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

   1 // shared_ptr and weak_ptr implementation -*- C++ -*-
   2
   3 // Copyright (C) 2007-2014 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.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_H
  50 #define _SHARED_PTR_H 1
  51
  52 #include <bits/shared_ptr_base.h>
  53
  54 namespace std _GLIBCXX_VISIBILITY(default)
  55 {
  56 _GLIBCXX_BEGIN_NAMESPACE_VERSION
  57
  58   /**
  59    * @addtogroup pointer_abstractions
  60    * @{
  61    */
  62
  63   /// 20.7.2.2.11 shared_ptr I/O
  64   template<typename _Ch, typename _Tr, typename _Tp, _Lock_policy _Lp>
  65     inline std::basic_ostream<_Ch, _Tr>&
  66     operator<<(std::basic_ostream<_Ch, _Tr>& __os,
  67                const __shared_ptr<_Tp, _Lp>& __p)
  68     {
  69       __os << __p.get();
  70       return __os;
  71     }
  72
  73   /// 20.7.2.2.10 shared_ptr get_deleter
  74   template<typename _Del, typename _Tp, _Lock_policy _Lp>
  75     inline _Del*
  76     get_deleter(const __shared_ptr<_Tp, _Lp>& __p) noexcept
  77     {
  78 #ifdef __GXX_RTTI
  79       return static_cast<_Del*>(__p._M_get_deleter(typeid(_Del)));
  80 #else
  81       return 0;
  82 #endif
  83     }
  84
  85
  86   /**
  87    *  @brief  A smart pointer with reference-counted copy semantics.
  88    *
  89    *  The object pointed to is deleted when the last shared_ptr pointing to
  90    *  it is destroyed or reset.
  91   */
  92   template<typename _Tp>
  93     class shared_ptr : public __shared_ptr<_Tp>
  94     {
  95     public:
  96       /**
  97        *  @brief  Construct an empty %shared_ptr.
  98        *  @post   use_count()==0 && get()==0
  99        */
 100       constexpr shared_ptr() noexcept
 101       : __shared_ptr<_Tp>() { }
 102
 103       shared_ptr(const shared_ptr&) noexcept = default;
 104
 105       /**
 106        *  @brief  Construct a %shared_ptr that owns the pointer @a __p.
 107        *  @param  __p  A pointer that is convertible to element_type*.
 108        *  @post   use_count() == 1 && get() == __p
 109        *  @throw  std::bad_alloc, in which case @c delete @a __p is called.
 110        */
 111       template<typename _Tp1>
 112         explicit shared_ptr(_Tp1* __p)
 113         : __shared_ptr<_Tp>(__p) { }
 114
 115       /**
 116        *  @brief  Construct a %shared_ptr that owns the pointer @a __p
 117        *          and the deleter @a __d.
 118        *  @param  __p  A pointer.
 119        *  @param  __d  A deleter.
 120        *  @post   use_count() == 1 && get() == __p
 121        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 122        *
 123        *  Requirements: _Deleter's copy constructor and destructor must
 124        *  not throw
 125        *
 126        *  __shared_ptr will release __p by calling __d(__p)
 127        */
 128       template<typename _Tp1, typename _Deleter>
 129         shared_ptr(_Tp1* __p, _Deleter __d)
 130         : __shared_ptr<_Tp>(__p, __d) { }
 131
 132       /**
 133        *  @brief  Construct a %shared_ptr that owns a null pointer
 134        *          and the deleter @a __d.
 135        *  @param  __p  A null pointer constant.
 136        *  @param  __d  A deleter.
 137        *  @post   use_count() == 1 && get() == __p
 138        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 139        *
 140        *  Requirements: _Deleter's copy constructor and destructor must
 141        *  not throw
 142        *
 143        *  The last owner will call __d(__p)
 144        */
 145       template<typename _Deleter>
 146         shared_ptr(nullptr_t __p, _Deleter __d)
 147         : __shared_ptr<_Tp>(__p, __d) { }
 148
 149       /**
 150        *  @brief  Construct a %shared_ptr that owns the pointer @a __p
 151        *          and the deleter @a __d.
 152        *  @param  __p  A pointer.
 153        *  @param  __d  A deleter.
 154        *  @param  __a  An allocator.
 155        *  @post   use_count() == 1 && get() == __p
 156        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 157        *
 158        *  Requirements: _Deleter's copy constructor and destructor must
 159        *  not throw _Alloc's copy constructor and destructor must not
 160        *  throw.
 161        *
 162        *  __shared_ptr will release __p by calling __d(__p)
 163        */
 164       template<typename _Tp1, typename _Deleter, typename _Alloc>
 165         shared_ptr(_Tp1* __p, _Deleter __d, _Alloc __a)
 166         : __shared_ptr<_Tp>(__p, __d, std::move(__a)) { }
 167
 168       /**
 169        *  @brief  Construct a %shared_ptr that owns a null pointer
 170        *          and the deleter @a __d.
 171        *  @param  __p  A null pointer constant.
 172        *  @param  __d  A deleter.
 173        *  @param  __a  An allocator.
 174        *  @post   use_count() == 1 && get() == __p
 175        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 176        *
 177        *  Requirements: _Deleter's copy constructor and destructor must
 178        *  not throw _Alloc's copy constructor and destructor must not
 179        *  throw.
 180        *
 181        *  The last owner will call __d(__p)
 182        */
 183       template<typename _Deleter, typename _Alloc>
 184         shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
 185         : __shared_ptr<_Tp>(__p, __d, std::move(__a)) { }
 186
 187       // Aliasing constructor
 188
 189       /**
 190        *  @brief  Constructs a %shared_ptr instance that stores @a __p
 191        *          and shares ownership with @a __r.
 192        *  @param  __r  A %shared_ptr.
 193        *  @param  __p  A pointer that will remain valid while @a *__r is valid.
 194        *  @post   get() == __p && use_count() == __r.use_count()
 195        *
 196        *  This can be used to construct a @c shared_ptr to a sub-object
 197        *  of an object managed by an existing @c shared_ptr.
 198        *
 199        * @code
 200        * shared_ptr< pair<int,int> > pii(new pair<int,int>());
 201        * shared_ptr<int> pi(pii, &pii->first);
 202        * assert(pii.use_count() == 2);
 203        * @endcode
 204        */
 205       template<typename _Tp1>
 206         shared_ptr(const shared_ptr<_Tp1>& __r, _Tp* __p) noexcept
 207         : __shared_ptr<_Tp>(__r, __p) { }
 208
 209       /**
 210        *  @brief  If @a __r is empty, constructs an empty %shared_ptr;
 211        *          otherwise construct a %shared_ptr that shares ownership
 212        *          with @a __r.
 213        *  @param  __r  A %shared_ptr.
 214        *  @post   get() == __r.get() && use_count() == __r.use_count()
 215        */
 216       template<typename _Tp1, typename = typename
 217                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 218         shared_ptr(const shared_ptr<_Tp1>& __r) noexcept
 219         : __shared_ptr<_Tp>(__r) { }
 220
 221       /**
 222        *  @brief  Move-constructs a %shared_ptr instance from @a __r.
 223        *  @param  __r  A %shared_ptr rvalue.
 224        *  @post   *this contains the old value of @a __r, @a __r is empty.
 225        */
 226       shared_ptr(shared_ptr&& __r) noexcept
 227       : __shared_ptr<_Tp>(std::move(__r)) { }
 228
 229       /**
 230        *  @brief  Move-constructs a %shared_ptr instance from @a __r.
 231        *  @param  __r  A %shared_ptr rvalue.
 232        *  @post   *this contains the old value of @a __r, @a __r is empty.
 233        */
 234       template<typename _Tp1, typename = typename
 235                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 236         shared_ptr(shared_ptr<_Tp1>&& __r) noexcept
 237         : __shared_ptr<_Tp>(std::move(__r)) { }
 238
 239       /**
 240        *  @brief  Constructs a %shared_ptr that shares ownership with @a __r
 241        *          and stores a copy of the pointer stored in @a __r.
 242        *  @param  __r  A weak_ptr.
 243        *  @post   use_count() == __r.use_count()
 244        *  @throw  bad_weak_ptr when __r.expired(),
 245        *          in which case the constructor has no effect.
 246        */
 247       template<typename _Tp1>
 248         explicit shared_ptr(const weak_ptr<_Tp1>& __r)
 249         : __shared_ptr<_Tp>(__r) { }
 250
 251 #if _GLIBCXX_USE_DEPRECATED
 252       template<typename _Tp1>
 253         shared_ptr(std::auto_ptr<_Tp1>&& __r);
 254 #endif
 255
 256       template<typename _Tp1, typename _Del>
 257         shared_ptr(std::unique_ptr<_Tp1, _Del>&& __r)
 258         : __shared_ptr<_Tp>(std::move(__r)) { }
 259
 260       /**
 261        *  @brief  Construct an empty %shared_ptr.
 262        *  @param  __p  A null pointer constant.
 263        *  @post   use_count() == 0 && get() == nullptr
 264        */
 265       constexpr shared_ptr(nullptr_t __p) noexcept
 266       : __shared_ptr<_Tp>(__p) { }
 267
 268       shared_ptr& operator=(const shared_ptr&) noexcept = default;
 269
 270       template<typename _Tp1>
 271         shared_ptr&
 272         operator=(const shared_ptr<_Tp1>& __r) noexcept
 273         {
 274           this->__shared_ptr<_Tp>::operator=(__r);
 275           return *this;
 276         }
 277
 278 #if _GLIBCXX_USE_DEPRECATED
 279       template<typename _Tp1>
 280         shared_ptr&
 281         operator=(std::auto_ptr<_Tp1>&& __r)
 282         {
 283           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 284           return *this;
 285         }
 286 #endif
 287
 288       shared_ptr&
 289       operator=(shared_ptr&& __r) noexcept
 290       {
 291         this->__shared_ptr<_Tp>::operator=(std::move(__r));
 292         return *this;
 293       }
 294
 295       template<class _Tp1>
 296         shared_ptr&
 297         operator=(shared_ptr<_Tp1>&& __r) noexcept
 298         {
 299           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 300           return *this;
 301         }
 302
 303       template<typename _Tp1, typename _Del>
 304         shared_ptr&
 305         operator=(std::unique_ptr<_Tp1, _Del>&& __r)
 306         {
 307           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 308           return *this;
 309         }
 310
 311     private:
 312       // This constructor is non-standard, it is used by allocate_shared.
 313       template<typename _Alloc, typename... _Args>
 314         shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
 315                    _Args&&... __args)
 316         : __shared_ptr<_Tp>(__tag, __a, std::forward<_Args>(__args)...)
 317         { }
 318
 319       template<typename _Tp1, typename _Alloc, typename... _Args>
 320         friend shared_ptr<_Tp1>
 321         allocate_shared(const _Alloc& __a, _Args&&... __args);
 322
 323       // This constructor is non-standard, it is used by weak_ptr::lock().
 324       shared_ptr(const weak_ptr<_Tp>& __r, std::nothrow_t)
 325       : __shared_ptr<_Tp>(__r, std::nothrow) { }
 326
 327       friend class weak_ptr<_Tp>;
 328     };
 329
 330   // 20.7.2.2.7 shared_ptr comparisons
 331   template<typename _Tp1, typename _Tp2>
 332     inline bool
 333     operator==(const shared_ptr<_Tp1>& __a,
 334                const shared_ptr<_Tp2>& __b) noexcept
 335     { return __a.get() == __b.get(); }
 336
 337   template<typename _Tp>
 338     inline bool
 339     operator==(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 340     { return !__a; }
 341
 342   template<typename _Tp>
 343     inline bool
 344     operator==(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 345     { return !__a; }
 346
 347   template<typename _Tp1, typename _Tp2>
 348     inline bool
 349     operator!=(const shared_ptr<_Tp1>& __a,
 350                const shared_ptr<_Tp2>& __b) noexcept
 351     { return __a.get() != __b.get(); }
 352
 353   template<typename _Tp>
 354     inline bool
 355     operator!=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 356     { return (bool)__a; }
 357
 358   template<typename _Tp>
 359     inline bool
 360     operator!=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 361     { return (bool)__a; }
 362
 363   template<typename _Tp1, typename _Tp2>
 364     inline bool
 365     operator<(const shared_ptr<_Tp1>& __a,
 366               const shared_ptr<_Tp2>& __b) noexcept
 367     {
 368       typedef typename std::common_type<_Tp1*, _Tp2*>::type _CT;
 369       return std::less<_CT>()(__a.get(), __b.get());
 370     }
 371
 372   template<typename _Tp>
 373     inline bool
 374     operator<(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 375     { return std::less<_Tp*>()(__a.get(), nullptr); }
 376
 377   template<typename _Tp>
 378     inline bool
 379     operator<(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 380     { return std::less<_Tp*>()(nullptr, __a.get()); }
 381
 382   template<typename _Tp1, typename _Tp2>
 383     inline bool
 384     operator<=(const shared_ptr<_Tp1>& __a,
 385                const shared_ptr<_Tp2>& __b) noexcept
 386     { return !(__b < __a); }
 387
 388   template<typename _Tp>
 389     inline bool
 390     operator<=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 391     { return !(nullptr < __a); }
 392
 393   template<typename _Tp>
 394     inline bool
 395     operator<=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 396     { return !(__a < nullptr); }
 397
 398   template<typename _Tp1, typename _Tp2>
 399     inline bool
 400     operator>(const shared_ptr<_Tp1>& __a,
 401               const shared_ptr<_Tp2>& __b) noexcept
 402     { return (__b < __a); }
 403
 404   template<typename _Tp>
 405     inline bool
 406     operator>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 407     { return std::less<_Tp*>()(nullptr, __a.get()); }
 408
 409   template<typename _Tp>
 410     inline bool
 411     operator>(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 412     { return std::less<_Tp*>()(__a.get(), nullptr); }
 413
 414   template<typename _Tp1, typename _Tp2>
 415     inline bool
 416     operator>=(const shared_ptr<_Tp1>& __a,
 417                const shared_ptr<_Tp2>& __b) noexcept
 418     { return !(__a < __b); }
 419
 420   template<typename _Tp>
 421     inline bool
 422     operator>=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 423     { return !(__a < nullptr); }
 424
 425   template<typename _Tp>
 426     inline bool
 427     operator>=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 428     { return !(nullptr < __a); }
 429
 430   template<typename _Tp>
 431     struct less<shared_ptr<_Tp>> : public _Sp_less<shared_ptr<_Tp>>
 432     { };
 433
 434   // 20.7.2.2.8 shared_ptr specialized algorithms.
 435   template<typename _Tp>
 436     inline void
 437     swap(shared_ptr<_Tp>& __a, shared_ptr<_Tp>& __b) noexcept
 438     { __a.swap(__b); }
 439
 440   // 20.7.2.2.9 shared_ptr casts.
 441   template<typename _Tp, typename _Tp1>
 442     inline shared_ptr<_Tp>
 443     static_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 444     { return shared_ptr<_Tp>(__r, static_cast<_Tp*>(__r.get())); }
 445
 446   template<typename _Tp, typename _Tp1>
 447     inline shared_ptr<_Tp>
 448     const_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 449     { return shared_ptr<_Tp>(__r, const_cast<_Tp*>(__r.get())); }
 450
 451   template<typename _Tp, typename _Tp1>
 452     inline shared_ptr<_Tp>
 453     dynamic_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 454     {
 455       if (_Tp* __p = dynamic_cast<_Tp*>(__r.get()))
 456         return shared_ptr<_Tp>(__r, __p);
 457       return shared_ptr<_Tp>();
 458     }
 459
 460
 461   /**
 462    *  @brief  A smart pointer with weak semantics.
 463    *
 464    *  With forwarding constructors and assignment operators.
 465    */
 466   template<typename _Tp>
 467     class weak_ptr : public __weak_ptr<_Tp>
 468     {
 469     public:
 470       constexpr weak_ptr() noexcept
 471       : __weak_ptr<_Tp>() { }
 472
 473       template<typename _Tp1, typename = typename
 474                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 475         weak_ptr(const weak_ptr<_Tp1>& __r) noexcept
 476         : __weak_ptr<_Tp>(__r) { }
 477
 478       template<typename _Tp1, typename = typename
 479                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 480         weak_ptr(const shared_ptr<_Tp1>& __r) noexcept
 481         : __weak_ptr<_Tp>(__r) { }
 482
 483       template<typename _Tp1>
 484         weak_ptr&
 485         operator=(const weak_ptr<_Tp1>& __r) noexcept
 486         {
 487           this->__weak_ptr<_Tp>::operator=(__r);
 488           return *this;
 489         }
 490
 491       template<typename _Tp1>
 492         weak_ptr&
 493         operator=(const shared_ptr<_Tp1>& __r) noexcept
 494         {
 495           this->__weak_ptr<_Tp>::operator=(__r);
 496           return *this;
 497         }
 498
 499       shared_ptr<_Tp>
 500       lock() const noexcept
 501       { return shared_ptr<_Tp>(*this, std::nothrow); }
 502     };
 503
 504   // 20.7.2.3.6 weak_ptr specialized algorithms.
 505   template<typename _Tp>
 506     inline void
 507     swap(weak_ptr<_Tp>& __a, weak_ptr<_Tp>& __b) noexcept
 508     { __a.swap(__b); }
 509
 510
 511   /// Primary template owner_less
 512   template<typename _Tp>
 513     struct owner_less;
 514
 515   /// Partial specialization of owner_less for shared_ptr.
 516   template<typename _Tp>
 517     struct owner_less<shared_ptr<_Tp>>
 518     : public _Sp_owner_less<shared_ptr<_Tp>, weak_ptr<_Tp>>
 519     { };
 520
 521   /// Partial specialization of owner_less for weak_ptr.
 522   template<typename _Tp>
 523     struct owner_less<weak_ptr<_Tp>>
 524     : public _Sp_owner_less<weak_ptr<_Tp>, shared_ptr<_Tp>>
 525     { };
 526
 527   /**
 528    *  @brief Base class allowing use of member function shared_from_this.
 529    */
 530   template<typename _Tp>
 531     class enable_shared_from_this
 532     {
 533     protected:
 534       constexpr enable_shared_from_this() noexcept { }
 535
 536       enable_shared_from_this(const enable_shared_from_this&) noexcept { }
 537
 538       enable_shared_from_this&
 539       operator=(const enable_shared_from_this&) noexcept
 540       { return *this; }
 541
 542       ~enable_shared_from_this() { }
 543
 544     public:
 545       shared_ptr<_Tp>
 546       shared_from_this()
 547       { return shared_ptr<_Tp>(this->_M_weak_this); }
 548
 549       shared_ptr<const _Tp>
 550       shared_from_this() const
 551       { return shared_ptr<const _Tp>(this->_M_weak_this); }
 552
 553     private:
 554       template<typename _Tp1>
 555         void
 556         _M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const noexcept
 557         { _M_weak_this._M_assign(__p, __n); }
 558
 559       template<typename _Tp1>
 560         friend void
 561         __enable_shared_from_this_helper(const __shared_count<>& __pn,
 562                                          const enable_shared_from_this* __pe,
 563                                          const _Tp1* __px) noexcept
 564         {
 565           if (__pe != 0)
 566             __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
 567         }
 568
 569       mutable weak_ptr<_Tp>  _M_weak_this;
 570     };
 571
 572   /**
 573    *  @brief  Create an object that is owned by a shared_ptr.
 574    *  @param  __a     An allocator.
 575    *  @param  __args  Arguments for the @a _Tp object's constructor.
 576    *  @return A shared_ptr that owns the newly created object.
 577    *  @throw  An exception thrown from @a _Alloc::allocate or from the
 578    *          constructor of @a _Tp.
 579    *
 580    *  A copy of @a __a will be used to allocate memory for the shared_ptr
 581    *  and the new object.
 582    */
 583   template<typename _Tp, typename _Alloc, typename... _Args>
 584     inline shared_ptr<_Tp>
 585     allocate_shared(const _Alloc& __a, _Args&&... __args)
 586     {
 587       return shared_ptr<_Tp>(_Sp_make_shared_tag(), __a,
 588                              std::forward<_Args>(__args)...);
 589     }
 590
 591   /**
 592    *  @brief  Create an object that is owned by a shared_ptr.
 593    *  @param  __args  Arguments for the @a _Tp object's constructor.
 594    *  @return A shared_ptr that owns the newly created object.
 595    *  @throw  std::bad_alloc, or an exception thrown from the
 596    *          constructor of @a _Tp.
 597    */
 598   template<typename _Tp, typename... _Args>
 599     inline shared_ptr<_Tp>
 600     make_shared(_Args&&... __args)
 601     {
 602       typedef typename std::remove_const<_Tp>::type _Tp_nc;
 603       return std::allocate_shared<_Tp>(std::allocator<_Tp_nc>(),
 604                                        std::forward<_Args>(__args)...);
 605     }
 606
 607   /// std::hash specialization for shared_ptr.
 608   template<typename _Tp>
 609     struct hash<shared_ptr<_Tp>>
 610     : public __hash_base<size_t, shared_ptr<_Tp>>
 611     {
 612       size_t
 613       operator()(const shared_ptr<_Tp>& __s) const noexcept
 614       { return std::hash<_Tp*>()(__s.get()); }
 615     };
 616
 617   // @} group pointer_abstractions
 618
 619 _GLIBCXX_END_NAMESPACE_VERSION
 620 } // namespace
 621
 622 #endif // _SHARED_PTR_H