>>
84  : std::integral_constant<size_t, sizeof...(_Types)> {};
85 
86  template<typename _Variant>
87  inline constexpr size_t variant_size_v = variant_size<_Variant>::value;
88 
89  template<typename... _Types>
90  inline constexpr size_t
91  variant_size_v<variant<_Types...>> = sizeof...(_Types);
92 
93  template<typename... _Types>
94  inline constexpr size_t
95  variant_size_v<const variant<_Types...>> = sizeof...(_Types);
96 
97  template<size_t _Np, typename _Variant>
98  struct variant_alternative;
99 
100  template<size_t _Np, typename... _Types>
101  struct variant_alternative<_Np, variant<_Types...>>
102  {
103  static_assert(_Np < sizeof...(_Types));
104 
105  using type = typename _Nth_type<_Np, _Types...>::type;
106  };
107 
108  template<size_t _Np, typename _Variant>
109  using variant_alternative_t =
110  typename variant_alternative<_Np, _Variant>::type;
111 
112  template<size_t _Np, typename _Variant>
113  struct variant_alternative<_Np, const _Variant>
114  { using type = const variant_alternative_t<_Np, _Variant>; };
115 
116  template<size_t _Np, typename _Variant>
117  struct variant_alternative<_Np, volatile _Variant>
118  { using type = volatile variant_alternative_t<_Np, _Variant>; };
119 
120  template<size_t _Np, typename _Variant>
121  struct variant_alternative<_Np, const volatile _Variant>
122  { using type = const volatile variant_alternative_t<_Np, _Variant>; };
123 
124  inline constexpr size_t variant_npos = -1;
125 
126  template<size_t _Np, typename... _Types>
127  constexpr variant_alternative_t<_Np, variant<_Types...>>&
128  get(variant<_Types...>&);
129 
130  template<size_t _Np, typename... _Types>
131  constexpr variant_alternative_t<_Np, variant<_Types...>>&&
132  get(variant<_Types...>&&);
133 
134  template<size_t _Np, typename... _Types>
135  constexpr variant_alternative_t<_Np, variant<_Types...>> const&
136  get(const variant<_Types...>&);
137 
138  template<size_t _Np, typename... _Types>
139  constexpr variant_alternative_t<_Np, variant<_Types...>> const&&
140  get(const variant<_Types...>&&);
141 
142  template<typename _Result_type, typename _Visitor, typename... _Variants>
143  constexpr decltype(auto)
144  __do_visit(_Visitor&& __visitor, _Variants&&... __variants);
145 
146  template <typename... _Types, typename _Tp>
147  _GLIBCXX20_CONSTEXPR
148  decltype(auto)
149  __variant_cast(_Tp&& __rhs)
150  {
151  if constexpr (is_lvalue_reference_v<_Tp>)
152  {
153  if constexpr (is_const_v<remove_reference_t<_Tp>>)
154  return static_cast<const variant<_Types...>&>(__rhs);
155  else
156  return static_cast<variant<_Types...>&>(__rhs);
157  }
158  else
159  return static_cast<variant<_Types...>&&>(__rhs);
160  }
161 
162 namespace __detail
163 {
164 namespace __variant
165 {
166  // used for raw visitation
167  struct __variant_cookie {};
168  // used for raw visitation with indices passed in
169  struct __variant_idx_cookie { using type = __variant_idx_cookie; };
170  // Used to enable deduction (and same-type checking) for std::visit:
171  template<typename _Tp> struct __deduce_visit_result { using type = _Tp; };
172 
173  // Visit variants that might be valueless.
174  template<typename _Visitor, typename... _Variants>
175  constexpr void
176  __raw_visit(_Visitor&& __visitor, _Variants&&... __variants)
177  {
178  std::__do_visit<__variant_cookie>(std::forward<_Visitor>(__visitor),
179  std::forward<_Variants>(__variants)...);
180  }
181 
182  // Visit variants that might be valueless, passing indices to the visitor.
183  template<typename _Visitor, typename... _Variants>
184  constexpr void
185  __raw_idx_visit(_Visitor&& __visitor, _Variants&&... __variants)
186  {
187  std::__do_visit<__variant_idx_cookie>(std::forward<_Visitor>(__visitor),
188  std::forward<_Variants>(__variants)...);
189  }
190 
191  // The __as function templates implement the exposition-only "as-variant"
192 
193  template<typename... _Types>
194  constexpr std::variant<_Types...>&
195  __as(std::variant<_Types...>& __v) noexcept
196  { return __v; }
197 
198  template<typename... _Types>
199  constexpr const std::variant<_Types...>&
200  __as(const std::variant<_Types...>& __v) noexcept
201  { return __v; }
202 
203  template<typename... _Types>
204  constexpr std::variant<_Types...>&&
205  __as(std::variant<_Types...>&& __v) noexcept
206  { return std::move(__v); }
207 
208  template<typename... _Types>
209  constexpr const std::variant<_Types...>&&
210  __as(const std::variant<_Types...>&& __v) noexcept
211  { return std::move(__v); }
212 
213 #if __cpp_lib_variant < 202106L
214  template<typename _Type, bool = std::is_trivially_destructible_v<_Type>>
215  struct _Uninitialized;
216 #else
217  template<typename _Type, bool /* unused */ = true>
218  struct _Uninitialized;
219 #endif
220 
221  // The primary template is used for trivially destructible types in C++17,
222  // and for all types in C++20.
223  template<typename _Type, bool>
224  struct _Uninitialized
225  {
226  template<typename... _Args>
227  constexpr
228  _Uninitialized(in_place_index_t<0>, _Args&&... __args)
229  : _M_storage(std::forward<_Args>(__args)...)
230  { }
231 
232 #if __cpp_lib_variant < 202106L
233  constexpr const _Type& _M_get() const & noexcept
234  { return _M_storage; }
235 
236  constexpr _Type& _M_get() & noexcept
237  { return _M_storage; }
238 
239  constexpr const _Type&& _M_get() const && noexcept
240  { return std::move(_M_storage); }
241 
242  constexpr _Type&& _M_get() && noexcept
243  { return std::move(_M_storage); }
244 #endif
245 
246  _Type _M_storage;
247  };
248 
249 #if __cpp_lib_variant < 202106L
250  // This partial specialization is used for non-trivially destructible types
251  // in C++17, so that _Uninitialized<T> is trivially destructible and can be
252  // used as a union member in _Variadic_union.
253  template<typename _Type>
254  struct _Uninitialized<_Type, false>
255  {
256  template<typename... _Args>
257  constexpr
258  _Uninitialized(in_place_index_t<0>, _Args&&... __args)
259  {
260  ::new ((void*)std::addressof(_M_storage))
261  _Type(std::forward<_Args>(__args)...);
262  }
263 
264  const _Type& _M_get() const & noexcept
265  { return *_M_storage._M_ptr(); }
266 
267  _Type& _M_get() & noexcept
268  { return *_M_storage._M_ptr(); }
269 
270  const _Type&& _M_get() const && noexcept
271  { return std::move(*_M_storage._M_ptr()); }
272 
273  _Type&& _M_get() && noexcept
274  { return std::move(*_M_storage._M_ptr()); }
275 
276  __gnu_cxx::__aligned_membuf<_Type> _M_storage;
277  };
278 
279  template<size_t _Np, typename _Union>
280  constexpr decltype(auto)
281  __get_n(_Union&& __u) noexcept
282  {
283  if constexpr (_Np == 0)
284  return std::forward<_Union>(__u)._M_first._M_get();
285  else if constexpr (_Np == 1)
286  return std::forward<_Union>(__u)._M_rest._M_first._M_get();
287  else if constexpr (_Np == 2)
288  return std::forward<_Union>(__u)._M_rest._M_rest._M_first._M_get();
289  else
290  return __variant::__get_n<_Np - 3>(
291  std::forward<_Union>(__u)._M_rest._M_rest._M_rest);
292  }
293 #else
294  template<size_t _Np, typename _Union>
295  constexpr auto&&
296  __get_n(_Union&& __u) noexcept
297  {
298  if constexpr (_Np == 0)
299  return std::forward<_Union>(__u)._M_first._M_storage;
300  else if constexpr (_Np == 1)
301  return std::forward<_Union>(__u)._M_rest._M_first._M_storage;
302  else if constexpr (_Np == 2)
303  return std::forward<_Union>(__u)._M_rest._M_rest._M_first._M_storage;
304  else
305  return __variant::__get_n<_Np - 3>(
306  std::forward<_Union>(__u)._M_rest._M_rest._M_rest);
307  }
308 #endif
309 
310  // Returns the typed storage for __v.
311  template<size_t _Np, typename _Variant>
312  constexpr decltype(auto)
313  __get(_Variant&& __v) noexcept
314  { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
315 
316  // Gets the _Uninitialized to construct into for __u.
317  template<size_t _Np, typename _Union>
318  constexpr decltype(auto)
319  __construct_n(_Union& __u) noexcept
320  {
321  if constexpr (_Np == 0)
322  return &__u._M_first;
323  else if constexpr (_Np == 1)
324  {
325  std::_Construct(&__u._M_rest);
326  return &__u._M_rest._M_first;
327  }
328  else if constexpr (_Np == 2)
329  {
330  std::_Construct(&__u._M_rest);
331  std::_Construct(&__u._M_rest._M_rest);
332  return &__u._M_rest._M_rest._M_first;
333  }
334  else
335  {
336  std::_Construct(&__u._M_rest);
337  std::_Construct(&__u._M_rest._M_rest);
338  std::_Construct(&__u._M_rest._M_rest._M_rest);
339  return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
340  }
341  }
342 
343  template<typename... _Types>
344  struct _Traits
345  {
346  static constexpr bool _S_default_ctor =
347  is_default_constructible_v<typename _Nth_type<0, _Types...>::type>;
348  static constexpr bool _S_copy_ctor =
349  (is_copy_constructible_v<_Types> && ...);
350  static constexpr bool _S_move_ctor =
351  (is_move_constructible_v<_Types> && ...);
352  static constexpr bool _S_copy_assign =
353  _S_copy_ctor
354  && (is_copy_assignable_v<_Types> && ...);
355  static constexpr bool _S_move_assign =
356  _S_move_ctor
357  && (is_move_assignable_v<_Types> && ...);
358 
359  static constexpr bool _S_trivial_dtor =
360  (is_trivially_destructible_v<_Types> && ...);
361  static constexpr bool _S_trivial_copy_ctor =
362  (is_trivially_copy_constructible_v<_Types> && ...);
363  static constexpr bool _S_trivial_move_ctor =
364  (is_trivially_move_constructible_v<_Types> && ...);
365  static constexpr bool _S_trivial_copy_assign =
366  _S_trivial_dtor && _S_trivial_copy_ctor
367  && (is_trivially_copy_assignable_v<_Types> && ...);
368  static constexpr bool _S_trivial_move_assign =
369  _S_trivial_dtor && _S_trivial_move_ctor
370  && (is_trivially_move_assignable_v<_Types> && ...);
371 
372  // The following nothrow traits are for non-trivial SMFs. Trivial SMFs
373  // are always nothrow.
374  static constexpr bool _S_nothrow_default_ctor =
375  is_nothrow_default_constructible_v<
376  typename _Nth_type<0, _Types...>::type>;
377  static constexpr bool _S_nothrow_copy_ctor = false;
378  static constexpr bool _S_nothrow_move_ctor =
379  (is_nothrow_move_constructible_v<_Types> && ...);
380  static constexpr bool _S_nothrow_copy_assign = false;
381  static constexpr bool _S_nothrow_move_assign =
382  _S_nothrow_move_ctor
383  && (is_nothrow_move_assignable_v<_Types> && ...);
384  };
385 
386  // Defines members and ctors.
387  template<bool __trivially_destructible, typename... _Types>
388  union _Variadic_union
389  {
390  _Variadic_union() = default;
391 
392  template<size_t _Np, typename... _Args>
393  _Variadic_union(in_place_index_t<_Np>, _Args&&...) = delete;
394  };
395 
396  template<bool __trivially_destructible, typename _First, typename... _Rest>
397  union _Variadic_union<__trivially_destructible, _First, _Rest...>
398  {
399  constexpr _Variadic_union() : _M_rest() { }
400 
401  template<typename... _Args>
402  constexpr
403  _Variadic_union(in_place_index_t<0>, _Args&&... __args)
404  : _M_first(in_place_index<0>, std::forward<_Args>(__args)...)
405  { }
406 
407  template<size_t _Np, typename... _Args>
408  constexpr
409  _Variadic_union(in_place_index_t<_Np>, _Args&&... __args)
410  : _M_rest(in_place_index<_Np-1>, std::forward<_Args>(__args)...)
411  { }
412 
413 #if __cpp_lib_variant >= 202106L
414  _Variadic_union(const _Variadic_union&) = default;
415  _Variadic_union(_Variadic_union&&) = default;
416  _Variadic_union& operator=(const _Variadic_union&) = default;
417  _Variadic_union& operator=(_Variadic_union&&) = default;
418 
419  ~_Variadic_union() = default;
420 
421  // If any alternative type is not trivially destructible then we need a
422  // user-provided destructor that does nothing. The active alternative
423  // will be destroyed by _Variant_storage::_M_reset() instead of here.
424  constexpr ~_Variadic_union()
425  requires (!__trivially_destructible)
426  { }
427 #endif
428 
429  _Uninitialized<_First> _M_first;
430  _Variadic_union<__trivially_destructible, _Rest...> _M_rest;
431  };
432 
433  // _Never_valueless_alt is true for variant alternatives that can
434  // always be placed in a variant without it becoming valueless.
435 
436  // For suitably-small, trivially copyable types we can create temporaries
437  // on the stack and then memcpy them into place.
438  template<typename _Tp>
439  struct _Never_valueless_alt
440  : __and_<bool_constant<sizeof(_Tp) <= 256>, is_trivially_copyable<_Tp>>
441  { };
442 
443  // Specialize _Never_valueless_alt for other types which have a
444  // non-throwing and cheap move construction and move assignment operator,
445  // so that emplacing the type will provide the strong exception-safety
446  // guarantee, by creating and moving a temporary.
447  // Whether _Never_valueless_alt<T> is true or not affects the ABI of a
448  // variant using that alternative, so we can't change the value later!
449 
450  // True if every alternative in _Types... can be emplaced in a variant
451  // without it becoming valueless. If this is true, variant<_Types...>
452  // can never be valueless, which enables some minor optimizations.
453  template <typename... _Types>
454  constexpr bool __never_valueless()
455  {
456  return _Traits<_Types...>::_S_move_assign
457  && (_Never_valueless_alt<_Types>::value && ...);
458  }
459 
460  // Defines index and the dtor, possibly trivial.
461  template<bool __trivially_destructible, typename... _Types>
462  struct _Variant_storage;
463 
464  template <typename... _Types>
465  using __select_index =
466  typename __select_int::_Select_int_base<sizeof...(_Types),
467  unsigned char,
468  unsigned short>::type::value_type;
469 
470  template<typename... _Types>
471  struct _Variant_storage<false, _Types...>
472  {
473  constexpr
474  _Variant_storage()
475  : _M_index(static_cast<__index_type>(variant_npos))
476  { }
477 
478  template<size_t _Np, typename... _Args>
479  constexpr
480  _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
481  : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
482  _M_index{_Np}
483  { }
484 
485  constexpr void
486  _M_reset()
487  {
488  if (!_M_valid()) [[__unlikely__]]
489  return;
490 
491  std::__do_visit<void>([](auto&& __this_mem) mutable
492  {
493  std::_Destroy(std::__addressof(__this_mem));
494  }, __variant_cast<_Types...>(*this));
495 
496  _M_index = static_cast<__index_type>(variant_npos);
497  }
498 
499  _GLIBCXX20_CONSTEXPR
500  ~_Variant_storage()
501  { _M_reset(); }
502 
503  constexpr bool
504  _M_valid() const noexcept
505  {
506  if constexpr (__variant::__never_valueless<_Types...>())
507  return true;
508  return this->_M_index != __index_type(variant_npos);
509  }
510 
511  _Variadic_union<false, _Types...> _M_u;
512  using __index_type = __select_index<_Types...>;
513  __index_type _M_index;
514  };
515 
516  template<typename... _Types>
517  struct _Variant_storage<true, _Types...>
518  {
519  constexpr
520  _Variant_storage()
521  : _M_index(static_cast<__index_type>(variant_npos))
522  { }
523 
524  template<size_t _Np, typename... _Args>
525  constexpr
526  _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
527  : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
528  _M_index{_Np}
529  { }
530 
531  constexpr void
532  _M_reset() noexcept
533  { _M_index = static_cast<__index_type>(variant_npos); }
534 
535  constexpr bool
536  _M_valid() const noexcept
537  {
538  if constexpr (__variant::__never_valueless<_Types...>())
539  return true;
540  // It would be nice if we could just return true for -fno-exceptions.
541  // It's possible (but inadvisable) that a std::variant could become
542  // valueless in a translation unit compiled with -fexceptions and then
543  // be passed to functions compiled with -fno-exceptions. We would need
544  // some #ifdef _GLIBCXX_NO_EXCEPTIONS_GLOBALLY property to elide all
545  // checks for valueless_by_exception().
546  return this->_M_index != static_cast<__index_type>(variant_npos);
547  }
548 
549  _Variadic_union<true, _Types...> _M_u;
550  using __index_type = __select_index<_Types...>;
551  __index_type _M_index;
552  };
553 
554  // Implementation of v.emplace<N>(args...).
555  template<size_t _Np, bool _Triv, typename... _Types, typename... _Args>
556  _GLIBCXX20_CONSTEXPR
557  inline void
558  __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
559  {
560  __v._M_reset();
561  auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
562  std::_Construct(__addr, in_place_index<0>,
563  std::forward<_Args>(__args)...);
564  // Construction didn't throw, so can set the new index now:
565  __v._M_index = _Np;
566  }
567 
568  template<typename... _Types>
569  using _Variant_storage_alias =
570  _Variant_storage<_Traits<_Types...>::_S_trivial_dtor, _Types...>;
571 
572  // The following are (Copy|Move) (ctor|assign) layers for forwarding
573  // triviality and handling non-trivial SMF behaviors.
574 
575  template<bool, typename... _Types>
576  struct _Copy_ctor_base : _Variant_storage_alias<_Types...>
577  {
578  using _Base = _Variant_storage_alias<_Types...>;
579  using _Base::_Base;
580 
581  _GLIBCXX20_CONSTEXPR
582  _Copy_ctor_base(const _Copy_ctor_base& __rhs)
583  noexcept(_Traits<_Types...>::_S_nothrow_copy_ctor)
584  {
585  __variant::__raw_idx_visit(
586  [this](auto&& __rhs_mem, auto __rhs_index) mutable
587  {
588  constexpr size_t __j = __rhs_index;
589  if constexpr (__j != variant_npos)
590  std::_Construct(std::__addressof(this->_M_u),
591  in_place_index<__j>, __rhs_mem);
592  }, __variant_cast<_Types...>(__rhs));
593  this->_M_index = __rhs._M_index;
594  }
595 
596  _Copy_ctor_base(_Copy_ctor_base&&) = default;
597  _Copy_ctor_base& operator=(const _Copy_ctor_base&) = default;
598  _Copy_ctor_base& operator=(_Copy_ctor_base&&) = default;
599  };
600 
601  template<typename... _Types>
602  struct _Copy_ctor_base<true, _Types...> : _Variant_storage_alias<_Types...>
603  {
604  using _Base = _Variant_storage_alias<_Types...>;
605  using _Base::_Base;
606  };
607 
608  template<typename... _Types>
609  using _Copy_ctor_alias =
610  _Copy_ctor_base<_Traits<_Types...>::_S_trivial_copy_ctor, _Types...>;
611 
612  template<bool, typename... _Types>
613  struct _Move_ctor_base : _Copy_ctor_alias<_Types...>
614  {
615  using _Base = _Copy_ctor_alias<_Types...>;
616  using _Base::_Base;
617 
618  _GLIBCXX20_CONSTEXPR
619  _Move_ctor_base(_Move_ctor_base&& __rhs)
620  noexcept(_Traits<_Types...>::_S_nothrow_move_ctor)
621  {
622  __variant::__raw_idx_visit(
623  [this](auto&& __rhs_mem, auto __rhs_index) mutable
624  {
625  constexpr size_t __j = __rhs_index;
626  if constexpr (__j != variant_npos)
627  std::_Construct(std::__addressof(this->_M_u),
628  in_place_index<__j>,
629  std::forward<decltype(__rhs_mem)>(__rhs_mem));
630  }, __variant_cast<_Types...>(std::move(__rhs)));
631  this->_M_index = __rhs._M_index;
632  }
633 
634  _Move_ctor_base(const _Move_ctor_base&) = default;
635  _Move_ctor_base& operator=(const _Move_ctor_base&) = default;
636  _Move_ctor_base& operator=(_Move_ctor_base&&) = default;
637  };
638 
639  template<typename... _Types>
640  struct _Move_ctor_base<true, _Types...> : _Copy_ctor_alias<_Types...>
641  {
642  using _Base = _Copy_ctor_alias<_Types...>;
643  using _Base::_Base;
644  };
645 
646  template<typename... _Types>
647  using _Move_ctor_alias =
648  _Move_ctor_base<_Traits<_Types...>::_S_trivial_move_ctor, _Types...>;
649 
650  template<bool, typename... _Types>
651  struct _Copy_assign_base : _Move_ctor_alias<_Types...>
652  {
653  using _Base = _Move_ctor_alias<_Types...>;
654  using _Base::_Base;
655 
656  _GLIBCXX20_CONSTEXPR
657  _Copy_assign_base&
658  operator=(const _Copy_assign_base& __rhs)
659  noexcept(_Traits<_Types...>::_S_nothrow_copy_assign)
660  {
661  __variant::__raw_idx_visit(
662  [this](auto&& __rhs_mem, auto __rhs_index) mutable
663  {
664  constexpr size_t __j = __rhs_index;
665  if constexpr (__j == variant_npos)
666  this->_M_reset(); // Make *this valueless.
667  else if (this->_M_index == __j)
668  __variant::__get<__j>(*this) = __rhs_mem;
669  else
670  {
671  using _Tj = typename _Nth_type<__j, _Types...>::type;
672  if constexpr (is_nothrow_copy_constructible_v<_Tj>
673  || !is_nothrow_move_constructible_v<_Tj>)
674  __variant::__emplace<__j>(*this, __rhs_mem);
675  else
676  {
677  using _Variant = variant<_Types...>;
678  _Variant& __self = __variant_cast<_Types...>(*this);
679  __self = _Variant(in_place_index<__j>, __rhs_mem);
680  }
681  }
682  }, __variant_cast<_Types...>(__rhs));
683  return *this;
684  }
685 
686  _Copy_assign_base(const _Copy_assign_base&) = default;
687  _Copy_assign_base(_Copy_assign_base&&) = default;
688  _Copy_assign_base& operator=(_Copy_assign_base&&) = default;
689  };
690 
691  template<typename... _Types>
692  struct _Copy_assign_base<true, _Types...> : _Move_ctor_alias<_Types...>
693  {
694  using _Base = _Move_ctor_alias<_Types...>;
695  using _Base::_Base;
696  };
697 
698  template<typename... _Types>
699  using _Copy_assign_alias =
700  _Copy_assign_base<_Traits<_Types...>::_S_trivial_copy_assign, _Types...>;
701 
702  template<bool, typename... _Types>
703  struct _Move_assign_base : _Copy_assign_alias<_Types...>
704  {
705  using _Base = _Copy_assign_alias<_Types...>;
706  using _Base::_Base;
707 
708  _GLIBCXX20_CONSTEXPR
709  _Move_assign_base&
710  operator=(_Move_assign_base&& __rhs)
711  noexcept(_Traits<_Types...>::_S_nothrow_move_assign)
712  {
713  __variant::__raw_idx_visit(
714  [this](auto&& __rhs_mem, auto __rhs_index) mutable
715  {
716  constexpr size_t __j = __rhs_index;
717  if constexpr (__j != variant_npos)
718  {
719  if (this->_M_index == __j)
720  __variant::__get<__j>(*this) = std::move(__rhs_mem);
721  else
722  {
723  using _Tj = typename _Nth_type<__j, _Types...>::type;
724  if constexpr (is_nothrow_move_constructible_v<_Tj>)
725  __variant::__emplace<__j>(*this, std::move(__rhs_mem));
726  else
727  {
728  using _Variant = variant<_Types...>;
729  _Variant& __self = __variant_cast<_Types...>(*this);
730  __self.template emplace<__j>(std::move(__rhs_mem));
731  }
732  }
733  }
734  else
735  this->_M_reset();
736  }, __variant_cast<_Types...>(__rhs));
737  return *this;
738  }
739 
740  _Move_assign_base(const _Move_assign_base&) = default;
741  _Move_assign_base(_Move_assign_base&&) = default;
742  _Move_assign_base& operator=(const _Move_assign_base&) = default;
743  };
744 
745  template<typename... _Types>
746  struct _Move_assign_base<true, _Types...> : _Copy_assign_alias<_Types...>
747  {
748  using _Base = _Copy_assign_alias<_Types...>;
749  using _Base::_Base;
750  };
751 
752  template<typename... _Types>
753  using _Move_assign_alias =
754  _Move_assign_base<_Traits<_Types...>::_S_trivial_move_assign, _Types...>;
755 
756  template<typename... _Types>
757  struct _Variant_base : _Move_assign_alias<_Types...>
758  {
759  using _Base = _Move_assign_alias<_Types...>;
760 
761  constexpr
762  _Variant_base() noexcept(_Traits<_Types...>::_S_nothrow_default_ctor)
763  : _Variant_base(in_place_index<0>) { }
764 
765  template<size_t _Np, typename... _Args>
766  constexpr explicit
767  _Variant_base(in_place_index_t<_Np> __i, _Args&&... __args)
768  : _Base(__i, std::forward<_Args>(__args)...)
769  { }
770 
771  _Variant_base(const _Variant_base&) = default;
772  _Variant_base(_Variant_base&&) = default;
773  _Variant_base& operator=(const _Variant_base&) = default;
774  _Variant_base& operator=(_Variant_base&&) = default;
775  };
776 
777  template<typename _Tp, typename... _Types>
778  inline constexpr bool __exactly_once
779  = std::__find_uniq_type_in_pack<_Tp, _Types...>() < sizeof...(_Types);
780 
781  // Helper used to check for valid conversions that don't involve narrowing.
782  template<typename _Ti> struct _Arr { _Ti _M_x[1]; };
783 
784  // "Build an imaginary function FUN(Ti) for each alternative type Ti"
785  template<size_t _Ind, typename _Tp, typename _Ti, typename = void>
786  struct _Build_FUN
787  {
788  // This function means 'using _Build_FUN<I, T, Ti>::_S_fun;' is valid,
789  // but only static functions will be considered in the call below.
790  void _S_fun() = delete;
791  };
792 
793  // "... for which Ti x[] = {std::forward<T>(t)}; is well-formed."
794  template<size_t _Ind, typename _Tp, typename _Ti>
795  struct _Build_FUN<_Ind, _Tp, _Ti,
796  void_t<decltype(_Arr<_Ti>{{std::declval<_Tp>()}})>>
797  {
798  // This is the FUN function for type _Ti, with index _Ind
799  static integral_constant<size_t, _Ind> _S_fun(_Ti);
800  };
801 
802  template<typename _Tp, typename _Variant,
803  typename = make_index_sequence<variant_size_v<_Variant>>>
804  struct _Build_FUNs;
805 
806  template<typename _Tp, typename... _Ti, size_t... _Ind>
807  struct _Build_FUNs<_Tp, variant<_Ti...>, index_sequence<_Ind...>>
808  : _Build_FUN<_Ind, _Tp, _Ti>...
809  {
810  using _Build_FUN<_Ind, _Tp, _Ti>::_S_fun...;
811  };
812 
813  // The index j of the overload FUN(Tj) selected by overload resolution
814  // for FUN(std::forward<_Tp>(t))
815  template<typename _Tp, typename _Variant>
816  using _FUN_type
817  = decltype(_Build_FUNs<_Tp, _Variant>::_S_fun(std::declval<_Tp>()));
818 
819  // The index selected for FUN(std::forward<T>(t)), or variant_npos if none.
820  template<typename _Tp, typename _Variant, typename = void>
821  inline constexpr size_t
822  __accepted_index = variant_npos;
823 
824  template<typename _Tp, typename _Variant>
825  inline constexpr size_t
826  __accepted_index<_Tp, _Variant, void_t<_FUN_type<_Tp, _Variant>>>
827  = _FUN_type<_Tp, _Variant>::value;
828 
829  template<typename _Maybe_variant_cookie, typename _Variant,
830  typename = __remove_cvref_t<_Variant>>
831  inline constexpr bool
832  __extra_visit_slot_needed = false;
833 
834  template<typename _Var, typename... _Types>
835  inline constexpr bool
836  __extra_visit_slot_needed<__variant_cookie, _Var, variant<_Types...>>
837  = !__variant::__never_valueless<_Types...>();
838 
839  template<typename _Var, typename... _Types>
840  inline constexpr bool
841  __extra_visit_slot_needed<__variant_idx_cookie, _Var, variant<_Types...>>
842  = !__variant::__never_valueless<_Types...>();
843 
844  // Used for storing a multi-dimensional vtable.
845  template<typename _Tp, size_t... _Dimensions>
846  struct _Multi_array;
847 
848  // Partial specialization with rank zero, stores a single _Tp element.
849  template<typename _Tp>
850  struct _Multi_array<_Tp>
851  {
852  template<typename>
853  struct __untag_result
854  : false_type
855  { using element_type = _Tp; };
856 
857 #pragma GCC diagnostic push
858 #pragma GCC diagnostic ignored "-Wignored-qualifiers"
859  template <typename... _Args>
860  struct __untag_result<const void(*)(_Args...)>
861  : false_type
862  { using element_type = void(*)(_Args...); };
863 #pragma GCC diagnostic pop
864 
865  template <typename... _Args>
866  struct __untag_result<__variant_cookie(*)(_Args...)>
867  : false_type
868  { using element_type = void(*)(_Args...); };
869 
870  template <typename... _Args>
871  struct __untag_result<__variant_idx_cookie(*)(_Args...)>
872  : false_type
873  { using element_type = void(*)(_Args...); };
874 
875  template <typename _Res, typename... _Args>
876  struct __untag_result<__deduce_visit_result<_Res>(*)(_Args...)>
877  : true_type
878  { using element_type = _Res(*)(_Args...); };
879 
880  using __result_is_deduced = __untag_result<_Tp>;
881 
882  constexpr const typename __untag_result<_Tp>::element_type&
883  _M_access() const
884  { return _M_data; }
885 
886  typename __untag_result<_Tp>::element_type _M_data;
887  };
888 
889  // Partial specialization with rank >= 1.
890  template<typename _Ret,
891  typename _Visitor,
892  typename... _Variants,
893  size_t __first, size_t... __rest>
894  struct _Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>
895  {
896  static constexpr size_t __index =
897  sizeof...(_Variants) - sizeof...(__rest) - 1;
898 
899  using _Variant = typename _Nth_type<__index, _Variants...>::type;
900 
901  static constexpr int __do_cookie =
902  __extra_visit_slot_needed<_Ret, _Variant> ? 1 : 0;
903 
904  using _Tp = _Ret(*)(_Visitor, _Variants...);
905 
906  template<typename... _Args>
907  constexpr decltype(auto)
908  _M_access(size_t __first_index, _Args... __rest_indices) const
909  {
910  return _M_arr[__first_index + __do_cookie]
911  ._M_access(__rest_indices...);
912  }
913 
914  _Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
915  };
916 
917  // Creates a multi-dimensional vtable recursively.
918  //
919  // For example,
920  // visit([](auto, auto){},
921  // variant<int, char>(), // typedef'ed as V1
922  // variant<float, double, long double>()) // typedef'ed as V2
923  // will trigger instantiations of:
924  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 2, 3>,
925  // tuple<V1&&, V2&&>, std::index_sequence<>>
926  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
927  // tuple<V1&&, V2&&>, std::index_sequence<0>>
928  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
929  // tuple<V1&&, V2&&>, std::index_sequence<0, 0>>
930  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
931  // tuple<V1&&, V2&&>, std::index_sequence<0, 1>>
932  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
933  // tuple<V1&&, V2&&>, std::index_sequence<0, 2>>
934  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
935  // tuple<V1&&, V2&&>, std::index_sequence<1>>
936  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
937  // tuple<V1&&, V2&&>, std::index_sequence<1, 0>>
938  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
939  // tuple<V1&&, V2&&>, std::index_sequence<1, 1>>
940  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
941  // tuple<V1&&, V2&&>, std::index_sequence<1, 2>>
942  // The returned multi-dimensional vtable can be fast accessed by the visitor
943  // using index calculation.
944  template<typename _Array_type, typename _Index_seq>
945  struct __gen_vtable_impl;
946 
947  // Defines the _S_apply() member that returns a _Multi_array populated
948  // with function pointers that perform the visitation expressions e(m)
949  // for each valid pack of indexes into the variant types _Variants.
950  //
951  // This partial specialization builds up the index sequences by recursively
952  // calling _S_apply() on the next specialization of __gen_vtable_impl.
953  // The base case of the recursion defines the actual function pointers.
954  template<typename _Result_type, typename _Visitor, size_t... __dimensions,
955  typename... _Variants, size_t... __indices>
956  struct __gen_vtable_impl<
957  _Multi_array<_Result_type (*)(_Visitor, _Variants...), __dimensions...>,
958  std::index_sequence<__indices...>>
959  {
960  using _Next =
961  remove_reference_t<typename _Nth_type<sizeof...(__indices),
962  _Variants...>::type>;
963  using _Array_type =
964  _Multi_array<_Result_type (*)(_Visitor, _Variants...),
965  __dimensions...>;
966 
967  static constexpr _Array_type
968  _S_apply()
969  {
970  _Array_type __vtable{};
971  _S_apply_all_alts(
972  __vtable, make_index_sequence<variant_size_v<_Next>>());
973  return __vtable;
974  }
975 
976  template<size_t... __var_indices>
977  static constexpr void
978  _S_apply_all_alts(_Array_type& __vtable,
979  std::index_sequence<__var_indices...>)
980  {
981  if constexpr (__extra_visit_slot_needed<_Result_type, _Next>)
982  (_S_apply_single_alt<true, __var_indices>(
983  __vtable._M_arr[__var_indices + 1],
984  &(__vtable._M_arr[0])), ...);
985  else
986  (_S_apply_single_alt<false, __var_indices>(
987  __vtable._M_arr[__var_indices]), ...);
988  }
989 
990  template<bool __do_cookie, size_t __index, typename _Tp>
991  static constexpr void
992  _S_apply_single_alt(_Tp& __element, _Tp* __cookie_element = nullptr)
993  {
994  if constexpr (__do_cookie)
995  {
996  __element = __gen_vtable_impl<
997  _Tp,
998  std::index_sequence<__indices..., __index>>::_S_apply();
999  *__cookie_element = __gen_vtable_impl<
1000  _Tp,
1001  std::index_sequence<__indices..., variant_npos>>::_S_apply();
1002  }
1003  else
1004  {
1005  auto __tmp_element = __gen_vtable_impl<
1006  remove_reference_t<decltype(__element)>,
1007  std::index_sequence<__indices..., __index>>::_S_apply();
1008  static_assert(is_same_v<_Tp, decltype(__tmp_element)>,
1009  "std::visit requires the visitor to have the same "
1010  "return type for all alternatives of a variant");
1011  __element = __tmp_element;
1012  }
1013  }
1014  };
1015 
1016  // This partial specialization is the base case for the recursion.
1017  // It populates a _Multi_array element with the address of a function
1018  // that invokes the visitor with the alternatives specified by __indices.
1019  template<typename _Result_type, typename _Visitor, typename... _Variants,
1020  size_t... __indices>
1021  struct __gen_vtable_impl<
1022  _Multi_array<_Result_type (*)(_Visitor, _Variants...)>,
1023  std::index_sequence<__indices...>>
1024  {
1025  using _Array_type =
1026  _Multi_array<_Result_type (*)(_Visitor, _Variants...)>;
1027 
1028  template<size_t __index, typename _Variant>
1029  static constexpr decltype(auto)
1030  __element_by_index_or_cookie(_Variant&& __var) noexcept
1031  {
1032  if constexpr (__index != variant_npos)
1033  return __variant::__get<__index>(std::forward<_Variant>(__var));
1034  else
1035  return __variant_cookie{};
1036  }
1037 
1038  static constexpr decltype(auto)
1039  __visit_invoke(_Visitor&& __visitor, _Variants... __vars)
1040  {
1041  if constexpr (is_same_v<_Result_type, __variant_idx_cookie>)
1042  // For raw visitation using indices, pass the indices to the visitor
1043  // and discard the return value:
1044  std::__invoke(std::forward<_Visitor>(__visitor),
1045  __element_by_index_or_cookie<__indices>(
1046  std::forward<_Variants>(__vars))...,
1047  integral_constant<size_t, __indices>()...);
1048  else if constexpr (is_same_v<_Result_type, __variant_cookie>)
1049  // For raw visitation without indices, and discard the return value:
1050  std::__invoke(std::forward<_Visitor>(__visitor),
1051  __element_by_index_or_cookie<__indices>(
1052  std::forward<_Variants>(__vars))...);
1053  else if constexpr (_Array_type::__result_is_deduced::value)
1054  // For the usual std::visit case deduce the return value:
1055  return std::__invoke(std::forward<_Visitor>(__visitor),
1056  __element_by_index_or_cookie<__indices>(
1057  std::forward<_Variants>(__vars))...);
1058  else // for std::visit<R> use INVOKE<R>
1059  return std::__invoke_r<_Result_type>(
1060  std::forward<_Visitor>(__visitor),
1061  __variant::__get<__indices>(std::forward<_Variants>(__vars))...);
1062  }
1063 
1064  static constexpr auto
1065  _S_apply()
1066  {
1067  if constexpr (_Array_type::__result_is_deduced::value)
1068  {
1069  constexpr bool __visit_ret_type_mismatch =
1070  !is_same_v<typename _Result_type::type,
1071  decltype(__visit_invoke(std::declval<_Visitor>(),
1072  std::declval<_Variants>()...))>;
1073  if constexpr (__visit_ret_type_mismatch)
1074  {
1075  struct __cannot_match {};
1076  return __cannot_match{};
1077  }
1078  else
1079  return _Array_type{&__visit_invoke};
1080  }
1081  else
1082  return _Array_type{&__visit_invoke};
1083  }
1084  };
1085 
1086  template<typename _Result_type, typename _Visitor, typename... _Variants>
1087  struct __gen_vtable
1088  {
1089  using _Array_type =
1090  _Multi_array<_Result_type (*)(_Visitor, _Variants...),
1091  variant_size_v<remove_reference_t<_Variants>>...>;
1092 
1093  static constexpr _Array_type _S_vtable
1094  = __gen_vtable_impl<_Array_type, std::index_sequence<>>::_S_apply();
1095  };
1096 
1097 #if ! _GLIBCXX_INLINE_VERSION
1098  template<size_t _Nm, typename _Tp>
1099  struct _Base_dedup : public _Tp { };
1100 
1101  template<typename _Variant, typename __indices>
1102  struct _Variant_hash_base;
1103 
1104  template<typename... _Types, size_t... __indices>
1105  struct _Variant_hash_base<variant<_Types...>,
1106  std::index_sequence<__indices...>>
1107  : _Base_dedup<__indices, __hash_empty_base<remove_const_t<_Types>>>...
1108  { };
1109 #endif
1110 
1111  // Equivalent to decltype(get<_Np>(as-variant(declval<_Variant>())))
1112  template<size_t _Np, typename _Variant,
1113  typename _AsV = decltype(__variant::__as(std::declval<_Variant>())),
1114  typename _Tp = variant_alternative_t<_Np, remove_reference_t<_AsV>>>
1115  using __get_t
1116  = __conditional_t<is_lvalue_reference_v<_Variant>, _Tp&, _Tp&&>;
1117 
1118  // Return type of std::visit.
1119  template<typename _Visitor, typename... _Variants>
1120  using __visit_result_t
1121  = invoke_result_t<_Visitor, __get_t<0, _Variants>...>;
1122 
1123  template<typename _Tp, typename... _Types>
1124  constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);
1125 
1126  template <typename _Visitor, typename _Variant, size_t... _Idxs>
1127  constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>)
1128  {
1129  return __same_types<
1130  invoke_result_t<_Visitor, __get_t<_Idxs, _Variant>>...
1131  >;
1132  }
1133 
1134 } // namespace __variant
1135 } // namespace __detail
1136 
1137  template<typename _Tp, typename... _Types>
1138  constexpr bool
1139  holds_alternative(const variant<_Types...>& __v) noexcept
1140  {
1141  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1142  "T must occur exactly once in alternatives");
1143  return __v.index() == std::__find_uniq_type_in_pack<_Tp, _Types...>();
1144  }
1145 
1146  template<typename _Tp, typename... _Types>
1147  constexpr _Tp&
1148  get(variant<_Types...>& __v)
1149  {
1150  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1151  "T must occur exactly once in alternatives");
1152  constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
1153  return std::get<__n>(__v);
1154  }
1155 
1156  template<typename _Tp, typename... _Types>
1157  constexpr _Tp&&
1158  get(variant<_Types...>&& __v)
1159  {
1160  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1161  "T must occur exactly once in alternatives");
1162  constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
1163  return std::get<__n>(std::move(__v));
1164  }
1165 
1166  template<typename _Tp, typename... _Types>
1167  constexpr const _Tp&
1168  get(const variant<_Types...>& __v)
1169  {
1170  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1171  "T must occur exactly once in alternatives");
1172  constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
1173  return std::get<__n>(__v);
1174  }
1175 
1176  template<typename _Tp, typename... _Types>
1177  constexpr const _Tp&&
1178  get(const variant<_Types...>&& __v)
1179  {
1180  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1181  "T must occur exactly once in alternatives");
1182  constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
1183  return std::get<__n>(std::move(__v));
1184  }
1185 
1186  template<size_t _Np, typename... _Types>
1187  constexpr add_pointer_t<variant_alternative_t<_Np, variant<_Types...>>>
1188  get_if(variant<_Types...>* __ptr) noexcept
1189  {
1190  using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
1191  static_assert(_Np < sizeof...(_Types),
1192  "The index must be in [0, number of alternatives)");
1193  static_assert(!is_void_v<_Alternative_type>, "_Tp must not be void");
1194  if (__ptr && __ptr->index() == _Np)
1195  return std::addressof(__detail::__variant::__get<_Np>(*__ptr));
1196  return nullptr;
1197  }
1198 
1199  template<size_t _Np, typename... _Types>
1200  constexpr
1201  add_pointer_t<const variant_alternative_t<_Np, variant<_Types...>>>
1202  get_if(const variant<_Types...>* __ptr) noexcept
1203  {
1204  using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
1205  static_assert(_Np < sizeof...(_Types),
1206  "The index must be in [0, number of alternatives)");
1207  static_assert(!is_void_v<_Alternative_type>, "_Tp must not be void");
1208  if (__ptr && __ptr->index() == _Np)
1209  return std::addressof(__detail::__variant::__get<_Np>(*__ptr));
1210  return nullptr;
1211  }
1212 
1213  template<typename _Tp, typename... _Types>
1214  constexpr add_pointer_t<_Tp>
1215  get_if(variant<_Types...>* __ptr) noexcept
1216  {
1217  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1218  "T must occur exactly once in alternatives");
1219  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1220  constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
1221  return std::get_if<__n>(__ptr);
1222  }
1223 
1224  template<typename _Tp, typename... _Types>
1225  constexpr add_pointer_t<const _Tp>
1226  get_if(const variant<_Types...>* __ptr) noexcept
1227  {
1228  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1229  "T must occur exactly once in alternatives");
1230  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1231  constexpr size_t __n = std::__find_uniq_type_in_pack<_Tp, _Types...>();
1232  return std::get_if<__n>(__ptr);
1233  }
1234 
1235 namespace __detail::__variant
1236 {
1237  template<typename _Ret, typename _Vp, typename _Op>
1238  constexpr _Ret
1239  __compare(_Ret __ret, const _Vp& __lhs, const _Vp& __rhs, _Op __op)
1240  {
1241  __variant::__raw_idx_visit(
1242  [&__ret, &__lhs, __op] (auto&& __rhs_mem, auto __rhs_index) mutable
1243  {
1244  if constexpr (__rhs_index != variant_npos)
1245  {
1246  if (__lhs.index() == __rhs_index.value)
1247  {
1248  auto& __this_mem = std::get<__rhs_index>(__lhs);
1249  __ret = __op(__this_mem, __rhs_mem);
1250  return;
1251  }
1252  }
1253  __ret = __op(__lhs.index() + 1, __rhs_index + 1);
1254  }, __rhs);
1255  return __ret;
1256  }
1257 } // namespace __detail::__variant
1258 
1259  template<typename... _Types>
1260 #if __cpp_lib_concepts
1261  requires ((requires (const _Types& __t) {
1262  { __t == __t } -> convertible_to<bool>; }) && ...)
1263 #endif
1264  constexpr bool
1265  operator== [[nodiscard]] (const variant<_Types...>& __lhs,
1266  const variant<_Types...>& __rhs)
1267  {
1268  namespace __variant = __detail::__variant;
1269  return __variant::__compare(true, __lhs, __rhs,
1270  [](auto&& __l, auto&& __r) -> bool {
1271  return __l == __r;
1272  });
1273  }
1274 
1275  template<typename... _Types>
1276 #if __cpp_lib_concepts
1277  requires ((requires (const _Types& __t) {
1278  { __t != __t } -> convertible_to<bool>; }) && ...)
1279 #endif
1280  constexpr bool
1281  operator!= [[nodiscard]] (const variant<_Types...>& __lhs,
1282  const variant<_Types...>& __rhs)
1283  {
1284  namespace __variant = __detail::__variant;
1285  return __variant::__compare(true, __lhs, __rhs,
1286  [](auto&& __l, auto&& __r) -> bool {
1287  return __l != __r;
1288  });
1289  }
1290 
1291  template<typename... _Types>
1292 #if __cpp_lib_concepts
1293  requires ((requires (const _Types& __t) {
1294  { __t < __t } -> convertible_to<bool>; }) && ...)
1295 #endif
1296  constexpr bool
1297  operator< [[nodiscard]] (const variant<_Types...>& __lhs,
1298  const variant<_Types...>& __rhs)
1299  {
1300  namespace __variant = __detail::__variant;
1301  return __variant::__compare(true, __lhs, __rhs,
1302  [](auto&& __l, auto&& __r) -> bool {
1303  return __l < __r;
1304  });
1305  }
1306 
1307  template<typename... _Types>
1308 #if __cpp_lib_concepts
1309  requires ((requires (const _Types& __t) {
1310  { __t <= __t } -> convertible_to<bool>; }) && ...)
1311 #endif
1312  constexpr bool
1313  operator<= [[nodiscard]] (const variant<_Types...>& __lhs,
1314  const variant<_Types...>& __rhs)
1315  {
1316  namespace __variant = __detail::__variant;
1317  return __variant::__compare(true, __lhs, __rhs,
1318  [](auto&& __l, auto&& __r) -> bool {
1319  return __l <= __r;
1320  });
1321  }
1322 
1323  template<typename... _Types>
1324 #if __cpp_lib_concepts
1325  requires ((requires (const _Types& __t) {
1326  { __t > __t } -> convertible_to<bool>; }) && ...)
1327 #endif
1328  constexpr bool
1329  operator> [[nodiscard]] (const variant<_Types...>& __lhs,
1330  const variant<_Types...>& __rhs)
1331  {
1332  namespace __variant = __detail::__variant;
1333  return __variant::__compare(true, __lhs, __rhs,
1334  [](auto&& __l, auto&& __r) -> bool {
1335  return __l > __r;
1336  });
1337  }
1338 
1339  template<typename... _Types>
1340 #if __cpp_lib_concepts
1341  requires ((requires (const _Types& __t) {
1342  { __t >= __t } -> convertible_to<bool>; }) && ...)
1343 #endif
1344  constexpr bool
1345  operator>= [[nodiscard]] (const variant<_Types...>& __lhs,
1346  const variant<_Types...>& __rhs)
1347  {
1348  namespace __variant = __detail::__variant;
1349  return __variant::__compare(true, __lhs, __rhs,
1350  [](auto&& __l, auto&& __r) -> bool {
1351  return __l >= __r;
1352  });
1353  }
1354 
1355 #ifdef __cpp_lib_three_way_comparison
1356  template<typename... _Types>
1357  requires (three_way_comparable<_Types> && ...)
1358  constexpr
1359  common_comparison_category_t<compare_three_way_result_t<_Types>...>
1360  operator<=>(const variant<_Types...>& __v, const variant<_Types...>& __w)
1361  {
1362  common_comparison_category_t<compare_three_way_result_t<_Types>...> __ret
1363  = strong_ordering::equal;
1364  namespace __variant = __detail::__variant;
1365  return __variant::__compare(__ret, __v, __w,
1366  [](auto&& __l, auto&& __r) {
1367  return __l <=> __r;
1368  });
1369  }
1370 #endif
1371 
1372  template<typename _Visitor, typename... _Variants>
1373  constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
1374  visit(_Visitor&&, _Variants&&...);
1375 
1376 #if __cplusplus > 201703L
1377  template<typename _Res, typename _Visitor, typename... _Variants>
1378  constexpr _Res
1379  visit(_Visitor&&, _Variants&&...);
1380 #endif
1381 
1382  template<typename... _Types>
1383  _GLIBCXX20_CONSTEXPR
1384  inline enable_if_t<(is_move_constructible_v<_Types> && ...)
1385  && (is_swappable_v<_Types> && ...)>
1386  swap(variant<_Types...>& __lhs, variant<_Types...>& __rhs)
1387  noexcept(noexcept(__lhs.swap(__rhs)))
1388  { __lhs.swap(__rhs); }
1389 
1390  template<typename... _Types>
1391  enable_if_t<!((is_move_constructible_v<_Types> && ...)
1392  && (is_swappable_v<_Types> && ...))>
1393  swap(variant<_Types...>&, variant<_Types...>&) = delete;
1394 
1395  [[noreturn]] void __throw_bad_variant_access(unsigned);
1396 
1397  class bad_variant_access : public exception
1398  {
1399  public:
1400  bad_variant_access() noexcept { }
1401 
1402  const char* what() const noexcept override
1403  { return _M_reason; }
1404 
1405  private:
1406  // Must only be called with a string literal
1407  bad_variant_access(const char* __reason) noexcept : _M_reason(__reason) { }
1408 
1409  // Must point to a string with static storage duration:
1410  const char* _M_reason = "bad variant access";
1411 
1412  friend void __throw_bad_variant_access([[maybe_unused]] unsigned __n)
1413  {
1414  [[maybe_unused]] static constexpr const char* __reasons[] = {
1415  "std::get: wrong index for variant",
1416  "std::get: variant is valueless",
1417  "std::visit: variant is valueless",
1418  "std::visit<R>: variant is valueless",
1419  };
1420  _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__reasons[__n % 4u]));
1421  }
1422  };
1423 
1424  template<typename... _Types>
1425  class variant
1426  : private __detail::__variant::_Variant_base<_Types...>,
1427  private _Enable_copy_move<
1428  __detail::__variant::_Traits<_Types...>::_S_copy_ctor,
1429  __detail::__variant::_Traits<_Types...>::_S_copy_assign,
1430  __detail::__variant::_Traits<_Types...>::_S_move_ctor,
1431  __detail::__variant::_Traits<_Types...>::_S_move_assign,
1432  variant<_Types...>>
1433  {
1434  private:
1435  template <typename... _UTypes, typename _Tp>
1436  friend _GLIBCXX20_CONSTEXPR decltype(auto)
1437  __variant_cast(_Tp&&);
1438 
1439  static_assert(sizeof...(_Types) > 0,
1440  "variant must have at least one alternative");
1441  static_assert(((std::is_object_v<_Types> && !is_array_v<_Types>) && ...),
1442  "variant alternatives must be non-array object types");
1443 
1444  using _Base = __detail::__variant::_Variant_base<_Types...>;
1445 
1446  template<typename _Tp>
1447  static constexpr bool __not_self
1448  = !is_same_v<__remove_cvref_t<_Tp>, variant>;
1449 
1450  template<typename _Tp>
1451  static constexpr bool
1452  __exactly_once = __detail::__variant::__exactly_once<_Tp, _Types...>;
1453 
1454  template<typename _Tp>
1455  static constexpr size_t __accepted_index
1456  = __detail::__variant::__accepted_index<_Tp, variant>;
1457 
1458  template<size_t _Np, typename = enable_if_t<(_Np < sizeof...(_Types))>>
1459  using __to_type = typename _Nth_type<_Np, _Types...>::type;
1460 
1461  template<typename _Tp, typename = enable_if_t<__not_self<_Tp>>>
1462  using __accepted_type = __to_type<__accepted_index<_Tp>>;
1463 
1464  template<typename _Tp>
1465  static constexpr size_t __index_of
1466  = std::__find_uniq_type_in_pack<_Tp, _Types...>();
1467 
1468  using _Traits = __detail::__variant::_Traits<_Types...>;
1469 
1470  template<typename _Tp>
1471  static constexpr bool __not_in_place_tag
1472  = !__is_in_place_type_v<__remove_cvref_t<_Tp>>
1473  && !__is_in_place_index_v<__remove_cvref_t<_Tp>>;
1474 
1475  public:
1476 #if __cpp_concepts
1477  variant() requires is_default_constructible_v<__to_type<0>> = default;
1478 #else
1479  template<typename _Tp0 = __to_type<0>,
1480  typename = enable_if_t<is_default_constructible_v<_Tp0>>>
1481  constexpr
1482  variant() noexcept(is_nothrow_default_constructible_v<__to_type<0>>)
1483  { }
1484 #endif
1485 
1486  variant(const variant& __rhs) = default;
1487  variant(variant&&) = default;
1488  variant& operator=(const variant&) = default;
1489  variant& operator=(variant&&) = default;
1490  _GLIBCXX20_CONSTEXPR ~variant() = default;
1491 
1492  template<typename _Tp,
1493  typename = enable_if_t<sizeof...(_Types) != 0>,
1494  typename = enable_if_t<__not_in_place_tag<_Tp>>,
1495  typename _Tj = __accepted_type<_Tp&&>,
1496  typename = enable_if_t<__exactly_once<_Tj>
1497  && is_constructible_v<_Tj, _Tp>>>
1498  constexpr
1499  variant(_Tp&& __t)
1500  noexcept(is_nothrow_constructible_v<_Tj, _Tp>)
1501  : variant(in_place_index<__accepted_index<_Tp>>,
1502  std::forward<_Tp>(__t))
1503  { }
1504 
1505  template<typename _Tp, typename... _Args,
1506  typename = enable_if_t<__exactly_once<_Tp>
1507  && is_constructible_v<_Tp, _Args...>>>
1508  constexpr explicit
1509  variant(in_place_type_t<_Tp>, _Args&&... __args)
1510  : variant(in_place_index<__index_of<_Tp>>,
1511  std::forward<_Args>(__args)...)
1512  { }
1513 
1514  template<typename _Tp, typename _Up, typename... _Args,
1515  typename = enable_if_t<__exactly_once<_Tp>
1516  && is_constructible_v<_Tp,
1517  initializer_list<_Up>&, _Args...>>>
1518  constexpr explicit
1519  variant(in_place_type_t<_Tp>, initializer_list<_Up> __il,
1520  _Args&&... __args)
1521  : variant(in_place_index<__index_of<_Tp>>, __il,
1522  std::forward<_Args>(__args)...)
1523  { }
1524 
1525  template<size_t _Np, typename... _Args,
1526  typename _Tp = __to_type<_Np>,
1527  typename = enable_if_t<is_constructible_v<_Tp, _Args...>>>
1528  constexpr explicit
1529  variant(in_place_index_t<_Np>, _Args&&... __args)
1530  : _Base(in_place_index<_Np>, std::forward<_Args>(__args)...)
1531  { }
1532 
1533  template<size_t _Np, typename _Up, typename... _Args,
1534  typename _Tp = __to_type<_Np>,
1535  typename = enable_if_t<is_constructible_v<_Tp,
1536  initializer_list<_Up>&,
1537  _Args...>>>
1538  constexpr explicit
1539  variant(in_place_index_t<_Np>, initializer_list<_Up> __il,
1540  _Args&&... __args)
1541  : _Base(in_place_index<_Np>, __il, std::forward<_Args>(__args)...)
1542  { }
1543 
1544  template<typename _Tp>
1545  _GLIBCXX20_CONSTEXPR
1546  enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
1547  && is_constructible_v<__accepted_type<_Tp&&>, _Tp>
1548  && is_assignable_v<__accepted_type<_Tp&&>&, _Tp>,
1549  variant&>
1550  operator=(_Tp&& __rhs)
1551  noexcept(is_nothrow_assignable_v<__accepted_type<_Tp&&>&, _Tp>
1552  && is_nothrow_constructible_v<__accepted_type<_Tp&&>, _Tp>)
1553  {
1554  constexpr auto __index = __accepted_index<_Tp>;
1555  if (index() == __index)
1556  std::get<__index>(*this) = std::forward<_Tp>(__rhs);
1557  else
1558  {
1559  using _Tj = __accepted_type<_Tp&&>;
1560  if constexpr (is_nothrow_constructible_v<_Tj, _Tp>
1561  || !is_nothrow_move_constructible_v<_Tj>)
1562  this->emplace<__index>(std::forward<_Tp>(__rhs));
1563  else
1564  // _GLIBCXX_RESOLVE_LIB_DEFECTS
1565  // 3585. converting assignment with immovable alternative
1566  this->emplace<__index>(_Tj(std::forward<_Tp>(__rhs)));
1567  }
1568  return *this;
1569  }
1570 
1571  template<typename _Tp, typename... _Args>
1572  _GLIBCXX20_CONSTEXPR
1573  enable_if_t<is_constructible_v<_Tp, _Args...> && __exactly_once<_Tp>,
1574  _Tp&>
1575  emplace(_Args&&... __args)
1576  {
1577  constexpr size_t __index = __index_of<_Tp>;
1578  return this->emplace<__index>(std::forward<_Args>(__args)...);
1579  }
1580 
1581  template<typename _Tp, typename _Up, typename... _Args>
1582  _GLIBCXX20_CONSTEXPR
1583  enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>
1584  && __exactly_once<_Tp>,
1585  _Tp&>
1586  emplace(initializer_list<_Up> __il, _Args&&... __args)
1587  {
1588  constexpr size_t __index = __index_of<_Tp>;
1589  return this->emplace<__index>(__il, std::forward<_Args>(__args)...);
1590  }
1591 
1592  template<size_t _Np, typename... _Args>
1593  _GLIBCXX20_CONSTEXPR
1594  enable_if_t<is_constructible_v<__to_type<_Np>, _Args...>,
1595  __to_type<_Np>&>
1596  emplace(_Args&&... __args)
1597  {
1598  namespace __variant = std::__detail::__variant;
1599  using type = typename _Nth_type<_Np, _Types...>::type;
1600  // Provide the strong exception-safety guarantee when possible,
1601  // to avoid becoming valueless.
1602  if constexpr (is_nothrow_constructible_v<type, _Args...>)
1603  {
1604  __variant::__emplace<_Np>(*this, std::forward<_Args>(__args)...);
1605  }
1606  else if constexpr (is_scalar_v<type>)
1607  {
1608  // This might invoke a potentially-throwing conversion operator:
1609  const type __tmp(std::forward<_Args>(__args)...);
1610  // But this won't throw:
1611  __variant::__emplace<_Np>(*this, __tmp);
1612  }
1613  else if constexpr (__variant::_Never_valueless_alt<type>()
1614  && _Traits::_S_move_assign)
1615  {
1616  // This construction might throw:
1617  variant __tmp(in_place_index<_Np>,
1618  std::forward<_Args>(__args)...);
1619  // But _Never_valueless_alt<type> means this won't:
1620  *this = std::move(__tmp);
1621  }
1622  else
1623  {
1624  // This case only provides the basic exception-safety guarantee,
1625  // i.e. the variant can become valueless.
1626  __variant::__emplace<_Np>(*this, std::forward<_Args>(__args)...);
1627  }
1628  return std::get<_Np>(*this);
1629  }
1630 
1631  template<size_t _Np, typename _Up, typename... _Args>
1632  _GLIBCXX20_CONSTEXPR
1633  enable_if_t<is_constructible_v<__to_type<_Np>,
1634  initializer_list<_Up>&, _Args...>,
1635  __to_type<_Np>&>
1636  emplace(initializer_list<_Up> __il, _Args&&... __args)
1637  {
1638  namespace __variant = std::__detail::__variant;
1639  using type = typename _Nth_type<_Np, _Types...>::type;
1640  // Provide the strong exception-safety guarantee when possible,
1641  // to avoid becoming valueless.
1642  if constexpr (is_nothrow_constructible_v<type,
1643  initializer_list<_Up>&,
1644  _Args...>)
1645  {
1646  __variant::__emplace<_Np>(*this, __il,
1647  std::forward<_Args>(__args)...);
1648  }
1649  else if constexpr (__variant::_Never_valueless_alt<type>()
1650  && _Traits::_S_move_assign)
1651  {
1652  // This construction might throw:
1653  variant __tmp(in_place_index<_Np>, __il,
1654  std::forward<_Args>(__args)...);
1655  // But _Never_valueless_alt<type> means this won't:
1656  *this = std::move(__tmp);
1657  }
1658  else
1659  {
1660  // This case only provides the basic exception-safety guarantee,
1661  // i.e. the variant can become valueless.
1662  __variant::__emplace<_Np>(*this, __il,
1663  std::forward<_Args>(__args)...);
1664  }
1665  return std::get<_Np>(*this);
1666  }
1667 
1668  template<size_t _Np, typename... _Args>
1669  enable_if_t<!(_Np < sizeof...(_Types))> emplace(_Args&&...) = delete;
1670 
1671  template<typename _Tp, typename... _Args>
1672  enable_if_t<!__exactly_once<_Tp>> emplace(_Args&&...) = delete;
1673 
1674  constexpr bool valueless_by_exception() const noexcept
1675  { return !this->_M_valid(); }
1676 
1677  constexpr size_t index() const noexcept
1678  {
1679  using __index_type = typename _Base::__index_type;
1680  if constexpr (__detail::__variant::__never_valueless<_Types...>())
1681  return this->_M_index;
1682  else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2)
1683  return make_signed_t<__index_type>(this->_M_index);
1684  else
1685  return size_t(__index_type(this->_M_index + 1)) - 1;
1686  }
1687 
1688  _GLIBCXX20_CONSTEXPR
1689  void
1690  swap(variant& __rhs)
1691  noexcept((__is_nothrow_swappable<_Types>::value && ...)
1692  && is_nothrow_move_constructible_v<variant>)
1693  {
1694  static_assert((is_move_constructible_v<_Types> && ...));
1695 
1696  // Handle this here to simplify the visitation.
1697  if (__rhs.valueless_by_exception()) [[__unlikely__]]
1698  {
1699  if (!this->valueless_by_exception()) [[__likely__]]
1700  __rhs.swap(*this);
1701  return;
1702  }
1703 
1704  namespace __variant = __detail::__variant;
1705 
1706  __variant::__raw_idx_visit(
1707  [this, &__rhs](auto&& __rhs_mem, auto __rhs_index) mutable
1708  {
1709  constexpr size_t __j = __rhs_index;
1710  if constexpr (__j != variant_npos)
1711  {
1712  if (this->index() == __j)
1713  {
1714  using std::swap;
1715  swap(std::get<__j>(*this), __rhs_mem);
1716  }
1717  else
1718  {
1719  auto __tmp(std::move(__rhs_mem));
1720 
1721  if constexpr (_Traits::_S_trivial_move_assign)
1722  __rhs = std::move(*this);
1723  else
1724  __variant::__raw_idx_visit(
1725  [&__rhs](auto&& __this_mem, auto __this_index) mutable
1726  {
1727  constexpr size_t __k = __this_index;
1728  if constexpr (__k != variant_npos)
1729  __variant::__emplace<__k>(__rhs,
1730  std::move(__this_mem));
1731  }, *this);
1732 
1733  __variant::__emplace<__j>(*this, std::move(__tmp));
1734  }
1735  }
1736  }, __rhs);
1737  }
1738 
1739 #if __cpp_lib_variant >= 202306L // >= C++26
1740  // [variant.visit], visitation
1741 
1742  /** Simple visitation for a single variant
1743  *
1744  * To visit a single variant you can use `var.visit(visitor)`
1745  * instead of `std::visit(visitor, var)`.
1746  *
1747  * @since C++26
1748  */
1749  template<int = 0, typename _Self, typename _Visitor>
1750  constexpr decltype(auto)
1751  visit(this _Self&& __self, _Visitor&& __vis)
1752  {
1753  using _CVar = __conditional_t<is_const_v<remove_reference_t<_Self>>,
1754  const variant, variant>;
1755  using _Var = __conditional_t<is_rvalue_reference_v<_Self&&>,
1756  _CVar&&, _CVar&>;
1757  return std::visit(std::forward<_Visitor>(__vis), (_Var)__self);
1758  }
1759 
1760  /** Simple visitation for a single variant, with explicit return type
1761  *
1762  * To visit a single variant you can use `var.visit<R>(visitor)`
1763  * instead of `std::visit<R>(visitor, var)`.
1764  *
1765  * @since C++26
1766  */
1767  template<typename _Res, typename _Self, typename _Visitor>
1768  constexpr _Res
1769  visit(this _Self&& __self, _Visitor&& __vis)
1770  {
1771  using _CVar = __conditional_t<is_const_v<remove_reference_t<_Self>>,
1772  const variant, variant>;
1773  using _Var = __conditional_t<is_rvalue_reference_v<_Self&&>,
1774  _CVar&&, _CVar&>;
1775  return std::visit<_Res>(std::forward<_Visitor>(__vis), (_Var)__self);
1776  }
1777 #endif
1778 
1779  private:
1780  template<size_t _Np, typename _Vp>
1781  friend constexpr decltype(auto)
1782  __detail::__variant::__get(_Vp&& __v) noexcept;
1783  };
1784 
1785  template<size_t _Np, typename... _Types>
1786  constexpr variant_alternative_t<_Np, variant<_Types...>>&
1787  get(variant<_Types...>& __v)
1788  {
1789  static_assert(_Np < sizeof...(_Types),
1790  "The index must be in [0, number of alternatives)");
1791  if (__v.index() != _Np)
1792  __throw_bad_variant_access(__v.valueless_by_exception());
1793  return __detail::__variant::__get<_Np>(__v);
1794  }
1795 
1796  template<size_t _Np, typename... _Types>
1797  constexpr variant_alternative_t<_Np, variant<_Types...>>&&
1798  get(variant<_Types...>&& __v)
1799  {
1800  static_assert(_Np < sizeof...(_Types),
1801  "The index must be in [0, number of alternatives)");
1802  if (__v.index() != _Np)
1803  __throw_bad_variant_access(__v.valueless_by_exception());
1804  return __detail::__variant::__get<_Np>(std::move(__v));
1805  }
1806 
1807  template<size_t _Np, typename... _Types>
1808  constexpr const variant_alternative_t<_Np, variant<_Types...>>&
1809  get(const variant<_Types...>& __v)
1810  {
1811  static_assert(_Np < sizeof...(_Types),
1812  "The index must be in [0, number of alternatives)");
1813  if (__v.index() != _Np)
1814  __throw_bad_variant_access(__v.valueless_by_exception());
1815  return __detail::__variant::__get<_Np>(__v);
1816  }
1817 
1818  template<size_t _Np, typename... _Types>
1819  constexpr const variant_alternative_t<_Np, variant<_Types...>>&&
1820  get(const variant<_Types...>&& __v)
1821  {
1822  static_assert(_Np < sizeof...(_Types),
1823  "The index must be in [0, number of alternatives)");
1824  if (__v.index() != _Np)
1825  __throw_bad_variant_access(__v.valueless_by_exception());
1826  return __detail::__variant::__get<_Np>(std::move(__v));
1827  }
1828 
1829  /// @cond undocumented
1830  template<typename _Result_type, typename _Visitor, typename... _Variants>
1831  constexpr decltype(auto)
1832  __do_visit(_Visitor&& __visitor, _Variants&&... __variants)
1833  {
1834  // Get the silly case of visiting no variants out of the way first.
1835  if constexpr (sizeof...(_Variants) == 0)
1836  {
1837  if constexpr (is_void_v<_Result_type>)
1838  return (void) std::forward<_Visitor>(__visitor)();
1839  else
1840  return std::forward<_Visitor>(__visitor)();
1841  }
1842  else
1843  {
1844  constexpr size_t __max = 11; // "These go to eleven."
1845 
1846  // The type of the first variant in the pack.
1847  using _V0 = typename _Nth_type<0, _Variants...>::type;
1848  // The number of alternatives in that first variant.
1849  constexpr auto __n = variant_size_v<remove_reference_t<_V0>>;
1850 
1851  if constexpr (sizeof...(_Variants) > 1 || __n > __max)
1852  {
1853  // Use a jump table for the general case.
1854  constexpr auto& __vtable = __detail::__variant::__gen_vtable<
1855  _Result_type, _Visitor&&, _Variants&&...>::_S_vtable;
1856 
1857  auto __func_ptr = __vtable._M_access(__variants.index()...);
1858  return (*__func_ptr)(std::forward<_Visitor>(__visitor),
1859  std::forward<_Variants>(__variants)...);
1860  }
1861  else // We have a single variant with a small number of alternatives.
1862  {
1863  // A name for the first variant in the pack.
1864  _V0& __v0
1865  = [](_V0& __v, ...) -> _V0& { return __v; }(__variants...);
1866 
1867  using __detail::__variant::_Multi_array;
1868  using __detail::__variant::__gen_vtable_impl;
1869  using _Ma = _Multi_array<_Result_type (*)(_Visitor&&, _V0&&)>;
1870 
1871 #ifdef _GLIBCXX_DEBUG
1872 # define _GLIBCXX_VISIT_UNREACHABLE __builtin_trap
1873 #else
1874 # define _GLIBCXX_VISIT_UNREACHABLE __builtin_unreachable
1875 #endif
1876 
1877 #define _GLIBCXX_VISIT_CASE(N) \
1878  case N: \
1879  { \
1880  if constexpr (N < __n) \
1881  { \
1882  return __gen_vtable_impl<_Ma, index_sequence<N>>:: \
1883  __visit_invoke(std::forward<_Visitor>(__visitor), \
1884  std::forward<_V0>(__v0)); \
1885  } \
1886  else _GLIBCXX_VISIT_UNREACHABLE(); \
1887  }
1888 
1889  switch (__v0.index())
1890  {
1891  _GLIBCXX_VISIT_CASE(0)
1892  _GLIBCXX_VISIT_CASE(1)
1893  _GLIBCXX_VISIT_CASE(2)
1894  _GLIBCXX_VISIT_CASE(3)
1895  _GLIBCXX_VISIT_CASE(4)
1896  _GLIBCXX_VISIT_CASE(5)
1897  _GLIBCXX_VISIT_CASE(6)
1898  _GLIBCXX_VISIT_CASE(7)
1899  _GLIBCXX_VISIT_CASE(8)
1900  _GLIBCXX_VISIT_CASE(9)
1901  _GLIBCXX_VISIT_CASE(10)
1902  case variant_npos:
1903  using __detail::__variant::__variant_idx_cookie;
1904  using __detail::__variant::__variant_cookie;
1905  if constexpr (is_same_v<_Result_type, __variant_idx_cookie>
1906  || is_same_v<_Result_type, __variant_cookie>)
1907  {
1908  using _Npos = index_sequence<variant_npos>;
1909  return __gen_vtable_impl<_Ma, _Npos>::
1910  __visit_invoke(std::forward<_Visitor>(__visitor),
1911  std::forward<_V0>(__v0));
1912  }
1913  else
1914  _GLIBCXX_VISIT_UNREACHABLE();
1915  default:
1916  _GLIBCXX_VISIT_UNREACHABLE();
1917  }
1918 #undef _GLIBCXX_VISIT_CASE
1919 #undef _GLIBCXX_VISIT_UNREACHABLE
1920  }
1921  }
1922  }
1923  /// @endcond
1924 
1925  template<typename _Visitor, typename... _Variants>
1926  constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...>
1927  visit(_Visitor&& __visitor, _Variants&&... __variants)
1928  {
1929  namespace __variant = std::__detail::__variant;
1930 
1931  if ((__variant::__as(__variants).valueless_by_exception() || ...))
1932  __throw_bad_variant_access(2);
1933 
1934  using _Result_type
1935  = __detail::__variant::__visit_result_t<_Visitor, _Variants...>;
1936 
1937  using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>;
1938 
1939  if constexpr (sizeof...(_Variants) == 1)
1940  {
1941  using _Vp = decltype(__variant::__as(std::declval<_Variants>()...));
1942 
1943  constexpr bool __visit_rettypes_match = __detail::__variant::
1944  __check_visitor_results<_Visitor, _Vp>(
1945  make_index_sequence<variant_size_v<remove_reference_t<_Vp>>>());
1946  if constexpr (!__visit_rettypes_match)
1947  {
1948  static_assert(__visit_rettypes_match,
1949  "std::visit requires the visitor to have the same "
1950  "return type for all alternatives of a variant");
1951  return;
1952  }
1953  else
1954  return std::__do_visit<_Tag>(
1955  std::forward<_Visitor>(__visitor),
1956  static_cast<_Vp>(__variants)...);
1957  }
1958  else
1959  return std::__do_visit<_Tag>(
1960  std::forward<_Visitor>(__visitor),
1961  __variant::__as(std::forward<_Variants>(__variants))...);
1962  }
1963 
1964 #if __cplusplus > 201703L
1965  template<typename _Res, typename _Visitor, typename... _Variants>
1966  constexpr _Res
1967  visit(_Visitor&& __visitor, _Variants&&... __variants)
1968  {
1969  namespace __variant = std::__detail::__variant;
1970 
1971  if ((__variant::__as(__variants).valueless_by_exception() || ...))
1972  __throw_bad_variant_access(3);
1973 
1974  return std::__do_visit<_Res>(std::forward<_Visitor>(__visitor),
1975  __variant::__as(std::forward<_Variants>(__variants))...);
1976  }
1977 #endif
1978 
1979  /// @cond undocumented
1980  template<typename... _Types>
1981  struct __variant_hash
1982  {
1983 #if __cplusplus < 202002L
1984  using result_type [[__deprecated__]] = size_t;
1985  using argument_type [[__deprecated__]] = variant<_Types...>;
1986 #endif
1987 
1988  size_t
1989  operator()(const variant<_Types...>& __t) const
1990  noexcept((is_nothrow_invocable_v<hash<decay_t<_Types>>, _Types> && ...))
1991  {
1992  size_t __ret;
1993  __detail::__variant::__raw_visit(
1994  [&__t, &__ret](auto&& __t_mem) mutable
1995  {
1996  using _Type = __remove_cvref_t<decltype(__t_mem)>;
1997  if constexpr (!is_same_v<_Type,
1998  __detail::__variant::__variant_cookie>)
1999  __ret = std::hash<size_t>{}(__t.index())
2000  + std::hash<_Type>{}(__t_mem);
2001  else
2002  __ret = std::hash<size_t>{}(__t.index());
2003  }, __t);
2004  return __ret;
2005  }
2006  };
2007  /// @endcond
2008 
2009  template<typename... _Types>
2010  struct hash<variant<_Types...>>
2011  : __conditional_t<(__is_hash_enabled_for<remove_const_t<_Types>> && ...),
2012  __variant_hash<_Types...>,
2013  __hash_not_enabled<variant<_Types...>>>
2014 #if ! _GLIBCXX_INLINE_VERSION
2015  , __detail::__variant::_Variant_hash_base<variant<_Types...>,
2016  index_sequence_for<_Types...>>
2017 #endif
2018  { };
2019 
2020  template<typename... _Types>
2021  struct __is_fast_hash<hash<variant<_Types...>>>
2022  : bool_constant<(__is_fast_hash<_Types>::value && ...)>
2023  { };
2024 
2025 _GLIBCXX_END_NAMESPACE_VERSION
2026 } // namespace std
2027 
2028 #endif // __cpp_lib_variant
2029 #endif // _GLIBCXX_VARIANT