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

   1 // shared_ptr and weak_ptr implementation -*- 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.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   /// 2.2.3.7 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   /// 2.2.3.10 shared_ptr get_deleter (experimental)
  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
 324   // 20.8.13.2.7 shared_ptr comparisons
 325   template<typename _Tp1, typename _Tp2>
 326     inline bool
 327     operator==(const shared_ptr<_Tp1>& __a,
 328                const shared_ptr<_Tp2>& __b) noexcept
 329     { return __a.get() == __b.get(); }
 330
 331   template<typename _Tp>
 332     inline bool
 333     operator==(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 334     { return !__a; }
 335
 336   template<typename _Tp>
 337     inline bool
 338     operator==(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 339     { return !__a; }
 340
 341   template<typename _Tp1, typename _Tp2>
 342     inline bool
 343     operator!=(const shared_ptr<_Tp1>& __a,
 344                const shared_ptr<_Tp2>& __b) noexcept
 345     { return __a.get() != __b.get(); }
 346
 347   template<typename _Tp>
 348     inline bool
 349     operator!=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 350     { return (bool)__a; }
 351
 352   template<typename _Tp>
 353     inline bool
 354     operator!=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 355     { return (bool)__a; }
 356
 357   template<typename _Tp1, typename _Tp2>
 358     inline bool
 359     operator<(const shared_ptr<_Tp1>& __a,
 360               const shared_ptr<_Tp2>& __b) noexcept
 361     {
 362       typedef typename std::common_type<_Tp1*, _Tp2*>::type _CT;
 363       return std::less<_CT>()(__a.get(), __b.get());
 364     }
 365
 366   template<typename _Tp>
 367     inline bool
 368     operator<(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 369     { return std::less<_Tp*>()(__a.get(), nullptr); }
 370
 371   template<typename _Tp>
 372     inline bool
 373     operator<(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 374     { return std::less<_Tp*>()(nullptr, __a.get()); }
 375
 376   template<typename _Tp1, typename _Tp2>
 377     inline bool
 378     operator<=(const shared_ptr<_Tp1>& __a,
 379                const shared_ptr<_Tp2>& __b) noexcept
 380     { return !(__b < __a); }
 381
 382   template<typename _Tp>
 383     inline bool
 384     operator<=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 385     { return !(nullptr < __a); }
 386
 387   template<typename _Tp>
 388     inline bool
 389     operator<=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 390     { return !(__a < nullptr); }
 391
 392   template<typename _Tp1, typename _Tp2>
 393     inline bool
 394     operator>(const shared_ptr<_Tp1>& __a,
 395               const shared_ptr<_Tp2>& __b) noexcept
 396     { return (__b < __a); }
 397
 398   template<typename _Tp>
 399     inline bool
 400     operator>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 401     { return std::less<_Tp*>()(nullptr, __a.get()); }
 402
 403   template<typename _Tp>
 404     inline bool
 405     operator>(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 406     { return std::less<_Tp*>()(__a.get(), nullptr); }
 407
 408   template<typename _Tp1, typename _Tp2>
 409     inline bool
 410     operator>=(const shared_ptr<_Tp1>& __a,
 411                const shared_ptr<_Tp2>& __b) noexcept
 412     { return !(__a < __b); }
 413
 414   template<typename _Tp>
 415     inline bool
 416     operator>=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 417     { return !(__a < nullptr); }
 418
 419   template<typename _Tp>
 420     inline bool
 421     operator>=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 422     { return !(nullptr < __a); }
 423
 424   template<typename _Tp>
 425     struct less<shared_ptr<_Tp>> : public _Sp_less<shared_ptr<_Tp>>
 426     { };
 427
 428   // 20.8.13.2.9 shared_ptr specialized algorithms.
 429   template<typename _Tp>
 430     inline void
 431     swap(shared_ptr<_Tp>& __a, shared_ptr<_Tp>& __b) noexcept
 432     { __a.swap(__b); }
 433
 434   // 20.8.13.2.10 shared_ptr casts.
 435   template<typename _Tp, typename _Tp1>
 436     inline shared_ptr<_Tp>
 437     static_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 438     { return shared_ptr<_Tp>(__r, static_cast<_Tp*>(__r.get())); }
 439
 440   template<typename _Tp, typename _Tp1>
 441     inline shared_ptr<_Tp>
 442     const_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 443     { return shared_ptr<_Tp>(__r, const_cast<_Tp*>(__r.get())); }
 444
 445   template<typename _Tp, typename _Tp1>
 446     inline shared_ptr<_Tp>
 447     dynamic_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 448     {
 449       if (_Tp* __p = dynamic_cast<_Tp*>(__r.get()))
 450         return shared_ptr<_Tp>(__r, __p);
 451       return shared_ptr<_Tp>();
 452     }
 453
 454
 455   /**
 456    *  @brief  A smart pointer with weak semantics.
 457    *
 458    *  With forwarding constructors and assignment operators.
 459    */
 460   template<typename _Tp>
 461     class weak_ptr : public __weak_ptr<_Tp>
 462     {
 463     public:
 464       constexpr weak_ptr() noexcept
 465       : __weak_ptr<_Tp>() { }
 466
 467       template<typename _Tp1, typename = typename
 468                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 469         weak_ptr(const weak_ptr<_Tp1>& __r) noexcept
 470         : __weak_ptr<_Tp>(__r) { }
 471
 472       template<typename _Tp1, typename = typename
 473                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 474         weak_ptr(const shared_ptr<_Tp1>& __r) noexcept
 475         : __weak_ptr<_Tp>(__r) { }
 476
 477       template<typename _Tp1>
 478         weak_ptr&
 479         operator=(const weak_ptr<_Tp1>& __r) noexcept
 480         {
 481           this->__weak_ptr<_Tp>::operator=(__r);
 482           return *this;
 483         }
 484
 485       template<typename _Tp1>
 486         weak_ptr&
 487         operator=(const shared_ptr<_Tp1>& __r) noexcept
 488         {
 489           this->__weak_ptr<_Tp>::operator=(__r);
 490           return *this;
 491         }
 492
 493       shared_ptr<_Tp>
 494       lock() const noexcept
 495       {
 496 #ifdef __GTHREADS
 497         if (this->expired())
 498           return shared_ptr<_Tp>();
 499
 500         __try
 501           {
 502             return shared_ptr<_Tp>(*this);
 503           }
 504         __catch(const bad_weak_ptr&)
 505           {
 506             return shared_ptr<_Tp>();
 507           }
 508 #else
 509         return this->expired() ? shared_ptr<_Tp>() : shared_ptr<_Tp>(*this);
 510 #endif
 511       }
 512     };
 513
 514   // 20.8.13.3.7 weak_ptr specialized algorithms.
 515   template<typename _Tp>
 516     inline void
 517     swap(weak_ptr<_Tp>& __a, weak_ptr<_Tp>& __b) noexcept
 518     { __a.swap(__b); }
 519
 520
 521   /// Primary template owner_less
 522   template<typename _Tp>
 523     struct owner_less;
 524
 525   /// Partial specialization of owner_less for shared_ptr.
 526   template<typename _Tp>
 527     struct owner_less<shared_ptr<_Tp>>
 528     : public _Sp_owner_less<shared_ptr<_Tp>, weak_ptr<_Tp>>
 529     { };
 530
 531   /// Partial specialization of owner_less for weak_ptr.
 532   template<typename _Tp>
 533     struct owner_less<weak_ptr<_Tp>>
 534     : public _Sp_owner_less<weak_ptr<_Tp>, shared_ptr<_Tp>>
 535     { };
 536
 537   /**
 538    *  @brief Base class allowing use of member function shared_from_this.
 539    */
 540   template<typename _Tp>
 541     class enable_shared_from_this
 542     {
 543     protected:
 544       constexpr enable_shared_from_this() noexcept { }
 545
 546       enable_shared_from_this(const enable_shared_from_this&) noexcept { }
 547
 548       enable_shared_from_this&
 549       operator=(const enable_shared_from_this&) noexcept
 550       { return *this; }
 551
 552       ~enable_shared_from_this() { }
 553
 554     public:
 555       shared_ptr<_Tp>
 556       shared_from_this()
 557       { return shared_ptr<_Tp>(this->_M_weak_this); }
 558
 559       shared_ptr<const _Tp>
 560       shared_from_this() const
 561       { return shared_ptr<const _Tp>(this->_M_weak_this); }
 562
 563     private:
 564       template<typename _Tp1>
 565         void
 566         _M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const noexcept
 567         { _M_weak_this._M_assign(__p, __n); }
 568
 569       template<typename _Tp1>
 570         friend void
 571         __enable_shared_from_this_helper(const __shared_count<>& __pn,
 572                                          const enable_shared_from_this* __pe,
 573                                          const _Tp1* __px) noexcept
 574         {
 575           if (__pe != 0)
 576             __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
 577         }
 578
 579       mutable weak_ptr<_Tp>  _M_weak_this;
 580     };
 581
 582   /**
 583    *  @brief  Create an object that is owned by a shared_ptr.
 584    *  @param  __a     An allocator.
 585    *  @param  __args  Arguments for the @a _Tp object's constructor.
 586    *  @return A shared_ptr that owns the newly created object.
 587    *  @throw  An exception thrown from @a _Alloc::allocate or from the
 588    *          constructor of @a _Tp.
 589    *
 590    *  A copy of @a __a will be used to allocate memory for the shared_ptr
 591    *  and the new object.
 592    */
 593   template<typename _Tp, typename _Alloc, typename... _Args>
 594     inline shared_ptr<_Tp>
 595     allocate_shared(const _Alloc& __a, _Args&&... __args)
 596     {
 597       return shared_ptr<_Tp>(_Sp_make_shared_tag(), __a,
 598                              std::forward<_Args>(__args)...);
 599     }
 600
 601   /**
 602    *  @brief  Create an object that is owned by a shared_ptr.
 603    *  @param  __args  Arguments for the @a _Tp object's constructor.
 604    *  @return A shared_ptr that owns the newly created object.
 605    *  @throw  std::bad_alloc, or an exception thrown from the
 606    *          constructor of @a _Tp.
 607    */
 608   template<typename _Tp, typename... _Args>
 609     inline shared_ptr<_Tp>
 610     make_shared(_Args&&... __args)
 611     {
 612       typedef typename std::remove_const<_Tp>::type _Tp_nc;
 613       return std::allocate_shared<_Tp>(std::allocator<_Tp_nc>(),
 614                                        std::forward<_Args>(__args)...);
 615     }
 616
 617   /// std::hash specialization for shared_ptr.
 618   template<typename _Tp>
 619     struct hash<shared_ptr<_Tp>>
 620     : public __hash_base<size_t, shared_ptr<_Tp>>
 621     {
 622       size_t
 623       operator()(const shared_ptr<_Tp>& __s) const noexcept
 624       { return std::hash<_Tp*>()(__s.get()); }
 625     };
 626
 627   // @} group pointer_abstractions
 628
 629 _GLIBCXX_END_NAMESPACE_VERSION
 630 } // namespace
 631
 632 #endif // _SHARED_PTR_H