libstdc++
variant
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1 // <variant> -*- C++ -*-
2 
3 // Copyright (C) 2016-2020 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 #pragma GCC system_header
33 
34 #if __cplusplus >= 201703L
35 
36 #include <type_traits>
37 #include <utility>
39 #include <bits/functexcept.h>
40 #include <bits/move.h>
41 #include <bits/functional_hash.h>
42 #include <bits/invoke.h>
43 #include <ext/aligned_buffer.h>
44 #include <bits/parse_numbers.h>
47 #include <bits/stl_construct.h>
48 #if __cplusplus > 201703L
49 # include <compare>
50 #endif
51 
52 namespace std _GLIBCXX_VISIBILITY(default)
53 {
54 _GLIBCXX_BEGIN_NAMESPACE_VERSION
55 
56 namespace __detail
57 {
58 namespace __variant
59 {
60  template<size_t _Np, typename... _Types>
61  struct _Nth_type;
62 
63  template<size_t _Np, typename _First, typename... _Rest>
64  struct _Nth_type<_Np, _First, _Rest...>
65  : _Nth_type<_Np-1, _Rest...> { };
66 
67  template<typename _First, typename... _Rest>
68  struct _Nth_type<0, _First, _Rest...>
69  { using type = _First; };
70 
71 } // namespace __variant
72 } // namespace __detail
73 
74 #define __cpp_lib_variant 201606L
75 
76  template<typename... _Types> class tuple;
77  template<typename... _Types> class variant;
78  template <typename> struct hash;
79 
80  template<typename _Variant>
81  struct variant_size;
82 
83  template<typename _Variant>
84  struct variant_size<const _Variant> : variant_size<_Variant> {};
85 
86  template<typename _Variant>
87  struct variant_size<volatile _Variant> : variant_size<_Variant> {};
88 
89  template<typename _Variant>
90  struct variant_size<const volatile _Variant> : variant_size<_Variant> {};
91 
92  template<typename... _Types>
93  struct variant_size<variant<_Types...>>
94  : std::integral_constant<size_t, sizeof...(_Types)> {};
95 
96  template<typename _Variant>
97  inline constexpr size_t variant_size_v = variant_size<_Variant>::value;
98 
99  template<size_t _Np, typename _Variant>
100  struct variant_alternative;
101 
102  template<size_t _Np, typename _First, typename... _Rest>
103  struct variant_alternative<_Np, variant<_First, _Rest...>>
104  : variant_alternative<_Np-1, variant<_Rest...>> {};
105 
106  template<typename _First, typename... _Rest>
107  struct variant_alternative<0, variant<_First, _Rest...>>
108  { using type = _First; };
109 
110  template<size_t _Np, typename _Variant>
111  using variant_alternative_t =
112  typename variant_alternative<_Np, _Variant>::type;
113 
114  template<size_t _Np, typename _Variant>
115  struct variant_alternative<_Np, const _Variant>
116  { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
117 
118  template<size_t _Np, typename _Variant>
119  struct variant_alternative<_Np, volatile _Variant>
120  { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
121 
122  template<size_t _Np, typename _Variant>
123  struct variant_alternative<_Np, const volatile _Variant>
124  { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
125 
126  inline constexpr size_t variant_npos = -1;
127 
128  template<size_t _Np, typename... _Types>
129  constexpr variant_alternative_t<_Np, variant<_Types...>>&
130  get(variant<_Types...>&);
131 
132  template<size_t _Np, typename... _Types>
133  constexpr variant_alternative_t<_Np, variant<_Types...>>&&
134  get(variant<_Types...>&&);
135 
136  template<size_t _Np, typename... _Types>
137  constexpr variant_alternative_t<_Np, variant<_Types...>> const&
138  get(const variant<_Types...>&);
139 
140  template<size_t _Np, typename... _Types>
141  constexpr variant_alternative_t<_Np, variant<_Types...>> const&&
142  get(const variant<_Types...>&&);
143 
144  template<typename _Result_type, typename _Visitor, typename... _Variants>
145  constexpr decltype(auto)
146  __do_visit(_Visitor&& __visitor, _Variants&&... __variants);
147 
148  template <typename... _Types, typename _Tp>
149  decltype(auto)
150  __variant_cast(_Tp&& __rhs)
151  {
152  if constexpr (is_lvalue_reference_v<_Tp>)
153  {
154  if constexpr (is_const_v<remove_reference_t<_Tp>>)
155  return static_cast<const variant<_Types...>&>(__rhs);
156  else
157  return static_cast<variant<_Types...>&>(__rhs);
158  }
159  else
160  return static_cast<variant<_Types...>&&>(__rhs);
161  }
162 
163 namespace __detail
164 {
165 namespace __variant
166 {
167  // Returns the first appearence of _Tp in _Types.
168  // Returns sizeof...(_Types) if _Tp is not in _Types.
169  template<typename _Tp, typename... _Types>
170  struct __index_of : std::integral_constant<size_t, 0> {};
171 
172  template<typename _Tp, typename... _Types>
173  inline constexpr size_t __index_of_v = __index_of<_Tp, _Types...>::value;
174 
175  template<typename _Tp, typename _First, typename... _Rest>
176  struct __index_of<_Tp, _First, _Rest...> :
177  std::integral_constant<size_t, is_same_v<_Tp, _First>
178  ? 0 : __index_of_v<_Tp, _Rest...> + 1> {};
179 
180  // used for raw visitation
181  struct __variant_cookie {};
182  // used for raw visitation with indices passed in
183  struct __variant_idx_cookie { using type = __variant_idx_cookie; };
184  // Used to enable deduction (and same-type checking) for std::visit:
185  template<typename> struct __deduce_visit_result { };
186 
187  // Visit variants that might be valueless.
188  template<typename _Visitor, typename... _Variants>
189  constexpr void
190  __raw_visit(_Visitor&& __visitor, _Variants&&... __variants)
191  {
192  std::__do_visit<__variant_cookie>(std::forward<_Visitor>(__visitor),
193  std::forward<_Variants>(__variants)...);
194  }
195 
196  // Visit variants that might be valueless, passing indices to the visitor.
197  template<typename _Visitor, typename... _Variants>
198  constexpr void
199  __raw_idx_visit(_Visitor&& __visitor, _Variants&&... __variants)
200  {
201  std::__do_visit<__variant_idx_cookie>(std::forward<_Visitor>(__visitor),
202  std::forward<_Variants>(__variants)...);
203  }
204 
205  // _Uninitialized<T> is guaranteed to be a trivially destructible type,
206  // even if T is not.
207  template<typename _Type, bool = std::is_trivially_destructible_v<_Type>>
208  struct _Uninitialized;
209 
210  template<typename _Type>
211  struct _Uninitialized<_Type, true>
212  {
213  template<typename... _Args>
214  constexpr
215  _Uninitialized(in_place_index_t<0>, _Args&&... __args)
216  : _M_storage(std::forward<_Args>(__args)...)
217  { }
218 
219  constexpr const _Type& _M_get() const & noexcept
220  { return _M_storage; }
221 
222  constexpr _Type& _M_get() & noexcept
223  { return _M_storage; }
224 
225  constexpr const _Type&& _M_get() const && noexcept
226  { return std::move(_M_storage); }
227 
228  constexpr _Type&& _M_get() && noexcept
229  { return std::move(_M_storage); }
230 
231  _Type _M_storage;
232  };
233 
234  template<typename _Type>
235  struct _Uninitialized<_Type, false>
236  {
237  template<typename... _Args>
238  constexpr
239  _Uninitialized(in_place_index_t<0>, _Args&&... __args)
240  {
241  ::new ((void*)std::addressof(_M_storage))
242  _Type(std::forward<_Args>(__args)...);
243  }
244 
245  const _Type& _M_get() const & noexcept
246  { return *_M_storage._M_ptr(); }
247 
248  _Type& _M_get() & noexcept
249  { return *_M_storage._M_ptr(); }
250 
251  const _Type&& _M_get() const && noexcept
252  { return std::move(*_M_storage._M_ptr()); }
253 
254  _Type&& _M_get() && noexcept
255  { return std::move(*_M_storage._M_ptr()); }
256 
257  __gnu_cxx::__aligned_membuf<_Type> _M_storage;
258  };
259 
260  template<typename _Union>
261  constexpr decltype(auto)
262  __get(in_place_index_t<0>, _Union&& __u) noexcept
263  { return std::forward<_Union>(__u)._M_first._M_get(); }
264 
265  template<size_t _Np, typename _Union>
266  constexpr decltype(auto)
267  __get(in_place_index_t<_Np>, _Union&& __u) noexcept
268  {
269  return __variant::__get(in_place_index<_Np-1>,
270  std::forward<_Union>(__u)._M_rest);
271  }
272 
273  // Returns the typed storage for __v.
274  template<size_t _Np, typename _Variant>
275  constexpr decltype(auto)
276  __get(_Variant&& __v) noexcept
277  {
278  return __variant::__get(std::in_place_index<_Np>,
279  std::forward<_Variant>(__v)._M_u);
280  }
281 
282  template<typename... _Types>
283  struct _Traits
284  {
285  static constexpr bool _S_default_ctor =
286  is_default_constructible_v<typename _Nth_type<0, _Types...>::type>;
287  static constexpr bool _S_copy_ctor =
288  (is_copy_constructible_v<_Types> && ...);
289  static constexpr bool _S_move_ctor =
290  (is_move_constructible_v<_Types> && ...);
291  static constexpr bool _S_copy_assign =
292  _S_copy_ctor
293  && (is_copy_assignable_v<_Types> && ...);
294  static constexpr bool _S_move_assign =
295  _S_move_ctor
296  && (is_move_assignable_v<_Types> && ...);
297 
298  static constexpr bool _S_trivial_dtor =
299  (is_trivially_destructible_v<_Types> && ...);
300  static constexpr bool _S_trivial_copy_ctor =
301  (is_trivially_copy_constructible_v<_Types> && ...);
302  static constexpr bool _S_trivial_move_ctor =
303  (is_trivially_move_constructible_v<_Types> && ...);
304  static constexpr bool _S_trivial_copy_assign =
305  _S_trivial_dtor && _S_trivial_copy_ctor
306  && (is_trivially_copy_assignable_v<_Types> && ...);
307  static constexpr bool _S_trivial_move_assign =
308  _S_trivial_dtor && _S_trivial_move_ctor
309  && (is_trivially_move_assignable_v<_Types> && ...);
310 
311  // The following nothrow traits are for non-trivial SMFs. Trivial SMFs
312  // are always nothrow.
313  static constexpr bool _S_nothrow_default_ctor =
314  is_nothrow_default_constructible_v<
315  typename _Nth_type<0, _Types...>::type>;
316  static constexpr bool _S_nothrow_copy_ctor = false;
317  static constexpr bool _S_nothrow_move_ctor =
318  (is_nothrow_move_constructible_v<_Types> && ...);
319  static constexpr bool _S_nothrow_copy_assign = false;
320  static constexpr bool _S_nothrow_move_assign =
321  _S_nothrow_move_ctor
322  && (is_nothrow_move_assignable_v<_Types> && ...);
323  };
324 
325  // Defines members and ctors.
326  template<typename... _Types>
327  union _Variadic_union { };
328 
329  template<typename _First, typename... _Rest>
330  union _Variadic_union<_First, _Rest...>
331  {
332  constexpr _Variadic_union() : _M_rest() { }
333 
334  template<typename... _Args>
335  constexpr _Variadic_union(in_place_index_t<0>, _Args&&... __args)
336  : _M_first(in_place_index<0>, std::forward<_Args>(__args)...)
337  { }
338 
339  template<size_t _Np, typename... _Args>
340  constexpr _Variadic_union(in_place_index_t<_Np>, _Args&&... __args)
341  : _M_rest(in_place_index<_Np-1>, std::forward<_Args>(__args)...)
342  { }
343 
344  _Uninitialized<_First> _M_first;
345  _Variadic_union<_Rest...> _M_rest;
346  };
347 
348  // _Never_valueless_alt is true for variant alternatives that can
349  // always be placed in a variant without it becoming valueless.
350 
351  // For suitably-small, trivially copyable types we can create temporaries
352  // on the stack and then memcpy them into place.
353  template<typename _Tp>
354  struct _Never_valueless_alt
355  : __and_<bool_constant<sizeof(_Tp) <= 256>, is_trivially_copyable<_Tp>>
356  { };
357 
358  // Specialize _Never_valueless_alt for other types which have a
359  // non-throwing and cheap move construction and move assignment operator,
360  // so that emplacing the type will provide the strong exception-safety
361  // guarantee, by creating and moving a temporary.
362  // Whether _Never_valueless_alt<T> is true or not affects the ABI of a
363  // variant using that alternative, so we can't change the value later!
364 
365  // True if every alternative in _Types... can be emplaced in a variant
366  // without it becoming valueless. If this is true, variant<_Types...>
367  // can never be valueless, which enables some minor optimizations.
368  template <typename... _Types>
369  constexpr bool __never_valueless()
370  {
371  return _Traits<_Types...>::_S_move_assign
372  && (_Never_valueless_alt<_Types>::value && ...);
373  }
374 
375  // Defines index and the dtor, possibly trivial.
376  template<bool __trivially_destructible, typename... _Types>
377  struct _Variant_storage;
378 
379  template <typename... _Types>
380  using __select_index =
381  typename __select_int::_Select_int_base<sizeof...(_Types),
382  unsigned char,
383  unsigned short>::type::value_type;
384 
385  template<typename... _Types>
386  struct _Variant_storage<false, _Types...>
387  {
388  constexpr
389  _Variant_storage()
390  : _M_index(static_cast<__index_type>(variant_npos))
391  { }
392 
393  template<size_t _Np, typename... _Args>
394  constexpr
395  _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
396  : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
397  _M_index{_Np}
398  { }
399 
400  void _M_reset()
401  {
402  if (!_M_valid()) [[unlikely]]
403  return;
404 
405  std::__do_visit<void>([](auto&& __this_mem) mutable
406  {
407  std::_Destroy(std::__addressof(__this_mem));
408  }, __variant_cast<_Types...>(*this));
409 
410  _M_index = static_cast<__index_type>(variant_npos);
411  }
412 
413  ~_Variant_storage()
414  { _M_reset(); }
415 
416  void*
417  _M_storage() const noexcept
418  {
419  return const_cast<void*>(static_cast<const void*>(
420  std::addressof(_M_u)));
421  }
422 
423  constexpr bool
424  _M_valid() const noexcept
425  {
426  if constexpr (__variant::__never_valueless<_Types...>())
427  return true;
428  return this->_M_index != __index_type(variant_npos);
429  }
430 
431  _Variadic_union<_Types...> _M_u;
432  using __index_type = __select_index<_Types...>;
433  __index_type _M_index;
434  };
435 
436  template<typename... _Types>
437  struct _Variant_storage<true, _Types...>
438  {
439  constexpr
440  _Variant_storage()
441  : _M_index(static_cast<__index_type>(variant_npos))
442  { }
443 
444  template<size_t _Np, typename... _Args>
445  constexpr
446  _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
447  : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
448  _M_index{_Np}
449  { }
450 
451  void _M_reset() noexcept
452  { _M_index = static_cast<__index_type>(variant_npos); }
453 
454  void*
455  _M_storage() const noexcept
456  {
457  return const_cast<void*>(static_cast<const void*>(
458  std::addressof(_M_u)));
459  }
460 
461  constexpr bool
462  _M_valid() const noexcept
463  {
464  if constexpr (__variant::__never_valueless<_Types...>())
465  return true;
466  return this->_M_index != static_cast<__index_type>(variant_npos);
467  }
468 
469  _Variadic_union<_Types...> _M_u;
470  using __index_type = __select_index<_Types...>;
471  __index_type _M_index;
472  };
473 
474  template<typename... _Types>
475  using _Variant_storage_alias =
476  _Variant_storage<_Traits<_Types...>::_S_trivial_dtor, _Types...>;
477 
478  template<typename _Tp, typename _Up>
479  void __variant_construct_single(_Tp&& __lhs, _Up&& __rhs_mem)
480  {
481  void* __storage = std::addressof(__lhs._M_u);
482  using _Type = remove_reference_t<decltype(__rhs_mem)>;
483  if constexpr (!is_same_v<_Type, __variant_cookie>)
484  ::new (__storage)
485  _Type(std::forward<decltype(__rhs_mem)>(__rhs_mem));
486  }
487 
488  template<typename... _Types, typename _Tp, typename _Up>
489  void __variant_construct(_Tp&& __lhs, _Up&& __rhs)
490  {
491  __lhs._M_index = __rhs._M_index;
492  __variant::__raw_visit([&__lhs](auto&& __rhs_mem) mutable
493  {
494  __variant_construct_single(std::forward<_Tp>(__lhs),
495  std::forward<decltype(__rhs_mem)>(__rhs_mem));
496  }, __variant_cast<_Types...>(std::forward<_Up>(__rhs)));
497  }
498 
499  // The following are (Copy|Move) (ctor|assign) layers for forwarding
500  // triviality and handling non-trivial SMF behaviors.
501 
502  template<bool, typename... _Types>
503  struct _Copy_ctor_base : _Variant_storage_alias<_Types...>
504  {
505  using _Base = _Variant_storage_alias<_Types...>;
506  using _Base::_Base;
507 
508  _Copy_ctor_base(const _Copy_ctor_base& __rhs)
509  noexcept(_Traits<_Types...>::_S_nothrow_copy_ctor)
510  {
511  __variant_construct<_Types...>(*this, __rhs);
512  }
513 
514  _Copy_ctor_base(_Copy_ctor_base&&) = default;
515  _Copy_ctor_base& operator=(const _Copy_ctor_base&) = default;
516  _Copy_ctor_base& operator=(_Copy_ctor_base&&) = default;
517  };
518 
519  template<typename... _Types>
520  struct _Copy_ctor_base<true, _Types...> : _Variant_storage_alias<_Types...>
521  {
522  using _Base = _Variant_storage_alias<_Types...>;
523  using _Base::_Base;
524  };
525 
526  template<typename... _Types>
527  using _Copy_ctor_alias =
528  _Copy_ctor_base<_Traits<_Types...>::_S_trivial_copy_ctor, _Types...>;
529 
530  template<bool, typename... _Types>
531  struct _Move_ctor_base : _Copy_ctor_alias<_Types...>
532  {
533  using _Base = _Copy_ctor_alias<_Types...>;
534  using _Base::_Base;
535 
536  _Move_ctor_base(_Move_ctor_base&& __rhs)
537  noexcept(_Traits<_Types...>::_S_nothrow_move_ctor)
538  {
539  __variant_construct<_Types...>(*this, std::move(__rhs));
540  }
541 
542  template<typename _Up>
543  void _M_destructive_move(unsigned short __rhs_index, _Up&& __rhs)
544  {
545  this->_M_reset();
546  __variant_construct_single(*this, std::forward<_Up>(__rhs));
547  this->_M_index = __rhs_index;
548  }
549 
550  template<typename _Up>
551  void _M_destructive_copy(unsigned short __rhs_index, const _Up& __rhs)
552  {
553  this->_M_reset();
554  __variant_construct_single(*this, __rhs);
555  this->_M_index = __rhs_index;
556  }
557 
558  _Move_ctor_base(const _Move_ctor_base&) = default;
559  _Move_ctor_base& operator=(const _Move_ctor_base&) = default;
560  _Move_ctor_base& operator=(_Move_ctor_base&&) = default;
561  };
562 
563  template<typename... _Types>
564  struct _Move_ctor_base<true, _Types...> : _Copy_ctor_alias<_Types...>
565  {
566  using _Base = _Copy_ctor_alias<_Types...>;
567  using _Base::_Base;
568 
569  template<typename _Up>
570  void _M_destructive_move(unsigned short __rhs_index, _Up&& __rhs)
571  {
572  this->_M_reset();
573  __variant_construct_single(*this, std::forward<_Up>(__rhs));
574  this->_M_index = __rhs_index;
575  }
576 
577  template<typename _Up>
578  void _M_destructive_copy(unsigned short __rhs_index, const _Up& __rhs)
579  {
580  this->_M_reset();
581  __variant_construct_single(*this, __rhs);
582  this->_M_index = __rhs_index;
583  }
584  };
585 
586  template<typename... _Types>
587  using _Move_ctor_alias =
588  _Move_ctor_base<_Traits<_Types...>::_S_trivial_move_ctor, _Types...>;
589 
590  template<bool, typename... _Types>
591  struct _Copy_assign_base : _Move_ctor_alias<_Types...>
592  {
593  using _Base = _Move_ctor_alias<_Types...>;
594  using _Base::_Base;
595 
596  _Copy_assign_base&
597  operator=(const _Copy_assign_base& __rhs)
598  noexcept(_Traits<_Types...>::_S_nothrow_copy_assign)
599  {
600  __variant::__raw_idx_visit(
601  [this](auto&& __rhs_mem, auto __rhs_index) mutable
602  {
603  if constexpr (__rhs_index != variant_npos)
604  {
605  if (this->_M_index == __rhs_index)
606  __variant::__get<__rhs_index>(*this) = __rhs_mem;
607  else
608  {
609  using __rhs_type = __remove_cvref_t<decltype(__rhs_mem)>;
610  if constexpr (is_nothrow_copy_constructible_v<__rhs_type>
611  || !is_nothrow_move_constructible_v<__rhs_type>)
612  // The standard says this->emplace<__rhs_type>(__rhs_mem)
613  // should be used here, but _M_destructive_copy is
614  // equivalent in this case. Either copy construction
615  // doesn't throw, so _M_destructive_copy gives strong
616  // exception safety guarantee, or both copy construction
617  // and move construction can throw, so emplace only gives
618  // basic exception safety anyway.
619  this->_M_destructive_copy(__rhs_index, __rhs_mem);
620  else
621  __variant_cast<_Types...>(*this)
622  = variant<_Types...>(std::in_place_index<__rhs_index>,
623  __rhs_mem);
624  }
625  }
626  else
627  this->_M_reset();
628  }, __variant_cast<_Types...>(__rhs));
629  return *this;
630  }
631 
632  _Copy_assign_base(const _Copy_assign_base&) = default;
633  _Copy_assign_base(_Copy_assign_base&&) = default;
634  _Copy_assign_base& operator=(_Copy_assign_base&&) = default;
635  };
636 
637  template<typename... _Types>
638  struct _Copy_assign_base<true, _Types...> : _Move_ctor_alias<_Types...>
639  {
640  using _Base = _Move_ctor_alias<_Types...>;
641  using _Base::_Base;
642  };
643 
644  template<typename... _Types>
645  using _Copy_assign_alias =
646  _Copy_assign_base<_Traits<_Types...>::_S_trivial_copy_assign, _Types...>;
647 
648  template<bool, typename... _Types>
649  struct _Move_assign_base : _Copy_assign_alias<_Types...>
650  {
651  using _Base = _Copy_assign_alias<_Types...>;
652  using _Base::_Base;
653 
654  _Move_assign_base&
655  operator=(_Move_assign_base&& __rhs)
656  noexcept(_Traits<_Types...>::_S_nothrow_move_assign)
657  {
658  __variant::__raw_idx_visit(
659  [this](auto&& __rhs_mem, auto __rhs_index) mutable
660  {
661  if constexpr (__rhs_index != variant_npos)
662  {
663  if (this->_M_index == __rhs_index)
664  __variant::__get<__rhs_index>(*this) = std::move(__rhs_mem);
665  else
666  __variant_cast<_Types...>(*this)
667  .template emplace<__rhs_index>(std::move(__rhs_mem));
668  }
669  else
670  this->_M_reset();
671  }, __variant_cast<_Types...>(__rhs));
672  return *this;
673  }
674 
675  _Move_assign_base(const _Move_assign_base&) = default;
676  _Move_assign_base(_Move_assign_base&&) = default;
677  _Move_assign_base& operator=(const _Move_assign_base&) = default;
678  };
679 
680  template<typename... _Types>
681  struct _Move_assign_base<true, _Types...> : _Copy_assign_alias<_Types...>
682  {
683  using _Base = _Copy_assign_alias<_Types...>;
684  using _Base::_Base;
685  };
686 
687  template<typename... _Types>
688  using _Move_assign_alias =
689  _Move_assign_base<_Traits<_Types...>::_S_trivial_move_assign, _Types...>;
690 
691  template<typename... _Types>
692  struct _Variant_base : _Move_assign_alias<_Types...>
693  {
694  using _Base = _Move_assign_alias<_Types...>;
695 
696  constexpr
697  _Variant_base()
698  noexcept(_Traits<_Types...>::_S_nothrow_default_ctor)
699  : _Variant_base(in_place_index<0>) { }
700 
701  template<size_t _Np, typename... _Args>
702  constexpr explicit
703  _Variant_base(in_place_index_t<_Np> __i, _Args&&... __args)
704  : _Base(__i, std::forward<_Args>(__args)...)
705  { }
706 
707  _Variant_base(const _Variant_base&) = default;
708  _Variant_base(_Variant_base&&) = default;
709  _Variant_base& operator=(const _Variant_base&) = default;
710  _Variant_base& operator=(_Variant_base&&) = default;
711  };
712 
713  // For how many times does _Tp appear in _Tuple?
714  template<typename _Tp, typename _Tuple>
715  struct __tuple_count;
716 
717  template<typename _Tp, typename _Tuple>
718  inline constexpr size_t __tuple_count_v =
719  __tuple_count<_Tp, _Tuple>::value;
720 
721  template<typename _Tp, typename... _Types>
722  struct __tuple_count<_Tp, tuple<_Types...>>
723  : integral_constant<size_t, 0> { };
724 
725  template<typename _Tp, typename _First, typename... _Rest>
726  struct __tuple_count<_Tp, tuple<_First, _Rest...>>
727  : integral_constant<
728  size_t,
729  __tuple_count_v<_Tp, tuple<_Rest...>> + is_same_v<_Tp, _First>> { };
730 
731  // TODO: Reuse this in <tuple> ?
732  template<typename _Tp, typename... _Types>
733  inline constexpr bool __exactly_once =
734  __tuple_count_v<_Tp, tuple<_Types...>> == 1;
735 
736  // Helper used to check for valid conversions that don't involve narrowing.
737  template<typename _Ti> struct _Arr { _Ti _M_x[1]; };
738 
739  // Build an imaginary function FUN(Ti) for each alternative type Ti
740  template<size_t _Ind, typename _Tp, typename _Ti,
741  bool _Ti_is_cv_bool = is_same_v<remove_cv_t<_Ti>, bool>,
742  typename = void>
743  struct _Build_FUN
744  {
745  // This function means 'using _Build_FUN<I, T, Ti>::_S_fun;' is valid,
746  // but only static functions will be considered in the call below.
747  void _S_fun();
748  };
749 
750  // ... for which Ti x[] = {std::forward<T>(t)}; is well-formed,
751  template<size_t _Ind, typename _Tp, typename _Ti>
752  struct _Build_FUN<_Ind, _Tp, _Ti, false,
753  void_t<decltype(_Arr<_Ti>{{std::declval<_Tp>()}})>>
754  {
755  // This is the FUN function for type _Ti, with index _Ind
756  static integral_constant<size_t, _Ind> _S_fun(_Ti);
757  };
758 
759  // ... and if Ti is cv bool, remove_cvref_t<T> is bool.
760  template<size_t _Ind, typename _Tp, typename _Ti>
761  struct _Build_FUN<_Ind, _Tp, _Ti, true,
762  enable_if_t<is_same_v<__remove_cvref_t<_Tp>, bool>>>
763  {
764  // This is the FUN function for when _Ti is cv bool, with index _Ind
765  static integral_constant<size_t, _Ind> _S_fun(_Ti);
766  };
767 
768  template<typename _Tp, typename _Variant,
769  typename = make_index_sequence<variant_size_v<_Variant>>>
770  struct _Build_FUNs;
771 
772  template<typename _Tp, typename... _Ti, size_t... _Ind>
773  struct _Build_FUNs<_Tp, variant<_Ti...>, index_sequence<_Ind...>>
774  : _Build_FUN<_Ind, _Tp, _Ti>...
775  {
776  using _Build_FUN<_Ind, _Tp, _Ti>::_S_fun...;
777  };
778 
779  // The index j of the overload FUN(Tj) selected by overload resolution
780  // for FUN(std::forward<_Tp>(t))
781  template<typename _Tp, typename _Variant>
782  using _FUN_type
783  = decltype(_Build_FUNs<_Tp, _Variant>::_S_fun(std::declval<_Tp>()));
784 
785  // The index selected for FUN(std::forward<T>(t)), or variant_npos if none.
786  template<typename _Tp, typename _Variant, typename = void>
787  struct __accepted_index
788  : integral_constant<size_t, variant_npos>
789  { };
790 
791  template<typename _Tp, typename _Variant>
792  struct __accepted_index<_Tp, _Variant, void_t<_FUN_type<_Tp, _Variant>>>
793  : _FUN_type<_Tp, _Variant>
794  { };
795 
796  // Returns the raw storage for __v.
797  template<typename _Variant>
798  void* __get_storage(_Variant&& __v) noexcept
799  { return __v._M_storage(); }
800 
801  template <typename _Maybe_variant_cookie, typename _Variant>
802  struct _Extra_visit_slot_needed
803  {
804  template <typename> struct _Variant_never_valueless;
805 
806  template <typename... _Types>
807  struct _Variant_never_valueless<variant<_Types...>>
808  : bool_constant<__variant::__never_valueless<_Types...>()> {};
809 
810  static constexpr bool value =
811  (is_same_v<_Maybe_variant_cookie, __variant_cookie>
812  || is_same_v<_Maybe_variant_cookie, __variant_idx_cookie>)
813  && !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
814  };
815 
816  // Used for storing a multi-dimensional vtable.
817  template<typename _Tp, size_t... _Dimensions>
818  struct _Multi_array;
819 
820  // Partial specialization with rank zero, stores a single _Tp element.
821  template<typename _Tp>
822  struct _Multi_array<_Tp>
823  {
824  template<typename>
825  struct __untag_result
826  : false_type
827  { using element_type = _Tp; };
828 
829  template <typename... _Args>
830  struct __untag_result<const void(*)(_Args...)>
831  : false_type
832  { using element_type = void(*)(_Args...); };
833 
834  template <typename... _Args>
835  struct __untag_result<__variant_cookie(*)(_Args...)>
836  : false_type
837  { using element_type = void(*)(_Args...); };
838 
839  template <typename... _Args>
840  struct __untag_result<__variant_idx_cookie(*)(_Args...)>
841  : false_type
842  { using element_type = void(*)(_Args...); };
843 
844  template <typename _Res, typename... _Args>
845  struct __untag_result<__deduce_visit_result<_Res>(*)(_Args...)>
846  : true_type
847  { using element_type = _Res(*)(_Args...); };
848 
849  using __result_is_deduced = __untag_result<_Tp>;
850 
851  constexpr const typename __untag_result<_Tp>::element_type&
852  _M_access() const
853  { return _M_data; }
854 
855  typename __untag_result<_Tp>::element_type _M_data;
856  };
857 
858  // Partial specialization with rank >= 1.
859  template<typename _Ret,
860  typename _Visitor,
861  typename... _Variants,
862  size_t __first, size_t... __rest>
863  struct _Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>
864  {
865  static constexpr size_t __index =
866  sizeof...(_Variants) - sizeof...(__rest) - 1;
867 
868  using _Variant = typename _Nth_type<__index, _Variants...>::type;
869 
870  static constexpr int __do_cookie =
871  _Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
872 
873  using _Tp = _Ret(*)(_Visitor, _Variants...);
874 
875  template<typename... _Args>
876  constexpr decltype(auto)
877  _M_access(size_t __first_index, _Args... __rest_indices) const
878  {
879  return _M_arr[__first_index + __do_cookie]
880  ._M_access(__rest_indices...);
881  }
882 
883  _Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
884  };
885 
886  // Creates a multi-dimensional vtable recursively.
887  //
888  // For example,
889  // visit([](auto, auto){},
890  // variant<int, char>(), // typedef'ed as V1
891  // variant<float, double, long double>()) // typedef'ed as V2
892  // will trigger instantiations of:
893  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 2, 3>,
894  // tuple<V1&&, V2&&>, std::index_sequence<>>
895  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
896  // tuple<V1&&, V2&&>, std::index_sequence<0>>
897  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
898  // tuple<V1&&, V2&&>, std::index_sequence<0, 0>>
899  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
900  // tuple<V1&&, V2&&>, std::index_sequence<0, 1>>
901  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
902  // tuple<V1&&, V2&&>, std::index_sequence<0, 2>>
903  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
904  // tuple<V1&&, V2&&>, std::index_sequence<1>>
905  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
906  // tuple<V1&&, V2&&>, std::index_sequence<1, 0>>
907  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
908  // tuple<V1&&, V2&&>, std::index_sequence<1, 1>>
909  // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
910  // tuple<V1&&, V2&&>, std::index_sequence<1, 2>>
911  // The returned multi-dimensional vtable can be fast accessed by the visitor
912  // using index calculation.
913  template<typename _Array_type, typename _Index_seq>
914  struct __gen_vtable_impl;
915 
916  // Defines the _S_apply() member that returns a _Multi_array populated
917  // with function pointers that perform the visitation expressions e(m)
918  // for each valid pack of indexes into the variant types _Variants.
919  //
920  // This partial specialization builds up the index sequences by recursively
921  // calling _S_apply() on the next specialization of __gen_vtable_impl.
922  // The base case of the recursion defines the actual function pointers.
923  template<typename _Result_type, typename _Visitor, size_t... __dimensions,
924  typename... _Variants, size_t... __indices>
925  struct __gen_vtable_impl<
926  _Multi_array<_Result_type (*)(_Visitor, _Variants...), __dimensions...>,
927  std::index_sequence<__indices...>>
928  {
929  using _Next =
930  remove_reference_t<typename _Nth_type<sizeof...(__indices),
931  _Variants...>::type>;
932  using _Array_type =
933  _Multi_array<_Result_type (*)(_Visitor, _Variants...),
934  __dimensions...>;
935 
936  static constexpr _Array_type
937  _S_apply()
938  {
939  _Array_type __vtable{};
940  _S_apply_all_alts(
941  __vtable, make_index_sequence<variant_size_v<_Next>>());
942  return __vtable;
943  }
944 
945  template<size_t... __var_indices>
946  static constexpr void
947  _S_apply_all_alts(_Array_type& __vtable,
949  {
950  if constexpr (_Extra_visit_slot_needed<_Result_type, _Next>::value)
951  (_S_apply_single_alt<true, __var_indices>(
952  __vtable._M_arr[__var_indices + 1],
953  &(__vtable._M_arr[0])), ...);
954  else
955  (_S_apply_single_alt<false, __var_indices>(
956  __vtable._M_arr[__var_indices]), ...);
957  }
958 
959  template<bool __do_cookie, size_t __index, typename _Tp>
960  static constexpr void
961  _S_apply_single_alt(_Tp& __element, _Tp* __cookie_element = nullptr)
962  {
963  if constexpr (__do_cookie)
964  {
965  __element = __gen_vtable_impl<
966  _Tp,
967  std::index_sequence<__indices..., __index>>::_S_apply();
968  *__cookie_element = __gen_vtable_impl<
969  _Tp,
970  std::index_sequence<__indices..., variant_npos>>::_S_apply();
971  }
972  else
973  {
974  __element = __gen_vtable_impl<
975  remove_reference_t<decltype(__element)>,
976  std::index_sequence<__indices..., __index>>::_S_apply();
977  }
978  }
979  };
980 
981  // This partial specialization is the base case for the recursion.
982  // It populates a _Multi_array element with the address of a function
983  // that invokes the visitor with the alternatives specified by __indices.
984  template<typename _Result_type, typename _Visitor, typename... _Variants,
985  size_t... __indices>
986  struct __gen_vtable_impl<
987  _Multi_array<_Result_type (*)(_Visitor, _Variants...)>,
988  std::index_sequence<__indices...>>
989  {
990  using _Array_type =
991  _Multi_array<_Result_type (*)(_Visitor, _Variants...)>;
992 
993  template<size_t __index, typename _Variant>
994  static constexpr decltype(auto)
995  __element_by_index_or_cookie(_Variant&& __var) noexcept
996  {
997  if constexpr (__index != variant_npos)
998  return __variant::__get<__index>(std::forward<_Variant>(__var));
999  else
1000  return __variant_cookie{};
1001  }
1002 
1003  static constexpr decltype(auto)
1004  __visit_invoke(_Visitor&& __visitor, _Variants... __vars)
1005  {
1006  if constexpr (is_same_v<_Result_type, __variant_idx_cookie>)
1007  // For raw visitation using indices, pass the indices to the visitor
1008  // and discard the return value:
1009  std::__invoke(std::forward<_Visitor>(__visitor),
1010  __element_by_index_or_cookie<__indices>(
1011  std::forward<_Variants>(__vars))...,
1012  integral_constant<size_t, __indices>()...);
1013  else if constexpr (is_same_v<_Result_type, __variant_cookie>)
1014  // For raw visitation without indices, and discard the return value:
1015  std::__invoke(std::forward<_Visitor>(__visitor),
1016  __element_by_index_or_cookie<__indices>(
1017  std::forward<_Variants>(__vars))...);
1018  else if constexpr (_Array_type::__result_is_deduced::value)
1019  // For the usual std::visit case deduce the return value:
1020  return std::__invoke(std::forward<_Visitor>(__visitor),
1021  __element_by_index_or_cookie<__indices>(
1022  std::forward<_Variants>(__vars))...);
1023  else // for std::visit<R> use INVOKE<R>
1024  return std::__invoke_r<_Result_type>(
1025  std::forward<_Visitor>(__visitor),
1026  __variant::__get<__indices>(std::forward<_Variants>(__vars))...);
1027  }
1028 
1029  static constexpr auto
1030  _S_apply()
1031  { return _Array_type{&__visit_invoke}; }
1032  };
1033 
1034  template<typename _Result_type, typename _Visitor, typename... _Variants>
1035  struct __gen_vtable
1036  {
1037  using _Array_type =
1038  _Multi_array<_Result_type (*)(_Visitor, _Variants...),
1039  variant_size_v<remove_reference_t<_Variants>>...>;
1040 
1041  static constexpr _Array_type _S_vtable
1042  = __gen_vtable_impl<_Array_type, std::index_sequence<>>::_S_apply();
1043  };
1044 
1045  template<size_t _Np, typename _Tp>
1046  struct _Base_dedup : public _Tp { };
1047 
1048  template<typename _Variant, typename __indices>
1049  struct _Variant_hash_base;
1050 
1051  template<typename... _Types, size_t... __indices>
1052  struct _Variant_hash_base<variant<_Types...>,
1053  std::index_sequence<__indices...>>
1054  : _Base_dedup<__indices, __poison_hash<remove_const_t<_Types>>>... { };
1055 
1056 } // namespace __variant
1057 } // namespace __detail
1058 
1059  template<size_t _Np, typename _Variant, typename... _Args>
1060  void __variant_construct_by_index(_Variant& __v, _Args&&... __args)
1061  {
1062  __v._M_index = _Np;
1063  auto&& __storage = __detail::__variant::__get<_Np>(__v);
1064  ::new ((void*)std::addressof(__storage))
1065  remove_reference_t<decltype(__storage)>
1066  (std::forward<_Args>(__args)...);
1067  }
1068 
1069  template<typename _Tp, typename... _Types>
1070  constexpr bool
1071  holds_alternative(const variant<_Types...>& __v) noexcept
1072  {
1073  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1074  "T must occur exactly once in alternatives");
1075  return __v.index() == __detail::__variant::__index_of_v<_Tp, _Types...>;
1076  }
1077 
1078  template<typename _Tp, typename... _Types>
1079  constexpr _Tp& get(variant<_Types...>& __v)
1080  {
1081  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1082  "T must occur exactly once in alternatives");
1083  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1084  return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
1085  }
1086 
1087  template<typename _Tp, typename... _Types>
1088  constexpr _Tp&& get(variant<_Types...>&& __v)
1089  {
1090  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1091  "T must occur exactly once in alternatives");
1092  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1093  return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
1094  std::move(__v));
1095  }
1096 
1097  template<typename _Tp, typename... _Types>
1098  constexpr const _Tp& get(const variant<_Types...>& __v)
1099  {
1100  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1101  "T must occur exactly once in alternatives");
1102  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1103  return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
1104  }
1105 
1106  template<typename _Tp, typename... _Types>
1107  constexpr const _Tp&& get(const variant<_Types...>&& __v)
1108  {
1109  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1110  "T must occur exactly once in alternatives");
1111  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1112  return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
1113  std::move(__v));
1114  }
1115 
1116  template<size_t _Np, typename... _Types>
1117  constexpr add_pointer_t<variant_alternative_t<_Np, variant<_Types...>>>
1118  get_if(variant<_Types...>* __ptr) noexcept
1119  {
1120  using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
1121  static_assert(_Np < sizeof...(_Types),
1122  "The index must be in [0, number of alternatives)");
1123  static_assert(!is_void_v<_Alternative_type>, "_Tp must not be void");
1124  if (__ptr && __ptr->index() == _Np)
1125  return std::addressof(__detail::__variant::__get<_Np>(*__ptr));
1126  return nullptr;
1127  }
1128 
1129  template<size_t _Np, typename... _Types>
1130  constexpr
1131  add_pointer_t<const variant_alternative_t<_Np, variant<_Types...>>>
1132  get_if(const variant<_Types...>* __ptr) noexcept
1133  {
1134  using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
1135  static_assert(_Np < sizeof...(_Types),
1136  "The index must be in [0, number of alternatives)");
1137  static_assert(!is_void_v<_Alternative_type>, "_Tp must not be void");
1138  if (__ptr && __ptr->index() == _Np)
1139  return std::addressof(__detail::__variant::__get<_Np>(*__ptr));
1140  return nullptr;
1141  }
1142 
1143  template<typename _Tp, typename... _Types>
1144  constexpr add_pointer_t<_Tp>
1145  get_if(variant<_Types...>* __ptr) noexcept
1146  {
1147  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1148  "T must occur exactly once in alternatives");
1149  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1150  return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
1151  __ptr);
1152  }
1153 
1154  template<typename _Tp, typename... _Types>
1155  constexpr add_pointer_t<const _Tp>
1156  get_if(const variant<_Types...>* __ptr) noexcept
1157  {
1158  static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
1159  "T must occur exactly once in alternatives");
1160  static_assert(!is_void_v<_Tp>, "_Tp must not be void");
1161  return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
1162  __ptr);
1163  }
1164 
1165  struct monostate { };
1166 
1167 #define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
1168  template<typename... _Types> \
1169  constexpr bool operator __OP(const variant<_Types...>& __lhs, \
1170  const variant<_Types...>& __rhs) \
1171  { \
1172  bool __ret = true; \
1173  __detail::__variant::__raw_idx_visit( \
1174  [&__ret, &__lhs] (auto&& __rhs_mem, auto __rhs_index) mutable \
1175  { \
1176  if constexpr (__rhs_index != variant_npos) \
1177  { \
1178  if (__lhs.index() == __rhs_index) \
1179  { \
1180  auto& __this_mem = std::get<__rhs_index>(__lhs); \
1181  __ret = __this_mem __OP __rhs_mem; \
1182  } \
1183  else \
1184  __ret = (__lhs.index() + 1) __OP (__rhs_index + 1); \
1185  } \
1186  else \
1187  __ret = (__lhs.index() + 1) __OP (__rhs_index + 1); \
1188  }, __rhs); \
1189  return __ret; \
1190  }
1191 
1192  _VARIANT_RELATION_FUNCTION_TEMPLATE(<, less)
1193  _VARIANT_RELATION_FUNCTION_TEMPLATE(<=, less_equal)
1194  _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
1195  _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
1196  _VARIANT_RELATION_FUNCTION_TEMPLATE(>=, greater_equal)
1197  _VARIANT_RELATION_FUNCTION_TEMPLATE(>, greater)
1198 
1199 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
1200 
1201  constexpr bool operator==(monostate, monostate) noexcept { return true; }
1202 
1203 #ifdef __cpp_lib_three_way_comparison
1204  template<typename... _Types>
1205  requires (three_way_comparable<_Types> && ...)
1206  constexpr
1207  common_comparison_category_t<compare_three_way_result_t<_Types>...>
1208  operator<=>(const variant<_Types...>& __v, const variant<_Types...>& __w)
1209  {
1210  common_comparison_category_t<compare_three_way_result_t<_Types>...> __ret
1211  = strong_ordering::equal;
1212 
1213  __detail::__variant::__raw_idx_visit(
1214  [&__ret, &__v] (auto&& __w_mem, auto __w_index) mutable
1215  {
1216  if constexpr (__w_index != variant_npos)
1217  {
1218  if (__v.index() == __w_index)
1219  {
1220  auto& __this_mem = std::get<__w_index>(__v);
1221  __ret = __this_mem <=> __w_mem;
1222  return;
1223  }
1224  }
1225  __ret = (__v.index() + 1) <=> (__w_index + 1);
1226  }, __w);
1227  return __ret;
1228  }
1229 
1230  constexpr strong_ordering
1231  operator<=>(monostate, monostate) noexcept { return strong_ordering::equal; }
1232 #else
1233  constexpr bool operator!=(monostate, monostate) noexcept { return false; }
1234  constexpr bool operator<(monostate, monostate) noexcept { return false; }
1235  constexpr bool operator>(monostate, monostate) noexcept { return false; }
1236  constexpr bool operator<=(monostate, monostate) noexcept { return true; }
1237  constexpr bool operator>=(monostate, monostate) noexcept { return true; }
1238 #endif
1239 
1240  template<typename _Visitor, typename... _Variants>
1241  constexpr decltype(auto) visit(_Visitor&&, _Variants&&...);
1242 
1243  template<typename... _Types>
1244  inline enable_if_t<(is_move_constructible_v<_Types> && ...)
1245  && (is_swappable_v<_Types> && ...)>
1246  swap(variant<_Types...>& __lhs, variant<_Types...>& __rhs)
1247  noexcept(noexcept(__lhs.swap(__rhs)))
1248  { __lhs.swap(__rhs); }
1249 
1250  template<typename... _Types>
1251  enable_if_t<!((is_move_constructible_v<_Types> && ...)
1252  && (is_swappable_v<_Types> && ...))>
1253  swap(variant<_Types...>&, variant<_Types...>&) = delete;
1254 
1255  class bad_variant_access : public exception
1256  {
1257  public:
1258  bad_variant_access() noexcept { }
1259 
1260  const char* what() const noexcept override
1261  { return _M_reason; }
1262 
1263  private:
1264  bad_variant_access(const char* __reason) noexcept : _M_reason(__reason) { }
1265 
1266  // Must point to a string with static storage duration:
1267  const char* _M_reason = "bad variant access";
1268 
1269  friend void __throw_bad_variant_access(const char* __what);
1270  };
1271 
1272  // Must only be called with a string literal
1273  inline void
1274  __throw_bad_variant_access(const char* __what)
1275  { _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__what)); }
1276 
1277  inline void
1278  __throw_bad_variant_access(bool __valueless)
1279  {
1280  if (__valueless) [[__unlikely__]]
1281  __throw_bad_variant_access("std::get: variant is valueless");
1282  else
1283  __throw_bad_variant_access("std::get: wrong index for variant");
1284  }
1285 
1286  template<typename... _Types>
1287  class variant
1288  : private __detail::__variant::_Variant_base<_Types...>,
1289  private _Enable_default_constructor<
1290  __detail::__variant::_Traits<_Types...>::_S_default_ctor,
1291  variant<_Types...>>,
1292  private _Enable_copy_move<
1293  __detail::__variant::_Traits<_Types...>::_S_copy_ctor,
1294  __detail::__variant::_Traits<_Types...>::_S_copy_assign,
1295  __detail::__variant::_Traits<_Types...>::_S_move_ctor,
1296  __detail::__variant::_Traits<_Types...>::_S_move_assign,
1297  variant<_Types...>>
1298  {
1299  private:
1300  template <typename... _UTypes, typename _Tp>
1301  friend decltype(auto) __variant_cast(_Tp&&);
1302  template<size_t _Np, typename _Variant, typename... _Args>
1303  friend void __variant_construct_by_index(_Variant& __v,
1304  _Args&&... __args);
1305 
1306  static_assert(sizeof...(_Types) > 0,
1307  "variant must have at least one alternative");
1308  static_assert(!(std::is_reference_v<_Types> || ...),
1309  "variant must have no reference alternative");
1310  static_assert(!(std::is_void_v<_Types> || ...),
1311  "variant must have no void alternative");
1312 
1313  using _Base = __detail::__variant::_Variant_base<_Types...>;
1314  using _Default_ctor_enabler =
1315  _Enable_default_constructor<
1316  __detail::__variant::_Traits<_Types...>::_S_default_ctor,
1317  variant<_Types...>>;
1318 
1319  template<typename _Tp>
1320  static constexpr bool __not_self
1321  = !is_same_v<__remove_cvref_t<_Tp>, variant>;
1322 
1323  template<typename _Tp>
1324  static constexpr bool
1325  __exactly_once = __detail::__variant::__exactly_once<_Tp, _Types...>;
1326 
1327  template<typename _Tp>
1328  static constexpr size_t __accepted_index
1329  = __detail::__variant::__accepted_index<_Tp, variant>::value;
1330 
1331  template<size_t _Np, typename = enable_if_t<(_Np < sizeof...(_Types))>>
1332  using __to_type = variant_alternative_t<_Np, variant>;
1333 
1334  template<typename _Tp, typename = enable_if_t<__not_self<_Tp>>>
1335  using __accepted_type = __to_type<__accepted_index<_Tp>>;
1336 
1337  template<typename _Tp>
1338  static constexpr size_t __index_of =
1339  __detail::__variant::__index_of_v<_Tp, _Types...>;
1340 
1341  using _Traits = __detail::__variant::_Traits<_Types...>;
1342 
1343  template<typename _Tp>
1344  struct __is_in_place_tag : false_type { };
1345  template<typename _Tp>
1346  struct __is_in_place_tag<in_place_type_t<_Tp>> : true_type { };
1347  template<size_t _Np>
1348  struct __is_in_place_tag<in_place_index_t<_Np>> : true_type { };
1349 
1350  template<typename _Tp>
1351  static constexpr bool __not_in_place_tag
1352  = !__is_in_place_tag<__remove_cvref_t<_Tp>>::value;
1353 
1354  public:
1355  variant() = default;
1356  variant(const variant& __rhs) = default;
1357  variant(variant&&) = default;
1358  variant& operator=(const variant&) = default;
1359  variant& operator=(variant&&) = default;
1360  ~variant() = default;
1361 
1362  template<typename _Tp,
1363  typename = enable_if_t<sizeof...(_Types) != 0>,
1364  typename = enable_if_t<__not_in_place_tag<_Tp>>,
1365  typename _Tj = __accepted_type<_Tp&&>,
1366  typename = enable_if_t<__exactly_once<_Tj>
1367  && is_constructible_v<_Tj, _Tp>>>
1368  constexpr
1369  variant(_Tp&& __t)
1370  noexcept(is_nothrow_constructible_v<_Tj, _Tp>)
1371  : variant(in_place_index<__accepted_index<_Tp>>,
1372  std::forward<_Tp>(__t))
1373  { }
1374 
1375  template<typename _Tp, typename... _Args,
1376  typename = enable_if_t<__exactly_once<_Tp>
1377  && is_constructible_v<_Tp, _Args...>>>
1378  constexpr explicit
1379  variant(in_place_type_t<_Tp>, _Args&&... __args)
1380  : variant(in_place_index<__index_of<_Tp>>,
1381  std::forward<_Args>(__args)...)
1382  { }
1383 
1384  template<typename _Tp, typename _Up, typename... _Args,
1385  typename = enable_if_t<__exactly_once<_Tp>
1386  && is_constructible_v<_Tp,
1387  initializer_list<_Up>&, _Args...>>>
1388  constexpr explicit
1389  variant(in_place_type_t<_Tp>, initializer_list<_Up> __il,
1390  _Args&&... __args)
1391  : variant(in_place_index<__index_of<_Tp>>, __il,
1392  std::forward<_Args>(__args)...)
1393  { }
1394 
1395  template<size_t _Np, typename... _Args,
1396  typename _Tp = __to_type<_Np>,
1397  typename = enable_if_t<is_constructible_v<_Tp, _Args...>>>
1398  constexpr explicit
1399  variant(in_place_index_t<_Np>, _Args&&... __args)
1400  : _Base(in_place_index<_Np>, std::forward<_Args>(__args)...),
1401  _Default_ctor_enabler(_Enable_default_constructor_tag{})
1402  { }
1403 
1404  template<size_t _Np, typename _Up, typename... _Args,
1405  typename _Tp = __to_type<_Np>,
1406  typename = enable_if_t<is_constructible_v<_Tp,
1407  initializer_list<_Up>&,
1408  _Args...>>>
1409  constexpr explicit
1410  variant(in_place_index_t<_Np>, initializer_list<_Up> __il,
1411  _Args&&... __args)
1412  : _Base(in_place_index<_Np>, __il, std::forward<_Args>(__args)...),
1413  _Default_ctor_enabler(_Enable_default_constructor_tag{})
1414  { }
1415 
1416  template<typename _Tp>
1417  enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
1418  && is_constructible_v<__accepted_type<_Tp&&>, _Tp>
1419  && is_assignable_v<__accepted_type<_Tp&&>&, _Tp>,
1420  variant&>
1421  operator=(_Tp&& __rhs)
1422  noexcept(is_nothrow_assignable_v<__accepted_type<_Tp&&>&, _Tp>
1423  && is_nothrow_constructible_v<__accepted_type<_Tp&&>, _Tp>)
1424  {
1425  constexpr auto __index = __accepted_index<_Tp>;
1426  if (index() == __index)
1427  std::get<__index>(*this) = std::forward<_Tp>(__rhs);
1428  else
1429  {
1430  using _Tj = __accepted_type<_Tp&&>;
1431  if constexpr (is_nothrow_constructible_v<_Tj, _Tp>
1432  || !is_nothrow_move_constructible_v<_Tj>)
1433  this->emplace<__index>(std::forward<_Tp>(__rhs));
1434  else
1435  operator=(variant(std::forward<_Tp>(__rhs)));
1436  }
1437  return *this;
1438  }
1439 
1440  template<typename _Tp, typename... _Args>
1441  enable_if_t<is_constructible_v<_Tp, _Args...> && __exactly_once<_Tp>,
1442  _Tp&>
1443  emplace(_Args&&... __args)
1444  {
1445  constexpr size_t __index = __index_of<_Tp>;
1446  return this->emplace<__index>(std::forward<_Args>(__args)...);
1447  }
1448 
1449  template<typename _Tp, typename _Up, typename... _Args>
1450  enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>
1451  && __exactly_once<_Tp>,
1452  _Tp&>
1453  emplace(initializer_list<_Up> __il, _Args&&... __args)
1454  {
1455  constexpr size_t __index = __index_of<_Tp>;
1456  return this->emplace<__index>(__il, std::forward<_Args>(__args)...);
1457  }
1458 
1459  template<size_t _Np, typename... _Args>
1460  enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,
1461  _Args...>,
1462  variant_alternative_t<_Np, variant>&>
1463  emplace(_Args&&... __args)
1464  {
1465  static_assert(_Np < sizeof...(_Types),
1466  "The index must be in [0, number of alternatives)");
1467  using type = variant_alternative_t<_Np, variant>;
1468  // Provide the strong exception-safety guarantee when possible,
1469  // to avoid becoming valueless.
1470  if constexpr (is_nothrow_constructible_v<type, _Args...>)
1471  {
1472  this->_M_reset();
1473  __variant_construct_by_index<_Np>(*this,
1474  std::forward<_Args>(__args)...);
1475  }
1476  else if constexpr (is_scalar_v<type>)
1477  {
1478  // This might invoke a potentially-throwing conversion operator:
1479  const type __tmp(std::forward<_Args>(__args)...);
1480  // But these steps won't throw:
1481  this->_M_reset();
1482  __variant_construct_by_index<_Np>(*this, __tmp);
1483  }
1484  else if constexpr (__detail::__variant::_Never_valueless_alt<type>()
1485  && _Traits::_S_move_assign)
1486  {
1487  // This construction might throw:
1488  variant __tmp(in_place_index<_Np>,
1489  std::forward<_Args>(__args)...);
1490  // But _Never_valueless_alt<type> means this won't:
1491  *this = std::move(__tmp);
1492  }
1493  else
1494  {
1495  // This case only provides the basic exception-safety guarantee,
1496  // i.e. the variant can become valueless.
1497  this->_M_reset();
1498  __try
1499  {
1500  __variant_construct_by_index<_Np>(*this,
1501  std::forward<_Args>(__args)...);
1502  }
1503  __catch (...)
1504  {
1505  this->_M_index = variant_npos;
1506  __throw_exception_again;
1507  }
1508  }
1509  return std::get<_Np>(*this);
1510  }
1511 
1512  template<size_t _Np, typename _Up, typename... _Args>
1513  enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,
1514  initializer_list<_Up>&, _Args...>,
1515  variant_alternative_t<_Np, variant>&>
1516  emplace(initializer_list<_Up> __il, _Args&&... __args)
1517  {
1518  static_assert(_Np < sizeof...(_Types),
1519  "The index must be in [0, number of alternatives)");
1520  using type = variant_alternative_t<_Np, variant>;
1521  // Provide the strong exception-safety guarantee when possible,
1522  // to avoid becoming valueless.
1523  if constexpr (is_nothrow_constructible_v<type,
1524  initializer_list<_Up>&,
1525  _Args...>)
1526  {
1527  this->_M_reset();
1528  __variant_construct_by_index<_Np>(*this, __il,
1529  std::forward<_Args>(__args)...);
1530  }
1531  else if constexpr (__detail::__variant::_Never_valueless_alt<type>()
1532  && _Traits::_S_move_assign)
1533  {
1534  // This construction might throw:
1535  variant __tmp(in_place_index<_Np>, __il,
1536  std::forward<_Args>(__args)...);
1537  // But _Never_valueless_alt<type> means this won't:
1538  *this = std::move(__tmp);
1539  }
1540  else
1541  {
1542  // This case only provides the basic exception-safety guarantee,
1543  // i.e. the variant can become valueless.
1544  this->_M_reset();
1545  __try
1546  {
1547  __variant_construct_by_index<_Np>(*this, __il,
1548  std::forward<_Args>(__args)...);
1549  }
1550  __catch (...)
1551  {
1552  this->_M_index = variant_npos;
1553  __throw_exception_again;
1554  }
1555  }
1556  return std::get<_Np>(*this);
1557  }
1558 
1559  constexpr bool valueless_by_exception() const noexcept
1560  { return !this->_M_valid(); }
1561 
1562  constexpr size_t index() const noexcept
1563  {
1564  using __index_type = typename _Base::__index_type;
1565  if constexpr (__detail::__variant::__never_valueless<_Types...>())
1566  return this->_M_index;
1567  else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2)
1568  return make_signed_t<__index_type>(this->_M_index);
1569  else
1570  return size_t(__index_type(this->_M_index + 1)) - 1;
1571  }
1572 
1573  void
1574  swap(variant& __rhs)
1575  noexcept((__is_nothrow_swappable<_Types>::value && ...)
1576  && is_nothrow_move_constructible_v<variant>)
1577  {
1578  __detail::__variant::__raw_idx_visit(
1579  [this, &__rhs](auto&& __rhs_mem, auto __rhs_index) mutable
1580  {
1581  if constexpr (__rhs_index != variant_npos)
1582  {
1583  if (this->index() == __rhs_index)
1584  {
1585  auto& __this_mem =
1586  std::get<__rhs_index>(*this);
1587  using std::swap;
1588  swap(__this_mem, __rhs_mem);
1589  }
1590  else
1591  {
1592  if (!this->valueless_by_exception()) [[__likely__]]
1593  {
1594  auto __tmp(std::move(__rhs_mem));
1595  __rhs = std::move(*this);
1596  this->_M_destructive_move(__rhs_index,
1597  std::move(__tmp));
1598  }
1599  else
1600  {
1601  this->_M_destructive_move(__rhs_index,
1602  std::move(__rhs_mem));
1603  __rhs._M_reset();
1604  }
1605  }
1606  }
1607  else
1608  {
1609  if (!this->valueless_by_exception()) [[__likely__]]
1610  {
1611  __rhs = std::move(*this);
1612  this->_M_reset();
1613  }
1614  }
1615  }, __rhs);
1616  }
1617 
1618  private:
1619 
1620 #if defined(__clang__) && __clang_major__ <= 7
1621  public:
1622  using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
1623  private:
1624 #endif
1625 
1626  template<size_t _Np, typename _Vp>
1627  friend constexpr decltype(auto)
1628  __detail::__variant::__get(_Vp&& __v) noexcept;
1629 
1630  template<typename _Vp>
1631  friend void*
1632  __detail::__variant::__get_storage(_Vp&& __v) noexcept;
1633 
1634 #define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP) \
1635  template<typename... _Tp> \
1636  friend constexpr bool \
1637  operator __OP(const variant<_Tp...>& __lhs, \
1638  const variant<_Tp...>& __rhs);
1639 
1640  _VARIANT_RELATION_FUNCTION_TEMPLATE(<)
1641  _VARIANT_RELATION_FUNCTION_TEMPLATE(<=)
1642  _VARIANT_RELATION_FUNCTION_TEMPLATE(==)
1643  _VARIANT_RELATION_FUNCTION_TEMPLATE(!=)
1644  _VARIANT_RELATION_FUNCTION_TEMPLATE(>=)
1645  _VARIANT_RELATION_FUNCTION_TEMPLATE(>)
1646 
1647 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
1648  };
1649 
1650  template<size_t _Np, typename... _Types>
1651  constexpr variant_alternative_t<_Np, variant<_Types...>>&
1652  get(variant<_Types...>& __v)
1653  {
1654  static_assert(_Np < sizeof...(_Types),
1655  "The index must be in [0, number of alternatives)");
1656  if (__v.index() != _Np)
1657  __throw_bad_variant_access(__v.valueless_by_exception());
1658  return __detail::__variant::__get<_Np>(__v);
1659  }
1660 
1661  template<size_t _Np, typename... _Types>
1662  constexpr variant_alternative_t<_Np, variant<_Types...>>&&
1663  get(variant<_Types...>&& __v)
1664  {
1665  static_assert(_Np < sizeof...(_Types),
1666  "The index must be in [0, number of alternatives)");
1667  if (__v.index() != _Np)
1668  __throw_bad_variant_access(__v.valueless_by_exception());
1669  return __detail::__variant::__get<_Np>(std::move(__v));
1670  }
1671 
1672  template<size_t _Np, typename... _Types>
1673  constexpr const variant_alternative_t<_Np, variant<_Types...>>&
1674  get(const variant<_Types...>& __v)
1675  {
1676  static_assert(_Np < sizeof...(_Types),
1677  "The index must be in [0, number of alternatives)");
1678  if (__v.index() != _Np)
1679  __throw_bad_variant_access(__v.valueless_by_exception());
1680  return __detail::__variant::__get<_Np>(__v);
1681  }
1682 
1683  template<size_t _Np, typename... _Types>
1684  constexpr const variant_alternative_t<_Np, variant<_Types...>>&&
1685  get(const variant<_Types...>&& __v)
1686  {
1687  static_assert(_Np < sizeof...(_Types),
1688  "The index must be in [0, number of alternatives)");
1689  if (__v.index() != _Np)
1690  __throw_bad_variant_access(__v.valueless_by_exception());
1691  return __detail::__variant::__get<_Np>(std::move(__v));
1692  }
1693 
1694  template<typename _Result_type, typename _Visitor, typename... _Variants>
1695  constexpr decltype(auto)
1696  __do_visit(_Visitor&& __visitor, _Variants&&... __variants)
1697  {
1698  constexpr auto& __vtable = __detail::__variant::__gen_vtable<
1699  _Result_type, _Visitor&&, _Variants&&...>::_S_vtable;
1700 
1701  auto __func_ptr = __vtable._M_access(__variants.index()...);
1702  return (*__func_ptr)(std::forward<_Visitor>(__visitor),
1703  std::forward<_Variants>(__variants)...);
1704  }
1705 
1706  template<typename _Visitor, typename... _Variants>
1707  constexpr decltype(auto)
1708  visit(_Visitor&& __visitor, _Variants&&... __variants)
1709  {
1710  if ((__variants.valueless_by_exception() || ...))
1711  __throw_bad_variant_access("std::visit: variant is valueless");
1712 
1713  using _Result_type = std::invoke_result_t<_Visitor,
1714  decltype(std::get<0>(std::declval<_Variants>()))...>;
1715 
1716  using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>;
1717 
1718  return std::__do_visit<_Tag>(std::forward<_Visitor>(__visitor),
1719  std::forward<_Variants>(__variants)...);
1720  }
1721 
1722 #if __cplusplus > 201703L
1723  template<typename _Res, typename _Visitor, typename... _Variants>
1724  constexpr _Res
1725  visit(_Visitor&& __visitor, _Variants&&... __variants)
1726  {
1727  if ((__variants.valueless_by_exception() || ...))
1728  __throw_bad_variant_access("std::visit<R>: variant is valueless");
1729 
1730  return std::__do_visit<_Res>(std::forward<_Visitor>(__visitor),
1731  std::forward<_Variants>(__variants)...);
1732  }
1733 #endif
1734 
1735  template<bool, typename... _Types>
1736  struct __variant_hash_call_base_impl
1737  {
1738  size_t
1739  operator()(const variant<_Types...>& __t) const
1740  noexcept((is_nothrow_invocable_v<hash<decay_t<_Types>>, _Types> && ...))
1741  {
1742  size_t __ret;
1743  __detail::__variant::__raw_visit(
1744  [&__t, &__ret](auto&& __t_mem) mutable
1745  {
1746  using _Type = __remove_cvref_t<decltype(__t_mem)>;
1747  if constexpr (!is_same_v<_Type,
1748  __detail::__variant::__variant_cookie>)
1749  __ret = std::hash<size_t>{}(__t.index())
1750  + std::hash<_Type>{}(__t_mem);
1751  else
1752  __ret = std::hash<size_t>{}(__t.index());
1753  }, __t);
1754  return __ret;
1755  }
1756  };
1757 
1758  template<typename... _Types>
1759  struct __variant_hash_call_base_impl<false, _Types...> {};
1760 
1761  template<typename... _Types>
1762  using __variant_hash_call_base =
1763  __variant_hash_call_base_impl<(__poison_hash<remove_const_t<_Types>>::
1764  __enable_hash_call &&...), _Types...>;
1765 
1766  template<typename... _Types>
1767  struct hash<variant<_Types...>>
1768  : private __detail::__variant::_Variant_hash_base<
1769  variant<_Types...>, std::index_sequence_for<_Types...>>,
1770  public __variant_hash_call_base<_Types...>
1771  {
1772  using result_type [[__deprecated__]] = size_t;
1773  using argument_type [[__deprecated__]] = variant<_Types...>;
1774  };
1775 
1776  template<>
1777  struct hash<monostate>
1778  {
1779  using result_type [[__deprecated__]] = size_t;
1780  using argument_type [[__deprecated__]] = monostate;
1781 
1782  size_t
1783  operator()(const monostate&) const noexcept
1784  {
1785  constexpr size_t __magic_monostate_hash = -7777;
1786  return __magic_monostate_hash;
1787  }
1788  };
1789 
1790  template<typename... _Types>
1791  struct __is_fast_hash<hash<variant<_Types...>>>
1792  : bool_constant<(__is_fast_hash<_Types>::value && ...)>
1793  { };
1794 
1795 _GLIBCXX_END_NAMESPACE_VERSION
1796 } // namespace std
1797 
1798 #endif // C++17
1799 
1800 #endif // _GLIBCXX_VARIANT
constexpr _Tp * addressof(_Tp &__r) noexcept
Returns the actual address of the object or function referenced by r, even in the presence of an over...
Definition: move.h:140
integer_sequence< size_t, _Idx...> index_sequence
Alias template index_sequence.
Definition: utility:345
integral_constant
Definition: type_traits:57
make_integer_sequence< size_t, _Num > make_index_sequence
Alias template make_index_sequence.
Definition: utility:349
Class template integer_sequence.
Definition: utility:326
integral_constant< bool, false > false_type
The type used as a compile-time boolean with false value.
Definition: type_traits:78
void void_t
A metafunction that always yields void, used for detecting valid types.
Definition: type_traits:2576
typename make_signed< _Tp >::type make_signed_t
Alias template for make_signed.
Definition: type_traits:1961
constexpr bool is_swappable_v
is_swappable_v
Definition: type_traits:2739
typename add_pointer< _Tp >::type add_pointer_t
Alias template for add_pointer.
Definition: type_traits:2044
constexpr _Tp && forward(typename std::remove_reference< _Tp >::type &__t) noexcept
Forward an lvalue.
Definition: move.h:76
Primary class template hash.
Definition: typeindex:110
typename remove_reference< _Tp >::type remove_reference_t
Alias template for remove_reference.
Definition: type_traits:1635
constexpr std::remove_reference< _Tp >::type && move(_Tp &&__t) noexcept
Convert a value to an rvalue.
Definition: move.h:101
typename enable_if< _Cond, _Tp >::type enable_if_t
Alias template for enable_if.
Definition: type_traits:2554
integral_constant< bool, true > true_type
The type used as a compile-time boolean with true value.
Definition: type_traits:75