libstdc++-v3/include/experimental/optional

   1 // <optional> -*- C++ -*-
   2
   3 // Copyright (C) 2013-2022 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 experimental/optional
  26  *  This is a TS C++ Library header.
  27  *  @ingroup libfund-ts
  28  */
  29
  30 #ifndef _GLIBCXX_EXPERIMENTAL_OPTIONAL
  31 #define _GLIBCXX_EXPERIMENTAL_OPTIONAL 1
  32
  33 #if __cplusplus >= 201402L
  34
  35 #include <type_traits>
  36 #include <stdexcept>
  37 #include <new>
  38 #include <initializer_list>
  39 #include <bits/functexcept.h>
  40 #include <bits/functional_hash.h>
  41 #include <bits/enable_special_members.h>
  42 #include <bits/move.h>
  43 #include <experimental/bits/lfts_config.h>
  44
  45 namespace std _GLIBCXX_VISIBILITY(default)
  46 {
  47 _GLIBCXX_BEGIN_NAMESPACE_VERSION
  48
  49 namespace experimental
  50 {
  51 inline namespace fundamentals_v1
  52 {
  53   /**
  54    * @defgroup optional Optional values
  55    * @ingroup libfund-ts
  56    *
  57    * Class template for optional values and surrounding facilities, as
  58    * described in n3793 "A proposal to add a utility class to represent
  59    * optional objects (Revision 5)".
  60    *
  61    * @{
  62    */
  63
  64 #define __cpp_lib_experimental_optional 201411
  65
  66   // All subsequent [X.Y.n] references are against n3793.
  67
  68   // [X.Y.4]
  69   template<typename _Tp>
  70     class optional;
  71
  72   // [X.Y.5]
  73   /// Tag type for in-place construction.
  74   struct in_place_t { };
  75
  76   /// Tag for in-place construction.
  77   constexpr in_place_t in_place { };
  78
  79   // [X.Y.6]
  80   /// Tag type to disengage optional objects.
  81   struct nullopt_t
  82   {
  83     // Do not user-declare default constructor at all for
  84     // optional_value = {} syntax to work.
  85     // nullopt_t() = delete;
  86
  87     // Used for constructing nullopt.
  88     enum class _Construct { _Token };
  89
  90     // Must be constexpr for nullopt_t to be literal.
  91     explicit constexpr nullopt_t(_Construct) { }
  92   };
  93
  94   // [X.Y.6]
  95   /// Tag to disengage optional objects.
  96   constexpr nullopt_t nullopt { nullopt_t::_Construct::_Token };
  97
  98   // [X.Y.7]
  99   /**
 100    *  @brief Exception class thrown when a disengaged optional object is
 101    *  dereferenced.
 102    *  @ingroup exceptions
 103    */
 104   class bad_optional_access : public logic_error
 105   {
 106   public:
 107     bad_optional_access() : logic_error("bad optional access") { }
 108
 109     // XXX This constructor is non-standard. Should not be inline
 110     explicit bad_optional_access(const char* __arg) : logic_error(__arg) { }
 111
 112     virtual ~bad_optional_access() noexcept = default;
 113   };
 114
 115   /// @cond undocumented
 116
 117   // XXX Does not belong here.
 118   [[noreturn]] inline void
 119   __throw_bad_optional_access(const char* __s)
 120   { _GLIBCXX_THROW_OR_ABORT(bad_optional_access(__s)); }
 121
 122   /**
 123     * @brief Class template that holds the necessary state for @ref optional
 124     * and that has the responsibility for construction and the special members.
 125     *
 126     * Such a separate base class template is necessary in order to
 127     * conditionally enable the special members (e.g. copy/move constructors).
 128     * Note that this means that @ref _Optional_base implements the
 129     * functionality for copy and move assignment, but not for converting
 130     * assignment.
 131     *
 132     * @see optional, _Enable_special_members
 133     */
 134   template<typename _Tp, bool _ShouldProvideDestructor =
 135            !is_trivially_destructible<_Tp>::value>
 136     class _Optional_base
 137     {
 138     private:
 139       // Remove const to avoid prohibition of reusing object storage for
 140       // const-qualified types in [3.8/9]. This is strictly internal
 141       // and even optional itself is oblivious to it.
 142       using _Stored_type = remove_const_t<_Tp>;
 143
 144     public:
 145       // [X.Y.4.1] Constructors.
 146
 147       // Constructors for disengaged optionals.
 148       constexpr _Optional_base() noexcept
 149       : _M_empty{} { }
 150
 151       constexpr _Optional_base(nullopt_t) noexcept
 152       : _Optional_base{} { }
 153
 154       // Constructors for engaged optionals.
 155       template<typename... _Args>
 156         constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
 157         : _M_payload(std::forward<_Args>(__args)...), _M_engaged(true) { }
 158
 159       template<typename _Up, typename... _Args,
 160                enable_if_t<is_constructible<_Tp,
 161                                             initializer_list<_Up>&,
 162                                             _Args&&...>::value,
 163                            int>...>
 164         constexpr explicit _Optional_base(in_place_t,
 165                                           initializer_list<_Up> __il,
 166                                           _Args&&... __args)
 167         : _M_payload(__il, std::forward<_Args>(__args)...),
 168           _M_engaged(true) { }
 169
 170       // Copy and move constructors.
 171       _Optional_base(const _Optional_base& __other)
 172       {
 173         if (__other._M_engaged)
 174           this->_M_construct(__other._M_get());
 175       }
 176
 177       _Optional_base(_Optional_base&& __other)
 178       noexcept(is_nothrow_move_constructible<_Tp>())
 179       {
 180         if (__other._M_engaged)
 181           this->_M_construct(std::move(__other._M_get()));
 182       }
 183
 184       // [X.Y.4.3] (partly) Assignment.
 185       _Optional_base&
 186       operator=(const _Optional_base& __other)
 187       {
 188         if (this->_M_engaged && __other._M_engaged)
 189           this->_M_get() = __other._M_get();
 190         else
 191           {
 192             if (__other._M_engaged)
 193               this->_M_construct(__other._M_get());
 194             else
 195               this->_M_reset();
 196           }
 197
 198         return *this;
 199       }
 200
 201       _Optional_base&
 202       operator=(_Optional_base&& __other)
 203       noexcept(__and_<is_nothrow_move_constructible<_Tp>,
 204                       is_nothrow_move_assignable<_Tp>>())
 205       {
 206         if (this->_M_engaged && __other._M_engaged)
 207           this->_M_get() = std::move(__other._M_get());
 208         else
 209           {
 210             if (__other._M_engaged)
 211               this->_M_construct(std::move(__other._M_get()));
 212             else
 213               this->_M_reset();
 214           }
 215         return *this;
 216       }
 217
 218       // [X.Y.4.2] Destructor.
 219       ~_Optional_base()
 220       {
 221         if (this->_M_engaged)
 222           this->_M_payload.~_Stored_type();
 223       }
 224
 225       // The following functionality is also needed by optional, hence the
 226       // protected accessibility.
 227     protected:
 228       constexpr bool _M_is_engaged() const noexcept
 229       { return this->_M_engaged; }
 230
 231       // The _M_get operations have _M_engaged as a precondition.
 232       constexpr _Tp&
 233       _M_get() noexcept
 234       { return _M_payload; }
 235
 236       constexpr const _Tp&
 237       _M_get() const noexcept
 238       { return _M_payload; }
 239
 240       // The _M_construct operation has !_M_engaged as a precondition
 241       // while _M_destruct has _M_engaged as a precondition.
 242       template<typename... _Args>
 243         void
 244         _M_construct(_Args&&... __args)
 245         noexcept(is_nothrow_constructible<_Stored_type, _Args...>())
 246         {
 247           ::new (std::__addressof(this->_M_payload))
 248             _Stored_type(std::forward<_Args>(__args)...);
 249           this->_M_engaged = true;
 250         }
 251
 252       void
 253       _M_destruct()
 254       {
 255         this->_M_engaged = false;
 256         this->_M_payload.~_Stored_type();
 257       }
 258
 259       // _M_reset is a 'safe' operation with no precondition.
 260       void
 261       _M_reset()
 262       {
 263         if (this->_M_engaged)
 264           this->_M_destruct();
 265       }
 266
 267     private:
 268       struct _Empty_byte { };
 269       union {
 270           _Empty_byte _M_empty;
 271           _Stored_type _M_payload;
 272       };
 273       bool _M_engaged = false;
 274     };
 275
 276   /// Partial specialization that is exactly identical to the primary template
 277   /// save for not providing a destructor, to fulfill triviality requirements.
 278   template<typename _Tp>
 279     class _Optional_base<_Tp, false>
 280     {
 281     private:
 282       using _Stored_type = remove_const_t<_Tp>;
 283
 284     public:
 285       constexpr _Optional_base() noexcept
 286       : _M_empty{} { }
 287
 288       constexpr _Optional_base(nullopt_t) noexcept
 289       : _Optional_base{} { }
 290
 291       template<typename... _Args>
 292         constexpr explicit _Optional_base(in_place_t, _Args&&... __args)
 293         : _M_payload(std::forward<_Args>(__args)...), _M_engaged(true) { }
 294
 295       template<typename _Up, typename... _Args,
 296                enable_if_t<is_constructible<_Tp,
 297                                             initializer_list<_Up>&,
 298                                             _Args&&...>::value,
 299                            int>...>
 300         constexpr explicit _Optional_base(in_place_t,
 301                                           initializer_list<_Up> __il,
 302                                           _Args&&... __args)
 303         : _M_payload(__il, std::forward<_Args>(__args)...),
 304           _M_engaged(true) { }
 305
 306       _Optional_base(const _Optional_base& __other)
 307       {
 308         if (__other._M_engaged)
 309           this->_M_construct(__other._M_get());
 310       }
 311
 312       _Optional_base(_Optional_base&& __other)
 313       noexcept(is_nothrow_move_constructible<_Tp>())
 314       {
 315         if (__other._M_engaged)
 316           this->_M_construct(std::move(__other._M_get()));
 317       }
 318
 319       _Optional_base&
 320       operator=(const _Optional_base& __other)
 321       {
 322         if (this->_M_engaged && __other._M_engaged)
 323           this->_M_get() = __other._M_get();
 324         else
 325           {
 326             if (__other._M_engaged)
 327               this->_M_construct(__other._M_get());
 328             else
 329               this->_M_reset();
 330           }
 331         return *this;
 332       }
 333
 334       _Optional_base&
 335       operator=(_Optional_base&& __other)
 336       noexcept(__and_<is_nothrow_move_constructible<_Tp>,
 337                       is_nothrow_move_assignable<_Tp>>())
 338       {
 339         if (this->_M_engaged && __other._M_engaged)
 340           this->_M_get() = std::move(__other._M_get());
 341         else
 342           {
 343             if (__other._M_engaged)
 344               this->_M_construct(std::move(__other._M_get()));
 345             else
 346               this->_M_reset();
 347           }
 348         return *this;
 349       }
 350
 351       // Sole difference
 352       // ~_Optional_base() noexcept = default;
 353
 354     protected:
 355       constexpr bool _M_is_engaged() const noexcept
 356       { return this->_M_engaged; }
 357
 358       _Tp&
 359       _M_get() noexcept
 360       { return _M_payload; }
 361
 362       constexpr const _Tp&
 363       _M_get() const noexcept
 364       { return _M_payload; }
 365
 366       template<typename... _Args>
 367         void
 368         _M_construct(_Args&&... __args)
 369         noexcept(is_nothrow_constructible<_Stored_type, _Args...>())
 370         {
 371           ::new (std::__addressof(this->_M_payload))
 372             _Stored_type(std::forward<_Args>(__args)...);
 373           this->_M_engaged = true;
 374         }
 375
 376       void
 377       _M_destruct()
 378       {
 379         this->_M_engaged = false;
 380         this->_M_payload.~_Stored_type();
 381       }
 382
 383       void
 384       _M_reset()
 385       {
 386         if (this->_M_engaged)
 387           this->_M_destruct();
 388       }
 389
 390     private:
 391       struct _Empty_byte { };
 392       union
 393       {
 394         _Empty_byte _M_empty;
 395         _Stored_type _M_payload;
 396       };
 397       bool _M_engaged = false;
 398     };
 399
 400   template<typename _Tp, typename _Up>
 401     using __converts_from_optional =
 402       __or_<is_constructible<_Tp, const optional<_Up>&>,
 403             is_constructible<_Tp, optional<_Up>&>,
 404             is_constructible<_Tp, const optional<_Up>&&>,
 405             is_constructible<_Tp, optional<_Up>&&>,
 406             is_convertible<const optional<_Up>&, _Tp>,
 407             is_convertible<optional<_Up>&, _Tp>,
 408             is_convertible<const optional<_Up>&&, _Tp>,
 409             is_convertible<optional<_Up>&&, _Tp>>;
 410
 411   template<typename _Tp, typename _Up>
 412     using __assigns_from_optional =
 413       __or_<is_assignable<_Tp&, const optional<_Up>&>,
 414             is_assignable<_Tp&, optional<_Up>&>,
 415             is_assignable<_Tp&, const optional<_Up>&&>,
 416             is_assignable<_Tp&, optional<_Up>&&>>;
 417
 418   /// @endcond
 419
 420   /**
 421     * @brief Class template for optional values.
 422     */
 423   template<typename _Tp>
 424     class optional
 425     : private _Optional_base<_Tp>,
 426       private _Enable_copy_move<
 427         // Copy constructor.
 428         is_copy_constructible<_Tp>::value,
 429         // Copy assignment.
 430         __and_<is_copy_constructible<_Tp>, is_copy_assignable<_Tp>>::value,
 431         // Move constructor.
 432         is_move_constructible<_Tp>::value,
 433         // Move assignment.
 434         __and_<is_move_constructible<_Tp>, is_move_assignable<_Tp>>::value,
 435         // Unique tag type.
 436         optional<_Tp>>
 437     {
 438       static_assert(__and_<__not_<is_same<remove_cv_t<_Tp>, nullopt_t>>,
 439                            __not_<is_same<remove_cv_t<_Tp>, in_place_t>>,
 440                            __not_<is_reference<_Tp>>>(),
 441                     "Invalid instantiation of optional<T>");
 442
 443     private:
 444       using _Base = _Optional_base<_Tp>;
 445
 446     public:
 447       using value_type = _Tp;
 448
 449       // _Optional_base has the responsibility for construction.
 450       using _Base::_Base;
 451
 452       constexpr optional() = default;
 453       // Converting constructors for engaged optionals.
 454       template <typename _Up = _Tp,
 455                 enable_if_t<__and_<
 456                               __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
 457                               is_constructible<_Tp, _Up&&>,
 458                               is_convertible<_Up&&, _Tp>
 459                               >::value, bool> = true>
 460       constexpr optional(_Up&& __t)
 461         : _Base(in_place, std::forward<_Up>(__t)) { }
 462
 463       template <typename _Up = _Tp,
 464                 enable_if_t<__and_<
 465                               __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
 466                               is_constructible<_Tp, _Up&&>,
 467                               __not_<is_convertible<_Up&&, _Tp>>
 468                               >::value, bool> = false>
 469       explicit constexpr optional(_Up&& __t)
 470         : _Base(in_place, std::forward<_Up>(__t)) { }
 471
 472       template <typename _Up,
 473                 enable_if_t<__and_<
 474                             __not_<is_same<_Tp, _Up>>,
 475                             is_constructible<_Tp, const _Up&>,
 476                             is_convertible<const _Up&, _Tp>,
 477                             __not_<__converts_from_optional<_Tp, _Up>>
 478                             >::value, bool> = true>
 479       constexpr optional(const optional<_Up>& __t)
 480       {
 481         if (__t)
 482           emplace(*__t);
 483       }
 484
 485       template <typename _Up,
 486                  enable_if_t<__and_<
 487                                __not_<is_same<_Tp, _Up>>,
 488                                is_constructible<_Tp, const _Up&>,
 489                                __not_<is_convertible<const _Up&, _Tp>>,
 490                                __not_<__converts_from_optional<_Tp, _Up>>
 491                                >::value, bool> = false>
 492       explicit constexpr optional(const optional<_Up>& __t)
 493       {
 494         if (__t)
 495           emplace(*__t);
 496       }
 497
 498       template <typename _Up,
 499                 enable_if_t<__and_<
 500                               __not_<is_same<_Tp, _Up>>,
 501                               is_constructible<_Tp, _Up&&>,
 502                               is_convertible<_Up&&, _Tp>,
 503                               __not_<__converts_from_optional<_Tp, _Up>>
 504                               >::value, bool> = true>
 505       constexpr optional(optional<_Up>&& __t)
 506       {
 507         if (__t)
 508           emplace(std::move(*__t));
 509       }
 510
 511       template <typename _Up,
 512                 enable_if_t<__and_<
 513                             __not_<is_same<_Tp, _Up>>,
 514                             is_constructible<_Tp, _Up&&>,
 515                             __not_<is_convertible<_Up&&, _Tp>>,
 516                             __not_<__converts_from_optional<_Tp, _Up>>
 517                             >::value, bool> = false>
 518       explicit constexpr optional(optional<_Up>&& __t)
 519       {
 520         if (__t)
 521           emplace(std::move(*__t));
 522       }
 523
 524       // [X.Y.4.3] (partly) Assignment.
 525       optional&
 526       operator=(nullopt_t) noexcept
 527       {
 528         this->_M_reset();
 529         return *this;
 530       }
 531
 532       template<typename _Up = _Tp>
 533         enable_if_t<__and_<
 534                       __not_<is_same<optional<_Tp>, decay_t<_Up>>>,
 535                       is_constructible<_Tp, _Up>,
 536                       __not_<__and_<is_scalar<_Tp>,
 537                                     is_same<_Tp, decay_t<_Up>>>>,
 538                       is_assignable<_Tp&, _Up>>::value,
 539                     optional&>
 540         operator=(_Up&& __u)
 541         {
 542           if (this->_M_is_engaged())
 543             this->_M_get() = std::forward<_Up>(__u);
 544           else
 545             this->_M_construct(std::forward<_Up>(__u));
 546
 547           return *this;
 548         }
 549
 550       template<typename _Up>
 551         enable_if_t<__and_<
 552                       __not_<is_same<_Tp, _Up>>,
 553                       is_constructible<_Tp, const _Up&>,
 554                       is_assignable<_Tp&, _Up>,
 555                       __not_<__converts_from_optional<_Tp, _Up>>,
 556                       __not_<__assigns_from_optional<_Tp, _Up>>
 557                       >::value,
 558                     optional&>
 559         operator=(const optional<_Up>& __u)
 560         {
 561           if (__u)
 562             {
 563               if (this->_M_is_engaged())
 564                 this->_M_get() = *__u;
 565               else
 566                 this->_M_construct(*__u);
 567             }
 568           else
 569             {
 570               this->_M_reset();
 571             }
 572           return *this;
 573         }
 574
 575       template<typename _Up>
 576         enable_if_t<__and_<
 577                       __not_<is_same<_Tp, _Up>>,
 578                       is_constructible<_Tp, _Up>,
 579                       is_assignable<_Tp&, _Up>,
 580                       __not_<__converts_from_optional<_Tp, _Up>>,
 581                       __not_<__assigns_from_optional<_Tp, _Up>>
 582                       >::value,
 583                     optional&>
 584         operator=(optional<_Up>&& __u)
 585         {
 586           if (__u)
 587             {
 588               if (this->_M_is_engaged())
 589                 this->_M_get() = std::move(*__u);
 590               else
 591                 this->_M_construct(std::move(*__u));
 592             }
 593           else
 594             {
 595               this->_M_reset();
 596             }
 597
 598           return *this;
 599         }
 600
 601       template<typename... _Args>
 602         enable_if_t<is_constructible<_Tp, _Args&&...>::value>
 603         emplace(_Args&&... __args)
 604         {
 605           this->_M_reset();
 606           this->_M_construct(std::forward<_Args>(__args)...);
 607         }
 608
 609       template<typename _Up, typename... _Args>
 610         enable_if_t<is_constructible<_Tp, initializer_list<_Up>&,
 611                                      _Args&&...>::value>
 612         emplace(initializer_list<_Up> __il, _Args&&... __args)
 613         {
 614           this->_M_reset();
 615           this->_M_construct(__il, std::forward<_Args>(__args)...);
 616         }
 617
 618       // [X.Y.4.2] Destructor is implicit, implemented in _Optional_base.
 619
 620       // [X.Y.4.4] Swap.
 621       void
 622       swap(optional& __other)
 623       noexcept(is_nothrow_move_constructible<_Tp>()
 624                && __is_nothrow_swappable<_Tp>::value)
 625       {
 626         using std::swap;
 627
 628         if (this->_M_is_engaged() && __other._M_is_engaged())
 629           swap(this->_M_get(), __other._M_get());
 630         else if (this->_M_is_engaged())
 631           {
 632             __other._M_construct(std::move(this->_M_get()));
 633             this->_M_destruct();
 634           }
 635         else if (__other._M_is_engaged())
 636           {
 637             this->_M_construct(std::move(__other._M_get()));
 638             __other._M_destruct();
 639           }
 640       }
 641
 642       // [X.Y.4.5] Observers.
 643       constexpr const _Tp*
 644       operator->() const
 645       { return std::__addressof(this->_M_get()); }
 646
 647       _Tp*
 648       operator->()
 649       { return std::__addressof(this->_M_get()); }
 650
 651       constexpr const _Tp&
 652       operator*() const&
 653       { return this->_M_get(); }
 654
 655       constexpr _Tp&
 656       operator*()&
 657       { return this->_M_get(); }
 658
 659       constexpr _Tp&&
 660       operator*()&&
 661       { return std::move(this->_M_get()); }
 662
 663       constexpr const _Tp&&
 664       operator*() const&&
 665       { return std::move(this->_M_get()); }
 666
 667       constexpr explicit operator bool() const noexcept
 668       { return this->_M_is_engaged(); }
 669
 670       constexpr const _Tp&
 671       value() const&
 672       {
 673         if (this->_M_is_engaged())
 674           return this->_M_get();
 675         __throw_bad_optional_access("Attempt to access value of a "
 676                                     "disengaged optional object");
 677       }
 678
 679       constexpr _Tp&
 680       value()&
 681       {
 682         if (this->_M_is_engaged())
 683           return this->_M_get();
 684         __throw_bad_optional_access("Attempt to access value of a "
 685                                     "disengaged optional object");
 686       }
 687
 688       constexpr _Tp&&
 689       value()&&
 690       {
 691         if (this->_M_is_engaged())
 692           return std::move(this->_M_get());
 693         __throw_bad_optional_access("Attempt to access value of a "
 694                                     "disengaged optional object");
 695       }
 696
 697       constexpr const _Tp&&
 698       value() const&&
 699       {
 700         if (this->_M_is_engaged())
 701           return std::move(this->_M_get());
 702         __throw_bad_optional_access("Attempt to access value of a "
 703                                     "disengaged optional object");
 704       }
 705
 706       template<typename _Up>
 707         constexpr _Tp
 708         value_or(_Up&& __u) const&
 709         {
 710           static_assert(__and_<is_copy_constructible<_Tp>,
 711                                is_convertible<_Up&&, _Tp>>(),
 712                         "Cannot return value");
 713
 714           if (this->_M_is_engaged())
 715             return this->_M_get();
 716           else
 717             return static_cast<_Tp>(std::forward<_Up>(__u));
 718         }
 719
 720       template<typename _Up>
 721         _Tp
 722         value_or(_Up&& __u) &&
 723         {
 724           static_assert(__and_<is_move_constructible<_Tp>,
 725                                is_convertible<_Up&&, _Tp>>(),
 726                         "Cannot return value" );
 727
 728           if (this->_M_is_engaged())
 729             return std::move(this->_M_get());
 730           else
 731             return static_cast<_Tp>(std::forward<_Up>(__u));
 732         }
 733     };
 734
 735   /// @relates experimental::optional @{
 736
 737   // [X.Y.8] Comparisons between optional values.
 738   template<typename _Tp>
 739     constexpr bool
 740     operator==(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
 741     {
 742       return static_cast<bool>(__lhs) == static_cast<bool>(__rhs)
 743              && (!__lhs || *__lhs == *__rhs);
 744     }
 745
 746   template<typename _Tp>
 747     constexpr bool
 748     operator!=(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
 749     { return !(__lhs == __rhs); }
 750
 751   template<typename _Tp>
 752     constexpr bool
 753     operator<(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
 754     {
 755       return static_cast<bool>(__rhs) && (!__lhs || *__lhs < *__rhs);
 756     }
 757
 758   template<typename _Tp>
 759     constexpr bool
 760     operator>(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
 761     { return __rhs < __lhs; }
 762
 763   template<typename _Tp>
 764     constexpr bool
 765     operator<=(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
 766     { return !(__rhs < __lhs); }
 767
 768   template<typename _Tp>
 769     constexpr bool
 770     operator>=(const optional<_Tp>& __lhs, const optional<_Tp>& __rhs)
 771     { return !(__lhs < __rhs); }
 772
 773   // [X.Y.9] Comparisons with nullopt.
 774   template<typename _Tp>
 775     constexpr bool
 776     operator==(const optional<_Tp>& __lhs, nullopt_t) noexcept
 777     { return !__lhs; }
 778
 779   template<typename _Tp>
 780     constexpr bool
 781     operator==(nullopt_t, const optional<_Tp>& __rhs) noexcept
 782     { return !__rhs; }
 783
 784   template<typename _Tp>
 785     constexpr bool
 786     operator!=(const optional<_Tp>& __lhs, nullopt_t) noexcept
 787     { return static_cast<bool>(__lhs); }
 788
 789   template<typename _Tp>
 790     constexpr bool
 791     operator!=(nullopt_t, const optional<_Tp>& __rhs) noexcept
 792     { return static_cast<bool>(__rhs); }
 793
 794   template<typename _Tp>
 795     constexpr bool
 796     operator<(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
 797     { return false; }
 798
 799   template<typename _Tp>
 800     constexpr bool
 801     operator<(nullopt_t, const optional<_Tp>& __rhs) noexcept
 802     { return static_cast<bool>(__rhs); }
 803
 804   template<typename _Tp>
 805     constexpr bool
 806     operator>(const optional<_Tp>& __lhs, nullopt_t) noexcept
 807     { return static_cast<bool>(__lhs); }
 808
 809   template<typename _Tp>
 810     constexpr bool
 811     operator>(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
 812     { return false; }
 813
 814   template<typename _Tp>
 815     constexpr bool
 816     operator<=(const optional<_Tp>& __lhs, nullopt_t) noexcept
 817     { return !__lhs; }
 818
 819   template<typename _Tp>
 820     constexpr bool
 821     operator<=(nullopt_t, const optional<_Tp>& /* __rhs */) noexcept
 822     { return true; }
 823
 824   template<typename _Tp>
 825     constexpr bool
 826     operator>=(const optional<_Tp>& /* __lhs */, nullopt_t) noexcept
 827     { return true; }
 828
 829   template<typename _Tp>
 830     constexpr bool
 831     operator>=(nullopt_t, const optional<_Tp>& __rhs) noexcept
 832     { return !__rhs; }
 833
 834   // [X.Y.10] Comparisons with value type.
 835   template<typename _Tp>
 836     constexpr bool
 837     operator==(const optional<_Tp>& __lhs, const _Tp& __rhs)
 838     { return __lhs && *__lhs == __rhs; }
 839
 840   template<typename _Tp>
 841     constexpr bool
 842     operator==(const _Tp& __lhs, const optional<_Tp>& __rhs)
 843     { return __rhs && __lhs == *__rhs; }
 844
 845   template<typename _Tp>
 846     constexpr bool
 847     operator!=(const optional<_Tp>& __lhs, _Tp const& __rhs)
 848     { return !__lhs || !(*__lhs == __rhs); }
 849
 850   template<typename _Tp>
 851     constexpr bool
 852     operator!=(const _Tp& __lhs, const optional<_Tp>& __rhs)
 853     { return !__rhs || !(__lhs == *__rhs); }
 854
 855   template<typename _Tp>
 856     constexpr bool
 857     operator<(const optional<_Tp>& __lhs, const _Tp& __rhs)
 858     { return !__lhs || *__lhs < __rhs; }
 859
 860   template<typename _Tp>
 861     constexpr bool
 862     operator<(const _Tp& __lhs, const optional<_Tp>& __rhs)
 863     { return __rhs && __lhs < *__rhs; }
 864
 865   template<typename _Tp>
 866     constexpr bool
 867     operator>(const optional<_Tp>& __lhs, const _Tp& __rhs)
 868     { return __lhs && __rhs < *__lhs; }
 869
 870   template<typename _Tp>
 871     constexpr bool
 872     operator>(const _Tp& __lhs, const optional<_Tp>& __rhs)
 873     { return !__rhs || *__rhs < __lhs; }
 874
 875   template<typename _Tp>
 876     constexpr bool
 877     operator<=(const optional<_Tp>& __lhs, const _Tp& __rhs)
 878     { return !__lhs || !(__rhs < *__lhs); }
 879
 880   template<typename _Tp>
 881     constexpr bool
 882     operator<=(const _Tp& __lhs, const optional<_Tp>& __rhs)
 883     { return __rhs && !(*__rhs < __lhs); }
 884
 885   template<typename _Tp>
 886     constexpr bool
 887     operator>=(const optional<_Tp>& __lhs, const _Tp& __rhs)
 888     { return __lhs && !(*__lhs < __rhs); }
 889
 890   template<typename _Tp>
 891     constexpr bool
 892     operator>=(const _Tp& __lhs, const optional<_Tp>& __rhs)
 893     { return !__rhs || !(__lhs < *__rhs); }
 894
 895   // [X.Y.11]
 896   template<typename _Tp>
 897     inline void
 898     swap(optional<_Tp>& __lhs, optional<_Tp>& __rhs)
 899     noexcept(noexcept(__lhs.swap(__rhs)))
 900     { __lhs.swap(__rhs); }
 901
 902   template<typename _Tp>
 903     constexpr optional<decay_t<_Tp>>
 904     make_optional(_Tp&& __t)
 905     { return optional<decay_t<_Tp>> { std::forward<_Tp>(__t) }; }
 906
 907   /// @} relates experimental::optional
 908   /// @} group optional
 909 } // namespace fundamentals_v1
 910 } // namespace experimental
 911
 912   // [X.Y.12]
 913   /// std::hash partial specialization for experimental::optional
 914   /// @relates experimental::optional
 915   template<typename _Tp>
 916     struct hash<experimental::optional<_Tp>>
 917     {
 918       using result_type = size_t;
 919       using argument_type = experimental::optional<_Tp>;
 920
 921       size_t
 922       operator()(const experimental::optional<_Tp>& __t) const
 923       noexcept(noexcept(hash<_Tp> {}(*__t)))
 924       {
 925         // We pick an arbitrary hash for disengaged optionals which hopefully
 926         // usual values of _Tp won't typically hash to.
 927         constexpr size_t __magic_disengaged_hash = static_cast<size_t>(-3333);
 928         return __t ? hash<_Tp> {}(*__t) : __magic_disengaged_hash;
 929       }
 930     };
 931
 932 _GLIBCXX_END_NAMESPACE_VERSION
 933 } // namespace std
 934
 935 #endif // C++14
 936
 937 #endif // _GLIBCXX_EXPERIMENTAL_OPTIONAL