libstdc++-v3/include/bits/stl_pair.h

   1 // Pair implementation -*- C++ -*-
   2
   3 // Copyright (C) 2001-2024 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 /*
  26  *
  27  * Copyright (c) 1994
  28  * Hewlett-Packard Company
  29  *
  30  * Permission to use, copy, modify, distribute and sell this software
  31  * and its documentation for any purpose is hereby granted without fee,
  32  * provided that the above copyright notice appear in all copies and
  33  * that both that copyright notice and this permission notice appear
  34  * in supporting documentation.  Hewlett-Packard Company makes no
  35  * representations about the suitability of this software for any
  36  * purpose.  It is provided "as is" without express or implied warranty.
  37  *
  38  *
  39  * Copyright (c) 1996,1997
  40  * Silicon Graphics Computer Systems, Inc.
  41  *
  42  * Permission to use, copy, modify, distribute and sell this software
  43  * and its documentation for any purpose is hereby granted without fee,
  44  * provided that the above copyright notice appear in all copies and
  45  * that both that copyright notice and this permission notice appear
  46  * in supporting documentation.  Silicon Graphics makes no
  47  * representations about the suitability of this software for any
  48  * purpose.  It is provided "as is" without express or implied warranty.
  49  */
  50
  51 /** @file bits/stl_pair.h
  52  *  This is an internal header file, included by other library headers.
  53  *  Do not attempt to use it directly. @headername{utility}
  54  */
  55
  56 #ifndef _STL_PAIR_H
  57 #define _STL_PAIR_H 1
  58
  59 #if __cplusplus >= 201103L
  60 # include <type_traits>    // for std::__decay_and_strip
  61 # include <bits/move.h>    // for std::move / std::forward, and std::swap
  62 # include <bits/utility.h> // for std::tuple_element, std::tuple_size
  63 #endif
  64 #if __cplusplus >= 202002L
  65 # include <compare>
  66 #endif
  67
  68 namespace std _GLIBCXX_VISIBILITY(default)
  69 {
  70 _GLIBCXX_BEGIN_NAMESPACE_VERSION
  71
  72   /**
  73    *  @addtogroup utilities
  74    *  @{
  75    */
  76
  77 #if __cplusplus >= 201103L
  78   /// Tag type for piecewise construction of std::pair objects.
  79   struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
  80
  81   /// Tag for piecewise construction of std::pair objects.
  82   _GLIBCXX17_INLINE constexpr piecewise_construct_t piecewise_construct =
  83     piecewise_construct_t();
  84
  85   /// @cond undocumented
  86
  87   // Forward declarations.
  88   template<typename...>
  89     class tuple;
  90
  91   template<size_t...>
  92     struct _Index_tuple;
  93
  94 #if ! __cpp_lib_concepts
  95   // Concept utility functions, reused in conditionally-explicit
  96   // constructors.
  97   // See PR 70437, don't look at is_constructible or
  98   // is_convertible if the types are the same to
  99   // avoid querying those properties for incomplete types.
 100   template <bool, typename _T1, typename _T2>
 101     struct _PCC
 102     {
 103       template <typename _U1, typename _U2>
 104       static constexpr bool _ConstructiblePair()
 105       {
 106         return __and_<is_constructible<_T1, const _U1&>,
 107                       is_constructible<_T2, const _U2&>>::value;
 108       }
 109
 110       template <typename _U1, typename _U2>
 111       static constexpr bool _ImplicitlyConvertiblePair()
 112       {
 113         return __and_<is_convertible<const _U1&, _T1>,
 114                       is_convertible<const _U2&, _T2>>::value;
 115       }
 116
 117       template <typename _U1, typename _U2>
 118       static constexpr bool _MoveConstructiblePair()
 119       {
 120         return __and_<is_constructible<_T1, _U1&&>,
 121                       is_constructible<_T2, _U2&&>>::value;
 122       }
 123
 124       template <typename _U1, typename _U2>
 125       static constexpr bool _ImplicitlyMoveConvertiblePair()
 126       {
 127         return __and_<is_convertible<_U1&&, _T1>,
 128                       is_convertible<_U2&&, _T2>>::value;
 129       }
 130     };
 131
 132   template <typename _T1, typename _T2>
 133     struct _PCC<false, _T1, _T2>
 134     {
 135       template <typename _U1, typename _U2>
 136       static constexpr bool _ConstructiblePair()
 137       {
 138         return false;
 139       }
 140
 141       template <typename _U1, typename _U2>
 142       static constexpr bool _ImplicitlyConvertiblePair()
 143       {
 144         return false;
 145       }
 146
 147       template <typename _U1, typename _U2>
 148       static constexpr bool _MoveConstructiblePair()
 149       {
 150         return false;
 151       }
 152
 153       template <typename _U1, typename _U2>
 154       static constexpr bool _ImplicitlyMoveConvertiblePair()
 155       {
 156         return false;
 157       }
 158     };
 159 #endif // lib concepts
 160 #endif // C++11
 161
 162   template<typename _U1, typename _U2> class __pair_base
 163   {
 164 #if __cplusplus >= 201103L && ! __cpp_lib_concepts
 165     template<typename _T1, typename _T2> friend struct pair;
 166     __pair_base() = default;
 167     ~__pair_base() = default;
 168     __pair_base(const __pair_base&) = default;
 169     __pair_base& operator=(const __pair_base&) = delete;
 170 #endif // C++11
 171   };
 172
 173   /// @endcond
 174
 175  /**
 176    *  @brief Struct holding two objects of arbitrary type.
 177    *
 178    *  @tparam _T1  Type of first object.
 179    *  @tparam _T2  Type of second object.
 180    *
 181    *  <https://gcc.gnu.org/onlinedocs/libstdc++/manual/utilities.html>
 182    *
 183    * @headerfile utility
 184    */
 185   template<typename _T1, typename _T2>
 186     struct pair
 187     : public __pair_base<_T1, _T2>
 188     {
 189       typedef _T1 first_type;    ///< The type of the `first` member
 190       typedef _T2 second_type;   ///< The type of the `second` member
 191
 192       _T1 first;                 ///< The first member
 193       _T2 second;                ///< The second member
 194
 195 #if __cplusplus >= 201103L
 196       constexpr pair(const pair&) = default;    ///< Copy constructor
 197       constexpr pair(pair&&) = default;         ///< Move constructor
 198
 199       template<typename... _Args1, typename... _Args2>
 200         _GLIBCXX20_CONSTEXPR
 201         pair(piecewise_construct_t, tuple<_Args1...>, tuple<_Args2...>);
 202
 203       /// Swap the first members and then the second members.
 204       _GLIBCXX20_CONSTEXPR void
 205       swap(pair& __p)
 206       noexcept(__and_<__is_nothrow_swappable<_T1>,
 207                       __is_nothrow_swappable<_T2>>::value)
 208       {
 209         using std::swap;
 210         swap(first, __p.first);
 211         swap(second, __p.second);
 212       }
 213
 214 #if __glibcxx_ranges_zip // >= C++23
 215       // As an extension, we constrain the const swap member function in order
 216       // to continue accepting explicit instantiation of pairs whose elements
 217       // are not all const swappable.  Without this constraint, such an
 218       // explicit instantiation would also instantiate the ill-formed body of
 219       // this function and yield a hard error.  This constraint shouldn't
 220       // affect the behavior of valid programs.
 221       constexpr void
 222       swap(const pair& __p) const
 223       noexcept(__and_v<__is_nothrow_swappable<const _T1>,
 224                        __is_nothrow_swappable<const _T2>>)
 225       requires is_swappable_v<const _T1> && is_swappable_v<const _T2>
 226       {
 227         using std::swap;
 228         swap(first, __p.first);
 229         swap(second, __p.second);
 230       }
 231 #endif // C++23
 232
 233     private:
 234       template<typename... _Args1, size_t... _Indexes1,
 235                typename... _Args2, size_t... _Indexes2>
 236         _GLIBCXX20_CONSTEXPR
 237         pair(tuple<_Args1...>&, tuple<_Args2...>&,
 238              _Index_tuple<_Indexes1...>, _Index_tuple<_Indexes2...>);
 239     public:
 240
 241 #if __cpp_lib_concepts
 242       // C++20 implementation using concepts, explicit(bool), fully constexpr.
 243
 244       /// Default constructor
 245       constexpr
 246       explicit(__not_<__and_<__is_implicitly_default_constructible<_T1>,
 247                              __is_implicitly_default_constructible<_T2>>>())
 248       pair()
 249       requires is_default_constructible_v<_T1>
 250                && is_default_constructible_v<_T2>
 251       : first(), second()
 252       { }
 253
 254     private:
 255
 256       /// @cond undocumented
 257       template<typename _U1, typename _U2>
 258         static constexpr bool
 259         _S_constructible()
 260         {
 261           if constexpr (is_constructible_v<_T1, _U1>)
 262             return is_constructible_v<_T2, _U2>;
 263           return false;
 264         }
 265
 266       template<typename _U1, typename _U2>
 267         static constexpr bool
 268         _S_nothrow_constructible()
 269         {
 270           if constexpr (is_nothrow_constructible_v<_T1, _U1>)
 271             return is_nothrow_constructible_v<_T2, _U2>;
 272           return false;
 273         }
 274
 275       template<typename _U1, typename _U2>
 276         static constexpr bool
 277         _S_convertible()
 278         {
 279           if constexpr (is_convertible_v<_U1, _T1>)
 280             return is_convertible_v<_U2, _T2>;
 281           return false;
 282         }
 283
 284       // True if construction from _U1 and _U2 would create a dangling ref.
 285       template<typename _U1, typename _U2>
 286         static constexpr bool
 287         _S_dangles()
 288         {
 289 #if __has_builtin(__reference_constructs_from_temporary)
 290           if constexpr (__reference_constructs_from_temporary(_T1, _U1&&))
 291             return true;
 292           else
 293             return __reference_constructs_from_temporary(_T2, _U2&&);
 294 #else
 295           return false;
 296 #endif
 297         }
 298       /// @endcond
 299
 300     public:
 301
 302       /// Constructor accepting lvalues of `first_type` and `second_type`
 303       constexpr explicit(!_S_convertible<const _T1&, const _T2&>())
 304       pair(const _T1& __x, const _T2& __y)
 305       noexcept(_S_nothrow_constructible<const _T1&, const _T2&>())
 306       requires (_S_constructible<const _T1&, const _T2&>())
 307       : first(__x), second(__y)
 308       { }
 309
 310       /// Constructor accepting two values of arbitrary types
 311 #if __cplusplus > 202002L
 312       template<typename _U1 = _T1, typename _U2 = _T2>
 313 #else
 314       template<typename _U1, typename _U2>
 315 #endif
 316         requires (_S_constructible<_U1, _U2>()) && (!_S_dangles<_U1, _U2>())
 317         constexpr explicit(!_S_convertible<_U1, _U2>())
 318         pair(_U1&& __x, _U2&& __y)
 319         noexcept(_S_nothrow_constructible<_U1, _U2>())
 320         : first(std::forward<_U1>(__x)), second(std::forward<_U2>(__y))
 321         { }
 322
 323 #if __cplusplus > 202002L
 324       template<typename _U1 = _T1, typename _U2 = _T2>
 325 #else
 326       template<typename _U1, typename _U2>
 327 #endif
 328         requires (_S_constructible<_U1, _U2>()) && (_S_dangles<_U1, _U2>())
 329         constexpr explicit(!_S_convertible<_U1, _U2>())
 330         pair(_U1&&, _U2&&) = delete;
 331
 332       /// Converting constructor from a const `pair<U1, U2>` lvalue
 333       template<typename _U1, typename _U2>
 334         requires (_S_constructible<const _U1&, const _U2&>())
 335           && (!_S_dangles<_U1, _U2>())
 336         constexpr explicit(!_S_convertible<const _U1&, const _U2&>())
 337         pair(const pair<_U1, _U2>& __p)
 338         noexcept(_S_nothrow_constructible<const _U1&, const _U2&>())
 339         : first(__p.first), second(__p.second)
 340         { }
 341
 342       template<typename _U1, typename _U2>
 343         requires (_S_constructible<const _U1&, const _U2&>())
 344               && (_S_dangles<const _U1&, const _U2&>())
 345         constexpr explicit(!_S_convertible<const _U1&, const _U2&>())
 346         pair(const pair<_U1, _U2>&) = delete;
 347
 348       /// Converting constructor from a non-const `pair<U1, U2>` rvalue
 349       template<typename _U1, typename _U2>
 350         requires (_S_constructible<_U1, _U2>()) && (!_S_dangles<_U1, _U2>())
 351         constexpr explicit(!_S_convertible<_U1, _U2>())
 352         pair(pair<_U1, _U2>&& __p)
 353         noexcept(_S_nothrow_constructible<_U1, _U2>())
 354         : first(std::forward<_U1>(__p.first)),
 355           second(std::forward<_U2>(__p.second))
 356         { }
 357
 358       template<typename _U1, typename _U2>
 359         requires (_S_constructible<_U1, _U2>()) && (_S_dangles<_U1, _U2>())
 360         constexpr explicit(!_S_convertible<_U1, _U2>())
 361         pair(pair<_U1, _U2>&&) = delete;
 362
 363 #if __glibcxx_ranges_zip // >= C++23
 364       /// Converting constructor from a non-const `pair<U1, U2>` lvalue
 365       template<typename _U1, typename _U2>
 366         requires (_S_constructible<_U1&, _U2&>()) && (!_S_dangles<_U1&, _U2&>())
 367         constexpr explicit(!_S_convertible<_U1&, _U2&>())
 368         pair(pair<_U1, _U2>& __p)
 369         noexcept(_S_nothrow_constructible<_U1&, _U2&>())
 370         : first(__p.first), second(__p.second)
 371         { }
 372
 373       template<typename _U1, typename _U2>
 374         requires (_S_constructible<_U1&, _U2&>()) && (_S_dangles<_U1&, _U2&>())
 375         constexpr explicit(!_S_convertible<_U1&, _U2&>())
 376         pair(pair<_U1, _U2>&) = delete;
 377
 378       /// Converting constructor from a const `pair<U1, U2>` rvalue
 379       template<typename _U1, typename _U2>
 380         requires (_S_constructible<const _U1, const _U2>())
 381           && (!_S_dangles<const _U1, const _U2>())
 382         constexpr explicit(!_S_convertible<const _U1, const _U2>())
 383         pair(const pair<_U1, _U2>&& __p)
 384         noexcept(_S_nothrow_constructible<const _U1, const _U2>())
 385         : first(std::forward<const _U1>(__p.first)),
 386           second(std::forward<const _U2>(__p.second))
 387         { }
 388
 389       template<typename _U1, typename _U2>
 390         requires (_S_constructible<const _U1, const _U2>())
 391           && (_S_dangles<const _U1, const _U2>())
 392         constexpr explicit(!_S_convertible<const _U1, const _U2>())
 393         pair(const pair<_U1, _U2>&&) = delete;
 394 #endif // C++23
 395
 396   private:
 397       /// @cond undocumented
 398       template<typename _U1, typename _U2>
 399         static constexpr bool
 400         _S_assignable()
 401         {
 402           if constexpr (is_assignable_v<_T1&, _U1>)
 403             return is_assignable_v<_T2&, _U2>;
 404           return false;
 405         }
 406
 407       template<typename _U1, typename _U2>
 408         static constexpr bool
 409         _S_nothrow_assignable()
 410         {
 411           if constexpr (is_nothrow_assignable_v<_T1&, _U1>)
 412             return is_nothrow_assignable_v<_T2&, _U2>;
 413           return false;
 414         }
 415       /// @endcond
 416
 417   public:
 418
 419       pair& operator=(const pair&) = delete;
 420
 421       /// Copy assignment operator
 422       constexpr pair&
 423       operator=(const pair& __p)
 424       noexcept(_S_nothrow_assignable<const _T1&, const _T2&>())
 425       requires (_S_assignable<const _T1&, const _T2&>())
 426       {
 427         first = __p.first;
 428         second = __p.second;
 429         return *this;
 430       }
 431
 432       /// Move assignment operator
 433       constexpr pair&
 434       operator=(pair&& __p)
 435       noexcept(_S_nothrow_assignable<_T1, _T2>())
 436       requires (_S_assignable<_T1, _T2>())
 437       {
 438         first = std::forward<first_type>(__p.first);
 439         second = std::forward<second_type>(__p.second);
 440         return *this;
 441       }
 442
 443       /// Converting assignment from a const `pair<U1, U2>` lvalue
 444       template<typename _U1, typename _U2>
 445         constexpr pair&
 446         operator=(const pair<_U1, _U2>& __p)
 447         noexcept(_S_nothrow_assignable<const _U1&, const _U2&>())
 448         requires (_S_assignable<const _U1&, const _U2&>())
 449         {
 450           first = __p.first;
 451           second = __p.second;
 452           return *this;
 453         }
 454
 455       /// Converting assignment from a non-const `pair<U1, U2>` rvalue
 456       template<typename _U1, typename _U2>
 457         constexpr pair&
 458         operator=(pair<_U1, _U2>&& __p)
 459         noexcept(_S_nothrow_assignable<_U1, _U2>())
 460         requires (_S_assignable<_U1, _U2>())
 461         {
 462           first = std::forward<_U1>(__p.first);
 463           second = std::forward<_U2>(__p.second);
 464           return *this;
 465         }
 466
 467 #if __glibcxx_ranges_zip // >= C++23
 468       /// Copy assignment operator (const)
 469       constexpr const pair&
 470       operator=(const pair& __p) const
 471       requires is_copy_assignable_v<const first_type>
 472         && is_copy_assignable_v<const second_type>
 473       {
 474         first = __p.first;
 475         second = __p.second;
 476         return *this;
 477       }
 478
 479       /// Move assignment operator (const)
 480       constexpr const pair&
 481       operator=(pair&& __p) const
 482       requires is_assignable_v<const first_type&, first_type>
 483         && is_assignable_v<const second_type&, second_type>
 484       {
 485         first = std::forward<first_type>(__p.first);
 486         second = std::forward<second_type>(__p.second);
 487         return *this;
 488       }
 489
 490       /// Converting assignment from a const `pair<U1, U2>` lvalue
 491       template<typename _U1, typename _U2>
 492         constexpr const pair&
 493         operator=(const pair<_U1, _U2>& __p) const
 494         requires is_assignable_v<const first_type&, const _U1&>
 495           && is_assignable_v<const second_type&, const _U2&>
 496         {
 497           first = __p.first;
 498           second = __p.second;
 499           return *this;
 500         }
 501
 502       /// Converting assignment from a non-const `pair<U1, U2>` rvalue
 503       template<typename _U1, typename _U2>
 504         constexpr const pair&
 505         operator=(pair<_U1, _U2>&& __p) const
 506         requires is_assignable_v<const first_type&, _U1>
 507           && is_assignable_v<const second_type&, _U2>
 508         {
 509           first = std::forward<_U1>(__p.first);
 510           second = std::forward<_U2>(__p.second);
 511           return *this;
 512         }
 513 #endif // C++23
 514 #else // !__cpp_lib_concepts
 515       // C++11/14/17 implementation using enable_if, partially constexpr.
 516
 517       /// @cond undocumented
 518       // Error if construction from _U1 and _U2 would create a dangling ref.
 519 #if __has_builtin(__reference_constructs_from_temporary) \
 520       && defined _GLIBCXX_DEBUG
 521 # define __glibcxx_no_dangling_refs(_U1, _U2) \
 522   static_assert(!__reference_constructs_from_temporary(_T1, _U1) \
 523                && !__reference_constructs_from_temporary(_T2, _U2), \
 524                 "std::pair constructor creates a dangling reference")
 525 #else
 526 # define __glibcxx_no_dangling_refs(_U1, _U2)
 527 #endif
 528       /// @endcond
 529
 530       /** The default constructor creates @c first and @c second using their
 531        *  respective default constructors.  */
 532       template <typename _U1 = _T1,
 533                 typename _U2 = _T2,
 534                 typename enable_if<__and_<
 535                                      __is_implicitly_default_constructible<_U1>,
 536                                      __is_implicitly_default_constructible<_U2>>
 537                                    ::value, bool>::type = true>
 538       constexpr pair()
 539       : first(), second() { }
 540
 541       template <typename _U1 = _T1,
 542                 typename _U2 = _T2,
 543                 typename enable_if<__and_<
 544                        is_default_constructible<_U1>,
 545                        is_default_constructible<_U2>,
 546                        __not_<
 547                          __and_<__is_implicitly_default_constructible<_U1>,
 548                                 __is_implicitly_default_constructible<_U2>>>>
 549                                    ::value, bool>::type = false>
 550       explicit constexpr pair()
 551       : first(), second() { }
 552
 553       // Shortcut for constraining the templates that don't take pairs.
 554       /// @cond undocumented
 555       using _PCCP = _PCC<true, _T1, _T2>;
 556       /// @endcond
 557
 558       /// Construct from two const lvalues, allowing implicit conversions.
 559       template<typename _U1 = _T1, typename _U2=_T2, typename
 560                enable_if<_PCCP::template
 561                            _ConstructiblePair<_U1, _U2>()
 562                          && _PCCP::template
 563                            _ImplicitlyConvertiblePair<_U1, _U2>(),
 564                          bool>::type=true>
 565       constexpr pair(const _T1& __a, const _T2& __b)
 566       : first(__a), second(__b) { }
 567
 568       /// Construct from two const lvalues, disallowing implicit conversions.
 569        template<typename _U1 = _T1, typename _U2=_T2, typename
 570                 enable_if<_PCCP::template
 571                             _ConstructiblePair<_U1, _U2>()
 572                           && !_PCCP::template
 573                             _ImplicitlyConvertiblePair<_U1, _U2>(),
 574                          bool>::type=false>
 575       explicit constexpr pair(const _T1& __a, const _T2& __b)
 576       : first(__a), second(__b) { }
 577
 578       // Shortcut for constraining the templates that take pairs.
 579       /// @cond undocumented
 580       template <typename _U1, typename _U2>
 581         using _PCCFP = _PCC<!is_same<_T1, _U1>::value
 582                             || !is_same<_T2, _U2>::value,
 583                             _T1, _T2>;
 584       /// @endcond
 585
 586       template<typename _U1, typename _U2, typename
 587                enable_if<_PCCFP<_U1, _U2>::template
 588                            _ConstructiblePair<_U1, _U2>()
 589                          && _PCCFP<_U1, _U2>::template
 590                            _ImplicitlyConvertiblePair<_U1, _U2>(),
 591                           bool>::type=true>
 592         constexpr pair(const pair<_U1, _U2>& __p)
 593         : first(__p.first), second(__p.second)
 594         { __glibcxx_no_dangling_refs(const _U1&, const _U2&); }
 595
 596       template<typename _U1, typename _U2, typename
 597                enable_if<_PCCFP<_U1, _U2>::template
 598                            _ConstructiblePair<_U1, _U2>()
 599                          && !_PCCFP<_U1, _U2>::template
 600                            _ImplicitlyConvertiblePair<_U1, _U2>(),
 601                          bool>::type=false>
 602         explicit constexpr pair(const pair<_U1, _U2>& __p)
 603         : first(__p.first), second(__p.second)
 604         { __glibcxx_no_dangling_refs(const _U1&, const _U2&); }
 605
 606 #if _GLIBCXX_USE_DEPRECATED
 607 #if defined(__DEPRECATED)
 608 # define _GLIBCXX_DEPRECATED_PAIR_CTOR \
 609       __attribute__ ((__deprecated__ ("use 'nullptr' instead of '0' to " \
 610                                       "initialize std::pair of move-only " \
 611                                       "type and pointer")))
 612 #else
 613 # define _GLIBCXX_DEPRECATED_PAIR_CTOR
 614 #endif
 615
 616     private:
 617       /// @cond undocumented
 618
 619       // A type which can be constructed from literal zero, but not nullptr
 620       struct __zero_as_null_pointer_constant
 621       {
 622         __zero_as_null_pointer_constant(int __zero_as_null_pointer_constant::*)
 623         { }
 624         template<typename _Tp,
 625                  typename = __enable_if_t<is_null_pointer<_Tp>::value>>
 626         __zero_as_null_pointer_constant(_Tp) = delete;
 627       };
 628       /// @endcond
 629     public:
 630
 631       // Deprecated extensions to DR 811.
 632       // These allow construction from an rvalue and a literal zero,
 633       // in cases where the standard says the zero should be deduced as int
 634       template<typename _U1,
 635                __enable_if_t<__and_<__not_<is_reference<_U1>>,
 636                                     is_pointer<_T2>,
 637                                     is_constructible<_T1, _U1>,
 638                                     __not_<is_constructible<_T1, const _U1&>>,
 639                                     is_convertible<_U1, _T1>>::value,
 640                              bool> = true>
 641         _GLIBCXX_DEPRECATED_PAIR_CTOR
 642         constexpr
 643         pair(_U1&& __x, __zero_as_null_pointer_constant, ...)
 644         : first(std::forward<_U1>(__x)), second(nullptr)
 645         { __glibcxx_no_dangling_refs(_U1&&, std::nullptr_t); }
 646
 647       template<typename _U1,
 648                __enable_if_t<__and_<__not_<is_reference<_U1>>,
 649                                     is_pointer<_T2>,
 650                                     is_constructible<_T1, _U1>,
 651                                     __not_<is_constructible<_T1, const _U1&>>,
 652                                     __not_<is_convertible<_U1, _T1>>>::value,
 653                              bool> = false>
 654         _GLIBCXX_DEPRECATED_PAIR_CTOR
 655         explicit constexpr
 656         pair(_U1&& __x, __zero_as_null_pointer_constant, ...)
 657         : first(std::forward<_U1>(__x)), second(nullptr)
 658         { __glibcxx_no_dangling_refs(_U1&&, std::nullptr_t); }
 659
 660       template<typename _U2,
 661                __enable_if_t<__and_<is_pointer<_T1>,
 662                                     __not_<is_reference<_U2>>,
 663                                     is_constructible<_T2, _U2>,
 664                                     __not_<is_constructible<_T2, const _U2&>>,
 665                                     is_convertible<_U2, _T2>>::value,
 666                              bool> = true>
 667         _GLIBCXX_DEPRECATED_PAIR_CTOR
 668         constexpr
 669         pair(__zero_as_null_pointer_constant, _U2&& __y, ...)
 670         : first(nullptr), second(std::forward<_U2>(__y))
 671         { __glibcxx_no_dangling_refs(std::nullptr_t, _U2&&); }
 672
 673       template<typename _U2,
 674                __enable_if_t<__and_<is_pointer<_T1>,
 675                                     __not_<is_reference<_U2>>,
 676                                     is_constructible<_T2, _U2>,
 677                                     __not_<is_constructible<_T2, const _U2&>>,
 678                                     __not_<is_convertible<_U2, _T2>>>::value,
 679                              bool> = false>
 680         _GLIBCXX_DEPRECATED_PAIR_CTOR
 681         explicit constexpr
 682         pair(__zero_as_null_pointer_constant, _U2&& __y, ...)
 683         : first(nullptr), second(std::forward<_U2>(__y))
 684         { __glibcxx_no_dangling_refs(std::nullptr_t, _U2&&); }
 685 #undef _GLIBCXX_DEPRECATED_PAIR_CTOR
 686 #endif
 687
 688       template<typename _U1, typename _U2, typename
 689                enable_if<_PCCP::template
 690                            _MoveConstructiblePair<_U1, _U2>()
 691                           && _PCCP::template
 692                            _ImplicitlyMoveConvertiblePair<_U1, _U2>(),
 693                          bool>::type=true>
 694         constexpr pair(_U1&& __x, _U2&& __y)
 695         : first(std::forward<_U1>(__x)), second(std::forward<_U2>(__y))
 696         { __glibcxx_no_dangling_refs(_U1&&, _U2&&); }
 697
 698       template<typename _U1, typename _U2, typename
 699                enable_if<_PCCP::template
 700                            _MoveConstructiblePair<_U1, _U2>()
 701                           && !_PCCP::template
 702                            _ImplicitlyMoveConvertiblePair<_U1, _U2>(),
 703                          bool>::type=false>
 704         explicit constexpr pair(_U1&& __x, _U2&& __y)
 705         : first(std::forward<_U1>(__x)), second(std::forward<_U2>(__y))
 706         { __glibcxx_no_dangling_refs(_U1&&, _U2&&); }
 707
 708
 709       template<typename _U1, typename _U2, typename
 710                enable_if<_PCCFP<_U1, _U2>::template
 711                            _MoveConstructiblePair<_U1, _U2>()
 712                           && _PCCFP<_U1, _U2>::template
 713                            _ImplicitlyMoveConvertiblePair<_U1, _U2>(),
 714                          bool>::type=true>
 715         constexpr pair(pair<_U1, _U2>&& __p)
 716         : first(std::forward<_U1>(__p.first)),
 717           second(std::forward<_U2>(__p.second))
 718         { __glibcxx_no_dangling_refs(_U1&&, _U2&&); }
 719
 720       template<typename _U1, typename _U2, typename
 721                enable_if<_PCCFP<_U1, _U2>::template
 722                            _MoveConstructiblePair<_U1, _U2>()
 723                           && !_PCCFP<_U1, _U2>::template
 724                            _ImplicitlyMoveConvertiblePair<_U1, _U2>(),
 725                          bool>::type=false>
 726         explicit constexpr pair(pair<_U1, _U2>&& __p)
 727         : first(std::forward<_U1>(__p.first)),
 728           second(std::forward<_U2>(__p.second))
 729         { __glibcxx_no_dangling_refs(_U1&&, _U2&&); }
 730
 731 #undef __glibcxx_no_dangling_refs
 732
 733       pair&
 734       operator=(__conditional_t<__and_<is_copy_assignable<_T1>,
 735                                        is_copy_assignable<_T2>>::value,
 736                                 const pair&, const __nonesuch&> __p)
 737       {
 738         first = __p.first;
 739         second = __p.second;
 740         return *this;
 741       }
 742
 743       pair&
 744       operator=(__conditional_t<__and_<is_move_assignable<_T1>,
 745                                        is_move_assignable<_T2>>::value,
 746                                 pair&&, __nonesuch&&> __p)
 747       noexcept(__and_<is_nothrow_move_assignable<_T1>,
 748                       is_nothrow_move_assignable<_T2>>::value)
 749       {
 750         first = std::forward<first_type>(__p.first);
 751         second = std::forward<second_type>(__p.second);
 752         return *this;
 753       }
 754
 755       template<typename _U1, typename _U2>
 756         typename enable_if<__and_<is_assignable<_T1&, const _U1&>,
 757                                   is_assignable<_T2&, const _U2&>>::value,
 758                            pair&>::type
 759         operator=(const pair<_U1, _U2>& __p)
 760         {
 761           first = __p.first;
 762           second = __p.second;
 763           return *this;
 764         }
 765
 766       template<typename _U1, typename _U2>
 767         typename enable_if<__and_<is_assignable<_T1&, _U1&&>,
 768                                   is_assignable<_T2&, _U2&&>>::value,
 769                            pair&>::type
 770         operator=(pair<_U1, _U2>&& __p)
 771         {
 772           first = std::forward<_U1>(__p.first);
 773           second = std::forward<_U2>(__p.second);
 774           return *this;
 775         }
 776 #endif // lib concepts
 777 #else
 778       // C++03 implementation
 779
 780       // _GLIBCXX_RESOLVE_LIB_DEFECTS
 781       // 265.  std::pair::pair() effects overly restrictive
 782       /** The default constructor creates @c first and @c second using their
 783        *  respective default constructors.  */
 784       pair() : first(), second() { }
 785
 786       /// Two objects may be passed to a `pair` constructor to be copied.
 787       pair(const _T1& __a, const _T2& __b)
 788       : first(__a), second(__b) { }
 789
 790       /// Templated constructor to convert from other pairs.
 791       template<typename _U1, typename _U2>
 792         pair(const pair<_U1, _U2>& __p)
 793         : first(__p.first), second(__p.second)
 794         {
 795 #if __has_builtin(__reference_constructs_from_temporary)
 796 #pragma GCC diagnostic push
 797 #pragma GCC diagnostic ignored "-Wunused-local-typedefs"
 798           typedef int _DanglingCheck1[
 799             __reference_constructs_from_temporary(_T1, const _U1&) ? -1 : 1
 800                       ];
 801           typedef int _DanglingCheck2[
 802             __reference_constructs_from_temporary(_T2, const _U2&) ? -1 : 1
 803                       ];
 804 #pragma GCC diagnostic pop
 805 #endif
 806         }
 807 #endif // C++11
 808     };
 809
 810   /// @relates pair @{
 811
 812 #if __cpp_deduction_guides >= 201606
 813   template<typename _T1, typename _T2> pair(_T1, _T2) -> pair<_T1, _T2>;
 814 #endif
 815
 816   /// Two pairs of the same type are equal iff their members are equal.
 817   template<typename _T1, typename _T2>
 818     inline _GLIBCXX_CONSTEXPR bool
 819     operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 820     { return __x.first == __y.first && __x.second == __y.second; }
 821
 822 #if __cpp_lib_three_way_comparison && __cpp_lib_concepts
 823   template<typename _T1, typename _T2>
 824     constexpr common_comparison_category_t<__detail::__synth3way_t<_T1>,
 825                                            __detail::__synth3way_t<_T2>>
 826     operator<=>(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 827     {
 828       if (auto __c = __detail::__synth3way(__x.first, __y.first); __c != 0)
 829         return __c;
 830       return __detail::__synth3way(__x.second, __y.second);
 831     }
 832 #else
 833   /** Defines a lexicographical order for pairs.
 834    *
 835    * For two pairs of the same type, `P` is ordered before `Q` if
 836    * `P.first` is less than `Q.first`, or if `P.first` and `Q.first`
 837    * are equivalent (neither is less than the other) and `P.second` is less
 838    * than `Q.second`.
 839   */
 840   template<typename _T1, typename _T2>
 841     inline _GLIBCXX_CONSTEXPR bool
 842     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 843     { return __x.first < __y.first
 844              || (!(__y.first < __x.first) && __x.second < __y.second); }
 845
 846   /// Uses @c operator== to find the result.
 847   template<typename _T1, typename _T2>
 848     inline _GLIBCXX_CONSTEXPR bool
 849     operator!=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 850     { return !(__x == __y); }
 851
 852   /// Uses @c operator< to find the result.
 853   template<typename _T1, typename _T2>
 854     inline _GLIBCXX_CONSTEXPR bool
 855     operator>(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 856     { return __y < __x; }
 857
 858   /// Uses @c operator< to find the result.
 859   template<typename _T1, typename _T2>
 860     inline _GLIBCXX_CONSTEXPR bool
 861     operator<=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 862     { return !(__y < __x); }
 863
 864   /// Uses @c operator< to find the result.
 865   template<typename _T1, typename _T2>
 866     inline _GLIBCXX_CONSTEXPR bool
 867     operator>=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 868     { return !(__x < __y); }
 869 #endif // !(three_way_comparison && concepts)
 870
 871 #if __cplusplus >= 201103L
 872   /** Swap overload for pairs. Calls std::pair::swap().
 873    *
 874    * @note This std::swap overload is not declared in C++03 mode,
 875    * which has performance implications, e.g. see https://gcc.gnu.org/PR38466
 876   */
 877   template<typename _T1, typename _T2>
 878     _GLIBCXX20_CONSTEXPR inline
 879 #if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
 880     // Constrained free swap overload, see p0185r1
 881     typename enable_if<__and_<__is_swappable<_T1>,
 882                               __is_swappable<_T2>>::value>::type
 883 #else
 884     void
 885 #endif
 886     swap(pair<_T1, _T2>& __x, pair<_T1, _T2>& __y)
 887     noexcept(noexcept(__x.swap(__y)))
 888     { __x.swap(__y); }
 889
 890 #if __glibcxx_ranges_zip // >= C++23
 891   template<typename _T1, typename _T2>
 892     requires is_swappable_v<const _T1> && is_swappable_v<const _T2>
 893     constexpr void
 894     swap(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
 895     noexcept(noexcept(__x.swap(__y)))
 896     { __x.swap(__y); }
 897 #endif // C++23
 898
 899 #if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
 900   template<typename _T1, typename _T2>
 901     typename enable_if<!__and_<__is_swappable<_T1>,
 902                                __is_swappable<_T2>>::value>::type
 903     swap(pair<_T1, _T2>&, pair<_T1, _T2>&) = delete;
 904 #endif
 905 #endif // __cplusplus >= 201103L
 906
 907   /// @} relates pair
 908
 909   /**
 910    *  @brief A convenience wrapper for creating a pair from two objects.
 911    *  @param  __x  The first object.
 912    *  @param  __y  The second object.
 913    *  @return   A newly-constructed pair<> object of the appropriate type.
 914    *
 915    *  The C++98 standard says the objects are passed by reference-to-const,
 916    *  but C++03 says they are passed by value (this was LWG issue #181).
 917    *
 918    *  Since C++11 they have been passed by forwarding reference and then
 919    *  forwarded to the new members of the pair. To create a pair with a
 920    *  member of reference type, pass a `reference_wrapper` to this function.
 921    */
 922   // _GLIBCXX_RESOLVE_LIB_DEFECTS
 923   // 181.  make_pair() unintended behavior
 924 #if __cplusplus >= 201103L
 925   // NB: DR 706.
 926   template<typename _T1, typename _T2>
 927     constexpr pair<typename __decay_and_strip<_T1>::__type,
 928                    typename __decay_and_strip<_T2>::__type>
 929     make_pair(_T1&& __x, _T2&& __y)
 930     {
 931       typedef typename __decay_and_strip<_T1>::__type __ds_type1;
 932       typedef typename __decay_and_strip<_T2>::__type __ds_type2;
 933       typedef pair<__ds_type1, __ds_type2>            __pair_type;
 934       return __pair_type(std::forward<_T1>(__x), std::forward<_T2>(__y));
 935     }
 936 #else
 937   template<typename _T1, typename _T2>
 938     inline pair<_T1, _T2>
 939     make_pair(_T1 __x, _T2 __y)
 940     { return pair<_T1, _T2>(__x, __y); }
 941 #endif
 942
 943   /// @}
 944
 945 #if __cplusplus >= 201103L
 946   // Various functions which give std::pair a tuple-like interface.
 947
 948   /// @cond undocumented
 949   template<typename _T1, typename _T2>
 950     struct __is_tuple_like_impl<pair<_T1, _T2>> : true_type
 951     { };
 952   /// @endcond
 953
 954   /// Partial specialization for std::pair
 955   template<class _Tp1, class _Tp2>
 956     struct tuple_size<pair<_Tp1, _Tp2>>
 957     : public integral_constant<size_t, 2> { };
 958
 959   /// Partial specialization for std::pair
 960   template<class _Tp1, class _Tp2>
 961     struct tuple_element<0, pair<_Tp1, _Tp2>>
 962     { typedef _Tp1 type; };
 963
 964   /// Partial specialization for std::pair
 965   template<class _Tp1, class _Tp2>
 966     struct tuple_element<1, pair<_Tp1, _Tp2>>
 967     { typedef _Tp2 type; };
 968
 969 #if __cplusplus >= 201703L
 970   template<typename _Tp1, typename _Tp2>
 971     inline constexpr size_t tuple_size_v<pair<_Tp1, _Tp2>> = 2;
 972
 973   template<typename _Tp1, typename _Tp2>
 974     inline constexpr size_t tuple_size_v<const pair<_Tp1, _Tp2>> = 2;
 975
 976   template<typename _Tp>
 977     inline constexpr bool __is_pair = false;
 978
 979   template<typename _Tp, typename _Up>
 980     inline constexpr bool __is_pair<pair<_Tp, _Up>> = true;
 981 #endif
 982
 983   /// @cond undocumented
 984   template<size_t _Int>
 985     struct __pair_get;
 986
 987   template<>
 988     struct __pair_get<0>
 989     {
 990       template<typename _Tp1, typename _Tp2>
 991         static constexpr _Tp1&
 992         __get(pair<_Tp1, _Tp2>& __pair) noexcept
 993         { return __pair.first; }
 994
 995       template<typename _Tp1, typename _Tp2>
 996         static constexpr _Tp1&&
 997         __move_get(pair<_Tp1, _Tp2>&& __pair) noexcept
 998         { return std::forward<_Tp1>(__pair.first); }
 999
1000       template<typename _Tp1, typename _Tp2>
1001         static constexpr const _Tp1&
1002         __const_get(const pair<_Tp1, _Tp2>& __pair) noexcept
1003         { return __pair.first; }
1004
1005       template<typename _Tp1, typename _Tp2>
1006         static constexpr const _Tp1&&
1007         __const_move_get(const pair<_Tp1, _Tp2>&& __pair) noexcept
1008         { return std::forward<const _Tp1>(__pair.first); }
1009     };
1010
1011   template<>
1012     struct __pair_get<1>
1013     {
1014       template<typename _Tp1, typename _Tp2>
1015         static constexpr _Tp2&
1016         __get(pair<_Tp1, _Tp2>& __pair) noexcept
1017         { return __pair.second; }
1018
1019       template<typename _Tp1, typename _Tp2>
1020         static constexpr _Tp2&&
1021         __move_get(pair<_Tp1, _Tp2>&& __pair) noexcept
1022         { return std::forward<_Tp2>(__pair.second); }
1023
1024       template<typename _Tp1, typename _Tp2>
1025         static constexpr const _Tp2&
1026         __const_get(const pair<_Tp1, _Tp2>& __pair) noexcept
1027         { return __pair.second; }
1028
1029       template<typename _Tp1, typename _Tp2>
1030         static constexpr const _Tp2&&
1031         __const_move_get(const pair<_Tp1, _Tp2>&& __pair) noexcept
1032         { return std::forward<const _Tp2>(__pair.second); }
1033     };
1034   /// @endcond
1035
1036   /** @{
1037    * std::get overloads for accessing members of std::pair
1038    */
1039
1040   template<size_t _Int, class _Tp1, class _Tp2>
1041     constexpr typename tuple_element<_Int, pair<_Tp1, _Tp2>>::type&
1042     get(pair<_Tp1, _Tp2>& __in) noexcept
1043     { return __pair_get<_Int>::__get(__in); }
1044
1045   template<size_t _Int, class _Tp1, class _Tp2>
1046     constexpr typename tuple_element<_Int, pair<_Tp1, _Tp2>>::type&&
1047     get(pair<_Tp1, _Tp2>&& __in) noexcept
1048     { return __pair_get<_Int>::__move_get(std::move(__in)); }
1049
1050   template<size_t _Int, class _Tp1, class _Tp2>
1051     constexpr const typename tuple_element<_Int, pair<_Tp1, _Tp2>>::type&
1052     get(const pair<_Tp1, _Tp2>& __in) noexcept
1053     { return __pair_get<_Int>::__const_get(__in); }
1054
1055   template<size_t _Int, class _Tp1, class _Tp2>
1056     constexpr const typename tuple_element<_Int, pair<_Tp1, _Tp2>>::type&&
1057     get(const pair<_Tp1, _Tp2>&& __in) noexcept
1058     { return __pair_get<_Int>::__const_move_get(std::move(__in)); }
1059
1060
1061 #ifdef __glibcxx_tuples_by_type // C++ >= 14
1062   template <typename _Tp, typename _Up>
1063     constexpr _Tp&
1064     get(pair<_Tp, _Up>& __p) noexcept
1065     { return __p.first; }
1066
1067   template <typename _Tp, typename _Up>
1068     constexpr const _Tp&
1069     get(const pair<_Tp, _Up>& __p) noexcept
1070     { return __p.first; }
1071
1072   template <typename _Tp, typename _Up>
1073     constexpr _Tp&&
1074     get(pair<_Tp, _Up>&& __p) noexcept
1075     { return std::move(__p.first); }
1076
1077   template <typename _Tp, typename _Up>
1078     constexpr const _Tp&&
1079     get(const pair<_Tp, _Up>&& __p) noexcept
1080     { return std::move(__p.first); }
1081
1082   template <typename _Tp, typename _Up>
1083     constexpr _Tp&
1084     get(pair<_Up, _Tp>& __p) noexcept
1085     { return __p.second; }
1086
1087   template <typename _Tp, typename _Up>
1088     constexpr const _Tp&
1089     get(const pair<_Up, _Tp>& __p) noexcept
1090     { return __p.second; }
1091
1092   template <typename _Tp, typename _Up>
1093     constexpr _Tp&&
1094     get(pair<_Up, _Tp>&& __p) noexcept
1095     { return std::move(__p.second); }
1096
1097   template <typename _Tp, typename _Up>
1098     constexpr const _Tp&&
1099     get(const pair<_Up, _Tp>&& __p) noexcept
1100     { return std::move(__p.second); }
1101 #endif // __glibcxx_tuples_by_type
1102
1103
1104 #if __glibcxx_ranges_zip // >= C++23
1105   template<typename _T1, typename _T2, typename _U1, typename _U2,
1106            template<typename> class _TQual, template<typename> class _UQual>
1107     requires requires { typename pair<common_reference_t<_TQual<_T1>, _UQual<_U1>>,
1108                                       common_reference_t<_TQual<_T2>, _UQual<_U2>>>; }
1109   struct basic_common_reference<pair<_T1, _T2>, pair<_U1, _U2>, _TQual, _UQual>
1110   {
1111     using type = pair<common_reference_t<_TQual<_T1>, _UQual<_U1>>,
1112                       common_reference_t<_TQual<_T2>, _UQual<_U2>>>;
1113   };
1114
1115   template<typename _T1, typename _T2, typename _U1, typename _U2>
1116     requires requires { typename pair<common_type_t<_T1, _U1>, common_type_t<_T2, _U2>>; }
1117   struct common_type<pair<_T1, _T2>, pair<_U1, _U2>>
1118   { using type = pair<common_type_t<_T1, _U1>, common_type_t<_T2, _U2>>; };
1119 #endif // C++23
1120
1121   /// @}
1122 #endif // C++11
1123
1124 _GLIBCXX_END_NAMESPACE_VERSION
1125 } // namespace std
1126
1127 #endif /* _STL_PAIR_H */