libstdc++-v3/include/std/variant

   1 // <variant> -*- C++ -*-
   2
   3 // Copyright (C) 2016-2019 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>
  38 #include <bits/enable_special_members.h>
  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>
  45 #include <bits/stl_iterator_base_types.h>
  46 #include <bits/stl_iterator_base_funcs.h>
  47 #include <bits/stl_construct.h>
  48
  49 namespace std _GLIBCXX_VISIBILITY(default)
  50 {
  51 _GLIBCXX_BEGIN_NAMESPACE_VERSION
  52
  53 namespace __detail
  54 {
  55 namespace __variant
  56 {
  57   template<size_t _Np, typename... _Types>
  58     struct _Nth_type;
  59
  60   template<size_t _Np, typename _First, typename... _Rest>
  61     struct _Nth_type<_Np, _First, _Rest...>
  62     : _Nth_type<_Np-1, _Rest...> { };
  63
  64   template<typename _First, typename... _Rest>
  65     struct _Nth_type<0, _First, _Rest...>
  66     { using type = _First; };
  67
  68 } // namespace __variant
  69 } // namespace __detail
  70
  71 #define __cpp_lib_variant 201603
  72
  73   template<typename... _Types> class tuple;
  74   template<typename... _Types> class variant;
  75   template <typename> struct hash;
  76
  77   template<typename _Variant>
  78     struct variant_size;
  79
  80   template<typename _Variant>
  81     struct variant_size<const _Variant> : variant_size<_Variant> {};
  82
  83   template<typename _Variant>
  84     struct variant_size<volatile _Variant> : variant_size<_Variant> {};
  85
  86   template<typename _Variant>
  87     struct variant_size<const volatile _Variant> : variant_size<_Variant> {};
  88
  89   template<typename... _Types>
  90     struct variant_size<variant<_Types...>>
  91     : std::integral_constant<size_t, sizeof...(_Types)> {};
  92
  93   template<typename _Variant>
  94     inline constexpr size_t variant_size_v = variant_size<_Variant>::value;
  95
  96   template<size_t _Np, typename _Variant>
  97     struct variant_alternative;
  98
  99   template<size_t _Np, typename _First, typename... _Rest>
 100     struct variant_alternative<_Np, variant<_First, _Rest...>>
 101     : variant_alternative<_Np-1, variant<_Rest...>> {};
 102
 103   template<typename _First, typename... _Rest>
 104     struct variant_alternative<0, variant<_First, _Rest...>>
 105     { using type = _First; };
 106
 107   template<size_t _Np, typename _Variant>
 108     using variant_alternative_t =
 109       typename variant_alternative<_Np, _Variant>::type;
 110
 111   template<size_t _Np, typename _Variant>
 112     struct variant_alternative<_Np, const _Variant>
 113     { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
 114
 115   template<size_t _Np, typename _Variant>
 116     struct variant_alternative<_Np, volatile _Variant>
 117     { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
 118
 119   template<size_t _Np, typename _Variant>
 120     struct variant_alternative<_Np, const volatile _Variant>
 121     { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
 122
 123   inline constexpr size_t variant_npos = -1;
 124
 125   template<size_t _Np, typename... _Types>
 126     constexpr variant_alternative_t<_Np, variant<_Types...>>&
 127     get(variant<_Types...>&);
 128
 129   template<size_t _Np, typename... _Types>
 130     constexpr variant_alternative_t<_Np, variant<_Types...>>&&
 131     get(variant<_Types...>&&);
 132
 133   template<size_t _Np, typename... _Types>
 134     constexpr variant_alternative_t<_Np, variant<_Types...>> const&
 135     get(const variant<_Types...>&);
 136
 137   template<size_t _Np, typename... _Types>
 138     constexpr variant_alternative_t<_Np, variant<_Types...>> const&&
 139     get(const variant<_Types...>&&);
 140
 141 namespace __detail
 142 {
 143 namespace __variant
 144 {
 145   // Returns the first apparence of _Tp in _Types.
 146   // Returns sizeof...(_Types) if _Tp is not in _Types.
 147   template<typename _Tp, typename... _Types>
 148     struct __index_of : std::integral_constant<size_t, 0> {};
 149
 150   template<typename _Tp, typename... _Types>
 151     inline constexpr size_t __index_of_v = __index_of<_Tp, _Types...>::value;
 152
 153   template<typename _Tp, typename _First, typename... _Rest>
 154     struct __index_of<_Tp, _First, _Rest...> :
 155       std::integral_constant<size_t, is_same_v<_Tp, _First>
 156         ? 0 : __index_of_v<_Tp, _Rest...> + 1> {};
 157
 158   // _Uninitialized<T> is guaranteed to be a literal type, even if T is not.
 159   // We have to do this, because [basic.types]p10.5.3 (n4606) is not implemented
 160   // yet. When it's implemented, _Uninitialized<T> can be changed to the alias
 161   // to T, therefore equivalent to being removed entirely.
 162   //
 163   // Another reason we may not want to remove _Uninitialzied<T> may be that, we
 164   // want _Uninitialized<T> to be trivially destructible, no matter whether T
 165   // is; but we will see.
 166   template<typename _Type, bool = std::is_literal_type_v<_Type>>
 167     struct _Uninitialized;
 168
 169   template<typename _Type>
 170     struct _Uninitialized<_Type, true>
 171     {
 172       template<typename... _Args>
 173       constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
 174       : _M_storage(std::forward<_Args>(__args)...)
 175       { }
 176
 177       constexpr const _Type& _M_get() const &
 178       { return _M_storage; }
 179
 180       constexpr _Type& _M_get() &
 181       { return _M_storage; }
 182
 183       constexpr const _Type&& _M_get() const &&
 184       { return std::move(_M_storage); }
 185
 186       constexpr _Type&& _M_get() &&
 187       { return std::move(_M_storage); }
 188
 189       _Type _M_storage;
 190     };
 191
 192   template<typename _Type>
 193     struct _Uninitialized<_Type, false>
 194     {
 195       template<typename... _Args>
 196       constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
 197       { ::new (&_M_storage) _Type(std::forward<_Args>(__args)...); }
 198
 199       const _Type& _M_get() const &
 200       { return *_M_storage._M_ptr(); }
 201
 202       _Type& _M_get() &
 203       { return *_M_storage._M_ptr(); }
 204
 205       const _Type&& _M_get() const &&
 206       { return std::move(*_M_storage._M_ptr()); }
 207
 208       _Type&& _M_get() &&
 209       { return std::move(*_M_storage._M_ptr()); }
 210
 211       __gnu_cxx::__aligned_membuf<_Type> _M_storage;
 212     };
 213
 214   template<typename _Ref>
 215     _Ref __ref_cast(void* __ptr)
 216     {
 217       return static_cast<_Ref>(*static_cast<remove_reference_t<_Ref>*>(__ptr));
 218     }
 219
 220   template<typename _Union>
 221     constexpr decltype(auto) __get(in_place_index_t<0>, _Union&& __u)
 222     { return std::forward<_Union>(__u)._M_first._M_get(); }
 223
 224   template<size_t _Np, typename _Union>
 225     constexpr decltype(auto) __get(in_place_index_t<_Np>, _Union&& __u)
 226     {
 227       return __variant::__get(in_place_index<_Np-1>,
 228                               std::forward<_Union>(__u)._M_rest);
 229     }
 230
 231   // Returns the typed storage for __v.
 232   template<size_t _Np, typename _Variant>
 233     constexpr decltype(auto) __get(_Variant&& __v)
 234     {
 235       return __variant::__get(std::in_place_index<_Np>,
 236                               std::forward<_Variant>(__v)._M_u);
 237     }
 238
 239   // Various functions as "vtable" entries, where those vtables are used by
 240   // polymorphic operations.
 241   template<typename _Lhs, typename _Rhs>
 242     void
 243     __erased_ctor(void* __lhs, void* __rhs)
 244     {
 245       using _Type = remove_reference_t<_Lhs>;
 246       ::new (__lhs) _Type(__variant::__ref_cast<_Rhs>(__rhs));
 247     }
 248
 249   template<typename _Variant, size_t _Np>
 250     void
 251     __erased_dtor(_Variant&& __v)
 252     { std::_Destroy(std::__addressof(__variant::__get<_Np>(__v))); }
 253
 254   template<typename _Lhs, typename _Rhs>
 255     void
 256     __erased_assign(void* __lhs, void* __rhs)
 257     {
 258       __variant::__ref_cast<_Lhs>(__lhs) = __variant::__ref_cast<_Rhs>(__rhs);
 259     }
 260
 261   template<typename _Lhs, typename _Rhs>
 262     void
 263     __erased_swap(void* __lhs, void* __rhs)
 264     {
 265       using std::swap;
 266       swap(__variant::__ref_cast<_Lhs>(__lhs),
 267            __variant::__ref_cast<_Rhs>(__rhs));
 268     }
 269
 270 #define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
 271   template<typename _Variant, size_t _Np> \
 272     constexpr bool \
 273     __erased_##__NAME(const _Variant& __lhs, const _Variant& __rhs) \
 274     { \
 275       return __variant::__get<_Np>(std::forward<_Variant>(__lhs)) \
 276           __OP __variant::__get<_Np>(std::forward<_Variant>(__rhs)); \
 277     }
 278
 279   _VARIANT_RELATION_FUNCTION_TEMPLATE(<, less)
 280   _VARIANT_RELATION_FUNCTION_TEMPLATE(<=, less_equal)
 281   _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
 282   _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
 283   _VARIANT_RELATION_FUNCTION_TEMPLATE(>=, greater_equal)
 284   _VARIANT_RELATION_FUNCTION_TEMPLATE(>, greater)
 285
 286 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
 287
 288   template<typename _Tp>
 289     size_t
 290     __erased_hash(void* __t)
 291     {
 292       return std::hash<__remove_cvref_t<_Tp>>{}(
 293           __variant::__ref_cast<_Tp>(__t));
 294     }
 295
 296   template<typename... _Types>
 297     struct _Traits
 298     {
 299       static constexpr bool _S_default_ctor =
 300           is_default_constructible_v<typename _Nth_type<0, _Types...>::type>;
 301       static constexpr bool _S_copy_ctor =
 302           (is_copy_constructible_v<_Types> && ...);
 303       static constexpr bool _S_move_ctor =
 304           (is_move_constructible_v<_Types> && ...);
 305       static constexpr bool _S_copy_assign =
 306           _S_copy_ctor && _S_move_ctor
 307           && (is_copy_assignable_v<_Types> && ...);
 308       static constexpr bool _S_move_assign =
 309           _S_move_ctor
 310           && (is_move_assignable_v<_Types> && ...);
 311
 312       static constexpr bool _S_trivial_dtor =
 313           (is_trivially_destructible_v<_Types> && ...);
 314       static constexpr bool _S_trivial_copy_ctor =
 315           (is_trivially_copy_constructible_v<_Types> && ...);
 316       static constexpr bool _S_trivial_move_ctor =
 317           (is_trivially_move_constructible_v<_Types> && ...);
 318       static constexpr bool _S_trivial_copy_assign =
 319           _S_trivial_dtor && (is_trivially_copy_assignable_v<_Types> && ...);
 320       static constexpr bool _S_trivial_move_assign =
 321           _S_trivial_dtor && (is_trivially_move_assignable_v<_Types> && ...);
 322
 323       // The following nothrow traits are for non-trivial SMFs. Trivial SMFs
 324       // are always nothrow.
 325       static constexpr bool _S_nothrow_default_ctor =
 326           is_nothrow_default_constructible_v<
 327               typename _Nth_type<0, _Types...>::type>;
 328       static constexpr bool _S_nothrow_copy_ctor = false;
 329       static constexpr bool _S_nothrow_move_ctor =
 330           (is_nothrow_move_constructible_v<_Types> && ...);
 331       static constexpr bool _S_nothrow_copy_assign = false;
 332       static constexpr bool _S_nothrow_move_assign =
 333           _S_nothrow_move_ctor && (is_nothrow_move_assignable_v<_Types> && ...);
 334     };
 335
 336   // Defines members and ctors.
 337   template<typename... _Types>
 338     union _Variadic_union { };
 339
 340   template<typename _First, typename... _Rest>
 341     union _Variadic_union<_First, _Rest...>
 342     {
 343       constexpr _Variadic_union() : _M_rest() { }
 344
 345       template<typename... _Args>
 346         constexpr _Variadic_union(in_place_index_t<0>, _Args&&... __args)
 347         : _M_first(in_place_index<0>, std::forward<_Args>(__args)...)
 348         { }
 349
 350       template<size_t _Np, typename... _Args>
 351         constexpr _Variadic_union(in_place_index_t<_Np>, _Args&&... __args)
 352         : _M_rest(in_place_index<_Np-1>, std::forward<_Args>(__args)...)
 353         { }
 354
 355       _Uninitialized<_First> _M_first;
 356       _Variadic_union<_Rest...> _M_rest;
 357     };
 358
 359   // Defines index and the dtor, possibly trivial.
 360   template<bool __trivially_destructible, typename... _Types>
 361     struct _Variant_storage;
 362
 363   template <typename... _Types>
 364   using __select_index =
 365     typename __select_int::_Select_int_base<sizeof...(_Types),
 366                                             unsigned char,
 367                                             unsigned short>::type::value_type;
 368
 369   template<typename... _Types>
 370     struct _Variant_storage<false, _Types...>
 371     {
 372       template<size_t... __indices>
 373         static constexpr void (*_S_vtable[])(const _Variant_storage&) =
 374             { &__erased_dtor<const _Variant_storage&, __indices>... };
 375
 376       constexpr _Variant_storage() : _M_index(variant_npos) { }
 377
 378       template<size_t _Np, typename... _Args>
 379         constexpr _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
 380         : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
 381         _M_index(_Np)
 382         { }
 383
 384       template<size_t... __indices>
 385         constexpr void _M_reset_impl(std::index_sequence<__indices...>)
 386         {
 387           if (_M_index != __index_type(variant_npos))
 388             _S_vtable<__indices...>[_M_index](*this);
 389         }
 390
 391       void _M_reset()
 392       {
 393         _M_reset_impl(std::index_sequence_for<_Types...>{});
 394         _M_index = variant_npos;
 395       }
 396
 397       ~_Variant_storage()
 398       { _M_reset(); }
 399
 400       void*
 401       _M_storage() const
 402       {
 403         return const_cast<void*>(static_cast<const void*>(
 404             std::addressof(_M_u)));
 405       }
 406
 407       constexpr bool
 408       _M_valid() const noexcept
 409       {
 410         return this->_M_index != __index_type(variant_npos);
 411       }
 412
 413       _Variadic_union<_Types...> _M_u;
 414       using __index_type = __select_index<_Types...>;
 415       __index_type _M_index;
 416     };
 417
 418   template<typename... _Types>
 419     struct _Variant_storage<true, _Types...>
 420     {
 421       constexpr _Variant_storage() : _M_index(variant_npos) { }
 422
 423       template<size_t _Np, typename... _Args>
 424         constexpr _Variant_storage(in_place_index_t<_Np>, _Args&&... __args)
 425         : _M_u(in_place_index<_Np>, std::forward<_Args>(__args)...),
 426         _M_index(_Np)
 427         { }
 428
 429       void _M_reset()
 430       { _M_index = variant_npos; }
 431
 432       void*
 433       _M_storage() const
 434       {
 435         return const_cast<void*>(static_cast<const void*>(
 436             std::addressof(_M_u)));
 437       }
 438
 439       constexpr bool
 440       _M_valid() const noexcept
 441       {
 442         if constexpr ((is_scalar_v<_Types> && ...))
 443           return true;
 444         return this->_M_index != __index_type(variant_npos);
 445       }
 446
 447       _Variadic_union<_Types...> _M_u;
 448       using __index_type = __select_index<_Types...>;
 449       __index_type _M_index;
 450     };
 451
 452   template<typename... _Types>
 453     using _Variant_storage_alias =
 454         _Variant_storage<_Traits<_Types...>::_S_trivial_dtor, _Types...>;
 455
 456   // The following are (Copy|Move) (ctor|assign) layers for forwarding
 457   // triviality and handling non-trivial SMF behaviors.
 458
 459   template<bool, typename... _Types>
 460     struct _Copy_ctor_base : _Variant_storage_alias<_Types...>
 461     {
 462       using _Base = _Variant_storage_alias<_Types...>;
 463       using _Base::_Base;
 464
 465       _Copy_ctor_base(const _Copy_ctor_base& __rhs)
 466           noexcept(_Traits<_Types...>::_S_nothrow_copy_ctor)
 467       {
 468         if (__rhs._M_valid())
 469           {
 470             static constexpr void (*_S_vtable[])(void*, void*) =
 471               { &__erased_ctor<_Types&, const _Types&>... };
 472             _S_vtable[__rhs._M_index](this->_M_storage(), __rhs._M_storage());
 473             this->_M_index = __rhs._M_index;
 474           }
 475       }
 476
 477       _Copy_ctor_base(_Copy_ctor_base&&) = default;
 478       _Copy_ctor_base& operator=(const _Copy_ctor_base&) = default;
 479       _Copy_ctor_base& operator=(_Copy_ctor_base&&) = default;
 480     };
 481
 482   template<typename... _Types>
 483     struct _Copy_ctor_base<true, _Types...> : _Variant_storage_alias<_Types...>
 484     {
 485       using _Base = _Variant_storage_alias<_Types...>;
 486       using _Base::_Base;
 487     };
 488
 489   template<typename... _Types>
 490     using _Copy_ctor_alias =
 491         _Copy_ctor_base<_Traits<_Types...>::_S_trivial_copy_ctor, _Types...>;
 492
 493   template<bool, typename... _Types>
 494     struct _Move_ctor_base : _Copy_ctor_alias<_Types...>
 495     {
 496       using _Base = _Copy_ctor_alias<_Types...>;
 497       using _Base::_Base;
 498
 499       _Move_ctor_base(_Move_ctor_base&& __rhs)
 500           noexcept(_Traits<_Types...>::_S_nothrow_move_ctor)
 501       {
 502         if (__rhs._M_valid())
 503           {
 504             static constexpr void (*_S_vtable[])(void*, void*) =
 505               { &__erased_ctor<_Types&, _Types&&>... };
 506             _S_vtable[__rhs._M_index](this->_M_storage(), __rhs._M_storage());
 507             this->_M_index = __rhs._M_index;
 508           }
 509       }
 510
 511       void _M_destructive_move(_Move_ctor_base&& __rhs)
 512       {
 513         this->~_Move_ctor_base();
 514         __try
 515           {
 516             ::new (this) _Move_ctor_base(std::move(__rhs));
 517           }
 518         __catch (...)
 519           {
 520             this->_M_index = variant_npos;
 521             __throw_exception_again;
 522           }
 523       }
 524
 525       _Move_ctor_base(const _Move_ctor_base&) = default;
 526       _Move_ctor_base& operator=(const _Move_ctor_base&) = default;
 527       _Move_ctor_base& operator=(_Move_ctor_base&&) = default;
 528     };
 529
 530   template<typename... _Types>
 531     struct _Move_ctor_base<true, _Types...> : _Copy_ctor_alias<_Types...>
 532     {
 533       using _Base = _Copy_ctor_alias<_Types...>;
 534       using _Base::_Base;
 535
 536       void _M_destructive_move(_Move_ctor_base&& __rhs)
 537       {
 538         this->~_Move_ctor_base();
 539         ::new (this) _Move_ctor_base(std::move(__rhs));
 540       }
 541     };
 542
 543   template<typename... _Types>
 544     using _Move_ctor_alias =
 545         _Move_ctor_base<_Traits<_Types...>::_S_trivial_move_ctor, _Types...>;
 546
 547   template<bool, typename... _Types>
 548     struct _Copy_assign_base : _Move_ctor_alias<_Types...>
 549     {
 550       using _Base = _Move_ctor_alias<_Types...>;
 551       using _Base::_Base;
 552
 553       _Copy_assign_base&
 554       operator=(const _Copy_assign_base& __rhs)
 555           noexcept(_Traits<_Types...>::_S_nothrow_copy_assign)
 556       {
 557         if (this->_M_index == __rhs._M_index)
 558           {
 559             if (__rhs._M_valid())
 560               {
 561                 static constexpr void (*_S_vtable[])(void*, void*) =
 562                   { &__erased_assign<_Types&, const _Types&>... };
 563                 _S_vtable[__rhs._M_index](this->_M_storage(),
 564                                           __rhs._M_storage());
 565               }
 566           }
 567         else
 568           {
 569             _Copy_assign_base __tmp(__rhs);
 570             this->_M_destructive_move(std::move(__tmp));
 571           }
 572         __glibcxx_assert(this->_M_index == __rhs._M_index);
 573         return *this;
 574       }
 575
 576       _Copy_assign_base(const _Copy_assign_base&) = default;
 577       _Copy_assign_base(_Copy_assign_base&&) = default;
 578       _Copy_assign_base& operator=(_Copy_assign_base&&) = default;
 579     };
 580
 581   template<typename... _Types>
 582     struct _Copy_assign_base<true, _Types...> : _Move_ctor_alias<_Types...>
 583     {
 584       using _Base = _Move_ctor_alias<_Types...>;
 585       using _Base::_Base;
 586     };
 587
 588   template<typename... _Types>
 589     using _Copy_assign_alias =
 590         _Copy_assign_base<_Traits<_Types...>::_S_trivial_copy_assign,
 591                           _Types...>;
 592
 593   template<bool, typename... _Types>
 594     struct _Move_assign_base : _Copy_assign_alias<_Types...>
 595     {
 596       using _Base = _Copy_assign_alias<_Types...>;
 597       using _Base::_Base;
 598
 599       _Move_assign_base&
 600       operator=(_Move_assign_base&& __rhs)
 601           noexcept(_Traits<_Types...>::_S_nothrow_move_assign)
 602       {
 603         if (this->_M_index == __rhs._M_index)
 604           {
 605             if (__rhs._M_valid())
 606               {
 607                 static constexpr void (*_S_vtable[])(void*, void*) =
 608                   { &__erased_assign<_Types&, _Types&&>... };
 609                 _S_vtable[__rhs._M_index]
 610                   (this->_M_storage(), __rhs._M_storage());
 611               }
 612           }
 613         else
 614           {
 615             _Move_assign_base __tmp(std::move(__rhs));
 616             this->_M_destructive_move(std::move(__tmp));
 617           }
 618         __glibcxx_assert(this->_M_index == __rhs._M_index);
 619         return *this;
 620       }
 621
 622       _Move_assign_base(const _Move_assign_base&) = default;
 623       _Move_assign_base(_Move_assign_base&&) = default;
 624       _Move_assign_base& operator=(const _Move_assign_base&) = default;
 625     };
 626
 627   template<typename... _Types>
 628     struct _Move_assign_base<true, _Types...> : _Copy_assign_alias<_Types...>
 629     {
 630       using _Base = _Copy_assign_alias<_Types...>;
 631       using _Base::_Base;
 632     };
 633
 634   template<typename... _Types>
 635     using _Move_assign_alias =
 636         _Move_assign_base<_Traits<_Types...>::_S_trivial_move_assign,
 637                           _Types...>;
 638
 639   template<typename... _Types>
 640     struct _Variant_base : _Move_assign_alias<_Types...>
 641     {
 642       using _Base = _Move_assign_alias<_Types...>;
 643
 644       constexpr
 645       _Variant_base()
 646           noexcept(_Traits<_Types...>::_S_nothrow_default_ctor)
 647       : _Variant_base(in_place_index<0>) { }
 648
 649       template<size_t _Np, typename... _Args>
 650         constexpr explicit
 651         _Variant_base(in_place_index_t<_Np> __i, _Args&&... __args)
 652         : _Base(__i, std::forward<_Args>(__args)...)
 653         { }
 654
 655       _Variant_base(const _Variant_base&) = default;
 656       _Variant_base(_Variant_base&&) = default;
 657       _Variant_base& operator=(const _Variant_base&) = default;
 658       _Variant_base& operator=(_Variant_base&&) = default;
 659     };
 660
 661   // For how many times does _Tp appear in _Tuple?
 662   template<typename _Tp, typename _Tuple>
 663     struct __tuple_count;
 664
 665   template<typename _Tp, typename _Tuple>
 666     inline constexpr size_t __tuple_count_v =
 667       __tuple_count<_Tp, _Tuple>::value;
 668
 669   template<typename _Tp, typename... _Types>
 670     struct __tuple_count<_Tp, tuple<_Types...>>
 671     : integral_constant<size_t, 0> { };
 672
 673   template<typename _Tp, typename _First, typename... _Rest>
 674     struct __tuple_count<_Tp, tuple<_First, _Rest...>>
 675     : integral_constant<
 676         size_t,
 677         __tuple_count_v<_Tp, tuple<_Rest...>> + is_same_v<_Tp, _First>> { };
 678
 679   // TODO: Reuse this in <tuple> ?
 680   template<typename _Tp, typename... _Types>
 681     inline constexpr bool __exactly_once =
 682       __tuple_count_v<_Tp, tuple<_Types...>> == 1;
 683
 684   // Takes _Types and create an overloaded _S_fun for each type.
 685   // If a type appears more than once in _Types, create only one overload.
 686   template<typename... _Types>
 687     struct __overload_set
 688     { static void _S_fun(); };
 689
 690   template<typename _First, typename... _Rest>
 691     struct __overload_set<_First, _Rest...> : __overload_set<_Rest...>
 692     {
 693       using __overload_set<_Rest...>::_S_fun;
 694       static integral_constant<size_t, sizeof...(_Rest)> _S_fun(_First);
 695     };
 696
 697   template<typename... _Rest>
 698     struct __overload_set<void, _Rest...> : __overload_set<_Rest...>
 699     {
 700       using __overload_set<_Rest...>::_S_fun;
 701     };
 702
 703   // Helper for variant(_Tp&&) and variant::operator=(_Tp&&).
 704   // __accepted_index maps an arbitrary _Tp to an alternative type in _Variant
 705   // (or to variant_npos).
 706   template<typename _Tp, typename _Variant, typename = void>
 707     struct __accepted_index
 708     { static constexpr size_t value = variant_npos; };
 709
 710   template<typename _Tp, typename... _Types>
 711     struct __accepted_index<
 712       _Tp, variant<_Types...>,
 713       void_t<decltype(__overload_set<_Types...>::_S_fun(std::declval<_Tp>()))>>
 714     {
 715       static constexpr size_t value = sizeof...(_Types) - 1
 716         - decltype(__overload_set<_Types...>::
 717                    _S_fun(std::declval<_Tp>()))::value;
 718     };
 719
 720   // Returns the raw storage for __v.
 721   template<typename _Variant>
 722     void* __get_storage(_Variant&& __v)
 723     { return __v._M_storage(); }
 724
 725   // Used for storing multi-dimensional vtable.
 726   template<typename _Tp, size_t... _Dimensions>
 727     struct _Multi_array
 728     {
 729       constexpr const _Tp&
 730       _M_access() const
 731       { return _M_data; }
 732
 733       _Tp _M_data;
 734     };
 735
 736   template<typename _Tp, size_t __first, size_t... __rest>
 737     struct _Multi_array<_Tp, __first, __rest...>
 738     {
 739       template<typename... _Args>
 740         constexpr const _Tp&
 741         _M_access(size_t __first_index, _Args... __rest_indices) const
 742         { return _M_arr[__first_index]._M_access(__rest_indices...); }
 743
 744       _Multi_array<_Tp, __rest...> _M_arr[__first];
 745     };
 746
 747   // Creates a multi-dimensional vtable recursively.
 748   //
 749   // For example,
 750   // visit([](auto, auto){},
 751   //       variant<int, char>(),  // typedef'ed as V1
 752   //       variant<float, double, long double>())  // typedef'ed as V2
 753   // will trigger instantiations of:
 754   // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 2, 3>,
 755   //                   tuple<V1&&, V2&&>, std::index_sequence<>>
 756   //   __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
 757   //                     tuple<V1&&, V2&&>, std::index_sequence<0>>
 758   //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
 759   //                       tuple<V1&&, V2&&>, std::index_sequence<0, 0>>
 760   //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
 761   //                       tuple<V1&&, V2&&>, std::index_sequence<0, 1>>
 762   //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
 763   //                       tuple<V1&&, V2&&>, std::index_sequence<0, 2>>
 764   //   __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
 765   //                     tuple<V1&&, V2&&>, std::index_sequence<1>>
 766   //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
 767   //                       tuple<V1&&, V2&&>, std::index_sequence<1, 0>>
 768   //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
 769   //                       tuple<V1&&, V2&&>, std::index_sequence<1, 1>>
 770   //     __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
 771   //                       tuple<V1&&, V2&&>, std::index_sequence<1, 2>>
 772   // The returned multi-dimensional vtable can be fast accessed by the visitor
 773   // using index calculation.
 774   template<typename _Array_type, typename _Variant_tuple, typename _Index_seq>
 775     struct __gen_vtable_impl;
 776
 777   template<typename _Result_type, typename _Visitor, size_t... __dimensions,
 778            typename... _Variants, size_t... __indices>
 779     struct __gen_vtable_impl<
 780         _Multi_array<_Result_type (*)(_Visitor, _Variants...), __dimensions...>,
 781         tuple<_Variants...>, std::index_sequence<__indices...>>
 782     {
 783       using _Next =
 784           remove_reference_t<typename _Nth_type<sizeof...(__indices),
 785                              _Variants...>::type>;
 786       using _Array_type =
 787           _Multi_array<_Result_type (*)(_Visitor, _Variants...),
 788                        __dimensions...>;
 789
 790       static constexpr _Array_type
 791       _S_apply()
 792       {
 793         _Array_type __vtable{};
 794         _S_apply_all_alts(
 795           __vtable, make_index_sequence<variant_size_v<_Next>>());
 796         return __vtable;
 797       }
 798
 799       template<size_t... __var_indices>
 800         static constexpr void
 801         _S_apply_all_alts(_Array_type& __vtable,
 802                           std::index_sequence<__var_indices...>)
 803         {
 804           (_S_apply_single_alt<__var_indices>(
 805              __vtable._M_arr[__var_indices]), ...);
 806         }
 807
 808       template<size_t __index, typename _Tp>
 809         static constexpr void
 810         _S_apply_single_alt(_Tp& __element)
 811         {
 812           using _Alternative = variant_alternative_t<__index, _Next>;
 813           __element = __gen_vtable_impl<
 814             remove_reference_t<decltype(__element)>, tuple<_Variants...>,
 815             std::index_sequence<__indices..., __index>>::_S_apply();
 816         }
 817     };
 818
 819   template<typename _Result_type, typename _Visitor, typename... _Variants,
 820            size_t... __indices>
 821     struct __gen_vtable_impl<
 822       _Multi_array<_Result_type (*)(_Visitor, _Variants...)>,
 823                    tuple<_Variants...>, std::index_sequence<__indices...>>
 824     {
 825       using _Array_type =
 826           _Multi_array<_Result_type (*)(_Visitor&&, _Variants...)>;
 827
 828       static constexpr decltype(auto)
 829       __visit_invoke(_Visitor&& __visitor, _Variants... __vars)
 830       {
 831         return std::__invoke(std::forward<_Visitor>(__visitor),
 832             __variant::__get<__indices>(std::forward<_Variants>(__vars))...);
 833       }
 834
 835       static constexpr auto
 836       _S_apply()
 837       { return _Array_type{&__visit_invoke}; }
 838     };
 839
 840   template<typename _Result_type, typename _Visitor, typename... _Variants>
 841     struct __gen_vtable
 842     {
 843       using _Func_ptr = _Result_type (*)(_Visitor&&, _Variants...);
 844       using _Array_type =
 845           _Multi_array<_Func_ptr,
 846                        variant_size_v<remove_reference_t<_Variants>>...>;
 847
 848       static constexpr _Array_type
 849       _S_apply()
 850       {
 851         return __gen_vtable_impl<_Array_type, tuple<_Variants...>,
 852                                  std::index_sequence<>>::_S_apply();
 853       }
 854
 855       static constexpr auto _S_vtable = _S_apply();
 856     };
 857
 858   template<size_t _Np, typename _Tp>
 859     struct _Base_dedup : public _Tp { };
 860
 861   template<typename _Variant, typename __indices>
 862     struct _Variant_hash_base;
 863
 864   template<typename... _Types, size_t... __indices>
 865     struct _Variant_hash_base<variant<_Types...>,
 866                               std::index_sequence<__indices...>>
 867     : _Base_dedup<__indices, __poison_hash<remove_const_t<_Types>>>... { };
 868
 869 } // namespace __variant
 870 } // namespace __detail
 871
 872   template<typename _Tp, typename... _Types>
 873     constexpr bool
 874     holds_alternative(const variant<_Types...>& __v) noexcept
 875     {
 876       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 877                     "T should occur for exactly once in alternatives");
 878       return __v.index() == __detail::__variant::__index_of_v<_Tp, _Types...>;
 879     }
 880
 881   template<typename _Tp, typename... _Types>
 882     constexpr _Tp& get(variant<_Types...>& __v)
 883     {
 884       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 885                     "T should occur for exactly once in alternatives");
 886       static_assert(!is_void_v<_Tp>, "_Tp should not be void");
 887       return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
 888     }
 889
 890   template<typename _Tp, typename... _Types>
 891     constexpr _Tp&& get(variant<_Types...>&& __v)
 892     {
 893       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 894                     "T should occur for exactly once in alternatives");
 895       static_assert(!is_void_v<_Tp>, "_Tp should not be void");
 896       return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
 897         std::move(__v));
 898     }
 899
 900   template<typename _Tp, typename... _Types>
 901     constexpr const _Tp& get(const variant<_Types...>& __v)
 902     {
 903       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 904                     "T should occur for exactly once in alternatives");
 905       static_assert(!is_void_v<_Tp>, "_Tp should not be void");
 906       return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(__v);
 907     }
 908
 909   template<typename _Tp, typename... _Types>
 910     constexpr const _Tp&& get(const variant<_Types...>&& __v)
 911     {
 912       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 913                     "T should occur for exactly once in alternatives");
 914       static_assert(!is_void_v<_Tp>, "_Tp should not be void");
 915       return std::get<__detail::__variant::__index_of_v<_Tp, _Types...>>(
 916         std::move(__v));
 917     }
 918
 919   template<size_t _Np, typename... _Types>
 920     constexpr add_pointer_t<variant_alternative_t<_Np, variant<_Types...>>>
 921     get_if(variant<_Types...>* __ptr) noexcept
 922     {
 923       using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
 924       static_assert(_Np < sizeof...(_Types),
 925                     "The index should be in [0, number of alternatives)");
 926       static_assert(!is_void_v<_Alternative_type>, "_Tp should not be void");
 927       if (__ptr && __ptr->index() == _Np)
 928         return &__detail::__variant::__get<_Np>(*__ptr);
 929       return nullptr;
 930     }
 931
 932   template<size_t _Np, typename... _Types>
 933     constexpr
 934     add_pointer_t<const variant_alternative_t<_Np, variant<_Types...>>>
 935     get_if(const variant<_Types...>* __ptr) noexcept
 936     {
 937       using _Alternative_type = variant_alternative_t<_Np, variant<_Types...>>;
 938       static_assert(_Np < sizeof...(_Types),
 939                     "The index should be in [0, number of alternatives)");
 940       static_assert(!is_void_v<_Alternative_type>, "_Tp should not be void");
 941       if (__ptr && __ptr->index() == _Np)
 942         return &__detail::__variant::__get<_Np>(*__ptr);
 943       return nullptr;
 944     }
 945
 946   template<typename _Tp, typename... _Types>
 947     constexpr add_pointer_t<_Tp>
 948     get_if(variant<_Types...>* __ptr) noexcept
 949     {
 950       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 951                     "T should occur for exactly once in alternatives");
 952       static_assert(!is_void_v<_Tp>, "_Tp should not be void");
 953       return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
 954           __ptr);
 955     }
 956
 957   template<typename _Tp, typename... _Types>
 958     constexpr add_pointer_t<const _Tp>
 959     get_if(const variant<_Types...>* __ptr)
 960     noexcept
 961     {
 962       static_assert(__detail::__variant::__exactly_once<_Tp, _Types...>,
 963                     "T should occur for exactly once in alternatives");
 964       static_assert(!is_void_v<_Tp>, "_Tp should not be void");
 965       return std::get_if<__detail::__variant::__index_of_v<_Tp, _Types...>>(
 966           __ptr);
 967     }
 968
 969   struct monostate { };
 970
 971 #define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
 972   template<typename... _Types> \
 973     constexpr bool operator __OP(const variant<_Types...>& __lhs, \
 974                                  const variant<_Types...>& __rhs) \
 975     { \
 976       return __lhs._M_##__NAME(__rhs, std::index_sequence_for<_Types...>{}); \
 977     } \
 978 \
 979   constexpr bool operator __OP(monostate, monostate) noexcept \
 980   { return 0 __OP 0; }
 981
 982   _VARIANT_RELATION_FUNCTION_TEMPLATE(<, less)
 983   _VARIANT_RELATION_FUNCTION_TEMPLATE(<=, less_equal)
 984   _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
 985   _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
 986   _VARIANT_RELATION_FUNCTION_TEMPLATE(>=, greater_equal)
 987   _VARIANT_RELATION_FUNCTION_TEMPLATE(>, greater)
 988
 989 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
 990
 991   template<typename _Visitor, typename... _Variants>
 992     constexpr decltype(auto) visit(_Visitor&&, _Variants&&...);
 993
 994   template<typename... _Types>
 995     inline enable_if_t<(is_move_constructible_v<_Types> && ...)
 996                         && (is_swappable_v<_Types> && ...)>
 997     swap(variant<_Types...>& __lhs, variant<_Types...>& __rhs)
 998     noexcept(noexcept(__lhs.swap(__rhs)))
 999     { __lhs.swap(__rhs); }
1000
1001   template<typename... _Types>
1002     enable_if_t<!((is_move_constructible_v<_Types> && ...)
1003                    && (is_swappable_v<_Types> && ...))>
1004     swap(variant<_Types...>&, variant<_Types...>&) = delete;
1005
1006   class bad_variant_access : public exception
1007   {
1008   public:
1009     bad_variant_access() noexcept : _M_reason("Unknown reason") { }
1010     const char* what() const noexcept override
1011     { return _M_reason; }
1012
1013   private:
1014     bad_variant_access(const char* __reason) : _M_reason(__reason) { }
1015
1016     const char* _M_reason;
1017
1018     friend void __throw_bad_variant_access(const char* __what);
1019   };
1020
1021   inline void
1022   __throw_bad_variant_access(const char* __what)
1023   { _GLIBCXX_THROW_OR_ABORT(bad_variant_access(__what)); }
1024
1025   template<typename... _Types>
1026     class variant
1027     : private __detail::__variant::_Variant_base<_Types...>,
1028       private _Enable_default_constructor<
1029         __detail::__variant::_Traits<_Types...>::_S_default_ctor,
1030           variant<_Types...>>,
1031       private _Enable_copy_move<
1032         __detail::__variant::_Traits<_Types...>::_S_copy_ctor,
1033         __detail::__variant::_Traits<_Types...>::_S_copy_assign,
1034         __detail::__variant::_Traits<_Types...>::_S_move_ctor,
1035         __detail::__variant::_Traits<_Types...>::_S_move_assign,
1036         variant<_Types...>>
1037     {
1038     private:
1039       static_assert(sizeof...(_Types) > 0,
1040                     "variant must have at least one alternative");
1041       static_assert(!(std::is_reference_v<_Types> || ...),
1042                     "variant must have no reference alternative");
1043       static_assert(!(std::is_void_v<_Types> || ...),
1044                     "variant must have no void alternative");
1045
1046       using _Base = __detail::__variant::_Variant_base<_Types...>;
1047       using _Default_ctor_enabler =
1048         _Enable_default_constructor<
1049           __detail::__variant::_Traits<_Types...>::_S_default_ctor,
1050             variant<_Types...>>;
1051
1052       template<typename _Tp>
1053         static constexpr bool
1054         __exactly_once = __detail::__variant::__exactly_once<_Tp, _Types...>;
1055
1056       template<typename _Tp>
1057         static constexpr size_t __accepted_index =
1058           __detail::__variant::__accepted_index<_Tp&&, variant>::value;
1059
1060       template<size_t _Np, bool = _Np < sizeof...(_Types)>
1061         struct __to_type_impl;
1062
1063       template<size_t _Np>
1064         struct __to_type_impl<_Np, true>
1065         { using type = variant_alternative_t<_Np, variant>; };
1066
1067       template<size_t _Np>
1068         using __to_type = typename __to_type_impl<_Np>::type;
1069
1070       template<typename _Tp>
1071         using __accepted_type = __to_type<__accepted_index<_Tp>>;
1072
1073       template<typename _Tp>
1074         static constexpr size_t __index_of =
1075           __detail::__variant::__index_of_v<_Tp, _Types...>;
1076
1077       using _Traits = __detail::__variant::_Traits<_Types...>;
1078
1079     public:
1080       variant() = default;
1081       variant(const variant& __rhs) = default;
1082       variant(variant&&) = default;
1083       variant& operator=(const variant&) = default;
1084       variant& operator=(variant&&) = default;
1085       ~variant() = default;
1086
1087       template<typename _Tp,
1088                typename = enable_if_t<!is_same_v<decay_t<_Tp>, variant>>,
1089                typename = enable_if_t<(sizeof...(_Types)>0)>,
1090                typename = enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
1091                           && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>>>
1092         constexpr
1093         variant(_Tp&& __t)
1094         noexcept(is_nothrow_constructible_v<__accepted_type<_Tp&&>, _Tp&&>)
1095         : variant(in_place_index<__accepted_index<_Tp&&>>,
1096                   std::forward<_Tp>(__t))
1097         { __glibcxx_assert(holds_alternative<__accepted_type<_Tp&&>>(*this)); }
1098
1099       template<typename _Tp, typename... _Args,
1100                typename = enable_if_t<__exactly_once<_Tp>
1101                           && is_constructible_v<_Tp, _Args&&...>>>
1102         constexpr explicit
1103         variant(in_place_type_t<_Tp>, _Args&&... __args)
1104         : variant(in_place_index<__index_of<_Tp>>,
1105                   std::forward<_Args>(__args)...)
1106         { __glibcxx_assert(holds_alternative<_Tp>(*this)); }
1107
1108       template<typename _Tp, typename _Up, typename... _Args,
1109                typename = enable_if_t<__exactly_once<_Tp>
1110                           && is_constructible_v<
1111                             _Tp, initializer_list<_Up>&, _Args&&...>>>
1112         constexpr explicit
1113         variant(in_place_type_t<_Tp>, initializer_list<_Up> __il,
1114                 _Args&&... __args)
1115         : variant(in_place_index<__index_of<_Tp>>, __il,
1116                   std::forward<_Args>(__args)...)
1117         { __glibcxx_assert(holds_alternative<_Tp>(*this)); }
1118
1119       template<size_t _Np, typename... _Args,
1120                typename = enable_if_t<
1121                  is_constructible_v<__to_type<_Np>, _Args&&...>>>
1122         constexpr explicit
1123         variant(in_place_index_t<_Np>, _Args&&... __args)
1124         : _Base(in_place_index<_Np>, std::forward<_Args>(__args)...),
1125         _Default_ctor_enabler(_Enable_default_constructor_tag{})
1126         { __glibcxx_assert(index() == _Np); }
1127
1128       template<size_t _Np, typename _Up, typename... _Args,
1129                typename = enable_if_t<is_constructible_v<__to_type<_Np>,
1130                                       initializer_list<_Up>&, _Args&&...>>>
1131         constexpr explicit
1132         variant(in_place_index_t<_Np>, initializer_list<_Up> __il,
1133                 _Args&&... __args)
1134         : _Base(in_place_index<_Np>, __il, std::forward<_Args>(__args)...),
1135         _Default_ctor_enabler(_Enable_default_constructor_tag{})
1136         { __glibcxx_assert(index() == _Np); }
1137
1138       template<typename _Tp>
1139         enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
1140                     && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>
1141                     && is_assignable_v<__accepted_type<_Tp&&>&, _Tp&&>
1142                     && !is_same_v<decay_t<_Tp>, variant>, variant&>
1143         operator=(_Tp&& __rhs)
1144         noexcept(is_nothrow_assignable_v<__accepted_type<_Tp&&>&, _Tp&&>
1145                  && is_nothrow_constructible_v<__accepted_type<_Tp&&>, _Tp&&>)
1146         {
1147           constexpr auto __index = __accepted_index<_Tp&&>;
1148           if (index() == __index)
1149             std::get<__index>(*this) = std::forward<_Tp>(__rhs);
1150           else
1151             this->emplace<__index>(std::forward<_Tp>(__rhs));
1152           __glibcxx_assert(holds_alternative<__accepted_type<_Tp&&>>(*this));
1153           return *this;
1154         }
1155
1156       template<typename _Tp, typename... _Args>
1157         enable_if_t<is_constructible_v<_Tp, _Args...> && __exactly_once<_Tp>,
1158                     _Tp&>
1159         emplace(_Args&&... __args)
1160         {
1161           auto& ret =
1162             this->emplace<__index_of<_Tp>>(std::forward<_Args>(__args)...);
1163           __glibcxx_assert(holds_alternative<_Tp>(*this));
1164           return ret;
1165         }
1166
1167       template<typename _Tp, typename _Up, typename... _Args>
1168         enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...>
1169                     && __exactly_once<_Tp>,
1170                     _Tp&>
1171         emplace(initializer_list<_Up> __il, _Args&&... __args)
1172         {
1173           auto& ret =
1174             this->emplace<__index_of<_Tp>>(__il,
1175                                            std::forward<_Args>(__args)...);
1176           __glibcxx_assert(holds_alternative<_Tp>(*this));
1177           return ret;
1178         }
1179
1180       template<size_t _Np, typename... _Args>
1181         enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,
1182                                        _Args...>,
1183                     variant_alternative_t<_Np, variant>&>
1184         emplace(_Args&&... __args)
1185         {
1186           static_assert(_Np < sizeof...(_Types),
1187                         "The index should be in [0, number of alternatives)");
1188           this->~variant();
1189           __try
1190             {
1191               ::new (this) variant(in_place_index<_Np>,
1192                                    std::forward<_Args>(__args)...);
1193             }
1194           __catch (...)
1195             {
1196               this->_M_index = variant_npos;
1197               __throw_exception_again;
1198             }
1199           __glibcxx_assert(index() == _Np);
1200           return std::get<_Np>(*this);
1201         }
1202
1203       template<size_t _Np, typename _Up, typename... _Args>
1204         enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,
1205                                        initializer_list<_Up>&, _Args...>,
1206                     variant_alternative_t<_Np, variant>&>
1207         emplace(initializer_list<_Up> __il, _Args&&... __args)
1208         {
1209           static_assert(_Np < sizeof...(_Types),
1210                         "The index should be in [0, number of alternatives)");
1211           this->~variant();
1212           __try
1213             {
1214               ::new (this) variant(in_place_index<_Np>, __il,
1215                                    std::forward<_Args>(__args)...);
1216             }
1217           __catch (...)
1218             {
1219               this->_M_index = variant_npos;
1220               __throw_exception_again;
1221             }
1222           __glibcxx_assert(index() == _Np);
1223           return std::get<_Np>(*this);
1224         }
1225
1226       constexpr bool valueless_by_exception() const noexcept
1227       { return !this->_M_valid(); }
1228
1229       constexpr size_t index() const noexcept
1230       {
1231         if (this->_M_index ==
1232             typename _Base::__index_type(variant_npos))
1233           return variant_npos;
1234         return this->_M_index;
1235       }
1236
1237       void
1238       swap(variant& __rhs)
1239       noexcept((__is_nothrow_swappable<_Types>::value && ...)
1240                && is_nothrow_move_constructible_v<variant>)
1241       {
1242         if (this->index() == __rhs.index())
1243           {
1244             if (this->_M_valid())
1245               {
1246                 static constexpr void (*_S_vtable[])(void*, void*) =
1247                   { &__detail::__variant::__erased_swap<_Types&, _Types&>... };
1248                 _S_vtable[__rhs._M_index](this->_M_storage(),
1249                                           __rhs._M_storage());
1250               }
1251           }
1252         else if (!this->_M_valid())
1253           {
1254             this->_M_destructive_move(std::move(__rhs));
1255             __rhs._M_reset();
1256           }
1257         else if (!__rhs._M_valid())
1258           {
1259             __rhs._M_destructive_move(std::move(*this));
1260             this->_M_reset();
1261           }
1262         else
1263           {
1264             auto __tmp = std::move(__rhs);
1265             __rhs._M_destructive_move(std::move(*this));
1266             this->_M_destructive_move(std::move(__tmp));
1267           }
1268       }
1269
1270     private:
1271 #define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP, __NAME) \
1272       template<size_t... __indices> \
1273         static constexpr bool \
1274         (*_S_erased_##__NAME[])(const variant&, const variant&) = \
1275           { &__detail::__variant::__erased_##__NAME< \
1276                 const variant&, __indices>... }; \
1277       template<size_t... __indices> \
1278         constexpr bool \
1279         _M_##__NAME(const variant& __rhs, \
1280                     std::index_sequence<__indices...>) const \
1281         { \
1282           auto __lhs_index = this->index(); \
1283           auto __rhs_index = __rhs.index(); \
1284           if (__lhs_index != __rhs_index || valueless_by_exception()) \
1285             /* Modulo addition. */ \
1286             return __lhs_index + 1 __OP __rhs_index + 1; \
1287           return _S_erased_##__NAME<__indices...>[__lhs_index](*this, __rhs); \
1288         }
1289
1290       _VARIANT_RELATION_FUNCTION_TEMPLATE(<, less)
1291       _VARIANT_RELATION_FUNCTION_TEMPLATE(<=, less_equal)
1292       _VARIANT_RELATION_FUNCTION_TEMPLATE(==, equal)
1293       _VARIANT_RELATION_FUNCTION_TEMPLATE(!=, not_equal)
1294       _VARIANT_RELATION_FUNCTION_TEMPLATE(>=, greater_equal)
1295       _VARIANT_RELATION_FUNCTION_TEMPLATE(>, greater)
1296
1297 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
1298
1299 #if defined(__clang__) && __clang_major__ <= 7
1300     public:
1301       using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
1302     private:
1303 #endif
1304
1305       template<size_t _Np, typename _Vp>
1306         friend constexpr decltype(auto) __detail::__variant::__get(_Vp&& __v);
1307
1308       template<typename _Vp>
1309         friend void* __detail::__variant::__get_storage(_Vp&& __v);
1310
1311 #define _VARIANT_RELATION_FUNCTION_TEMPLATE(__OP) \
1312       template<typename... _Tp> \
1313         friend constexpr bool \
1314         operator __OP(const variant<_Tp...>& __lhs, \
1315                       const variant<_Tp...>& __rhs);
1316
1317       _VARIANT_RELATION_FUNCTION_TEMPLATE(<)
1318       _VARIANT_RELATION_FUNCTION_TEMPLATE(<=)
1319       _VARIANT_RELATION_FUNCTION_TEMPLATE(==)
1320       _VARIANT_RELATION_FUNCTION_TEMPLATE(!=)
1321       _VARIANT_RELATION_FUNCTION_TEMPLATE(>=)
1322       _VARIANT_RELATION_FUNCTION_TEMPLATE(>)
1323
1324 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
1325     };
1326
1327   template<size_t _Np, typename... _Types>
1328     constexpr variant_alternative_t<_Np, variant<_Types...>>&
1329     get(variant<_Types...>& __v)
1330     {
1331       static_assert(_Np < sizeof...(_Types),
1332                     "The index should be in [0, number of alternatives)");
1333       if (__v.index() != _Np)
1334         __throw_bad_variant_access("Unexpected index");
1335       return __detail::__variant::__get<_Np>(__v);
1336     }
1337
1338   template<size_t _Np, typename... _Types>
1339     constexpr variant_alternative_t<_Np, variant<_Types...>>&&
1340     get(variant<_Types...>&& __v)
1341     {
1342       static_assert(_Np < sizeof...(_Types),
1343                     "The index should be in [0, number of alternatives)");
1344       if (__v.index() != _Np)
1345         __throw_bad_variant_access("Unexpected index");
1346       return __detail::__variant::__get<_Np>(std::move(__v));
1347     }
1348
1349   template<size_t _Np, typename... _Types>
1350     constexpr const variant_alternative_t<_Np, variant<_Types...>>&
1351     get(const variant<_Types...>& __v)
1352     {
1353       static_assert(_Np < sizeof...(_Types),
1354                     "The index should be in [0, number of alternatives)");
1355       if (__v.index() != _Np)
1356         __throw_bad_variant_access("Unexpected index");
1357       return __detail::__variant::__get<_Np>(__v);
1358     }
1359
1360   template<size_t _Np, typename... _Types>
1361     constexpr const variant_alternative_t<_Np, variant<_Types...>>&&
1362     get(const variant<_Types...>&& __v)
1363     {
1364       static_assert(_Np < sizeof...(_Types),
1365                     "The index should be in [0, number of alternatives)");
1366       if (__v.index() != _Np)
1367         __throw_bad_variant_access("Unexpected index");
1368       return __detail::__variant::__get<_Np>(std::move(__v));
1369     }
1370
1371   template<typename _Visitor, typename... _Variants>
1372     constexpr decltype(auto)
1373     visit(_Visitor&& __visitor, _Variants&&... __variants)
1374     {
1375       if ((__variants.valueless_by_exception() || ...))
1376         __throw_bad_variant_access("Unexpected index");
1377
1378       using _Result_type =
1379         decltype(std::forward<_Visitor>(__visitor)(
1380             std::get<0>(std::forward<_Variants>(__variants))...));
1381
1382       constexpr auto& __vtable = __detail::__variant::__gen_vtable<
1383         _Result_type, _Visitor&&, _Variants&&...>::_S_vtable;
1384
1385       auto __func_ptr = __vtable._M_access(__variants.index()...);
1386       return (*__func_ptr)(std::forward<_Visitor>(__visitor),
1387                            std::forward<_Variants>(__variants)...);
1388     }
1389
1390   template<bool, typename... _Types>
1391     struct __variant_hash_call_base_impl
1392     {
1393       size_t
1394       operator()(const variant<_Types...>& __t) const
1395       noexcept((is_nothrow_invocable_v<hash<decay_t<_Types>>, _Types> && ...))
1396       {
1397         if (!__t.valueless_by_exception())
1398           {
1399             namespace __edv = __detail::__variant;
1400             static constexpr size_t (*_S_vtable[])(void*) =
1401               { &__edv::__erased_hash<const _Types&>... };
1402             return hash<size_t>{}(__t.index())
1403               + _S_vtable[__t.index()](__edv::__get_storage(__t));
1404           }
1405         return hash<size_t>{}(__t.index());
1406       }
1407     };
1408
1409   template<typename... _Types>
1410     struct __variant_hash_call_base_impl<false, _Types...> {};
1411
1412   template<typename... _Types>
1413     using __variant_hash_call_base =
1414     __variant_hash_call_base_impl<(__poison_hash<remove_const_t<_Types>>::
1415                                    __enable_hash_call &&...), _Types...>;
1416
1417   template<typename... _Types>
1418     struct hash<variant<_Types...>>
1419     : private __detail::__variant::_Variant_hash_base<
1420         variant<_Types...>, std::index_sequence_for<_Types...>>,
1421       public __variant_hash_call_base<_Types...>
1422     {
1423       using result_type [[__deprecated__]] = size_t;
1424       using argument_type [[__deprecated__]] = variant<_Types...>;
1425     };
1426
1427   template<>
1428     struct hash<monostate>
1429     {
1430       using result_type [[__deprecated__]] = size_t;
1431       using argument_type [[__deprecated__]] = monostate;
1432
1433       size_t
1434       operator()(const monostate& __t) const noexcept
1435       {
1436         constexpr size_t __magic_monostate_hash = -7777;
1437         return __magic_monostate_hash;
1438       }
1439     };
1440
1441   template<typename... _Types>
1442     struct __is_fast_hash<hash<variant<_Types...>>>
1443     : bool_constant<(__is_fast_hash<_Types>::value && ...)>
1444     { };
1445
1446 _GLIBCXX_END_NAMESPACE_VERSION
1447 } // namespace std
1448
1449 #endif // C++17
1450
1451 #endif // _GLIBCXX_VARIANT