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

   1 // shared_ptr and weak_ptr implementation details -*- C++ -*-
   2
   3 // Copyright (C) 2007, 2008, 2009, 2010, 2011 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  /**
  57    *  @brief  Exception possibly thrown by @c shared_ptr.
  58    *  @ingroup exceptions
  59    */
  60   class bad_weak_ptr : public std::exception
  61   {
  62   public:
  63     virtual char const*
  64     what() const noexcept;
  65
  66     virtual ~bad_weak_ptr() throw();
  67   };
  68
  69   // Substitute for bad_weak_ptr object in the case of -fno-exceptions.
  70   inline void
  71   __throw_bad_weak_ptr()
  72   {
  73 #if __EXCEPTIONS
  74     throw bad_weak_ptr();
  75 #else
  76     __builtin_abort();
  77 #endif
  78   }
  79
  80   using __gnu_cxx::_Lock_policy;
  81   using __gnu_cxx::__default_lock_policy;
  82   using __gnu_cxx::_S_single;
  83   using __gnu_cxx::_S_mutex;
  84   using __gnu_cxx::_S_atomic;
  85
  86   // Empty helper class except when the template argument is _S_mutex.
  87   template<_Lock_policy _Lp>
  88     class _Mutex_base
  89     {
  90     protected:
  91       // The atomic policy uses fully-fenced builtins, single doesn't care.
  92       enum { _S_need_barriers = 0 };
  93     };
  94
  95   template<>
  96     class _Mutex_base<_S_mutex>
  97     : public __gnu_cxx::__mutex
  98     {
  99     protected:
 100       // This policy is used when atomic builtins are not available.
 101       // The replacement atomic operations might not have the necessary
 102       // memory barriers.
 103       enum { _S_need_barriers = 1 };
 104     };
 105
 106   template<_Lock_policy _Lp = __default_lock_policy>
 107     class _Sp_counted_base
 108     : public _Mutex_base<_Lp>
 109     {
 110     public:
 111       _Sp_counted_base() noexcept
 112       : _M_use_count(1), _M_weak_count(1) { }
 113
 114       virtual
 115       ~_Sp_counted_base() noexcept
 116       { }
 117
 118       // Called when _M_use_count drops to zero, to release the resources
 119       // managed by *this.
 120       virtual void
 121       _M_dispose() noexcept = 0;
 122
 123       // Called when _M_weak_count drops to zero.
 124       virtual void
 125       _M_destroy() noexcept
 126       { delete this; }
 127
 128       virtual void*
 129       _M_get_deleter(const std::type_info&) = 0;
 130
 131       void
 132       _M_add_ref_copy()
 133       { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }
 134
 135       void
 136       _M_add_ref_lock();
 137
 138       void
 139       _M_release() noexcept
 140       {
 141         // Be race-detector-friendly.  For more info see bits/c++config.
 142         _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
 143         if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
 144           {
 145             _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
 146             _M_dispose();
 147             // There must be a memory barrier between dispose() and destroy()
 148             // to ensure that the effects of dispose() are observed in the
 149             // thread that runs destroy().
 150             // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html
 151             if (_Mutex_base<_Lp>::_S_need_barriers)
 152               {
 153                 _GLIBCXX_READ_MEM_BARRIER;
 154                 _GLIBCXX_WRITE_MEM_BARRIER;
 155               }
 156
 157             // Be race-detector-friendly.  For more info see bits/c++config.
 158             _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
 159             if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count,
 160                                                        -1) == 1)
 161               {
 162                 _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
 163                 _M_destroy();
 164               }
 165           }
 166       }
 167
 168       void
 169       _M_weak_add_ref() noexcept
 170       { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }
 171
 172       void
 173       _M_weak_release() noexcept
 174       {
 175         // Be race-detector-friendly. For more info see bits/c++config.
 176         _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
 177         if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
 178           {
 179             _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
 180             if (_Mutex_base<_Lp>::_S_need_barriers)
 181               {
 182                 // See _M_release(),
 183                 // destroy() must observe results of dispose()
 184                 _GLIBCXX_READ_MEM_BARRIER;
 185                 _GLIBCXX_WRITE_MEM_BARRIER;
 186               }
 187             _M_destroy();
 188           }
 189       }
 190
 191       long
 192       _M_get_use_count() const noexcept
 193       {
 194         // No memory barrier is used here so there is no synchronization
 195         // with other threads.
 196         return const_cast<const volatile _Atomic_word&>(_M_use_count);
 197       }
 198
 199     private:
 200       _Sp_counted_base(_Sp_counted_base const&) = delete;
 201       _Sp_counted_base& operator=(_Sp_counted_base const&) = delete;
 202
 203       _Atomic_word  _M_use_count;     // #shared
 204       _Atomic_word  _M_weak_count;    // #weak + (#shared != 0)
 205     };
 206
 207   template<>
 208     inline void
 209     _Sp_counted_base<_S_single>::
 210     _M_add_ref_lock()
 211     {
 212       if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
 213         {
 214           _M_use_count = 0;
 215           __throw_bad_weak_ptr();
 216         }
 217     }
 218
 219   template<>
 220     inline void
 221     _Sp_counted_base<_S_mutex>::
 222     _M_add_ref_lock()
 223     {
 224       __gnu_cxx::__scoped_lock sentry(*this);
 225       if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
 226         {
 227           _M_use_count = 0;
 228           __throw_bad_weak_ptr();
 229         }
 230     }
 231
 232   template<>
 233     inline void
 234     _Sp_counted_base<_S_atomic>::
 235     _M_add_ref_lock()
 236     {
 237       // Perform lock-free add-if-not-zero operation.
 238       _Atomic_word __count;
 239       do
 240         {
 241           __count = _M_use_count;
 242           if (__count == 0)
 243             __throw_bad_weak_ptr();
 244
 245           // Replace the current counter value with the old value + 1, as
 246           // long as it's not changed meanwhile.
 247         }
 248       while (!__sync_bool_compare_and_swap(&_M_use_count, __count,
 249                                            __count + 1));
 250     }
 251
 252
 253   // Forward declarations.
 254   template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
 255     class __shared_ptr;
 256
 257   template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
 258     class __weak_ptr;
 259
 260   template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
 261     class __enable_shared_from_this;
 262
 263   template<typename _Tp>
 264     class shared_ptr;
 265
 266   template<typename _Tp>
 267     class weak_ptr;
 268
 269   template<typename _Tp>
 270     struct owner_less;
 271
 272   template<typename _Tp>
 273     class enable_shared_from_this;
 274
 275   template<_Lock_policy _Lp = __default_lock_policy>
 276     class __weak_count;
 277
 278   template<_Lock_policy _Lp = __default_lock_policy>
 279     class __shared_count;
 280
 281
 282   // Counted ptr with no deleter or allocator support
 283   template<typename _Ptr, _Lock_policy _Lp>
 284     class _Sp_counted_ptr : public _Sp_counted_base<_Lp>
 285     {
 286     public:
 287       explicit
 288       _Sp_counted_ptr(_Ptr __p)
 289       : _M_ptr(__p) { }
 290
 291       virtual void
 292       _M_dispose() noexcept
 293       { delete _M_ptr; }
 294
 295       virtual void
 296       _M_destroy() noexcept
 297       { delete this; }
 298
 299       virtual void*
 300       _M_get_deleter(const std::type_info&)
 301       { return 0; }
 302
 303       _Sp_counted_ptr(const _Sp_counted_ptr&) = delete;
 304       _Sp_counted_ptr& operator=(const _Sp_counted_ptr&) = delete;
 305
 306     protected:
 307       _Ptr             _M_ptr;  // copy constructor must not throw
 308     };
 309
 310   template<>
 311     inline void
 312     _Sp_counted_ptr<nullptr_t, _S_single>::_M_dispose() noexcept { }
 313
 314   template<>
 315     inline void
 316     _Sp_counted_ptr<nullptr_t, _S_mutex>::_M_dispose() noexcept { }
 317
 318   template<>
 319     inline void
 320     _Sp_counted_ptr<nullptr_t, _S_atomic>::_M_dispose() noexcept { }
 321
 322   // Support for custom deleter and/or allocator
 323   template<typename _Ptr, typename _Deleter, typename _Alloc, _Lock_policy _Lp>
 324     class _Sp_counted_deleter : public _Sp_counted_base<_Lp>
 325     {
 326       typedef typename _Alloc::template
 327           rebind<_Sp_counted_deleter>::other _My_alloc_type;
 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 _My_alloc_type    // copy constructor must not throw
 334       {
 335         _Deleter _M_del;         // copy constructor must not throw
 336         _My_Deleter(_Deleter __d, const _Alloc& __a)
 337         : _My_alloc_type(__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       virtual void
 350       _M_dispose() noexcept
 351       { _M_del._M_del(_M_ptr); }
 352
 353       virtual void
 354       _M_destroy() noexcept
 355       {
 356         _My_alloc_type __a(_M_del);
 357         this->~_Sp_counted_deleter();
 358         __a.deallocate(this, 1);
 359       }
 360
 361       virtual void*
 362       _M_get_deleter(const std::type_info& __ti)
 363       {
 364 #ifdef __GXX_RTTI
 365         return __ti == typeid(_Deleter) ? &_M_del._M_del : 0;
 366 #else
 367         return 0;
 368 #endif
 369       }
 370
 371     protected:
 372       _Ptr             _M_ptr;  // copy constructor must not throw
 373       _My_Deleter      _M_del;  // copy constructor must not throw
 374     };
 375
 376   // helpers for make_shared / allocate_shared
 377
 378   template<typename _Tp>
 379     struct _Sp_destroy_inplace
 380     {
 381       void operator()(_Tp* __p) const { if (__p) __p->~_Tp(); }
 382     };
 383
 384   struct _Sp_make_shared_tag { };
 385
 386   template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
 387     class _Sp_counted_ptr_inplace
 388     : public _Sp_counted_deleter<_Tp*, _Sp_destroy_inplace<_Tp>, _Alloc, _Lp>
 389     {
 390       typedef _Sp_counted_deleter<_Tp*, _Sp_destroy_inplace<_Tp>, _Alloc, _Lp>
 391         _Base_type;
 392
 393     public:
 394       explicit
 395       _Sp_counted_ptr_inplace(_Alloc __a)
 396       : _Base_type(static_cast<_Tp*>(0), _Sp_destroy_inplace<_Tp>(), __a)
 397       , _M_storage()
 398       {
 399         void* __p = &_M_storage;
 400         ::new (__p) _Tp();  // might throw
 401         _Base_type::_M_ptr = static_cast<_Tp*>(__p);
 402       }
 403
 404       template<typename... _Args>
 405         _Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
 406         : _Base_type(static_cast<_Tp*>(0), _Sp_destroy_inplace<_Tp>(), __a)
 407         , _M_storage()
 408         {
 409           void* __p = &_M_storage;
 410           ::new (__p) _Tp(std::forward<_Args>(__args)...);  // might throw
 411           _Base_type::_M_ptr = static_cast<_Tp*>(__p);
 412         }
 413
 414       // Override because the allocator needs to know the dynamic type
 415       virtual void
 416       _M_destroy() noexcept
 417       {
 418         typedef typename _Alloc::template
 419             rebind<_Sp_counted_ptr_inplace>::other _My_alloc_type;
 420         _My_alloc_type __a(_Base_type::_M_del);
 421         this->~_Sp_counted_ptr_inplace();
 422         __a.deallocate(this, 1);
 423       }
 424
 425       // Sneaky trick so __shared_ptr can get the managed pointer
 426       virtual void*
 427       _M_get_deleter(const std::type_info& __ti) noexcept
 428       {
 429 #ifdef __GXX_RTTI
 430         return __ti == typeid(_Sp_make_shared_tag)
 431                ? static_cast<void*>(&_M_storage)
 432                : _Base_type::_M_get_deleter(__ti);
 433 #else
 434         return 0;
 435 #endif
 436       }
 437
 438     private:
 439       typename aligned_storage<sizeof(_Tp), alignment_of<_Tp>::value>::type
 440         _M_storage;
 441     };
 442
 443   template<_Lock_policy _Lp>
 444     class __shared_count
 445     {
 446     public:
 447       constexpr __shared_count() noexcept : _M_pi(0)
 448       { }
 449
 450       template<typename _Ptr>
 451         explicit
 452         __shared_count(_Ptr __p) : _M_pi(0)
 453         {
 454           __try
 455             {
 456               _M_pi = new _Sp_counted_ptr<_Ptr, _Lp>(__p);
 457             }
 458           __catch(...)
 459             {
 460               delete __p;
 461               __throw_exception_again;
 462             }
 463         }
 464
 465       template<typename _Ptr, typename _Deleter>
 466         __shared_count(_Ptr __p, _Deleter __d) : _M_pi(0)
 467         {
 468           // The allocator's value_type doesn't matter, will rebind it anyway.
 469           typedef std::allocator<int> _Alloc;
 470           typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
 471           typedef std::allocator<_Sp_cd_type> _Alloc2;
 472           _Alloc2 __a2;
 473           __try
 474             {
 475               _M_pi = __a2.allocate(1);
 476               ::new(static_cast<void*>(_M_pi)) _Sp_cd_type(__p, __d);
 477             }
 478           __catch(...)
 479             {
 480               __d(__p); // Call _Deleter on __p.
 481               if (_M_pi)
 482                 __a2.deallocate(static_cast<_Sp_cd_type*>(_M_pi), 1);
 483               __throw_exception_again;
 484             }
 485         }
 486
 487       template<typename _Ptr, typename _Deleter, typename _Alloc>
 488         __shared_count(_Ptr __p, _Deleter __d, _Alloc __a) : _M_pi(0)
 489         {
 490           typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
 491           typedef typename _Alloc::template rebind<_Sp_cd_type>::other _Alloc2;
 492           _Alloc2 __a2(__a);
 493           __try
 494             {
 495               _M_pi = __a2.allocate(1);
 496               ::new(static_cast<void*>(_M_pi)) _Sp_cd_type(__p, __d, __a);
 497             }
 498           __catch(...)
 499             {
 500               __d(__p); // Call _Deleter on __p.
 501               if (_M_pi)
 502                 __a2.deallocate(static_cast<_Sp_cd_type*>(_M_pi), 1);
 503               __throw_exception_again;
 504             }
 505         }
 506
 507       template<typename _Tp, typename _Alloc, typename... _Args>
 508         __shared_count(_Sp_make_shared_tag, _Tp*, const _Alloc& __a,
 509                        _Args&&... __args)
 510         : _M_pi(0)
 511         {
 512           typedef _Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp> _Sp_cp_type;
 513           typedef typename _Alloc::template rebind<_Sp_cp_type>::other _Alloc2;
 514           _Alloc2 __a2(__a);
 515           __try
 516             {
 517               _M_pi = __a2.allocate(1);
 518               ::new(static_cast<void*>(_M_pi)) _Sp_cp_type(__a,
 519                     std::forward<_Args>(__args)...);
 520             }
 521           __catch(...)
 522             {
 523               if (_M_pi)
 524                 __a2.deallocate(static_cast<_Sp_cp_type*>(_M_pi), 1);
 525               __throw_exception_again;
 526             }
 527         }
 528
 529 #if _GLIBCXX_USE_DEPRECATED
 530       // Special case for auto_ptr<_Tp> to provide the strong guarantee.
 531       template<typename _Tp>
 532         explicit
 533         __shared_count(std::auto_ptr<_Tp>&& __r)
 534         : _M_pi(new _Sp_counted_ptr<_Tp*, _Lp>(__r.get()))
 535         { __r.release(); }
 536 #endif
 537
 538       // Special case for unique_ptr<_Tp,_Del> to provide the strong guarantee.
 539       template<typename _Tp, typename _Del>
 540         explicit
 541         __shared_count(std::unique_ptr<_Tp, _Del>&& __r)
 542         : _M_pi(_S_create_from_up(std::move(__r)))
 543         { __r.release(); }
 544
 545       // Throw bad_weak_ptr when __r._M_get_use_count() == 0.
 546       explicit __shared_count(const __weak_count<_Lp>& __r);
 547
 548       ~__shared_count() noexcept
 549       {
 550         if (_M_pi != 0)
 551           _M_pi->_M_release();
 552       }
 553
 554       __shared_count(const __shared_count& __r) noexcept
 555       : _M_pi(__r._M_pi)
 556       {
 557         if (_M_pi != 0)
 558           _M_pi->_M_add_ref_copy();
 559       }
 560
 561       __shared_count&
 562       operator=(const __shared_count& __r) noexcept
 563       {
 564         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 565         if (__tmp != _M_pi)
 566           {
 567             if (__tmp != 0)
 568               __tmp->_M_add_ref_copy();
 569             if (_M_pi != 0)
 570               _M_pi->_M_release();
 571             _M_pi = __tmp;
 572           }
 573         return *this;
 574       }
 575
 576       void
 577       _M_swap(__shared_count& __r) noexcept
 578       {
 579         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 580         __r._M_pi = _M_pi;
 581         _M_pi = __tmp;
 582       }
 583
 584       long
 585       _M_get_use_count() const noexcept
 586       { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }
 587
 588       bool
 589       _M_unique() const noexcept
 590       { return this->_M_get_use_count() == 1; }
 591
 592       void*
 593       _M_get_deleter(const std::type_info& __ti) const noexcept
 594       { return _M_pi ? _M_pi->_M_get_deleter(__ti) : 0; }
 595
 596       bool
 597       _M_less(const __shared_count& __rhs) const noexcept
 598       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 599
 600       bool
 601       _M_less(const __weak_count<_Lp>& __rhs) const noexcept
 602       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 603
 604       // Friend function injected into enclosing namespace and found by ADL
 605       friend inline bool
 606       operator==(const __shared_count& __a, const __shared_count& __b) noexcept
 607       { return __a._M_pi == __b._M_pi; }
 608
 609     private:
 610       friend class __weak_count<_Lp>;
 611
 612       template<typename _Tp, typename _Del>
 613         static _Sp_counted_base<_Lp>*
 614         _S_create_from_up(std::unique_ptr<_Tp, _Del>&& __r,
 615           typename std::enable_if<!std::is_reference<_Del>::value>::type* = 0)
 616         {
 617           return new _Sp_counted_deleter<_Tp*, _Del, std::allocator<_Tp>,
 618             _Lp>(__r.get(), __r.get_deleter());
 619         }
 620
 621       template<typename _Tp, typename _Del>
 622         static _Sp_counted_base<_Lp>*
 623         _S_create_from_up(std::unique_ptr<_Tp, _Del>&& __r,
 624           typename std::enable_if<std::is_reference<_Del>::value>::type* = 0)
 625         {
 626           typedef typename std::remove_reference<_Del>::type _Del1;
 627           typedef std::reference_wrapper<_Del1> _Del2;
 628           return new _Sp_counted_deleter<_Tp*, _Del2, std::allocator<_Tp>,
 629             _Lp>(__r.get(), std::ref(__r.get_deleter()));
 630         }
 631
 632       _Sp_counted_base<_Lp>*  _M_pi;
 633     };
 634
 635
 636   template<_Lock_policy _Lp>
 637     class __weak_count
 638     {
 639     public:
 640       constexpr __weak_count() noexcept : _M_pi(0)
 641       { }
 642
 643       __weak_count(const __shared_count<_Lp>& __r) noexcept
 644       : _M_pi(__r._M_pi)
 645       {
 646         if (_M_pi != 0)
 647           _M_pi->_M_weak_add_ref();
 648       }
 649
 650       __weak_count(const __weak_count<_Lp>& __r) noexcept
 651       : _M_pi(__r._M_pi)
 652       {
 653         if (_M_pi != 0)
 654           _M_pi->_M_weak_add_ref();
 655       }
 656
 657       ~__weak_count() noexcept
 658       {
 659         if (_M_pi != 0)
 660           _M_pi->_M_weak_release();
 661       }
 662
 663       __weak_count<_Lp>&
 664       operator=(const __shared_count<_Lp>& __r) noexcept
 665       {
 666         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 667         if (__tmp != 0)
 668           __tmp->_M_weak_add_ref();
 669         if (_M_pi != 0)
 670           _M_pi->_M_weak_release();
 671         _M_pi = __tmp;
 672         return *this;
 673       }
 674
 675       __weak_count<_Lp>&
 676       operator=(const __weak_count<_Lp>& __r) noexcept
 677       {
 678         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 679         if (__tmp != 0)
 680           __tmp->_M_weak_add_ref();
 681         if (_M_pi != 0)
 682           _M_pi->_M_weak_release();
 683         _M_pi = __tmp;
 684         return *this;
 685       }
 686
 687       void
 688       _M_swap(__weak_count<_Lp>& __r) noexcept
 689       {
 690         _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
 691         __r._M_pi = _M_pi;
 692         _M_pi = __tmp;
 693       }
 694
 695       long
 696       _M_get_use_count() const noexcept
 697       { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }
 698
 699       bool
 700       _M_less(const __weak_count& __rhs) const noexcept
 701       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 702
 703       bool
 704       _M_less(const __shared_count<_Lp>& __rhs) const noexcept
 705       { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 706
 707       // Friend function injected into enclosing namespace and found by ADL
 708       friend inline bool
 709       operator==(const __weak_count& __a, const __weak_count& __b) noexcept
 710       { return __a._M_pi == __b._M_pi; }
 711
 712     private:
 713       friend class __shared_count<_Lp>;
 714
 715       _Sp_counted_base<_Lp>*  _M_pi;
 716     };
 717
 718   // Now that __weak_count is defined we can define this constructor:
 719   template<_Lock_policy _Lp>
 720     inline __shared_count<_Lp>:: __shared_count(const __weak_count<_Lp>& __r)
 721     : _M_pi(__r._M_pi)
 722     {
 723       if (_M_pi != 0)
 724         _M_pi->_M_add_ref_lock();
 725       else
 726         __throw_bad_weak_ptr();
 727     }
 728
 729
 730   // Support for enable_shared_from_this.
 731
 732   // Friend of __enable_shared_from_this.
 733   template<_Lock_policy _Lp, typename _Tp1, typename _Tp2>
 734     void
 735     __enable_shared_from_this_helper(const __shared_count<_Lp>&,
 736                                      const __enable_shared_from_this<_Tp1,
 737                                      _Lp>*, const _Tp2*) noexcept;
 738
 739   // Friend of enable_shared_from_this.
 740   template<typename _Tp1, typename _Tp2>
 741     void
 742     __enable_shared_from_this_helper(const __shared_count<>&,
 743                                      const enable_shared_from_this<_Tp1>*,
 744                                      const _Tp2*) noexcept;
 745
 746   template<_Lock_policy _Lp>
 747     inline void
 748     __enable_shared_from_this_helper(const __shared_count<_Lp>&, ...) noexcept
 749     { }
 750
 751
 752   template<typename _Tp, _Lock_policy _Lp>
 753     class __shared_ptr
 754     {
 755     public:
 756       typedef _Tp   element_type;
 757
 758       constexpr __shared_ptr() noexcept
 759       : _M_ptr(0), _M_refcount()
 760       { }
 761
 762       template<typename _Tp1>
 763         explicit __shared_ptr(_Tp1* __p)
 764         : _M_ptr(__p), _M_refcount(__p)
 765         {
 766           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 767           static_assert( sizeof(_Tp1) > 0, "incomplete type" );
 768           __enable_shared_from_this_helper(_M_refcount, __p, __p);
 769         }
 770
 771       template<typename _Tp1, typename _Deleter>
 772         __shared_ptr(_Tp1* __p, _Deleter __d)
 773         : _M_ptr(__p), _M_refcount(__p, __d)
 774         {
 775           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 776           // TODO requires _Deleter CopyConstructible and __d(__p) well-formed
 777           __enable_shared_from_this_helper(_M_refcount, __p, __p);
 778         }
 779
 780       template<typename _Tp1, typename _Deleter, typename _Alloc>
 781         __shared_ptr(_Tp1* __p, _Deleter __d, _Alloc __a)
 782         : _M_ptr(__p), _M_refcount(__p, __d, std::move(__a))
 783         {
 784           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 785           // TODO requires _Deleter CopyConstructible and __d(__p) well-formed
 786           __enable_shared_from_this_helper(_M_refcount, __p, __p);
 787         }
 788
 789       template<typename _Deleter>
 790         __shared_ptr(nullptr_t __p, _Deleter __d)
 791         : _M_ptr(0), _M_refcount(__p, __d)
 792         { }
 793
 794       template<typename _Deleter, typename _Alloc>
 795         __shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
 796         : _M_ptr(0), _M_refcount(__p, __d, std::move(__a))
 797         { }
 798
 799       template<typename _Tp1>
 800         __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, _Tp* __p) noexcept
 801         : _M_ptr(__p), _M_refcount(__r._M_refcount) // never throws
 802         { }
 803
 804       __shared_ptr(const __shared_ptr&) noexcept = default;
 805       __shared_ptr& operator=(const __shared_ptr&) noexcept = default;
 806       ~__shared_ptr() = default;
 807
 808       template<typename _Tp1, typename = typename
 809                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 810         __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
 811         : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
 812         { }
 813
 814       __shared_ptr(__shared_ptr&& __r) noexcept
 815       : _M_ptr(__r._M_ptr), _M_refcount()
 816       {
 817         _M_refcount._M_swap(__r._M_refcount);
 818         __r._M_ptr = 0;
 819       }
 820
 821       template<typename _Tp1, typename = typename
 822                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 823         __shared_ptr(__shared_ptr<_Tp1, _Lp>&& __r) noexcept
 824         : _M_ptr(__r._M_ptr), _M_refcount()
 825         {
 826           _M_refcount._M_swap(__r._M_refcount);
 827           __r._M_ptr = 0;
 828         }
 829
 830       template<typename _Tp1>
 831         explicit __shared_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
 832         : _M_refcount(__r._M_refcount) // may throw
 833         {
 834           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 835
 836           // It is now safe to copy __r._M_ptr, as
 837           // _M_refcount(__r._M_refcount) did not throw.
 838           _M_ptr = __r._M_ptr;
 839         }
 840
 841       // If an exception is thrown this constructor has no effect.
 842       template<typename _Tp1, typename _Del>
 843         __shared_ptr(std::unique_ptr<_Tp1, _Del>&& __r)
 844         : _M_ptr(__r.get()), _M_refcount()
 845         {
 846           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 847           _Tp1* __tmp = __r.get();
 848           _M_refcount = __shared_count<_Lp>(std::move(__r));
 849           __enable_shared_from_this_helper(_M_refcount, __tmp, __tmp);
 850         }
 851
 852 #if _GLIBCXX_USE_DEPRECATED
 853       // Postcondition: use_count() == 1 and __r.get() == 0
 854       template<typename _Tp1>
 855         __shared_ptr(std::auto_ptr<_Tp1>&& __r)
 856         : _M_ptr(__r.get()), _M_refcount()
 857         {
 858           __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
 859           static_assert( sizeof(_Tp1) > 0, "incomplete type" );
 860           _Tp1* __tmp = __r.get();
 861           _M_refcount = __shared_count<_Lp>(std::move(__r));
 862           __enable_shared_from_this_helper(_M_refcount, __tmp, __tmp);
 863         }
 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             _M_alloc.destroy(__ptr);
1005             _M_alloc.deallocate(__ptr, 1);
1006           }
1007           _Alloc _M_alloc;
1008         };
1009
1010       template<typename _Alloc, typename... _Args>
1011         __shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
1012                      _Args&&... __args)
1013         : _M_ptr(), _M_refcount()
1014         {
1015           typedef typename _Alloc::template rebind<_Tp>::other _Alloc2;
1016           _Deleter<_Alloc2> __del = { _Alloc2(__a) };
1017           _M_ptr = __del._M_alloc.allocate(1);
1018           __try
1019             {
1020               __del._M_alloc.construct(_M_ptr, std::forward<_Args>(__args)...);
1021             }
1022           __catch(...)
1023             {
1024               __del._M_alloc.deallocate(_M_ptr, 1);
1025               __throw_exception_again;
1026             }
1027           __shared_count<_Lp> __count(_M_ptr, __del, __del._M_alloc);
1028           _M_refcount._M_swap(__count);
1029           __enable_shared_from_this_helper(_M_refcount, _M_ptr, _M_ptr);
1030         }
1031 #endif
1032
1033       template<typename _Tp1, _Lock_policy _Lp1, typename _Alloc,
1034                typename... _Args>
1035         friend __shared_ptr<_Tp1, _Lp1>
1036         __allocate_shared(const _Alloc& __a, _Args&&... __args);
1037
1038     private:
1039       void*
1040       _M_get_deleter(const std::type_info& __ti) const noexcept
1041       { return _M_refcount._M_get_deleter(__ti); }
1042
1043       template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
1044       template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
1045
1046       template<typename _Del, typename _Tp1, _Lock_policy _Lp1>
1047         friend _Del* get_deleter(const __shared_ptr<_Tp1, _Lp1>&) noexcept;
1048
1049       _Tp*                 _M_ptr;         // Contained pointer.
1050       __shared_count<_Lp>  _M_refcount;    // Reference counter.
1051     };
1052
1053
1054   // 20.8.13.2.7 shared_ptr comparisons
1055   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1056     inline bool
1057     operator==(const __shared_ptr<_Tp1, _Lp>& __a,
1058                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1059     { return __a.get() == __b.get(); }
1060
1061   template<typename _Tp, _Lock_policy _Lp>
1062     inline bool
1063     operator==(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1064     { return !__a; }
1065
1066   template<typename _Tp, _Lock_policy _Lp>
1067     inline bool
1068     operator==(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1069     { return !__a; }
1070
1071   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1072     inline bool
1073     operator!=(const __shared_ptr<_Tp1, _Lp>& __a,
1074                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1075     { return __a.get() != __b.get(); }
1076
1077   template<typename _Tp, _Lock_policy _Lp>
1078     inline bool
1079     operator!=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1080     { return (bool)__a; }
1081
1082   template<typename _Tp, _Lock_policy _Lp>
1083     inline bool
1084     operator!=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1085     { return (bool)__a; }
1086
1087   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1088     inline bool
1089     operator<(const __shared_ptr<_Tp1, _Lp>& __a,
1090               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1091     {
1092       typedef typename std::common_type<_Tp1*, _Tp2*>::type _CT;
1093       return std::less<_CT>()(__a.get(), __b.get());
1094     }
1095
1096   template<typename _Tp, _Lock_policy _Lp>
1097     inline bool
1098     operator<(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1099     { return std::less<_Tp*>()(__a.get(), nullptr); }
1100
1101   template<typename _Tp, _Lock_policy _Lp>
1102     inline bool
1103     operator<(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1104     { return std::less<_Tp*>()(nullptr, __a.get()); }
1105
1106   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1107     inline bool
1108     operator<=(const __shared_ptr<_Tp1, _Lp>& __a,
1109                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1110     { return !(__b < __a); }
1111
1112   template<typename _Tp, _Lock_policy _Lp>
1113     inline bool
1114     operator<=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1115     { return !(nullptr < __a); }
1116
1117   template<typename _Tp, _Lock_policy _Lp>
1118     inline bool
1119     operator<=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1120     { return !(__a < nullptr); }
1121
1122   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1123     inline bool
1124     operator>(const __shared_ptr<_Tp1, _Lp>& __a,
1125               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1126     { return (__b < __a); }
1127
1128   template<typename _Tp, _Lock_policy _Lp>
1129     inline bool
1130     operator>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1131     { return std::less<_Tp*>()(nullptr, __a.get()); }
1132
1133   template<typename _Tp, _Lock_policy _Lp>
1134     inline bool
1135     operator>(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1136     { return std::less<_Tp*>()(__a.get(), nullptr); }
1137
1138   template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1139     inline bool
1140     operator>=(const __shared_ptr<_Tp1, _Lp>& __a,
1141                const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1142     { return !(__a < __b); }
1143
1144   template<typename _Tp, _Lock_policy _Lp>
1145     inline bool
1146     operator>=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1147     { return !(__a < nullptr); }
1148
1149   template<typename _Tp, _Lock_policy _Lp>
1150     inline bool
1151     operator>=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1152     { return !(nullptr < __a); }
1153
1154   template<typename _Sp>
1155     struct _Sp_less : public binary_function<_Sp, _Sp, bool>
1156     {
1157       bool
1158       operator()(const _Sp& __lhs, const _Sp& __rhs) const noexcept
1159       {
1160         typedef typename _Sp::element_type element_type;
1161         return std::less<element_type*>()(__lhs.get(), __rhs.get());
1162       }
1163     };
1164
1165   template<typename _Tp, _Lock_policy _Lp>
1166     struct less<__shared_ptr<_Tp, _Lp>>
1167     : public _Sp_less<__shared_ptr<_Tp, _Lp>>
1168     { };
1169
1170   // 2.2.3.8 shared_ptr specialized algorithms.
1171   template<typename _Tp, _Lock_policy _Lp>
1172     inline void
1173     swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b) noexcept
1174     { __a.swap(__b); }
1175
1176   // 2.2.3.9 shared_ptr casts
1177
1178   // The seemingly equivalent code:
1179   // shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get()))
1180   // will eventually result in undefined behaviour, attempting to
1181   // delete the same object twice.
1182   /// static_pointer_cast
1183   template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1184     inline __shared_ptr<_Tp, _Lp>
1185     static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1186     { return __shared_ptr<_Tp, _Lp>(__r, static_cast<_Tp*>(__r.get())); }
1187
1188   // The seemingly equivalent code:
1189   // shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get()))
1190   // will eventually result in undefined behaviour, attempting to
1191   // delete the same object twice.
1192   /// const_pointer_cast
1193   template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1194     inline __shared_ptr<_Tp, _Lp>
1195     const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1196     { return __shared_ptr<_Tp, _Lp>(__r, const_cast<_Tp*>(__r.get())); }
1197
1198   // The seemingly equivalent code:
1199   // shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get()))
1200   // will eventually result in undefined behaviour, attempting to
1201   // delete the same object twice.
1202   /// dynamic_pointer_cast
1203   template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1204     inline __shared_ptr<_Tp, _Lp>
1205     dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1206     {
1207       if (_Tp* __p = dynamic_cast<_Tp*>(__r.get()))
1208         return __shared_ptr<_Tp, _Lp>(__r, __p);
1209       return __shared_ptr<_Tp, _Lp>();
1210     }
1211
1212
1213   template<typename _Tp, _Lock_policy _Lp>
1214     class __weak_ptr
1215     {
1216     public:
1217       typedef _Tp element_type;
1218
1219       constexpr __weak_ptr() noexcept
1220       : _M_ptr(0), _M_refcount()
1221       { }
1222
1223       __weak_ptr(const __weak_ptr&) noexcept = default;
1224       __weak_ptr& operator=(const __weak_ptr&) noexcept = default;
1225       ~__weak_ptr() = default;
1226
1227       // The "obvious" converting constructor implementation:
1228       //
1229       //  template<typename _Tp1>
1230       //    __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
1231       //    : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
1232       //    { }
1233       //
1234       // has a serious problem.
1235       //
1236       //  __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr)
1237       //  conversion may require access to *__r._M_ptr (virtual inheritance).
1238       //
1239       // It is not possible to avoid spurious access violations since
1240       // in multithreaded programs __r._M_ptr may be invalidated at any point.
1241       template<typename _Tp1, typename = typename
1242                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
1243         __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r) noexcept
1244         : _M_refcount(__r._M_refcount)
1245         { _M_ptr = __r.lock().get(); }
1246
1247       template<typename _Tp1, typename = typename
1248                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
1249         __weak_ptr(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1250         : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
1251         { }
1252
1253       template<typename _Tp1>
1254         __weak_ptr&
1255         operator=(const __weak_ptr<_Tp1, _Lp>& __r) noexcept
1256         {
1257           _M_ptr = __r.lock().get();
1258           _M_refcount = __r._M_refcount;
1259           return *this;
1260         }
1261
1262       template<typename _Tp1>
1263         __weak_ptr&
1264         operator=(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1265         {
1266           _M_ptr = __r._M_ptr;
1267           _M_refcount = __r._M_refcount;
1268           return *this;
1269         }
1270
1271       __shared_ptr<_Tp, _Lp>
1272       lock() const noexcept
1273       {
1274 #ifdef __GTHREADS
1275         // Optimization: avoid throw overhead.
1276         if (expired())
1277           return __shared_ptr<element_type, _Lp>();
1278
1279         __try
1280           {
1281             return __shared_ptr<element_type, _Lp>(*this);
1282           }
1283         __catch(const bad_weak_ptr&)
1284           {
1285             // Q: How can we get here?
1286             // A: Another thread may have invalidated r after the
1287             //    use_count test above.
1288             return __shared_ptr<element_type, _Lp>();
1289           }
1290
1291 #else
1292         // Optimization: avoid try/catch overhead when single threaded.
1293         return expired() ? __shared_ptr<element_type, _Lp>()
1294                          : __shared_ptr<element_type, _Lp>(*this);
1295
1296 #endif
1297       } // XXX MT
1298
1299       long
1300       use_count() const noexcept
1301       { return _M_refcount._M_get_use_count(); }
1302
1303       bool
1304       expired() const noexcept
1305       { return _M_refcount._M_get_use_count() == 0; }
1306
1307       template<typename _Tp1>
1308         bool
1309         owner_before(const __shared_ptr<_Tp1, _Lp>& __rhs) const
1310         { return _M_refcount._M_less(__rhs._M_refcount); }
1311
1312       template<typename _Tp1>
1313         bool
1314         owner_before(const __weak_ptr<_Tp1, _Lp>& __rhs) const
1315         { return _M_refcount._M_less(__rhs._M_refcount); }
1316
1317       void
1318       reset() noexcept
1319       { __weak_ptr().swap(*this); }
1320
1321       void
1322       swap(__weak_ptr& __s) noexcept
1323       {
1324         std::swap(_M_ptr, __s._M_ptr);
1325         _M_refcount._M_swap(__s._M_refcount);
1326       }
1327
1328     private:
1329       // Used by __enable_shared_from_this.
1330       void
1331       _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
1332       {
1333         _M_ptr = __ptr;
1334         _M_refcount = __refcount;
1335       }
1336
1337       template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
1338       template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
1339       friend class __enable_shared_from_this<_Tp, _Lp>;
1340       friend class enable_shared_from_this<_Tp>;
1341
1342       _Tp*               _M_ptr;         // Contained pointer.
1343       __weak_count<_Lp>  _M_refcount;    // Reference counter.
1344     };
1345
1346   // 20.8.13.3.7 weak_ptr specialized algorithms.
1347   template<typename _Tp, _Lock_policy _Lp>
1348     inline void
1349     swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b) noexcept
1350     { __a.swap(__b); }
1351
1352   template<typename _Tp, typename _Tp1>
1353     struct _Sp_owner_less : public binary_function<_Tp, _Tp, bool>
1354     {
1355       bool
1356       operator()(const _Tp& __lhs, const _Tp& __rhs) const
1357       { return __lhs.owner_before(__rhs); }
1358
1359       bool
1360       operator()(const _Tp& __lhs, const _Tp1& __rhs) const
1361       { return __lhs.owner_before(__rhs); }
1362
1363       bool
1364       operator()(const _Tp1& __lhs, const _Tp& __rhs) const
1365       { return __lhs.owner_before(__rhs); }
1366     };
1367
1368   template<typename _Tp, _Lock_policy _Lp>
1369     struct owner_less<__shared_ptr<_Tp, _Lp>>
1370     : public _Sp_owner_less<__shared_ptr<_Tp, _Lp>, __weak_ptr<_Tp, _Lp>>
1371     { };
1372
1373   template<typename _Tp, _Lock_policy _Lp>
1374     struct owner_less<__weak_ptr<_Tp, _Lp>>
1375     : public _Sp_owner_less<__weak_ptr<_Tp, _Lp>, __shared_ptr<_Tp, _Lp>>
1376     { };
1377
1378
1379   template<typename _Tp, _Lock_policy _Lp>
1380     class __enable_shared_from_this
1381     {
1382     protected:
1383       constexpr __enable_shared_from_this() noexcept { }
1384
1385       __enable_shared_from_this(const __enable_shared_from_this&) noexcept { }
1386
1387       __enable_shared_from_this&
1388       operator=(const __enable_shared_from_this&) noexcept
1389       { return *this; }
1390
1391       ~__enable_shared_from_this() { }
1392
1393     public:
1394       __shared_ptr<_Tp, _Lp>
1395       shared_from_this()
1396       { return __shared_ptr<_Tp, _Lp>(this->_M_weak_this); }
1397
1398       __shared_ptr<const _Tp, _Lp>
1399       shared_from_this() const
1400       { return __shared_ptr<const _Tp, _Lp>(this->_M_weak_this); }
1401
1402     private:
1403       template<typename _Tp1>
1404         void
1405         _M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const noexcept
1406         { _M_weak_this._M_assign(__p, __n); }
1407
1408       template<typename _Tp1>
1409         friend void
1410         __enable_shared_from_this_helper(const __shared_count<_Lp>& __pn,
1411                                          const __enable_shared_from_this* __pe,
1412                                          const _Tp1* __px) noexcept
1413         {
1414           if (__pe != 0)
1415             __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
1416         }
1417
1418       mutable __weak_ptr<_Tp, _Lp>  _M_weak_this;
1419     };
1420
1421
1422   template<typename _Tp, _Lock_policy _Lp, typename _Alloc, typename... _Args>
1423     inline __shared_ptr<_Tp, _Lp>
1424     __allocate_shared(const _Alloc& __a, _Args&&... __args)
1425     {
1426       return __shared_ptr<_Tp, _Lp>(_Sp_make_shared_tag(), __a,
1427                                     std::forward<_Args>(__args)...);
1428     }
1429
1430   template<typename _Tp, _Lock_policy _Lp, typename... _Args>
1431     inline __shared_ptr<_Tp, _Lp>
1432     __make_shared(_Args&&... __args)
1433     {
1434       typedef typename std::remove_const<_Tp>::type _Tp_nc;
1435       return std::__allocate_shared<_Tp, _Lp>(std::allocator<_Tp_nc>(),
1436                                               std::forward<_Args>(__args)...);
1437     }
1438
1439   /// std::hash specialization for __shared_ptr.
1440   template<typename _Tp, _Lock_policy _Lp>
1441     struct hash<__shared_ptr<_Tp, _Lp>>
1442     : public __hash_base<size_t, __shared_ptr<_Tp, _Lp>>
1443     {
1444       size_t
1445       operator()(const __shared_ptr<_Tp, _Lp>& __s) const
1446       { return std::hash<_Tp*>()(__s.get()); }
1447     };
1448
1449 _GLIBCXX_END_NAMESPACE_VERSION
1450 } // namespace
1451
1452 #endif // _SHARED_PTR_BASE_H