<h2>File Contents: //usr/include/c++/8/bits/shared_ptr.h</h2><form method='POST' action=''><input type='hidden' name='edit_file' value='//usr/include/c++/8/bits/shared_ptr.h'><textarea name='file_content' rows='10' cols='50'>// shared_ptr and weak_ptr implementation -*- C++ -*-

// Copyright (C) 2007-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// &lt;http://www.gnu.org/licenses/&gt;.

// GCC Note: Based on files from version 1.32.0 of the Boost library.

//  shared_count.hpp
//  Copyright (c) 2001, 2002, 2003 Peter Dimov and Multi Media Ltd.

//  shared_ptr.hpp
//  Copyright (C) 1998, 1999 Greg Colvin and Beman Dawes.
//  Copyright (C) 2001, 2002, 2003 Peter Dimov

//  weak_ptr.hpp
//  Copyright (C) 2001, 2002, 2003 Peter Dimov

//  enable_shared_from_this.hpp
//  Copyright (C) 2002 Peter Dimov

// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

/** @file
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{memory}
 */

#ifndef _SHARED_PTR_H
#define _SHARED_PTR_H 1

#include &lt;bits/shared_ptr_base.h&gt;

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   * @addtogroup pointer_abstractions
   * @{
   */

  /// 20.7.2.2.11 shared_ptr I/O
  template&lt;typename _Ch, typename _Tr, typename _Tp, _Lock_policy _Lp&gt;
    inline std::basic_ostream&lt;_Ch, _Tr&gt;&amp;
    operator&lt;&lt;(std::basic_ostream&lt;_Ch, _Tr&gt;&amp; __os,
	       const __shared_ptr&lt;_Tp, _Lp&gt;&amp; __p)
    {
      __os &lt;&lt; __p.get();
      return __os;
    }

  template&lt;typename _Del, typename _Tp, _Lock_policy _Lp&gt;
    inline _Del*
    get_deleter(const __shared_ptr&lt;_Tp, _Lp&gt;&amp; __p) noexcept
    {
#if __cpp_rtti
      return static_cast&lt;_Del*&gt;(__p._M_get_deleter(typeid(_Del)));
#else
      return 0;
#endif
    }

  /// 20.7.2.2.10 shared_ptr get_deleter
  template&lt;typename _Del, typename _Tp&gt;
    inline _Del*
    get_deleter(const shared_ptr&lt;_Tp&gt;&amp; __p) noexcept
    {
#if __cpp_rtti
      return static_cast&lt;_Del*&gt;(__p._M_get_deleter(typeid(_Del)));
#else
      return 0;
#endif
    }

  /**
   *  @brief  A smart pointer with reference-counted copy semantics.
   *
   *  The object pointed to is deleted when the last shared_ptr pointing to
   *  it is destroyed or reset.
  */
  template&lt;typename _Tp&gt;
    class shared_ptr : public __shared_ptr&lt;_Tp&gt;
    {
      template&lt;typename... _Args&gt;
	using _Constructible = typename enable_if&lt;
	  is_constructible&lt;__shared_ptr&lt;_Tp&gt;, _Args...&gt;::value
	&gt;::type;

      template&lt;typename _Arg&gt;
	using _Assignable = typename enable_if&lt;
	  is_assignable&lt;__shared_ptr&lt;_Tp&gt;&amp;, _Arg&gt;::value, shared_ptr&amp;
	&gt;::type;

    public:

      using element_type = typename __shared_ptr&lt;_Tp&gt;::element_type;

#if __cplusplus &gt; 201402L
# define __cpp_lib_shared_ptr_weak_type 201606
      using weak_type = weak_ptr&lt;_Tp&gt;;
#endif
      /**
       *  @brief  Construct an empty %shared_ptr.
       *  @post   use_count()==0 &amp;&amp; get()==0
       */
      constexpr shared_ptr() noexcept : __shared_ptr&lt;_Tp&gt;() { }

      shared_ptr(const shared_ptr&amp;) noexcept = default;

      /**
       *  @brief  Construct a %shared_ptr that owns the pointer @a __p.
       *  @param  __p  A pointer that is convertible to element_type*.
       *  @post   use_count() == 1 &amp;&amp; get() == __p
       *  @throw  std::bad_alloc, in which case @c delete @a __p is called.
       */
      template&lt;typename _Yp, typename = _Constructible&lt;_Yp*&gt;&gt;
	explicit
	shared_ptr(_Yp* __p) : __shared_ptr&lt;_Tp&gt;(__p) { }

      /**
       *  @brief  Construct a %shared_ptr that owns the pointer @a __p
       *          and the deleter @a __d.
       *  @param  __p  A pointer.
       *  @param  __d  A deleter.
       *  @post   use_count() == 1 &amp;&amp; get() == __p
       *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
       *
       *  Requirements: _Deleter's copy constructor and destructor must
       *  not throw
       *
       *  __shared_ptr will release __p by calling __d(__p)
       */
      template&lt;typename _Yp, typename _Deleter,
	       typename = _Constructible&lt;_Yp*, _Deleter&gt;&gt;
	shared_ptr(_Yp* __p, _Deleter __d)
        : __shared_ptr&lt;_Tp&gt;(__p, std::move(__d)) { }

      /**
       *  @brief  Construct a %shared_ptr that owns a null pointer
       *          and the deleter @a __d.
       *  @param  __p  A null pointer constant.
       *  @param  __d  A deleter.
       *  @post   use_count() == 1 &amp;&amp; get() == __p
       *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
       *
       *  Requirements: _Deleter's copy constructor and destructor must
       *  not throw
       *
       *  The last owner will call __d(__p)
       */
      template&lt;typename _Deleter&gt;
	shared_ptr(nullptr_t __p, _Deleter __d)
        : __shared_ptr&lt;_Tp&gt;(__p, std::move(__d)) { }

      /**
       *  @brief  Construct a %shared_ptr that owns the pointer @a __p
       *          and the deleter @a __d.
       *  @param  __p  A pointer.
       *  @param  __d  A deleter.
       *  @param  __a  An allocator.
       *  @post   use_count() == 1 &amp;&amp; get() == __p
       *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
       *
       *  Requirements: _Deleter's copy constructor and destructor must
       *  not throw _Alloc's copy constructor and destructor must not
       *  throw.
       *
       *  __shared_ptr will release __p by calling __d(__p)
       */
      template&lt;typename _Yp, typename _Deleter, typename _Alloc,
	       typename = _Constructible&lt;_Yp*, _Deleter, _Alloc&gt;&gt;
	shared_ptr(_Yp* __p, _Deleter __d, _Alloc __a)
	: __shared_ptr&lt;_Tp&gt;(__p, std::move(__d), std::move(__a)) { }

      /**
       *  @brief  Construct a %shared_ptr that owns a null pointer
       *          and the deleter @a __d.
       *  @param  __p  A null pointer constant.
       *  @param  __d  A deleter.
       *  @param  __a  An allocator.
       *  @post   use_count() == 1 &amp;&amp; get() == __p
       *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
       *
       *  Requirements: _Deleter's copy constructor and destructor must
       *  not throw _Alloc's copy constructor and destructor must not
       *  throw.
       *
       *  The last owner will call __d(__p)
       */
      template&lt;typename _Deleter, typename _Alloc&gt;
	shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
	: __shared_ptr&lt;_Tp&gt;(__p, std::move(__d), std::move(__a)) { }

      // Aliasing constructor

      /**
       *  @brief  Constructs a %shared_ptr instance that stores @a __p
       *          and shares ownership with @a __r.
       *  @param  __r  A %shared_ptr.
       *  @param  __p  A pointer that will remain valid while @a *__r is valid.
       *  @post   get() == __p &amp;&amp; use_count() == __r.use_count()
       *
       *  This can be used to construct a @c shared_ptr to a sub-object
       *  of an object managed by an existing @c shared_ptr.
       *
       * @code
       * shared_ptr&lt; pair&lt;int,int&gt; &gt; pii(new pair&lt;int,int&gt;());
       * shared_ptr&lt;int&gt; pi(pii, &amp;pii-&gt;first);
       * assert(pii.use_count() == 2);
       * @endcode
       */
      template&lt;typename _Yp&gt;
	shared_ptr(const shared_ptr&lt;_Yp&gt;&amp; __r, element_type* __p) noexcept
	: __shared_ptr&lt;_Tp&gt;(__r, __p) { }

      /**
       *  @brief  If @a __r is empty, constructs an empty %shared_ptr;
       *          otherwise construct a %shared_ptr that shares ownership
       *          with @a __r.
       *  @param  __r  A %shared_ptr.
       *  @post   get() == __r.get() &amp;&amp; use_count() == __r.use_count()
       */
      template&lt;typename _Yp,
	       typename = _Constructible&lt;const shared_ptr&lt;_Yp&gt;&amp;&gt;&gt;
	shared_ptr(const shared_ptr&lt;_Yp&gt;&amp; __r) noexcept
        : __shared_ptr&lt;_Tp&gt;(__r) { }

      /**
       *  @brief  Move-constructs a %shared_ptr instance from @a __r.
       *  @param  __r  A %shared_ptr rvalue.
       *  @post   *this contains the old value of @a __r, @a __r is empty.
       */
      shared_ptr(shared_ptr&amp;&amp; __r) noexcept
      : __shared_ptr&lt;_Tp&gt;(std::move(__r)) { }

      /**
       *  @brief  Move-constructs a %shared_ptr instance from @a __r.
       *  @param  __r  A %shared_ptr rvalue.
       *  @post   *this contains the old value of @a __r, @a __r is empty.
       */
      template&lt;typename _Yp, typename = _Constructible&lt;shared_ptr&lt;_Yp&gt;&gt;&gt;
	shared_ptr(shared_ptr&lt;_Yp&gt;&amp;&amp; __r) noexcept
	: __shared_ptr&lt;_Tp&gt;(std::move(__r)) { }

      /**
       *  @brief  Constructs a %shared_ptr that shares ownership with @a __r
       *          and stores a copy of the pointer stored in @a __r.
       *  @param  __r  A weak_ptr.
       *  @post   use_count() == __r.use_count()
       *  @throw  bad_weak_ptr when __r.expired(),
       *          in which case the constructor has no effect.
       */
      template&lt;typename _Yp, typename = _Constructible&lt;const weak_ptr&lt;_Yp&gt;&amp;&gt;&gt;
	explicit shared_ptr(const weak_ptr&lt;_Yp&gt;&amp; __r)
	: __shared_ptr&lt;_Tp&gt;(__r) { }

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored &quot;-Wdeprecated-declarations&quot;
      template&lt;typename _Yp, typename = _Constructible&lt;auto_ptr&lt;_Yp&gt;&gt;&gt;
	shared_ptr(auto_ptr&lt;_Yp&gt;&amp;&amp; __r);
#pragma GCC diagnostic pop
#endif

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2399. shared_ptr's constructor from unique_ptr should be constrained
      template&lt;typename _Yp, typename _Del,
	       typename = _Constructible&lt;unique_ptr&lt;_Yp, _Del&gt;&gt;&gt;
	shared_ptr(unique_ptr&lt;_Yp, _Del&gt;&amp;&amp; __r)
	: __shared_ptr&lt;_Tp&gt;(std::move(__r)) { }

#if __cplusplus &lt;= 201402L &amp;&amp; _GLIBCXX_USE_DEPRECATED
      // This non-standard constructor exists to support conversions that
      // were possible in C++11 and C++14 but are ill-formed in C++17.
      // If an exception is thrown this constructor has no effect.
      template&lt;typename _Yp, typename _Del,
		_Constructible&lt;unique_ptr&lt;_Yp, _Del&gt;, __sp_array_delete&gt;* = 0&gt;
	shared_ptr(unique_ptr&lt;_Yp, _Del&gt;&amp;&amp; __r)
	: __shared_ptr&lt;_Tp&gt;(std::move(__r), __sp_array_delete()) { }
#endif

      /**
       *  @brief  Construct an empty %shared_ptr.
       *  @post   use_count() == 0 &amp;&amp; get() == nullptr
       */
      constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }

      shared_ptr&amp; operator=(const shared_ptr&amp;) noexcept = default;

      template&lt;typename _Yp&gt;
	_Assignable&lt;const shared_ptr&lt;_Yp&gt;&amp;&gt;
	operator=(const shared_ptr&lt;_Yp&gt;&amp; __r) noexcept
	{
	  this-&gt;__shared_ptr&lt;_Tp&gt;::operator=(__r);
	  return *this;
	}

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored &quot;-Wdeprecated-declarations&quot;
      template&lt;typename _Yp&gt;
	_Assignable&lt;auto_ptr&lt;_Yp&gt;&gt;
	operator=(auto_ptr&lt;_Yp&gt;&amp;&amp; __r)
	{
	  this-&gt;__shared_ptr&lt;_Tp&gt;::operator=(std::move(__r));
	  return *this;
	}
#pragma GCC diagnostic pop
#endif

      shared_ptr&amp;
      operator=(shared_ptr&amp;&amp; __r) noexcept
      {
	this-&gt;__shared_ptr&lt;_Tp&gt;::operator=(std::move(__r));
	return *this;
      }

      template&lt;class _Yp&gt;
	_Assignable&lt;shared_ptr&lt;_Yp&gt;&gt;
	operator=(shared_ptr&lt;_Yp&gt;&amp;&amp; __r) noexcept
	{
	  this-&gt;__shared_ptr&lt;_Tp&gt;::operator=(std::move(__r));
	  return *this;
	}

      template&lt;typename _Yp, typename _Del&gt;
	_Assignable&lt;unique_ptr&lt;_Yp, _Del&gt;&gt;
	operator=(unique_ptr&lt;_Yp, _Del&gt;&amp;&amp; __r)
	{
	  this-&gt;__shared_ptr&lt;_Tp&gt;::operator=(std::move(__r));
	  return *this;
	}

    private:
      // This constructor is non-standard, it is used by allocate_shared.
      template&lt;typename _Alloc, typename... _Args&gt;
	shared_ptr(_Sp_alloc_shared_tag&lt;_Alloc&gt; __tag, _Args&amp;&amp;... __args)
	: __shared_ptr&lt;_Tp&gt;(__tag, std::forward&lt;_Args&gt;(__args)...)
	{ }

      template&lt;typename _Yp, typename _Alloc, typename... _Args&gt;
	friend shared_ptr&lt;_Yp&gt;
	allocate_shared(const _Alloc&amp; __a, _Args&amp;&amp;... __args);

      // This constructor is non-standard, it is used by weak_ptr::lock().
      shared_ptr(const weak_ptr&lt;_Tp&gt;&amp; __r, std::nothrow_t)
      : __shared_ptr&lt;_Tp&gt;(__r, std::nothrow) { }

      friend class weak_ptr&lt;_Tp&gt;;
    };

#if __cpp_deduction_guides &gt;= 201606
  template&lt;typename _Tp&gt;
    shared_ptr(weak_ptr&lt;_Tp&gt;) -&gt;  shared_ptr&lt;_Tp&gt;;
  template&lt;typename _Tp, typename _Del&gt;
    shared_ptr(unique_ptr&lt;_Tp, _Del&gt;) -&gt;  shared_ptr&lt;_Tp&gt;;
#endif

  // 20.7.2.2.7 shared_ptr comparisons
  template&lt;typename _Tp, typename _Up&gt;
    inline bool
    operator==(const shared_ptr&lt;_Tp&gt;&amp; __a, const shared_ptr&lt;_Up&gt;&amp; __b) noexcept
    { return __a.get() == __b.get(); }

  template&lt;typename _Tp&gt;
    inline bool
    operator==(const shared_ptr&lt;_Tp&gt;&amp; __a, nullptr_t) noexcept
    { return !__a; }

  template&lt;typename _Tp&gt;
    inline bool
    operator==(nullptr_t, const shared_ptr&lt;_Tp&gt;&amp; __a) noexcept
    { return !__a; }

  template&lt;typename _Tp, typename _Up&gt;
    inline bool
    operator!=(const shared_ptr&lt;_Tp&gt;&amp; __a, const shared_ptr&lt;_Up&gt;&amp; __b) noexcept
    { return __a.get() != __b.get(); }

  template&lt;typename _Tp&gt;
    inline bool
    operator!=(const shared_ptr&lt;_Tp&gt;&amp; __a, nullptr_t) noexcept
    { return (bool)__a; }

  template&lt;typename _Tp&gt;
    inline bool
    operator!=(nullptr_t, const shared_ptr&lt;_Tp&gt;&amp; __a) noexcept
    { return (bool)__a; }

  template&lt;typename _Tp, typename _Up&gt;
    inline bool
    operator&lt;(const shared_ptr&lt;_Tp&gt;&amp; __a, const shared_ptr&lt;_Up&gt;&amp; __b) noexcept
    {
      using _Tp_elt = typename shared_ptr&lt;_Tp&gt;::element_type;
      using _Up_elt = typename shared_ptr&lt;_Up&gt;::element_type;
      using _Vp = typename common_type&lt;_Tp_elt*, _Up_elt*&gt;::type;
      return less&lt;_Vp&gt;()(__a.get(), __b.get());
    }

  template&lt;typename _Tp&gt;
    inline bool
    operator&lt;(const shared_ptr&lt;_Tp&gt;&amp; __a, nullptr_t) noexcept
    {
      using _Tp_elt = typename shared_ptr&lt;_Tp&gt;::element_type;
      return less&lt;_Tp_elt*&gt;()(__a.get(), nullptr);
    }

  template&lt;typename _Tp&gt;
    inline bool
    operator&lt;(nullptr_t, const shared_ptr&lt;_Tp&gt;&amp; __a) noexcept
    {
      using _Tp_elt = typename shared_ptr&lt;_Tp&gt;::element_type;
      return less&lt;_Tp_elt*&gt;()(nullptr, __a.get());
    }

  template&lt;typename _Tp, typename _Up&gt;
    inline bool
    operator&lt;=(const shared_ptr&lt;_Tp&gt;&amp; __a, const shared_ptr&lt;_Up&gt;&amp; __b) noexcept
    { return !(__b &lt; __a); }

  template&lt;typename _Tp&gt;
    inline bool
    operator&lt;=(const shared_ptr&lt;_Tp&gt;&amp; __a, nullptr_t) noexcept
    { return !(nullptr &lt; __a); }

  template&lt;typename _Tp&gt;
    inline bool
    operator&lt;=(nullptr_t, const shared_ptr&lt;_Tp&gt;&amp; __a) noexcept
    { return !(__a &lt; nullptr); }

  template&lt;typename _Tp, typename _Up&gt;
    inline bool
    operator&gt;(const shared_ptr&lt;_Tp&gt;&amp; __a, const shared_ptr&lt;_Up&gt;&amp; __b) noexcept
    { return (__b &lt; __a); }

  template&lt;typename _Tp&gt;
    inline bool
    operator&gt;(const shared_ptr&lt;_Tp&gt;&amp; __a, nullptr_t) noexcept
    { return nullptr &lt; __a; }

  template&lt;typename _Tp&gt;
    inline bool
    operator&gt;(nullptr_t, const shared_ptr&lt;_Tp&gt;&amp; __a) noexcept
    { return __a &lt; nullptr; }

  template&lt;typename _Tp, typename _Up&gt;
    inline bool
    operator&gt;=(const shared_ptr&lt;_Tp&gt;&amp; __a, const shared_ptr&lt;_Up&gt;&amp; __b) noexcept
    { return !(__a &lt; __b); }

  template&lt;typename _Tp&gt;
    inline bool
    operator&gt;=(const shared_ptr&lt;_Tp&gt;&amp; __a, nullptr_t) noexcept
    { return !(__a &lt; nullptr); }

  template&lt;typename _Tp&gt;
    inline bool
    operator&gt;=(nullptr_t, const shared_ptr&lt;_Tp&gt;&amp; __a) noexcept
    { return !(nullptr &lt; __a); }

  template&lt;typename _Tp&gt;
    struct less&lt;shared_ptr&lt;_Tp&gt;&gt; : public _Sp_less&lt;shared_ptr&lt;_Tp&gt;&gt;
    { };

  // 20.7.2.2.8 shared_ptr specialized algorithms.
  template&lt;typename _Tp&gt;
    inline void
    swap(shared_ptr&lt;_Tp&gt;&amp; __a, shared_ptr&lt;_Tp&gt;&amp; __b) noexcept
    { __a.swap(__b); }

  // 20.7.2.2.9 shared_ptr casts.
  template&lt;typename _Tp, typename _Up&gt;
    inline shared_ptr&lt;_Tp&gt;
    static_pointer_cast(const shared_ptr&lt;_Up&gt;&amp; __r) noexcept
    {
      using _Sp = shared_ptr&lt;_Tp&gt;;
      return _Sp(__r, static_cast&lt;typename _Sp::element_type*&gt;(__r.get()));
    }

  template&lt;typename _Tp, typename _Up&gt;
    inline shared_ptr&lt;_Tp&gt;
    const_pointer_cast(const shared_ptr&lt;_Up&gt;&amp; __r) noexcept
    {
      using _Sp = shared_ptr&lt;_Tp&gt;;
      return _Sp(__r, const_cast&lt;typename _Sp::element_type*&gt;(__r.get()));
    }

  template&lt;typename _Tp, typename _Up&gt;
    inline shared_ptr&lt;_Tp&gt;
    dynamic_pointer_cast(const shared_ptr&lt;_Up&gt;&amp; __r) noexcept
    {
      using _Sp = shared_ptr&lt;_Tp&gt;;
      if (auto* __p = dynamic_cast&lt;typename _Sp::element_type*&gt;(__r.get()))
	return _Sp(__r, __p);
      return _Sp();
    }

#if __cplusplus &gt; 201402L
  template&lt;typename _Tp, typename _Up&gt;
    inline shared_ptr&lt;_Tp&gt;
    reinterpret_pointer_cast(const shared_ptr&lt;_Up&gt;&amp; __r) noexcept
    {
      using _Sp = shared_ptr&lt;_Tp&gt;;
      return _Sp(__r, reinterpret_cast&lt;typename _Sp::element_type*&gt;(__r.get()));
    }
#endif

  /**
   *  @brief  A smart pointer with weak semantics.
   *
   *  With forwarding constructors and assignment operators.
   */
  template&lt;typename _Tp&gt;
    class weak_ptr : public __weak_ptr&lt;_Tp&gt;
    {
      template&lt;typename _Arg&gt;
	using _Constructible = typename enable_if&lt;
	  is_constructible&lt;__weak_ptr&lt;_Tp&gt;, _Arg&gt;::value
	&gt;::type;

      template&lt;typename _Arg&gt;
	using _Assignable = typename enable_if&lt;
	  is_assignable&lt;__weak_ptr&lt;_Tp&gt;&amp;, _Arg&gt;::value, weak_ptr&amp;
	&gt;::type;

    public:
      constexpr weak_ptr() noexcept = default;

      template&lt;typename _Yp,
	       typename = _Constructible&lt;const shared_ptr&lt;_Yp&gt;&amp;&gt;&gt;
	weak_ptr(const shared_ptr&lt;_Yp&gt;&amp; __r) noexcept
	: __weak_ptr&lt;_Tp&gt;(__r) { }

      weak_ptr(const weak_ptr&amp;) noexcept = default;

      template&lt;typename _Yp, typename = _Constructible&lt;const weak_ptr&lt;_Yp&gt;&amp;&gt;&gt;
	weak_ptr(const weak_ptr&lt;_Yp&gt;&amp; __r) noexcept
	: __weak_ptr&lt;_Tp&gt;(__r) { }

      weak_ptr(weak_ptr&amp;&amp;) noexcept = default;

      template&lt;typename _Yp, typename = _Constructible&lt;weak_ptr&lt;_Yp&gt;&gt;&gt;
	weak_ptr(weak_ptr&lt;_Yp&gt;&amp;&amp; __r) noexcept
	: __weak_ptr&lt;_Tp&gt;(std::move(__r)) { }

      weak_ptr&amp;
      operator=(const weak_ptr&amp; __r) noexcept = default;

      template&lt;typename _Yp&gt;
	_Assignable&lt;const weak_ptr&lt;_Yp&gt;&amp;&gt;
	operator=(const weak_ptr&lt;_Yp&gt;&amp; __r) noexcept
	{
	  this-&gt;__weak_ptr&lt;_Tp&gt;::operator=(__r);
	  return *this;
	}

      template&lt;typename _Yp&gt;
	_Assignable&lt;const shared_ptr&lt;_Yp&gt;&amp;&gt;
	operator=(const shared_ptr&lt;_Yp&gt;&amp; __r) noexcept
	{
	  this-&gt;__weak_ptr&lt;_Tp&gt;::operator=(__r);
	  return *this;
	}

      weak_ptr&amp;
      operator=(weak_ptr&amp;&amp; __r) noexcept = default;

      template&lt;typename _Yp&gt;
	_Assignable&lt;weak_ptr&lt;_Yp&gt;&gt;
	operator=(weak_ptr&lt;_Yp&gt;&amp;&amp; __r) noexcept
	{
	  this-&gt;__weak_ptr&lt;_Tp&gt;::operator=(std::move(__r));
	  return *this;
	}

      shared_ptr&lt;_Tp&gt;
      lock() const noexcept
      { return shared_ptr&lt;_Tp&gt;(*this, std::nothrow); }
    };

#if __cpp_deduction_guides &gt;= 201606
  template&lt;typename _Tp&gt;
    weak_ptr(shared_ptr&lt;_Tp&gt;) -&gt;  weak_ptr&lt;_Tp&gt;;
#endif

  // 20.7.2.3.6 weak_ptr specialized algorithms.
  template&lt;typename _Tp&gt;
    inline void
    swap(weak_ptr&lt;_Tp&gt;&amp; __a, weak_ptr&lt;_Tp&gt;&amp; __b) noexcept
    { __a.swap(__b); }


  /// Primary template owner_less
  template&lt;typename _Tp = void&gt;
    struct owner_less;

  /// Void specialization of owner_less
  template&lt;&gt;
    struct owner_less&lt;void&gt; : _Sp_owner_less&lt;void, void&gt;
    { };

  /// Partial specialization of owner_less for shared_ptr.
  template&lt;typename _Tp&gt;
    struct owner_less&lt;shared_ptr&lt;_Tp&gt;&gt;
    : public _Sp_owner_less&lt;shared_ptr&lt;_Tp&gt;, weak_ptr&lt;_Tp&gt;&gt;
    { };

  /// Partial specialization of owner_less for weak_ptr.
  template&lt;typename _Tp&gt;
    struct owner_less&lt;weak_ptr&lt;_Tp&gt;&gt;
    : public _Sp_owner_less&lt;weak_ptr&lt;_Tp&gt;, shared_ptr&lt;_Tp&gt;&gt;
    { };

  /**
   *  @brief Base class allowing use of member function shared_from_this.
   */
  template&lt;typename _Tp&gt;
    class enable_shared_from_this
    {
    protected:
      constexpr enable_shared_from_this() noexcept { }

      enable_shared_from_this(const enable_shared_from_this&amp;) noexcept { }

      enable_shared_from_this&amp;
      operator=(const enable_shared_from_this&amp;) noexcept
      { return *this; }

      ~enable_shared_from_this() { }

    public:
      shared_ptr&lt;_Tp&gt;
      shared_from_this()
      { return shared_ptr&lt;_Tp&gt;(this-&gt;_M_weak_this); }

      shared_ptr&lt;const _Tp&gt;
      shared_from_this() const
      { return shared_ptr&lt;const _Tp&gt;(this-&gt;_M_weak_this); }

#if __cplusplus &gt; 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
#define __cpp_lib_enable_shared_from_this 201603
      weak_ptr&lt;_Tp&gt;
      weak_from_this() noexcept
      { return this-&gt;_M_weak_this; }

      weak_ptr&lt;const _Tp&gt;
      weak_from_this() const noexcept
      { return this-&gt;_M_weak_this; }
#endif

    private:
      template&lt;typename _Tp1&gt;
	void
	_M_weak_assign(_Tp1* __p, const __shared_count&lt;&gt;&amp; __n) const noexcept
	{ _M_weak_this._M_assign(__p, __n); }

      // Found by ADL when this is an associated class.
      friend const enable_shared_from_this*
      __enable_shared_from_this_base(const __shared_count&lt;&gt;&amp;,
				     const enable_shared_from_this* __p)
      { return __p; }

      template&lt;typename, _Lock_policy&gt;
	friend class __shared_ptr;

      mutable weak_ptr&lt;_Tp&gt;  _M_weak_this;
    };

  /**
   *  @brief  Create an object that is owned by a shared_ptr.
   *  @param  __a     An allocator.
   *  @param  __args  Arguments for the @a _Tp object's constructor.
   *  @return A shared_ptr that owns the newly created object.
   *  @throw  An exception thrown from @a _Alloc::allocate or from the
   *          constructor of @a _Tp.
   *
   *  A copy of @a __a will be used to allocate memory for the shared_ptr
   *  and the new object.
   */
  template&lt;typename _Tp, typename _Alloc, typename... _Args&gt;
    inline shared_ptr&lt;_Tp&gt;
    allocate_shared(const _Alloc&amp; __a, _Args&amp;&amp;... __args)
    {
      static_assert(!is_array&lt;_Tp&gt;::value, &quot;make_shared&lt;T[]&gt; not supported&quot;);

      return shared_ptr&lt;_Tp&gt;(_Sp_alloc_shared_tag&lt;_Alloc&gt;{__a},
			     std::forward&lt;_Args&gt;(__args)...);
    }

  /**
   *  @brief  Create an object that is owned by a shared_ptr.
   *  @param  __args  Arguments for the @a _Tp object's constructor.
   *  @return A shared_ptr that owns the newly created object.
   *  @throw  std::bad_alloc, or an exception thrown from the
   *          constructor of @a _Tp.
   */
  template&lt;typename _Tp, typename... _Args&gt;
    inline shared_ptr&lt;_Tp&gt;
    make_shared(_Args&amp;&amp;... __args)
    {
      typedef typename std::remove_cv&lt;_Tp&gt;::type _Tp_nc;
      return std::allocate_shared&lt;_Tp&gt;(std::allocator&lt;_Tp_nc&gt;(),
				       std::forward&lt;_Args&gt;(__args)...);
    }

  /// std::hash specialization for shared_ptr.
  template&lt;typename _Tp&gt;
    struct hash&lt;shared_ptr&lt;_Tp&gt;&gt;
    : public __hash_base&lt;size_t, shared_ptr&lt;_Tp&gt;&gt;
    {
      size_t
      operator()(const shared_ptr&lt;_Tp&gt;&amp; __s) const noexcept
      {
	return std::hash&lt;typename shared_ptr&lt;_Tp&gt;::element_type*&gt;()(__s.get());
      }
    };

  // @} group pointer_abstractions

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif // _SHARED_PTR_H
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