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        *  @post   use_count() == 0 && get() == nullptr
 263        */
 264       constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }
 265
 266       shared_ptr& operator=(const shared_ptr&) noexcept = default;
 267
 268       template<typename _Tp1>
 269         shared_ptr&
 270         operator=(const shared_ptr<_Tp1>& __r) noexcept
 271         {
 272           this->__shared_ptr<_Tp>::operator=(__r);
 273           return *this;
 274         }
 275
 276 #if _GLIBCXX_USE_DEPRECATED
 277       template<typename _Tp1>
 278         shared_ptr&
 279         operator=(std::auto_ptr<_Tp1>&& __r)
 280         {
 281           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 282           return *this;
 283         }
 284 #endif
 285
 286       shared_ptr&
 287       operator=(shared_ptr&& __r) noexcept
 288       {
 289         this->__shared_ptr<_Tp>::operator=(std::move(__r));
 290         return *this;
 291       }
 292
 293       template<class _Tp1>
 294         shared_ptr&
 295         operator=(shared_ptr<_Tp1>&& __r) noexcept
 296         {
 297           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 298           return *this;
 299         }
 300
 301       template<typename _Tp1, typename _Del>
 302         shared_ptr&
 303         operator=(std::unique_ptr<_Tp1, _Del>&& __r)
 304         {
 305           this->__shared_ptr<_Tp>::operator=(std::move(__r));
 306           return *this;
 307         }
 308
 309     private:
 310       // This constructor is non-standard, it is used by allocate_shared.
 311       template<typename _Alloc, typename... _Args>
 312         shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
 313                    _Args&&... __args)
 314         : __shared_ptr<_Tp>(__tag, __a, std::forward<_Args>(__args)...)
 315         { }
 316
 317       template<typename _Tp1, typename _Alloc, typename... _Args>
 318         friend shared_ptr<_Tp1>
 319         allocate_shared(const _Alloc& __a, _Args&&... __args);
 320
 321       // This constructor is non-standard, it is used by weak_ptr::lock().
 322       shared_ptr(const weak_ptr<_Tp>& __r, std::nothrow_t)
 323       : __shared_ptr<_Tp>(__r, std::nothrow) { }
 324
 325       friend class weak_ptr<_Tp>;
 326     };
 327
 328   // 20.7.2.2.7 shared_ptr comparisons
 329   template<typename _Tp1, typename _Tp2>
 330     inline bool
 331     operator==(const shared_ptr<_Tp1>& __a,
 332                const shared_ptr<_Tp2>& __b) noexcept
 333     { return __a.get() == __b.get(); }
 334
 335   template<typename _Tp>
 336     inline bool
 337     operator==(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 338     { return !__a; }
 339
 340   template<typename _Tp>
 341     inline bool
 342     operator==(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 343     { return !__a; }
 344
 345   template<typename _Tp1, typename _Tp2>
 346     inline bool
 347     operator!=(const shared_ptr<_Tp1>& __a,
 348                const shared_ptr<_Tp2>& __b) noexcept
 349     { return __a.get() != __b.get(); }
 350
 351   template<typename _Tp>
 352     inline bool
 353     operator!=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 354     { return (bool)__a; }
 355
 356   template<typename _Tp>
 357     inline bool
 358     operator!=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 359     { return (bool)__a; }
 360
 361   template<typename _Tp1, typename _Tp2>
 362     inline bool
 363     operator<(const shared_ptr<_Tp1>& __a,
 364               const shared_ptr<_Tp2>& __b) noexcept
 365     {
 366       typedef typename std::common_type<_Tp1*, _Tp2*>::type _CT;
 367       return std::less<_CT>()(__a.get(), __b.get());
 368     }
 369
 370   template<typename _Tp>
 371     inline bool
 372     operator<(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 373     { return std::less<_Tp*>()(__a.get(), nullptr); }
 374
 375   template<typename _Tp>
 376     inline bool
 377     operator<(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 378     { return std::less<_Tp*>()(nullptr, __a.get()); }
 379
 380   template<typename _Tp1, typename _Tp2>
 381     inline bool
 382     operator<=(const shared_ptr<_Tp1>& __a,
 383                const shared_ptr<_Tp2>& __b) noexcept
 384     { return !(__b < __a); }
 385
 386   template<typename _Tp>
 387     inline bool
 388     operator<=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 389     { return !(nullptr < __a); }
 390
 391   template<typename _Tp>
 392     inline bool
 393     operator<=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 394     { return !(__a < nullptr); }
 395
 396   template<typename _Tp1, typename _Tp2>
 397     inline bool
 398     operator>(const shared_ptr<_Tp1>& __a,
 399               const shared_ptr<_Tp2>& __b) noexcept
 400     { return (__b < __a); }
 401
 402   template<typename _Tp>
 403     inline bool
 404     operator>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 405     { return std::less<_Tp*>()(nullptr, __a.get()); }
 406
 407   template<typename _Tp>
 408     inline bool
 409     operator>(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 410     { return std::less<_Tp*>()(__a.get(), nullptr); }
 411
 412   template<typename _Tp1, typename _Tp2>
 413     inline bool
 414     operator>=(const shared_ptr<_Tp1>& __a,
 415                const shared_ptr<_Tp2>& __b) noexcept
 416     { return !(__a < __b); }
 417
 418   template<typename _Tp>
 419     inline bool
 420     operator>=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
 421     { return !(__a < nullptr); }
 422
 423   template<typename _Tp>
 424     inline bool
 425     operator>=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
 426     { return !(nullptr < __a); }
 427
 428   template<typename _Tp>
 429     struct less<shared_ptr<_Tp>> : public _Sp_less<shared_ptr<_Tp>>
 430     { };
 431
 432   // 20.7.2.2.8 shared_ptr specialized algorithms.
 433   template<typename _Tp>
 434     inline void
 435     swap(shared_ptr<_Tp>& __a, shared_ptr<_Tp>& __b) noexcept
 436     { __a.swap(__b); }
 437
 438   // 20.7.2.2.9 shared_ptr casts.
 439   template<typename _Tp, typename _Tp1>
 440     inline shared_ptr<_Tp>
 441     static_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 442     { return shared_ptr<_Tp>(__r, static_cast<_Tp*>(__r.get())); }
 443
 444   template<typename _Tp, typename _Tp1>
 445     inline shared_ptr<_Tp>
 446     const_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 447     { return shared_ptr<_Tp>(__r, const_cast<_Tp*>(__r.get())); }
 448
 449   template<typename _Tp, typename _Tp1>
 450     inline shared_ptr<_Tp>
 451     dynamic_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
 452     {
 453       if (_Tp* __p = dynamic_cast<_Tp*>(__r.get()))
 454         return shared_ptr<_Tp>(__r, __p);
 455       return shared_ptr<_Tp>();
 456     }
 457
 458
 459   /**
 460    *  @brief  A smart pointer with weak semantics.
 461    *
 462    *  With forwarding constructors and assignment operators.
 463    */
 464   template<typename _Tp>
 465     class weak_ptr : public __weak_ptr<_Tp>
 466     {
 467     public:
 468       constexpr weak_ptr() noexcept = default;
 469
 470       template<typename _Tp1, typename = typename
 471                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 472         weak_ptr(const shared_ptr<_Tp1>& __r) noexcept
 473         : __weak_ptr<_Tp>(__r) { }
 474
 475       weak_ptr(const weak_ptr&) noexcept = default;
 476
 477       template<typename _Tp1, typename = typename
 478                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 479         weak_ptr(const weak_ptr<_Tp1>& __r) noexcept
 480         : __weak_ptr<_Tp>(__r) { }
 481
 482       weak_ptr(weak_ptr&&) noexcept = default;
 483
 484       template<typename _Tp1, typename = typename
 485                std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
 486         weak_ptr(weak_ptr<_Tp1>&& __r) noexcept
 487         : __weak_ptr<_Tp>(std::move(__r)) { }
 488
 489       weak_ptr&
 490       operator=(const weak_ptr& __r) noexcept = default;
 491
 492       template<typename _Tp1>
 493         weak_ptr&
 494         operator=(const weak_ptr<_Tp1>& __r) noexcept
 495         {
 496           this->__weak_ptr<_Tp>::operator=(__r);
 497           return *this;
 498         }
 499
 500       template<typename _Tp1>
 501         weak_ptr&
 502         operator=(const shared_ptr<_Tp1>& __r) noexcept
 503         {
 504           this->__weak_ptr<_Tp>::operator=(__r);
 505           return *this;
 506         }
 507
 508       weak_ptr&
 509       operator=(weak_ptr&& __r) noexcept = default;
 510
 511       template<typename _Tp1>
 512         weak_ptr&
 513         operator=(weak_ptr<_Tp1>&& __r) noexcept
 514         {
 515           this->__weak_ptr<_Tp>::operator=(std::move(__r));
 516           return *this;
 517         }
 518
 519       shared_ptr<_Tp>
 520       lock() const noexcept
 521       { return shared_ptr<_Tp>(*this, std::nothrow); }
 522     };
 523
 524   // 20.7.2.3.6 weak_ptr specialized algorithms.
 525   template<typename _Tp>
 526     inline void
 527     swap(weak_ptr<_Tp>& __a, weak_ptr<_Tp>& __b) noexcept
 528     { __a.swap(__b); }
 529
 530
 531   /// Primary template owner_less
 532   template<typename _Tp>
 533     struct owner_less;
 534
 535   /// Partial specialization of owner_less for shared_ptr.
 536   template<typename _Tp>
 537     struct owner_less<shared_ptr<_Tp>>
 538     : public _Sp_owner_less<shared_ptr<_Tp>, weak_ptr<_Tp>>
 539     { };
 540
 541   /// Partial specialization of owner_less for weak_ptr.
 542   template<typename _Tp>
 543     struct owner_less<weak_ptr<_Tp>>
 544     : public _Sp_owner_less<weak_ptr<_Tp>, shared_ptr<_Tp>>
 545     { };
 546
 547   /**
 548    *  @brief Base class allowing use of member function shared_from_this.
 549    */
 550   template<typename _Tp>
 551     class enable_shared_from_this
 552     {
 553     protected:
 554       constexpr enable_shared_from_this() noexcept { }
 555
 556       enable_shared_from_this(const enable_shared_from_this&) noexcept { }
 557
 558       enable_shared_from_this&
 559       operator=(const enable_shared_from_this&) noexcept
 560       { return *this; }
 561
 562       ~enable_shared_from_this() { }
 563
 564     public:
 565       shared_ptr<_Tp>
 566       shared_from_this()
 567       { return shared_ptr<_Tp>(this->_M_weak_this); }
 568
 569       shared_ptr<const _Tp>
 570       shared_from_this() const
 571       { return shared_ptr<const _Tp>(this->_M_weak_this); }
 572
 573     private:
 574       template<typename _Tp1>
 575         void
 576         _M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const noexcept
 577         { _M_weak_this._M_assign(__p, __n); }
 578
 579       template<typename _Tp1>
 580         friend void
 581         __enable_shared_from_this_helper(const __shared_count<>& __pn,
 582                                          const enable_shared_from_this* __pe,
 583                                          const _Tp1* __px) noexcept
 584         {
 585           if (__pe != 0)
 586             __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
 587         }
 588
 589       mutable weak_ptr<_Tp>  _M_weak_this;
 590     };
 591
 592   /**
 593    *  @brief  Create an object that is owned by a shared_ptr.
 594    *  @param  __a     An allocator.
 595    *  @param  __args  Arguments for the @a _Tp object's constructor.
 596    *  @return A shared_ptr that owns the newly created object.
 597    *  @throw  An exception thrown from @a _Alloc::allocate or from the
 598    *          constructor of @a _Tp.
 599    *
 600    *  A copy of @a __a will be used to allocate memory for the shared_ptr
 601    *  and the new object.
 602    */
 603   template<typename _Tp, typename _Alloc, typename... _Args>
 604     inline shared_ptr<_Tp>
 605     allocate_shared(const _Alloc& __a, _Args&&... __args)
 606     {
 607       return shared_ptr<_Tp>(_Sp_make_shared_tag(), __a,
 608                              std::forward<_Args>(__args)...);
 609     }
 610
 611   /**
 612    *  @brief  Create an object that is owned by a shared_ptr.
 613    *  @param  __args  Arguments for the @a _Tp object's constructor.
 614    *  @return A shared_ptr that owns the newly created object.
 615    *  @throw  std::bad_alloc, or an exception thrown from the
 616    *          constructor of @a _Tp.
 617    */
 618   template<typename _Tp, typename... _Args>
 619     inline shared_ptr<_Tp>
 620     make_shared(_Args&&... __args)
 621     {
 622       typedef typename std::remove_const<_Tp>::type _Tp_nc;
 623       return std::allocate_shared<_Tp>(std::allocator<_Tp_nc>(),
 624                                        std::forward<_Args>(__args)...);
 625     }
 626
 627   /// std::hash specialization for shared_ptr.
 628   template<typename _Tp>
 629     struct hash<shared_ptr<_Tp>>
 630     : public __hash_base<size_t, shared_ptr<_Tp>>
 631     {
 632       size_t
 633       operator()(const shared_ptr<_Tp>& __s) const noexcept
 634       { return std::hash<_Tp*>()(__s.get()); }
 635     };
 636
 637   // @} group pointer_abstractions
 638
 639 _GLIBCXX_END_NAMESPACE_VERSION
 640 } // namespace
 641
 642 #endif // _SHARED_PTR_H