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

   1 // shared_ptr and weak_ptr implementation -*- C++ -*-
   2
   3 // Copyright (C) 2007-2015 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 #if __cpp_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       template<typename _Ptr>
  96         using _Convertible
  97           = typename enable_if<is_convertible<_Ptr, _Tp*>::value>::type;
  98
  99     public:
 100       /**
 101        *  @brief  Construct an empty %shared_ptr.
 102        *  @post   use_count()==0 && get()==0
 103        */
 104       constexpr shared_ptr() noexcept
 105       : __shared_ptr<_Tp>() { }
 106
 107       shared_ptr(const shared_ptr&) noexcept = default;
 108
 109       /**
 110        *  @brief  Construct a %shared_ptr that owns the pointer @a __p.
 111        *  @param  __p  A pointer that is convertible to element_type*.
 112        *  @post   use_count() == 1 && get() == __p
 113        *  @throw  std::bad_alloc, in which case @c delete @a __p is called.
 114        */
 115       template<typename _Tp1>
 116         explicit shared_ptr(_Tp1* __p)
 117         : __shared_ptr<_Tp>(__p) { }
 118
 119       /**
 120        *  @brief  Construct a %shared_ptr that owns the pointer @a __p
 121        *          and the deleter @a __d.
 122        *  @param  __p  A pointer.
 123        *  @param  __d  A deleter.
 124        *  @post   use_count() == 1 && get() == __p
 125        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 126        *
 127        *  Requirements: _Deleter's copy constructor and destructor must
 128        *  not throw
 129        *
 130        *  __shared_ptr will release __p by calling __d(__p)
 131        */
 132       template<typename _Tp1, typename _Deleter>
 133         shared_ptr(_Tp1* __p, _Deleter __d)
 134         : __shared_ptr<_Tp>(__p, __d) { }
 135
 136       /**
 137        *  @brief  Construct a %shared_ptr that owns a null pointer
 138        *          and the deleter @a __d.
 139        *  @param  __p  A null pointer constant.
 140        *  @param  __d  A deleter.
 141        *  @post   use_count() == 1 && get() == __p
 142        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 143        *
 144        *  Requirements: _Deleter's copy constructor and destructor must
 145        *  not throw
 146        *
 147        *  The last owner will call __d(__p)
 148        */
 149       template<typename _Deleter>
 150         shared_ptr(nullptr_t __p, _Deleter __d)
 151         : __shared_ptr<_Tp>(__p, __d) { }
 152
 153       /**
 154        *  @brief  Construct a %shared_ptr that owns the pointer @a __p
 155        *          and the deleter @a __d.
 156        *  @param  __p  A pointer.
 157        *  @param  __d  A deleter.
 158        *  @param  __a  An allocator.
 159        *  @post   use_count() == 1 && get() == __p
 160        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 161        *
 162        *  Requirements: _Deleter's copy constructor and destructor must
 163        *  not throw _Alloc's copy constructor and destructor must not
 164        *  throw.
 165        *
 166        *  __shared_ptr will release __p by calling __d(__p)
 167        */
 168       template<typename _Tp1, typename _Deleter, typename _Alloc>
 169         shared_ptr(_Tp1* __p, _Deleter __d, _Alloc __a)
 170         : __shared_ptr<_Tp>(__p, __d, std::move(__a)) { }
 171
 172       /**
 173        *  @brief  Construct a %shared_ptr that owns a null pointer
 174        *          and the deleter @a __d.
 175        *  @param  __p  A null pointer constant.
 176        *  @param  __d  A deleter.
 177        *  @param  __a  An allocator.
 178        *  @post   use_count() == 1 && get() == __p
 179        *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
 180        *
 181        *  Requirements: _Deleter's copy constructor and destructor must
 182        *  not throw _Alloc's copy constructor and destructor must not
 183        *  throw.
 184        *
 185        *  The last owner will call __d(__p)
 186        */
 187       template<typename _Deleter, typename _Alloc>
 188         shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
 189         : __shared_ptr<_Tp>(__p, __d, std::move(__a)) { }
 190
 191       // Aliasing constructor
 192
 193       /**
 194        *  @brief  Constructs a %shared_ptr instance that stores @a __p
 195        *          and shares ownership with @a __r.
 196        *  @param  __r  A %shared_ptr.
 197        *  @param  __p  A pointer that will remain valid while @a *__r is valid.
 198        *  @post   get() == __p && use_count() == __r.use_count()
 199        *
 200        *  This can be used to construct a @c shared_ptr to a sub-object
 201        *  of an object managed by an existing @c shared_ptr.
 202        *
 203        * @code
 204        * shared_ptr< pair<int,int> > pii(new pair<int,int>());
 205        * shared_ptr<int> pi(pii, &pii->first);
 206        * assert(pii.use_count() == 2);
 207        * @endcode
 208        */
 209       template<typename _Tp1>
 210         shared_ptr(const shared_ptr<_Tp1>& __r, _Tp* __p) noexcept
 211         : __shared_ptr<_Tp>(__r, __p) { }
 212
 213       /**
 214        *  @brief  If @a __r is empty, constructs an empty %shared_ptr;
 215        *          otherwise construct a %shared_ptr that shares ownership
 216        *          with @a __r.
 217        *  @param  __r  A %shared_ptr.
 218        *  @post   get() == __r.get() && use_count() == __r.use_count()
 219        */
 220       template<typename _Tp1, typename = _Convertible<_Tp1*>>
 221         shared_ptr(const shared_ptr<_Tp1>& __r) noexcept
 222         : __shared_ptr<_Tp>(__r) { }
 223
 224       /**
 225        *  @brief  Move-constructs a %shared_ptr instance from @a __r.
 226        *  @param  __r  A %shared_ptr rvalue.
 227        *  @post   *this contains the old value of @a __r, @a __r is empty.
 228        */
 229       shared_ptr(shared_ptr&& __r) noexcept
 230       : __shared_ptr<_Tp>(std::move(__r)) { }
 231
 232       /**
 233        *  @brief  Move-constructs a %shared_ptr instance from @a __r.
 234        *  @param  __r  A %shared_ptr rvalue.
 235        *  @post   *this contains the old value of @a __r, @a __r is empty.
 236        */
 237       template<typename _Tp1, typename = _Convertible<_Tp1*>>
 238         shared_ptr(shared_ptr<_Tp1>&& __r) noexcept
 239         : __shared_ptr<_Tp>(std::move(__r)) { }
 240
 241       /**
 242        *  @brief  Constructs a %shared_ptr that shares ownership with @a __r
 243        *          and stores a copy of the pointer stored in @a __r.
 244        *  @param  __r  A weak_ptr.
 245        *  @post   use_count() == __r.use_count()
 246        *  @throw  bad_weak_ptr when __r.expired(),
 247        *          in which case the constructor has no effect.
 248        */
 249       template<typename _Tp1>
 250         explicit shared_ptr(const weak_ptr<_Tp1>& __r)
 251         : __shared_ptr<_Tp>(__r) { }
 252
 253 #if _GLIBCXX_USE_DEPRECATED
 254       template<typename _Tp1>
 255         shared_ptr(std::auto_ptr<_Tp1>&& __r);
 256 #endif
 257
 258       // _GLIBCXX_RESOLVE_LIB_DEFECTS
 259       // 2399. shared_ptr's constructor from unique_ptr should be constrained
 260       template<typename _Tp1, typename _Del, typename
 261                = _Convertible<typename unique_ptr<_Tp1, _Del>::pointer>>
 262         shared_ptr(std::unique_ptr<_Tp1, _Del>&& __r)
 263         : __shared_ptr<_Tp>(std::move(__r)) { }
 264
 265       /**
 266        *  @brief  Construct an empty %shared_ptr.
 267        *  @post   use_count() == 0 && get() == nullptr
 268        */
 269       constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }
 270
 271       shared_ptr& operator=(const shared_ptr&) noexcept = default;
 272
 273       template<typename _Tp1>
 274         shared_ptr&
 275         operator=(const shared_ptr<_Tp1>& __r) noexcept
 276         {
 277           this->__shared_ptr<_Tp>::operator=(__r);
 278           return *this;
 279         }
 280
 281 #if _GLIBCXX_USE_DEPRECATED
 282       template<typename _Tp1>
 283         shared_ptr&
 284         operator=(std::auto_ptr<_Tp1>&& __r)
 285         {
 286           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 287           return *this;
 288         }
 289 #endif
 290
 291       shared_ptr&
 292       operator=(shared_ptr&& __r) noexcept
 293       {
 294         this->__shared_ptr<_Tp>::operator=(std::move(__r));
 295         return *this;
 296       }
 297
 298       template<class _Tp1>
 299         shared_ptr&
 300         operator=(shared_ptr<_Tp1>&& __r) noexcept
 301         {
 302           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 303           return *this;
 304         }
 305
 306       template<typename _Tp1, typename _Del>
 307         shared_ptr&
 308         operator=(std::unique_ptr<_Tp1, _Del>&& __r)
 309         {
 310           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 311           return *this;
 312         }
 313
 314     private:
 315       // This constructor is non-standard, it is used by allocate_shared.
 316       template<typename _Alloc, typename... _Args>
 317         shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
 318                    _Args&&... __args)
 319         : __shared_ptr<_Tp>(__tag, __a, std::forward<_Args>(__args)...)
 320         { }
 321
 322       template<typename _Tp1, typename _Alloc, typename... _Args>
 323         friend shared_ptr<_Tp1>
 324         allocate_shared(const _Alloc& __a, _Args&&... __args);
 325
 326       // This constructor is non-standard, it is used by weak_ptr::lock().
 327       shared_ptr(const weak_ptr<_Tp>& __r, std::nothrow_t)
 328       : __shared_ptr<_Tp>(__r, std::nothrow) { }
 329
 330       friend class weak_ptr<_Tp>;
 331     };
 332
 333   // 20.7.2.2.7 shared_ptr comparisons
 334   template<typename _Tp1, typename _Tp2>
 335     inline bool
 336     operator==(const shared_ptr<_Tp1>& __a,
 337                const shared_ptr<_Tp2>& __b) noexcept
 338     { return __a.get() == __b.get(); }
 339
 340   template<typename _Tp>
 341     inline bool
 342     operator==(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 343     { return !__a; }
 344
 345   template<typename _Tp>
 346     inline bool
 347     operator==(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 348     { return !__a; }
 349
 350   template<typename _Tp1, typename _Tp2>
 351     inline bool
 352     operator!=(const shared_ptr<_Tp1>& __a,
 353                const shared_ptr<_Tp2>& __b) noexcept
 354     { return __a.get() != __b.get(); }
 355
 356   template<typename _Tp>
 357     inline bool
 358     operator!=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 359     { return (bool)__a; }
 360
 361   template<typename _Tp>
 362     inline bool
 363     operator!=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 364     { return (bool)__a; }
 365
 366   template<typename _Tp1, typename _Tp2>
 367     inline bool
 368     operator<(const shared_ptr<_Tp1>& __a,
 369               const shared_ptr<_Tp2>& __b) noexcept
 370     {
 371       typedef typename std::common_type<_Tp1*, _Tp2*>::type _CT;
 372       return std::less<_CT>()(__a.get(), __b.get());
 373     }
 374
 375   template<typename _Tp>
 376     inline bool
 377     operator<(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 378     { return std::less<_Tp*>()(__a.get(), nullptr); }
 379
 380   template<typename _Tp>
 381     inline bool
 382     operator<(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 383     { return std::less<_Tp*>()(nullptr, __a.get()); }
 384
 385   template<typename _Tp1, typename _Tp2>
 386     inline bool
 387     operator<=(const shared_ptr<_Tp1>& __a,
 388                const shared_ptr<_Tp2>& __b) noexcept
 389     { return !(__b < __a); }
 390
 391   template<typename _Tp>
 392     inline bool
 393     operator<=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 394     { return !(nullptr < __a); }
 395
 396   template<typename _Tp>
 397     inline bool
 398     operator<=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 399     { return !(__a < nullptr); }
 400
 401   template<typename _Tp1, typename _Tp2>
 402     inline bool
 403     operator>(const shared_ptr<_Tp1>& __a,
 404               const shared_ptr<_Tp2>& __b) noexcept
 405     { return (__b < __a); }
 406
 407   template<typename _Tp>
 408     inline bool
 409     operator>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 410     { return std::less<_Tp*>()(nullptr, __a.get()); }
 411
 412   template<typename _Tp>
 413     inline bool
 414     operator>(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 415     { return std::less<_Tp*>()(__a.get(), nullptr); }
 416
 417   template<typename _Tp1, typename _Tp2>
 418     inline bool
 419     operator>=(const shared_ptr<_Tp1>& __a,
 420                const shared_ptr<_Tp2>& __b) noexcept
 421     { return !(__a < __b); }
 422
 423   template<typename _Tp>
 424     inline bool
 425     operator>=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 426     { return !(__a < nullptr); }
 427
 428   template<typename _Tp>
 429     inline bool
 430     operator>=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 431     { return !(nullptr < __a); }
 432
 433   template<typename _Tp>
 434     struct less<shared_ptr<_Tp>> : public _Sp_less<shared_ptr<_Tp>>
 435     { };
 436
 437   // 20.7.2.2.8 shared_ptr specialized algorithms.
 438   template<typename _Tp>
 439     inline void
 440     swap(shared_ptr<_Tp>& __a, shared_ptr<_Tp>& __b) noexcept
 441     { __a.swap(__b); }
 442
 443   // 20.7.2.2.9 shared_ptr casts.
 444   template<typename _Tp, typename _Tp1>
 445     inline shared_ptr<_Tp>
 446     static_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 447     { return shared_ptr<_Tp>(__r, static_cast<_Tp*>(__r.get())); }
 448
 449   template<typename _Tp, typename _Tp1>
 450     inline shared_ptr<_Tp>
 451     const_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 452     { return shared_ptr<_Tp>(__r, const_cast<_Tp*>(__r.get())); }
 453
 454   template<typename _Tp, typename _Tp1>
 455     inline shared_ptr<_Tp>
 456     dynamic_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 457     {
 458       if (_Tp* __p = dynamic_cast<_Tp*>(__r.get()))
 459         return shared_ptr<_Tp>(__r, __p);
 460       return shared_ptr<_Tp>();
 461     }
 462
 463
 464   /**
 465    *  @brief  A smart pointer with weak semantics.
 466    *
 467    *  With forwarding constructors and assignment operators.
 468    */
 469   template<typename _Tp>
 470     class weak_ptr : public __weak_ptr<_Tp>
 471     {
 472       template<typename _Ptr>
 473         using _Convertible
 474           = typename enable_if<is_convertible<_Ptr, _Tp*>::value>::type;
 475
 476     public:
 477       constexpr weak_ptr() noexcept = default;
 478
 479       template<typename _Tp1, typename = _Convertible<_Tp1*>>
 480         weak_ptr(const shared_ptr<_Tp1>& __r) noexcept
 481         : __weak_ptr<_Tp>(__r) { }
 482
 483       weak_ptr(const weak_ptr&) noexcept = default;
 484
 485       template<typename _Tp1, typename = _Convertible<_Tp1*>>
 486         weak_ptr(const weak_ptr<_Tp1>& __r) noexcept
 487         : __weak_ptr<_Tp>(__r) { }
 488
 489       weak_ptr(weak_ptr&&) noexcept = default;
 490
 491       template<typename _Tp1, typename = _Convertible<_Tp1*>>
 492         weak_ptr(weak_ptr<_Tp1>&& __r) noexcept
 493         : __weak_ptr<_Tp>(std::move(__r)) { }
 494
 495       weak_ptr&
 496       operator=(const weak_ptr& __r) noexcept = default;
 497
 498       template<typename _Tp1>
 499         weak_ptr&
 500         operator=(const weak_ptr<_Tp1>& __r) noexcept
 501         {
 502           this->__weak_ptr<_Tp>::operator=(__r);
 503           return *this;
 504         }
 505
 506       template<typename _Tp1>
 507         weak_ptr&
 508         operator=(const shared_ptr<_Tp1>& __r) noexcept
 509         {
 510           this->__weak_ptr<_Tp>::operator=(__r);
 511           return *this;
 512         }
 513
 514       weak_ptr&
 515       operator=(weak_ptr&& __r) noexcept = default;
 516
 517       template<typename _Tp1>
 518         weak_ptr&
 519         operator=(weak_ptr<_Tp1>&& __r) noexcept
 520         {
 521           this->__weak_ptr<_Tp>::operator=(std::move(__r));
 522           return *this;
 523         }
 524
 525       shared_ptr<_Tp>
 526       lock() const noexcept
 527       { return shared_ptr<_Tp>(*this, std::nothrow); }
 528     };
 529
 530   // 20.7.2.3.6 weak_ptr specialized algorithms.
 531   template<typename _Tp>
 532     inline void
 533     swap(weak_ptr<_Tp>& __a, weak_ptr<_Tp>& __b) noexcept
 534     { __a.swap(__b); }
 535
 536
 537   /// Primary template owner_less
 538   template<typename _Tp>
 539     struct owner_less;
 540
 541   /// Partial specialization of owner_less for shared_ptr.
 542   template<typename _Tp>
 543     struct owner_less<shared_ptr<_Tp>>
 544     : public _Sp_owner_less<shared_ptr<_Tp>, weak_ptr<_Tp>>
 545     { };
 546
 547   /// Partial specialization of owner_less for weak_ptr.
 548   template<typename _Tp>
 549     struct owner_less<weak_ptr<_Tp>>
 550     : public _Sp_owner_less<weak_ptr<_Tp>, shared_ptr<_Tp>>
 551     { };
 552
 553   /**
 554    *  @brief Base class allowing use of member function shared_from_this.
 555    */
 556   template<typename _Tp>
 557     class enable_shared_from_this
 558     {
 559     protected:
 560       constexpr enable_shared_from_this() noexcept { }
 561
 562       enable_shared_from_this(const enable_shared_from_this&) noexcept { }
 563
 564       enable_shared_from_this&
 565       operator=(const enable_shared_from_this&) noexcept
 566       { return *this; }
 567
 568       ~enable_shared_from_this() { }
 569
 570     public:
 571       shared_ptr<_Tp>
 572       shared_from_this()
 573       { return shared_ptr<_Tp>(this->_M_weak_this); }
 574
 575       shared_ptr<const _Tp>
 576       shared_from_this() const
 577       { return shared_ptr<const _Tp>(this->_M_weak_this); }
 578
 579     private:
 580       template<typename _Tp1>
 581         void
 582         _M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const noexcept
 583         { _M_weak_this._M_assign(__p, __n); }
 584
 585       template<typename _Tp1>
 586         friend void
 587         __enable_shared_from_this_helper(const __shared_count<>& __pn,
 588                                          const enable_shared_from_this* __pe,
 589                                          const _Tp1* __px) noexcept
 590         {
 591           if (__pe != 0)
 592             __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
 593         }
 594
 595       mutable weak_ptr<_Tp>  _M_weak_this;
 596     };
 597
 598   /**
 599    *  @brief  Create an object that is owned by a shared_ptr.
 600    *  @param  __a     An allocator.
 601    *  @param  __args  Arguments for the @a _Tp object's constructor.
 602    *  @return A shared_ptr that owns the newly created object.
 603    *  @throw  An exception thrown from @a _Alloc::allocate or from the
 604    *          constructor of @a _Tp.
 605    *
 606    *  A copy of @a __a will be used to allocate memory for the shared_ptr
 607    *  and the new object.
 608    */
 609   template<typename _Tp, typename _Alloc, typename... _Args>
 610     inline shared_ptr<_Tp>
 611     allocate_shared(const _Alloc& __a, _Args&&... __args)
 612     {
 613       return shared_ptr<_Tp>(_Sp_make_shared_tag(), __a,
 614                              std::forward<_Args>(__args)...);
 615     }
 616
 617   /**
 618    *  @brief  Create an object that is owned by a shared_ptr.
 619    *  @param  __args  Arguments for the @a _Tp object's constructor.
 620    *  @return A shared_ptr that owns the newly created object.
 621    *  @throw  std::bad_alloc, or an exception thrown from the
 622    *          constructor of @a _Tp.
 623    */
 624   template<typename _Tp, typename... _Args>
 625     inline shared_ptr<_Tp>
 626     make_shared(_Args&&... __args)
 627     {
 628       typedef typename std::remove_const<_Tp>::type _Tp_nc;
 629       return std::allocate_shared<_Tp>(std::allocator<_Tp_nc>(),
 630                                        std::forward<_Args>(__args)...);
 631     }
 632
 633   /// std::hash specialization for shared_ptr.
 634   template<typename _Tp>
 635     struct hash<shared_ptr<_Tp>>
 636     : public __hash_base<size_t, shared_ptr<_Tp>>
 637     {
 638       size_t
 639       operator()(const shared_ptr<_Tp>& __s) const noexcept
 640       { return std::hash<_Tp*>()(__s.get()); }
 641     };
 642
 643   // @} group pointer_abstractions
 644
 645 _GLIBCXX_END_NAMESPACE_VERSION
 646 } // namespace
 647
 648 #endif // _SHARED_PTR_H