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

   1 // shared_ptr and weak_ptr implementation details -*- C++ -*-
   2
   3 // Copyright (C) 2007-2012 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 // GCC Note: Based on files from version 1.32.0 of the Boost library.
  26
  27 //  shared_count.hpp
  28 //  Copyright (c) 2001, 2002, 2003 Peter Dimov and Multi Media Ltd.
  29
  30 //  shared_ptr.hpp
  31 //  Copyright (C) 1998, 1999 Greg Colvin and Beman Dawes.
  32 //  Copyright (C) 2001, 2002, 2003 Peter Dimov
  33
  34 //  weak_ptr.hpp
  35 //  Copyright (C) 2001, 2002, 2003 Peter Dimov
  36
  37 //  enable_shared_from_this.hpp
  38 //  Copyright (C) 2002 Peter Dimov
  39
  40 // Distributed under the Boost Software License, Version 1.0. (See
  41 // accompanying file LICENSE_1_0.txt or copy at
  42 // http://www.boost.org/LICENSE_1_0.txt)
  43
  44 /** @file bits/shared_ptr_base.h
  45  *  This is an internal header file, included by other library headers.
  46  *  Do not attempt to use it directly. @headername{memory}
  47  */
  48
  49 #ifndef _SHARED_PTR_BASE_H
  50 #define _SHARED_PTR_BASE_H 1
  51
  52 namespace std _GLIBCXX_VISIBILITY(default)
  53 {
  54 _GLIBCXX_BEGIN_NAMESPACE_VERSION
  55
  56 #if _GLIBCXX_USE_DEPRECATED
  57   template<typename> class auto_ptr;
  58 #endif
  59
  60  /**
  61    *  @brief  Exception possibly thrown by @c shared_ptr.
  62    *  @ingroup exceptions
  63    */
  64   class bad_weak_ptr : public std::exception
  65   {
  66   public:
  67     virtual char const*
  68     what() const noexcept;
  69
  70     virtual ~bad_weak_ptr() noexcept;
  71   };
  72
  73   // Substitute for bad_weak_ptr object in the case of -fno-exceptions.
  74   inline void
  75   __throw_bad_weak_ptr()
  76   {
  77 #if __EXCEPTIONS
  78     throw bad_weak_ptr();
  79 #else
  80     __builtin_abort();
  81 #endif
  82   }
  83
  84   using __gnu_cxx::_Lock_policy;
  85   using __gnu_cxx::__default_lock_policy;
  86   using __gnu_cxx::_S_single;
  87   using __gnu_cxx::_S_mutex;
  88   using __gnu_cxx::_S_atomic;
  89
  90   // Empty helper class except when the template argument is _S_mutex.
  91   template<_Lock_policy _Lp>
  92     class _Mutex_base
  93     {
  94     protected:
  95       // The atomic policy uses fully-fenced builtins, single doesn't care.
  96       enum { _S_need_barriers = 0 };
  97     };
  98
  99   template<>
 100     class _Mutex_base<_S_mutex>
 101     : public __gnu_cxx::__mutex
 102     {
 103     protected:
 104       // This policy is used when atomic builtins are not available.
 105       // The replacement atomic operations might not have the necessary
 106       // memory barriers.
 107       enum { _S_need_barriers = 1 };
 108     };
 109
 110   template<_Lock_policy _Lp = __default_lock_policy>
 111     class _Sp_counted_base
 112     : public _Mutex_base<_Lp>
 113     {
 114     public:
 115       _Sp_counted_base() noexcept
 116       : _M_use_count(1), _M_weak_count(1) { }
 117
 118       virtual
 119       ~_Sp_counted_base() noexcept
 120       { }
 121
 122       // Called when _M_use_count drops to zero, to release the resources
 123       // managed by *this.
 124       virtual void
 125       _M_dispose() noexcept = 0;
 126
 127       // Called when _M_weak_count drops to zero.
 128       virtual void
 129       _M_destroy() noexcept
 130       { delete this; }
 131
 132       virtual void*
 133       _M_get_deleter(const std::type_info&) = 0;
 134
 135       void
 136       _M_add_ref_copy()
 137       { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }
 138
 139       void
 140       _M_add_ref_lock();
 141
 142       void
 143       _M_release() noexcept
 144       {
 145         // Be race-detector-friendly.  For more info see bits/c++config.
 146         _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
 147         if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
 148           {
 149             _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
 150             _M_dispose();
 151             // There must be a memory barrier between dispose() and destroy()
 152             // to ensure that the effects of dispose() are observed in the
 153             // thread that runs destroy().
 154             // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html
 155             if (_Mutex_base<_Lp>::_S_need_barriers)
 156               {
 157                 _GLIBCXX_READ_MEM_BARRIER;
 158                 _GLIBCXX_WRITE_MEM_BARRIER;
 159               }
 160
 161             // Be race-detector-friendly.  For more info see bits/c++config.
 162             _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
 163             if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count,
 164                                                        -1) == 1)
 165               {
 166                 _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
 167                 _M_destroy();
 168               }
 169           }
 170       }
 171
 172       void
 173       _M_weak_add_ref() noexcept
 174       { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }
 175
 176       void
 177       _M_weak_release() noexcept
 178       {
 179         // Be race-detector-friendly. For more info see bits/c++config.
 180         _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
 181         if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
 182           {
 183             _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
 184             if (_Mutex_base<_Lp>::_S_need_barriers)
 185               {
 186                 // See _M_release(),
 187                 // destroy() must observe results of dispose()
 188                 _GLIBCXX_READ_MEM_BARRIER;
 189                 _GLIBCXX_WRITE_MEM_BARRIER;
 190               }
 191             _M_destroy();
 192           }
 193       }
 194
 195       long
 196       _M_get_use_count() const noexcept
 197       {
 198         // No memory barrier is used here so there is no synchronization
 199         // with other threads.
 200         return __atomic_load_n(&_M_use_count, __ATOMIC_RELAXED);
 201       }
 202
 203     private:
 204       _Sp_counted_base(_Sp_counted_base const&) = delete;
 205       _Sp_counted_base& operator=(_Sp_counted_base const&) = delete;
 206
 207       _Atomic_word  _M_use_count;     // #shared
 208       _Atomic_word  _M_weak_count;    // #weak + (#shared != 0)
 209     };
 210
 211   template<>
 212     inline void
 213     _Sp_counted_base<_S_single>::
 214     _M_add_ref_lock()
 215     {
 216       if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
 217         {
 218           _M_use_count = 0;
 219           __throw_bad_weak_ptr();
 220         }
 221     }
 222
 223   template<>
 224     inline void
 225     _Sp_counted_base<_S_mutex>::
 226     _M_add_ref_lock()
 227     {
 228       __gnu_cxx::__scoped_lock sentry(*this);
 229       if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
 230         {
 231           _M_use_count = 0;
 232           __throw_bad_weak_ptr();
 233         }
 234     }
 235
 236   template<>
 237     inline void
 238     _Sp_counted_base<_S_atomic>::
 239     _M_add_ref_lock()
 240     {
 241       // Perform lock-free add-if-not-zero operation.
 242       _Atomic_word __count = _M_use_count;
 243       do
 244         {
 245           if (__count == 0)
 246             __throw_bad_weak_ptr();
 247           // Replace the current counter value with the old value + 1, as
 248           // long as it's not changed meanwhile.
 249         }
 250       while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
 251                                           true, __ATOMIC_ACQ_REL,
 252                                           __ATOMIC_RELAXED));
 253     }
 254
 255
 256   // Forward declarations.
 257   template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
 258     class __shared_ptr;
 259
 260   template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
 261     class __weak_ptr;
 262
 263   template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
 264     class __enable_shared_from_this;
 265
 266   template<typename _Tp>
 267     class shared_ptr;
 268
 269   template<typename _Tp>
 270     class weak_ptr;
 271
 272   template<typename _Tp>
 273     struct owner_less;
 274
 275   template<typename _Tp>
 276     class enable_shared_from_this;
 277
 278   template<_Lock_policy _Lp = __default_lock_policy>
 279     class __weak_count;
 280
 281   template<_Lock_policy _Lp = __default_lock_policy>
 282     class __shared_count;
 283
 284
 285   // Counted ptr with no deleter or allocator support
 286   template<typename _Ptr, _Lock_policy _Lp>
 287     class _Sp_counted_ptr final : public _Sp_counted_base<_Lp>
 288     {
 289     public:
 290       explicit
 291       _Sp_counted_ptr(_Ptr __p)
 292       : _M_ptr(__p) { }
 293
 294       virtual void
 295       _M_dispose() noexcept
 296       { delete _M_ptr; }
 297
 298       virtual void
 299       _M_destroy() noexcept
 300       { delete this; }
 301
 302       virtual void*
 303       _M_get_deleter(const std::type_info&)
 304       { return 0; }
 305
 306       _Sp_counted_ptr(const _Sp_counted_ptr&) = delete;
 307       _Sp_counted_ptr& operator=(const _Sp_counted_ptr&) = delete;
 308
 309     protected:
 310       _Ptr             _M_ptr;  // copy constructor must not throw
 311     };
 312
 313   template<>
 314     inline void
 315     _Sp_counted_ptr<nullptr_t, _S_single>::_M_dispose() noexcept { }
 316
 317   template<>
 318     inline void
 319     _Sp_counted_ptr<nullptr_t, _S_mutex>::_M_dispose() noexcept { }
 320
 321   template<>
 322     inline void
 323     _Sp_counted_ptr<nullptr_t, _S_atomic>::_M_dispose() noexcept { }
 324
 325   // Support for custom deleter and/or allocator
 326   template<typename _Ptr, typename _Deleter, typename _Alloc, _Lock_policy _Lp>
 327     class _Sp_counted_deleter final : public _Sp_counted_base<_Lp>
 328     {
 329       // Helper class that stores the Deleter and also acts as an allocator.
 330       // Used to dispose of the owned pointer and the internal refcount
 331       // Requires that copies of _Alloc can free each other's memory.
 332       struct _My_Deleter
 333       : public _Alloc           // copy constructor must not throw
 334       {
 335         _Deleter _M_del;        // copy constructor must not throw
 336         _My_Deleter(_Deleter __d, const _Alloc& __a)
 337         : _Alloc(__a), _M_del(__d) { }
 338       };
 339
 340     public:
 341       // __d(__p) must not throw.
 342       _Sp_counted_deleter(_Ptr __p, _Deleter __d)
 343       : _M_ptr(__p), _M_del(__d, _Alloc()) { }
 344
 345       // __d(__p) must not throw.
 346       _Sp_counted_deleter(_Ptr __p, _Deleter __d, const _Alloc& __a)
 347       : _M_ptr(__p), _M_del(__d, __a) { }
 348
 349       ~_Sp_counted_deleter() noexcept { }
 350
 351       virtual void
 352       _M_dispose() noexcept
 353       { _M_del._M_del(_M_ptr); }
 354
 355       virtual void
 356       _M_destroy() noexcept
 357       {
 358         typedef typename allocator_traits<_Alloc>::template
 359           rebind_traits<_Sp_counted_deleter> _Alloc_traits;
 360         typename _Alloc_traits::allocator_type __a(_M_del);
 361         _Alloc_traits::destroy(__a, this);
 362         _Alloc_traits::deallocate(__a, this, 1);
 363       }
 364
 365       virtual void*
 366       _M_get_deleter(const std::type_info& __ti)
 367       {
 368 #ifdef __GXX_RTTI
 369         return __ti == typeid(_Deleter) ? &_M_del._M_del : 0;
 370 #else
 371         return 0;
 372 #endif
 373       }
 374
 375     protected:
 376       _Ptr             _M_ptr;  // copy constructor must not throw
 377       _My_Deleter      _M_del;  // copy constructor must not throw
 378     };
 379
 380   // helpers for make_shared / allocate_shared
 381
 382   struct _Sp_make_shared_tag { };
 383
 384   template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
 385     class _Sp_counted_ptr_inplace final : public _Sp_counted_base<_Lp>
 386     {
 387       // Helper class that stores the pointer and also acts as an allocator.
 388       // Used to dispose of the owned pointer and the internal refcount
 389       // Requires that copies of _Alloc can free each other's memory.
 390       struct _Impl
 391       : public _Alloc           // copy constructor must not throw
 392       {
 393         _Impl(_Alloc __a) : _Alloc(__a), _M_ptr() { }
 394         _Tp* _M_ptr;
 395       };
 396
 397     public:
 398       template<typename... _Args>
 399         _Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
 400         : _M_impl(__a), _M_storage()
 401         {
 402           _M_impl._M_ptr = static_cast<_Tp*>(static_cast<void*>(&_M_storage));
 403           // _GLIBCXX_RESOLVE_LIB_DEFECTS
 404           // 2070.  allocate_shared should use allocator_traits<A>::construct
 405           allocator_traits<_Alloc>::construct(__a, _M_impl._M_ptr,
 406               std::forward<_Args>(__args)...); // might throw
 407         }
 408
 409       ~_Sp_counted_ptr_inplace() noexcept { }
 410
 411       virtual void
 412       _M_dispose() noexcept
 413       { allocator_traits<_Alloc>::destroy(_M_impl, _M_impl._M_ptr); }
 414
 415       // Override because the allocator needs to know the dynamic type
 416       virtual void
 417       _M_destroy() noexcept
 418       {
 419         typedef typename allocator_traits<_Alloc>::template
 420           rebind_traits<_Sp_counted_ptr_inplace> _Alloc_traits;
 421         typename _Alloc_traits::allocator_type __a(_M_impl);
 422         _Alloc_traits::destroy(__a, this);
 423         _Alloc_traits::deallocate(__a, this, 1);
 424       }
 425
 426       // Sneaky trick so __shared_ptr can get the managed pointer
 427       virtual void*
 428       _M_get_deleter(const std::type_info& __ti) noexcept
 429       {
 430 #ifdef __GXX_RTTI
 431         return __ti == typeid(_Sp_make_shared_tag)
 432                ? static_cast<void*>(&_M_storage)
 433                : 0;
 434 #else
 435         return 0;
 436 #endif
 437       }
 438
 439     private:
 440       _Impl _M_impl;
 441       typename aligned_storage<sizeof(_Tp), alignment_of<_Tp>::value>::type
 442         _M_storage;
 443     };
 444
 445   template<_Lock_policy _Lp>
 446     class __shared_count
 447     {
 448     public:
 449       constexpr __shared_count() noexcept : _M_pi(0)
 450       { }
 451
 452       template<typename _Ptr>
 453         explicit
 454         __shared_count(_Ptr __p) : _M_pi(0)
 455         {
 456           __try
 457             {
 458               _M_pi = new _Sp_counted_ptr<_Ptr, _Lp>(__p);
 459             }
 460           __catch(...)
 461             {
 462               delete __p;
 463               __throw_exception_again;
 464             }
 465         }
 466
 467       template<typename _Ptr, typename _Deleter>
 468         __shared_count(_Ptr __p, _Deleter __d) : _M_pi(0)
 469         {
 470           // The allocator's value_type doesn't matter, will rebind it anyway.
 471           typedef std::allocator<int> _Alloc;
 472           typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
 473           typedef typename allocator_traits<_Alloc>::template
 474             rebind_traits<_Sp_cd_type> _Alloc_traits;
 475           typename _Alloc_traits::allocator_type __a;
 476           _Sp_cd_type* __mem = 0;
 477           __try
 478             {
 479               __mem = _Alloc_traits::allocate(__a, 1);
 480               _Alloc_traits::construct(__a, __mem, __p, std::move(__d));
 481               _M_pi = __mem;
 482             }
 483           __catch(...)
 484             {
 485               __d(__p); // Call _Deleter on __p.
 486               if (__mem)
 487                 _Alloc_traits::deallocate(__a, __mem, 1);
 488               __throw_exception_again;
 489             }
 490         }
 491
 492       template<typename _Ptr, typename _Deleter, typename _Alloc>
 493         __shared_count(_Ptr __p, _Deleter __d, _Alloc __a) : _M_pi(0)
 494         {
 495           typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
 496           typedef typename allocator_traits<_Alloc>::template
 497             rebind_traits<_Sp_cd_type> _Alloc_traits;
 498           typename _Alloc_traits::allocator_type __a2(__a);
 499           _Sp_cd_type* __mem = 0;
 500           __try
 501             {
 502               __mem = _Alloc_traits::allocate(__a2, 1);
 503               _Alloc_traits::construct(__a2, __mem,
 504                   __p, std::move(__d), std::move(__a));
 505               _M_pi = __mem;
 506             }
 507           __catch(...)
 508             {
 509               __d(__p); // Call _Deleter on __p.
 510               if (__mem)
 511                 _Alloc_traits::deallocate(__a2, __mem, 1);
 512               __throw_exception_again;
 513             }
 514         }
 515
 516       template<typename _Tp, typename _Alloc, typename... _Args>
 517         __shared_count(_Sp_make_shared_tag, _Tp*, const _Alloc& __a,
 518                        _Args&&... __args)
 519         : _M_pi(0)
 520         {
 521           typedef _Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp> _Sp_cp_type;
 522           typedef typename allocator_traits<_Alloc>::template
 523             rebind_traits<_Sp_cp_type> _Alloc_traits;
 524           typename _Alloc_traits::allocator_type __a2(__a);
 525           _Sp_cp_type* __mem = _Alloc_traits::allocate(__a2, 1);
 526           __try
 527             {
 528               _Alloc_traits::construct(__a2, __mem, std::move(__a),
 529                     std::forward<_Args>(__args)...);
 530               _M_pi = __mem;
 531             }
 532           __catch(...)
 533             {
 534               _Alloc_traits::deallocate(__a2, __mem, 1);
 535               __throw_exception_again;
 536             }
 537         }
 538
 539 #if _GLIBCXX_USE_DEPRECATED
 540       // Special case for auto_ptr<_Tp> to provide the strong guarantee.
 541       template<typename _Tp>
 542         explicit
 543         __shared_count(std::auto_ptr<_Tp>&& __r);
 544 #endif
 545
 546       // Special case for unique_ptr<_Tp,_Del> to provide the strong guarantee.
 547       template<typename _Tp, typename _Del>
 548         explicit
 549         __shared_count(std::unique_ptr<_Tp, _Del>&& __r)
 550         : _M_pi(_S_create_from_up(std::move(__r)))
 551         { __r.release(); }
 552
 553       // Throw bad_weak_ptr when __r._M_get_use_count() == 0.
 554       explicit __shared_count(const __weak_count<_Lp>& __r);
 555
 556       ~__shared_count() noexcept
 557       {
 558         if (_M_pi != 0)
 559           _M_pi->_M_release();
 560       }
 561
 562       __shared_count(const __shared_count& __r) noexcept
 563       : _M_pi(__r._M_pi)
 564       {
 565         if (_M_pi != 0)
 566           _M_pi->_M_add_ref_copy();
 567       }
 568
 569       __shared_count&
 570       operator=(const __shared_count& __r) noexcept
 571       {
 572         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 573         if (__tmp != _M_pi)
 574           {
 575             if (__tmp != 0)
 576               __tmp->_M_add_ref_copy();
 577             if (_M_pi != 0)
 578               _M_pi->_M_release();
 579             _M_pi = __tmp;
 580           }
 581         return *this;
 582       }
 583
 584       void
 585       _M_swap(__shared_count& __r) noexcept
 586       {
 587         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 588         __r._M_pi = _M_pi;
 589         _M_pi = __tmp;
 590       }
 591
 592       long
 593       _M_get_use_count() const noexcept
 594       { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }
 595
 596       bool
 597       _M_unique() const noexcept
 598       { return this->_M_get_use_count() == 1; }
 599
 600       void*
 601       _M_get_deleter(const std::type_info& __ti) const noexcept
 602       { return _M_pi ? _M_pi->_M_get_deleter(__ti) : 0; }
 603
 604       bool
 605       _M_less(const __shared_count& __rhs) const noexcept
 606       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 607
 608       bool
 609       _M_less(const __weak_count<_Lp>& __rhs) const noexcept
 610       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 611
 612       // Friend function injected into enclosing namespace and found by ADL
 613       friend inline bool
 614       operator==(const __shared_count& __a, const __shared_count& __b) noexcept
 615       { return __a._M_pi == __b._M_pi; }
 616
 617     private:
 618       friend class __weak_count<_Lp>;
 619
 620       template<typename _Tp, typename _Del>
 621         static _Sp_counted_base<_Lp>*
 622         _S_create_from_up(std::unique_ptr<_Tp, _Del>&& __r,
 623           typename std::enable_if<!std::is_reference<_Del>::value>::type* = 0)
 624         {
 625           return new _Sp_counted_deleter<_Tp*, _Del, std::allocator<_Tp>,
 626             _Lp>(__r.get(), __r.get_deleter());
 627         }
 628
 629       template<typename _Tp, typename _Del>
 630         static _Sp_counted_base<_Lp>*
 631         _S_create_from_up(std::unique_ptr<_Tp, _Del>&& __r,
 632           typename std::enable_if<std::is_reference<_Del>::value>::type* = 0)
 633         {
 634           typedef typename std::remove_reference<_Del>::type _Del1;
 635           typedef std::reference_wrapper<_Del1> _Del2;
 636           return new _Sp_counted_deleter<_Tp*, _Del2, std::allocator<_Tp>,
 637             _Lp>(__r.get(), std::ref(__r.get_deleter()));
 638         }
 639
 640       _Sp_counted_base<_Lp>*  _M_pi;
 641     };
 642
 643
 644   template<_Lock_policy _Lp>
 645     class __weak_count
 646     {
 647     public:
 648       constexpr __weak_count() noexcept : _M_pi(0)
 649       { }
 650
 651       __weak_count(const __shared_count<_Lp>& __r) noexcept
 652       : _M_pi(__r._M_pi)
 653       {
 654         if (_M_pi != 0)
 655           _M_pi->_M_weak_add_ref();
 656       }
 657
 658       __weak_count(const __weak_count<_Lp>& __r) noexcept
 659       : _M_pi(__r._M_pi)
 660       {
 661         if (_M_pi != 0)
 662           _M_pi->_M_weak_add_ref();
 663       }
 664
 665       ~__weak_count() noexcept
 666       {
 667         if (_M_pi != 0)
 668           _M_pi->_M_weak_release();
 669       }
 670
 671       __weak_count<_Lp>&
 672       operator=(const __shared_count<_Lp>& __r) noexcept
 673       {
 674         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 675         if (__tmp != 0)
 676           __tmp->_M_weak_add_ref();
 677         if (_M_pi != 0)
 678           _M_pi->_M_weak_release();
 679         _M_pi = __tmp;
 680         return *this;
 681       }
 682
 683       __weak_count<_Lp>&
 684       operator=(const __weak_count<_Lp>& __r) noexcept
 685       {
 686         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 687         if (__tmp != 0)
 688           __tmp->_M_weak_add_ref();
 689         if (_M_pi != 0)
 690           _M_pi->_M_weak_release();
 691         _M_pi = __tmp;
 692         return *this;
 693       }
 694
 695       void
 696       _M_swap(__weak_count<_Lp>& __r) noexcept
 697       {
 698         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 699         __r._M_pi = _M_pi;
 700         _M_pi = __tmp;
 701       }
 702
 703       long
 704       _M_get_use_count() const noexcept
 705       { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }
 706
 707       bool
 708       _M_less(const __weak_count& __rhs) const noexcept
 709       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 710
 711       bool
 712       _M_less(const __shared_count<_Lp>& __rhs) const noexcept
 713       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 714
 715       // Friend function injected into enclosing namespace and found by ADL
 716       friend inline bool
 717       operator==(const __weak_count& __a, const __weak_count& __b) noexcept
 718       { return __a._M_pi == __b._M_pi; }
 719
 720     private:
 721       friend class __shared_count<_Lp>;
 722
 723       _Sp_counted_base<_Lp>*  _M_pi;
 724     };
 725
 726   // Now that __weak_count is defined we can define this constructor:
 727   template<_Lock_policy _Lp>
 728     inline __shared_count<_Lp>:: __shared_count(const __weak_count<_Lp>& __r)
 729     : _M_pi(__r._M_pi)
 730     {
 731       if (_M_pi != 0)
 732         _M_pi->_M_add_ref_lock();
 733       else
 734         __throw_bad_weak_ptr();
 735     }
 736
 737
 738   // Support for enable_shared_from_this.
 739
 740   // Friend of __enable_shared_from_this.
 741   template<_Lock_policy _Lp, typename _Tp1, typename _Tp2>
 742     void
 743     __enable_shared_from_this_helper(const __shared_count<_Lp>&,
 744                                      const __enable_shared_from_this<_Tp1,
 745                                      _Lp>*, const _Tp2*) noexcept;
 746
 747   // Friend of enable_shared_from_this.
 748   template<typename _Tp1, typename _Tp2>
 749     void
 750     __enable_shared_from_this_helper(const __shared_count<>&,
 751                                      const enable_shared_from_this<_Tp1>*,
 752                                      const _Tp2*) noexcept;
 753
 754   template<_Lock_policy _Lp>
 755     inline void
 756     __enable_shared_from_this_helper(const __shared_count<_Lp>&, ...) noexcept
 757     { }
 758
 759
 760   template<typename _Tp, _Lock_policy _Lp>
 761     class __shared_ptr
 762     {
 763     public:
 764       typedef _Tp   element_type;
 765
 766       constexpr __shared_ptr() noexcept
 767       : _M_ptr(0), _M_refcount()
 768       { }
 769
 770       template<typename _Tp1>
 771         explicit __shared_ptr(_Tp1* __p)
 772         : _M_ptr(__p), _M_refcount(__p)
 773         {
 774           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 775           static_assert( sizeof(_Tp1) > 0, "incomplete type" );
 776           __enable_shared_from_this_helper(_M_refcount, __p, __p);
 777         }
 778
 779       template<typename _Tp1, typename _Deleter>
 780         __shared_ptr(_Tp1* __p, _Deleter __d)
 781         : _M_ptr(__p), _M_refcount(__p, __d)
 782         {
 783           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 784           // TODO requires _Deleter CopyConstructible and __d(__p) well-formed
 785           __enable_shared_from_this_helper(_M_refcount, __p, __p);
 786         }
 787
 788       template<typename _Tp1, typename _Deleter, typename _Alloc>
 789         __shared_ptr(_Tp1* __p, _Deleter __d, _Alloc __a)
 790         : _M_ptr(__p), _M_refcount(__p, __d, std::move(__a))
 791         {
 792           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 793           // TODO requires _Deleter CopyConstructible and __d(__p) well-formed
 794           __enable_shared_from_this_helper(_M_refcount, __p, __p);
 795         }
 796
 797       template<typename _Deleter>
 798         __shared_ptr(nullptr_t __p, _Deleter __d)
 799         : _M_ptr(0), _M_refcount(__p, __d)
 800         { }
 801
 802       template<typename _Deleter, typename _Alloc>
 803         __shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
 804         : _M_ptr(0), _M_refcount(__p, __d, std::move(__a))
 805         { }
 806
 807       template<typename _Tp1>
 808         __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, _Tp* __p) noexcept
 809         : _M_ptr(__p), _M_refcount(__r._M_refcount) // never throws
 810         { }
 811
 812       __shared_ptr(const __shared_ptr&) noexcept = default;
 813       __shared_ptr& operator=(const __shared_ptr&) noexcept = default;
 814       ~__shared_ptr() = default;
 815
 816       template<typename _Tp1, typename = typename
 817                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 818         __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
 819         : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
 820         { }
 821
 822       __shared_ptr(__shared_ptr&& __r) noexcept
 823       : _M_ptr(__r._M_ptr), _M_refcount()
 824       {
 825         _M_refcount._M_swap(__r._M_refcount);
 826         __r._M_ptr = 0;
 827       }
 828
 829       template<typename _Tp1, typename = typename
 830                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 831         __shared_ptr(__shared_ptr<_Tp1, _Lp>&& __r) noexcept
 832         : _M_ptr(__r._M_ptr), _M_refcount()
 833         {
 834           _M_refcount._M_swap(__r._M_refcount);
 835           __r._M_ptr = 0;
 836         }
 837
 838       template<typename _Tp1>
 839         explicit __shared_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
 840         : _M_refcount(__r._M_refcount) // may throw
 841         {
 842           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 843
 844           // It is now safe to copy __r._M_ptr, as
 845           // _M_refcount(__r._M_refcount) did not throw.
 846           _M_ptr = __r._M_ptr;
 847         }
 848
 849       // If an exception is thrown this constructor has no effect.
 850       template<typename _Tp1, typename _Del>
 851         __shared_ptr(std::unique_ptr<_Tp1, _Del>&& __r)
 852         : _M_ptr(__r.get()), _M_refcount()
 853         {
 854           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 855           _Tp1* __tmp = __r.get();
 856           _M_refcount = __shared_count<_Lp>(std::move(__r));
 857           __enable_shared_from_this_helper(_M_refcount, __tmp, __tmp);
 858         }
 859
 860 #if _GLIBCXX_USE_DEPRECATED
 861       // Postcondition: use_count() == 1 and __r.get() == 0
 862       template<typename _Tp1>
 863         __shared_ptr(std::auto_ptr<_Tp1>&& __r);
 864 #endif
 865
 866       /* TODO: use delegating constructor */
 867       constexpr __shared_ptr(nullptr_t) noexcept
 868       : _M_ptr(0), _M_refcount()
 869       { }
 870
 871       template<typename _Tp1>
 872         __shared_ptr&
 873         operator=(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
 874         {
 875           _M_ptr = __r._M_ptr;
 876           _M_refcount = __r._M_refcount; // __shared_count::op= doesn't throw
 877           return *this;
 878         }
 879
 880 #if _GLIBCXX_USE_DEPRECATED
 881       template<typename _Tp1>
 882         __shared_ptr&
 883         operator=(std::auto_ptr<_Tp1>&& __r)
 884         {
 885           __shared_ptr(std::move(__r)).swap(*this);
 886           return *this;
 887         }
 888 #endif
 889
 890       __shared_ptr&
 891       operator=(__shared_ptr&& __r) noexcept
 892       {
 893         __shared_ptr(std::move(__r)).swap(*this);
 894         return *this;
 895       }
 896
 897       template<class _Tp1>
 898         __shared_ptr&
 899         operator=(__shared_ptr<_Tp1, _Lp>&& __r) noexcept
 900         {
 901           __shared_ptr(std::move(__r)).swap(*this);
 902           return *this;
 903         }
 904
 905       template<typename _Tp1, typename _Del>
 906         __shared_ptr&
 907         operator=(std::unique_ptr<_Tp1, _Del>&& __r)
 908         {
 909           __shared_ptr(std::move(__r)).swap(*this);
 910           return *this;
 911         }
 912
 913       void
 914       reset() noexcept
 915       { __shared_ptr().swap(*this); }
 916
 917       template<typename _Tp1>
 918         void
 919         reset(_Tp1* __p) // _Tp1 must be complete.
 920         {
 921           // Catch self-reset errors.
 922           _GLIBCXX_DEBUG_ASSERT(__p == 0 || __p != _M_ptr);
 923           __shared_ptr(__p).swap(*this);
 924         }
 925
 926       template<typename _Tp1, typename _Deleter>
 927         void
 928         reset(_Tp1* __p, _Deleter __d)
 929         { __shared_ptr(__p, __d).swap(*this); }
 930
 931       template<typename _Tp1, typename _Deleter, typename _Alloc>
 932         void
 933         reset(_Tp1* __p, _Deleter __d, _Alloc __a)
 934         { __shared_ptr(__p, __d, std::move(__a)).swap(*this); }
 935
 936       // Allow class instantiation when _Tp is [cv-qual] void.
 937       typename std::add_lvalue_reference<_Tp>::type
 938       operator*() const noexcept
 939       {
 940         _GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
 941         return *_M_ptr;
 942       }
 943
 944       _Tp*
 945       operator->() const noexcept
 946       {
 947         _GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
 948         return _M_ptr;
 949       }
 950
 951       _Tp*
 952       get() const noexcept
 953       { return _M_ptr; }
 954
 955       explicit operator bool() const // never throws
 956       { return _M_ptr == 0 ? false : true; }
 957
 958       bool
 959       unique() const noexcept
 960       { return _M_refcount._M_unique(); }
 961
 962       long
 963       use_count() const noexcept
 964       { return _M_refcount._M_get_use_count(); }
 965
 966       void
 967       swap(__shared_ptr<_Tp, _Lp>& __other) noexcept
 968       {
 969         std::swap(_M_ptr, __other._M_ptr);
 970         _M_refcount._M_swap(__other._M_refcount);
 971       }
 972
 973       template<typename _Tp1>
 974         bool
 975         owner_before(__shared_ptr<_Tp1, _Lp> const& __rhs) const
 976         { return _M_refcount._M_less(__rhs._M_refcount); }
 977
 978       template<typename _Tp1>
 979         bool
 980         owner_before(__weak_ptr<_Tp1, _Lp> const& __rhs) const
 981         { return _M_refcount._M_less(__rhs._M_refcount); }
 982
 983 #ifdef __GXX_RTTI
 984     protected:
 985       // This constructor is non-standard, it is used by allocate_shared.
 986       template<typename _Alloc, typename... _Args>
 987         __shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
 988                      _Args&&... __args)
 989         : _M_ptr(), _M_refcount(__tag, (_Tp*)0, __a,
 990                                 std::forward<_Args>(__args)...)
 991         {
 992           // _M_ptr needs to point to the newly constructed object.
 993           // This relies on _Sp_counted_ptr_inplace::_M_get_deleter.
 994           void* __p = _M_refcount._M_get_deleter(typeid(__tag));
 995           _M_ptr = static_cast<_Tp*>(__p);
 996           __enable_shared_from_this_helper(_M_refcount, _M_ptr, _M_ptr);
 997         }
 998 #else
 999       template<typename _Alloc>
1000         struct _Deleter
1001         {
1002           void operator()(_Tp* __ptr)
1003           {
1004             typedef allocator_traits<_Alloc> _Alloc_traits;
1005             _Alloc_traits::destroy(_M_alloc, __ptr);
1006             _Alloc_traits::deallocate(_M_alloc, __ptr, 1);
1007           }
1008           _Alloc _M_alloc;
1009         };
1010
1011       template<typename _Alloc, typename... _Args>
1012         __shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
1013                      _Args&&... __args)
1014         : _M_ptr(), _M_refcount()
1015         {
1016           typedef typename _Alloc::template rebind<_Tp>::other _Alloc2;
1017           _Deleter<_Alloc2> __del = { _Alloc2(__a) };
1018           typedef allocator_traits<_Alloc2> __traits;
1019           _M_ptr = __traits::allocate(__del._M_alloc, 1);
1020           __try
1021             {
1022               // _GLIBCXX_RESOLVE_LIB_DEFECTS
1023               // 2070. allocate_shared should use allocator_traits<A>::construct
1024               __traits::construct(__del._M_alloc, _M_ptr,
1025                                   std::forward<_Args>(__args)...);
1026             }
1027           __catch(...)
1028             {
1029               __traits::deallocate(__del._M_alloc, _M_ptr, 1);
1030               __throw_exception_again;
1031             }
1032           __shared_count<_Lp> __count(_M_ptr, __del, __del._M_alloc);
1033           _M_refcount._M_swap(__count);
1034           __enable_shared_from_this_helper(_M_refcount, _M_ptr, _M_ptr);
1035         }
1036 #endif
1037
1038       template<typename _Tp1, _Lock_policy _Lp1, typename _Alloc,
1039                typename... _Args>
1040         friend __shared_ptr<_Tp1, _Lp1>
1041         __allocate_shared(const _Alloc& __a, _Args&&... __args);
1042
1043     private:
1044       void*
1045       _M_get_deleter(const std::type_info& __ti) const noexcept
1046       { return _M_refcount._M_get_deleter(__ti); }
1047
1048       template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
1049       template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
1050
1051       template<typename _Del, typename _Tp1, _Lock_policy _Lp1>
1052         friend _Del* get_deleter(const __shared_ptr<_Tp1, _Lp1>&) noexcept;
1053
1054       _Tp*                 _M_ptr;         // Contained pointer.
1055       __shared_count<_Lp>  _M_refcount;    // Reference counter.
1056     };
1057
1058
1059   // 20.8.13.2.7 shared_ptr comparisons
1060   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1061     inline bool
1062     operator==(const __shared_ptr<_Tp1, _Lp>& __a,
1063                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1064     { return __a.get() == __b.get(); }
1065
1066   template<typename _Tp, _Lock_policy _Lp>
1067     inline bool
1068     operator==(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1069     { return !__a; }
1070
1071   template<typename _Tp, _Lock_policy _Lp>
1072     inline bool
1073     operator==(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1074     { return !__a; }
1075
1076   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1077     inline bool
1078     operator!=(const __shared_ptr<_Tp1, _Lp>& __a,
1079                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1080     { return __a.get() != __b.get(); }
1081
1082   template<typename _Tp, _Lock_policy _Lp>
1083     inline bool
1084     operator!=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1085     { return (bool)__a; }
1086
1087   template<typename _Tp, _Lock_policy _Lp>
1088     inline bool
1089     operator!=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1090     { return (bool)__a; }
1091
1092   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1093     inline bool
1094     operator<(const __shared_ptr<_Tp1, _Lp>& __a,
1095               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1096     {
1097       typedef typename std::common_type<_Tp1*, _Tp2*>::type _CT;
1098       return std::less<_CT>()(__a.get(), __b.get());
1099     }
1100
1101   template<typename _Tp, _Lock_policy _Lp>
1102     inline bool
1103     operator<(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1104     { return std::less<_Tp*>()(__a.get(), nullptr); }
1105
1106   template<typename _Tp, _Lock_policy _Lp>
1107     inline bool
1108     operator<(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1109     { return std::less<_Tp*>()(nullptr, __a.get()); }
1110
1111   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1112     inline bool
1113     operator<=(const __shared_ptr<_Tp1, _Lp>& __a,
1114                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1115     { return !(__b < __a); }
1116
1117   template<typename _Tp, _Lock_policy _Lp>
1118     inline bool
1119     operator<=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1120     { return !(nullptr < __a); }
1121
1122   template<typename _Tp, _Lock_policy _Lp>
1123     inline bool
1124     operator<=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1125     { return !(__a < nullptr); }
1126
1127   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1128     inline bool
1129     operator>(const __shared_ptr<_Tp1, _Lp>& __a,
1130               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1131     { return (__b < __a); }
1132
1133   template<typename _Tp, _Lock_policy _Lp>
1134     inline bool
1135     operator>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1136     { return std::less<_Tp*>()(nullptr, __a.get()); }
1137
1138   template<typename _Tp, _Lock_policy _Lp>
1139     inline bool
1140     operator>(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1141     { return std::less<_Tp*>()(__a.get(), nullptr); }
1142
1143   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1144     inline bool
1145     operator>=(const __shared_ptr<_Tp1, _Lp>& __a,
1146                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1147     { return !(__a < __b); }
1148
1149   template<typename _Tp, _Lock_policy _Lp>
1150     inline bool
1151     operator>=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1152     { return !(__a < nullptr); }
1153
1154   template<typename _Tp, _Lock_policy _Lp>
1155     inline bool
1156     operator>=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1157     { return !(nullptr < __a); }
1158
1159   template<typename _Sp>
1160     struct _Sp_less : public binary_function<_Sp, _Sp, bool>
1161     {
1162       bool
1163       operator()(const _Sp& __lhs, const _Sp& __rhs) const noexcept
1164       {
1165         typedef typename _Sp::element_type element_type;
1166         return std::less<element_type*>()(__lhs.get(), __rhs.get());
1167       }
1168     };
1169
1170   template<typename _Tp, _Lock_policy _Lp>
1171     struct less<__shared_ptr<_Tp, _Lp>>
1172     : public _Sp_less<__shared_ptr<_Tp, _Lp>>
1173     { };
1174
1175   // 2.2.3.8 shared_ptr specialized algorithms.
1176   template<typename _Tp, _Lock_policy _Lp>
1177     inline void
1178     swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b) noexcept
1179     { __a.swap(__b); }
1180
1181   // 2.2.3.9 shared_ptr casts
1182
1183   // The seemingly equivalent code:
1184   // shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get()))
1185   // will eventually result in undefined behaviour, attempting to
1186   // delete the same object twice.
1187   /// static_pointer_cast
1188   template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1189     inline __shared_ptr<_Tp, _Lp>
1190     static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1191     { return __shared_ptr<_Tp, _Lp>(__r, static_cast<_Tp*>(__r.get())); }
1192
1193   // The seemingly equivalent code:
1194   // shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get()))
1195   // will eventually result in undefined behaviour, attempting to
1196   // delete the same object twice.
1197   /// const_pointer_cast
1198   template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1199     inline __shared_ptr<_Tp, _Lp>
1200     const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1201     { return __shared_ptr<_Tp, _Lp>(__r, const_cast<_Tp*>(__r.get())); }
1202
1203   // The seemingly equivalent code:
1204   // shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get()))
1205   // will eventually result in undefined behaviour, attempting to
1206   // delete the same object twice.
1207   /// dynamic_pointer_cast
1208   template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1209     inline __shared_ptr<_Tp, _Lp>
1210     dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1211     {
1212       if (_Tp* __p = dynamic_cast<_Tp*>(__r.get()))
1213         return __shared_ptr<_Tp, _Lp>(__r, __p);
1214       return __shared_ptr<_Tp, _Lp>();
1215     }
1216
1217
1218   template<typename _Tp, _Lock_policy _Lp>
1219     class __weak_ptr
1220     {
1221     public:
1222       typedef _Tp element_type;
1223
1224       constexpr __weak_ptr() noexcept
1225       : _M_ptr(0), _M_refcount()
1226       { }
1227
1228       __weak_ptr(const __weak_ptr&) noexcept = default;
1229       __weak_ptr& operator=(const __weak_ptr&) noexcept = default;
1230       ~__weak_ptr() = default;
1231
1232       // The "obvious" converting constructor implementation:
1233       //
1234       //  template<typename _Tp1>
1235       //    __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
1236       //    : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
1237       //    { }
1238       //
1239       // has a serious problem.
1240       //
1241       //  __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr)
1242       //  conversion may require access to *__r._M_ptr (virtual inheritance).
1243       //
1244       // It is not possible to avoid spurious access violations since
1245       // in multithreaded programs __r._M_ptr may be invalidated at any point.
1246       template<typename _Tp1, typename = typename
1247                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
1248         __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r) noexcept
1249         : _M_refcount(__r._M_refcount)
1250         { _M_ptr = __r.lock().get(); }
1251
1252       template<typename _Tp1, typename = typename
1253                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
1254         __weak_ptr(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1255         : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
1256         { }
1257
1258       template<typename _Tp1>
1259         __weak_ptr&
1260         operator=(const __weak_ptr<_Tp1, _Lp>& __r) noexcept
1261         {
1262           _M_ptr = __r.lock().get();
1263           _M_refcount = __r._M_refcount;
1264           return *this;
1265         }
1266
1267       template<typename _Tp1>
1268         __weak_ptr&
1269         operator=(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1270         {
1271           _M_ptr = __r._M_ptr;
1272           _M_refcount = __r._M_refcount;
1273           return *this;
1274         }
1275
1276       __shared_ptr<_Tp, _Lp>
1277       lock() const noexcept
1278       {
1279 #ifdef __GTHREADS
1280         // Optimization: avoid throw overhead.
1281         if (expired())
1282           return __shared_ptr<element_type, _Lp>();
1283
1284         __try
1285           {
1286             return __shared_ptr<element_type, _Lp>(*this);
1287           }
1288         __catch(const bad_weak_ptr&)
1289           {
1290             // Q: How can we get here?
1291             // A: Another thread may have invalidated r after the
1292             //    use_count test above.
1293             return __shared_ptr<element_type, _Lp>();
1294           }
1295
1296 #else
1297         // Optimization: avoid try/catch overhead when single threaded.
1298         return expired() ? __shared_ptr<element_type, _Lp>()
1299                          : __shared_ptr<element_type, _Lp>(*this);
1300
1301 #endif
1302       } // XXX MT
1303
1304       long
1305       use_count() const noexcept
1306       { return _M_refcount._M_get_use_count(); }
1307
1308       bool
1309       expired() const noexcept
1310       { return _M_refcount._M_get_use_count() == 0; }
1311
1312       template<typename _Tp1>
1313         bool
1314         owner_before(const __shared_ptr<_Tp1, _Lp>& __rhs) const
1315         { return _M_refcount._M_less(__rhs._M_refcount); }
1316
1317       template<typename _Tp1>
1318         bool
1319         owner_before(const __weak_ptr<_Tp1, _Lp>& __rhs) const
1320         { return _M_refcount._M_less(__rhs._M_refcount); }
1321
1322       void
1323       reset() noexcept
1324       { __weak_ptr().swap(*this); }
1325
1326       void
1327       swap(__weak_ptr& __s) noexcept
1328       {
1329         std::swap(_M_ptr, __s._M_ptr);
1330         _M_refcount._M_swap(__s._M_refcount);
1331       }
1332
1333     private:
1334       // Used by __enable_shared_from_this.
1335       void
1336       _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
1337       {
1338         _M_ptr = __ptr;
1339         _M_refcount = __refcount;
1340       }
1341
1342       template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
1343       template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
1344       friend class __enable_shared_from_this<_Tp, _Lp>;
1345       friend class enable_shared_from_this<_Tp>;
1346
1347       _Tp*               _M_ptr;         // Contained pointer.
1348       __weak_count<_Lp>  _M_refcount;    // Reference counter.
1349     };
1350
1351   // 20.8.13.3.7 weak_ptr specialized algorithms.
1352   template<typename _Tp, _Lock_policy _Lp>
1353     inline void
1354     swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b) noexcept
1355     { __a.swap(__b); }
1356
1357   template<typename _Tp, typename _Tp1>
1358     struct _Sp_owner_less : public binary_function<_Tp, _Tp, bool>
1359     {
1360       bool
1361       operator()(const _Tp& __lhs, const _Tp& __rhs) const
1362       { return __lhs.owner_before(__rhs); }
1363
1364       bool
1365       operator()(const _Tp& __lhs, const _Tp1& __rhs) const
1366       { return __lhs.owner_before(__rhs); }
1367
1368       bool
1369       operator()(const _Tp1& __lhs, const _Tp& __rhs) const
1370       { return __lhs.owner_before(__rhs); }
1371     };
1372
1373   template<typename _Tp, _Lock_policy _Lp>
1374     struct owner_less<__shared_ptr<_Tp, _Lp>>
1375     : public _Sp_owner_less<__shared_ptr<_Tp, _Lp>, __weak_ptr<_Tp, _Lp>>
1376     { };
1377
1378   template<typename _Tp, _Lock_policy _Lp>
1379     struct owner_less<__weak_ptr<_Tp, _Lp>>
1380     : public _Sp_owner_less<__weak_ptr<_Tp, _Lp>, __shared_ptr<_Tp, _Lp>>
1381     { };
1382
1383
1384   template<typename _Tp, _Lock_policy _Lp>
1385     class __enable_shared_from_this
1386     {
1387     protected:
1388       constexpr __enable_shared_from_this() noexcept { }
1389
1390       __enable_shared_from_this(const __enable_shared_from_this&) noexcept { }
1391
1392       __enable_shared_from_this&
1393       operator=(const __enable_shared_from_this&) noexcept
1394       { return *this; }
1395
1396       ~__enable_shared_from_this() { }
1397
1398     public:
1399       __shared_ptr<_Tp, _Lp>
1400       shared_from_this()
1401       { return __shared_ptr<_Tp, _Lp>(this->_M_weak_this); }
1402
1403       __shared_ptr<const _Tp, _Lp>
1404       shared_from_this() const
1405       { return __shared_ptr<const _Tp, _Lp>(this->_M_weak_this); }
1406
1407     private:
1408       template<typename _Tp1>
1409         void
1410         _M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const noexcept
1411         { _M_weak_this._M_assign(__p, __n); }
1412
1413       template<typename _Tp1>
1414         friend void
1415         __enable_shared_from_this_helper(const __shared_count<_Lp>& __pn,
1416                                          const __enable_shared_from_this* __pe,
1417                                          const _Tp1* __px) noexcept
1418         {
1419           if (__pe != 0)
1420             __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
1421         }
1422
1423       mutable __weak_ptr<_Tp, _Lp>  _M_weak_this;
1424     };
1425
1426
1427   template<typename _Tp, _Lock_policy _Lp, typename _Alloc, typename... _Args>
1428     inline __shared_ptr<_Tp, _Lp>
1429     __allocate_shared(const _Alloc& __a, _Args&&... __args)
1430     {
1431       return __shared_ptr<_Tp, _Lp>(_Sp_make_shared_tag(), __a,
1432                                     std::forward<_Args>(__args)...);
1433     }
1434
1435   template<typename _Tp, _Lock_policy _Lp, typename... _Args>
1436     inline __shared_ptr<_Tp, _Lp>
1437     __make_shared(_Args&&... __args)
1438     {
1439       typedef typename std::remove_const<_Tp>::type _Tp_nc;
1440       return std::__allocate_shared<_Tp, _Lp>(std::allocator<_Tp_nc>(),
1441                                               std::forward<_Args>(__args)...);
1442     }
1443
1444   /// std::hash specialization for __shared_ptr.
1445   template<typename _Tp, _Lock_policy _Lp>
1446     struct hash<__shared_ptr<_Tp, _Lp>>
1447     : public __hash_base<size_t, __shared_ptr<_Tp, _Lp>>
1448     {
1449       size_t
1450       operator()(const __shared_ptr<_Tp, _Lp>& __s) const noexcept
1451       { return std::hash<_Tp*>()(__s.get()); }
1452     };
1453
1454 _GLIBCXX_END_NAMESPACE_VERSION
1455 } // namespace
1456
1457 #endif // _SHARED_PTR_BASE_H