shared_ptr_base.h

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// shared_ptr and weak_ptr implementation details -*- C++ -*-
 
// Copyright (C) 2007-2026 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
// <http://www.gnu.org/licenses/>.
 
// 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 bits/shared_ptr_base.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{memory}
 */
 
#ifndef _SHARED_PTR_BASE_H
#define _SHARED_PTR_BASE_H 1
 
#include <typeinfo>
#include <bits/allocated_ptr.h>
#include <bits/allocator.h>
#include <bits/exception_defines.h>
#include <bits/functional_hash.h>
#include <bits/refwrap.h>
#include <bits/stl_function.h>  // std::less
#include <bits/unique_ptr.h>
#include <ext/aligned_buffer.h>
#include <ext/atomicity.h>
#include <ext/concurrence.h>
#if __cplusplus >= 202002L
# include <compare>
# include <bits/align.h> // std::align
# include <bits/stl_uninitialized.h>
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
  template<typename> class auto_ptr;
#pragma GCC diagnostic pop
#endif
 
 /**
   *  @brief  Exception possibly thrown by @c shared_ptr.
   *  @ingroup exceptions
   */
  class bad_weak_ptr : public std::exception
  {
  public:
    virtual char const* what() const noexcept;
 
    virtual ~bad_weak_ptr() noexcept;
  };
 
  // Substitute for bad_weak_ptr object in the case of -fno-exceptions.
  inline void
  __throw_bad_weak_ptr()
  { _GLIBCXX_THROW_OR_ABORT(bad_weak_ptr()); }
 
  using __gnu_cxx::_Lock_policy;
  using __gnu_cxx::__default_lock_policy;
  using __gnu_cxx::_S_single;
  using __gnu_cxx::_S_mutex;
  using __gnu_cxx::_S_atomic;
 
  // Empty helper class except when the template argument is _S_mutex.
  template<_Lock_policy _Lp>
    class _Mutex_base
    {
    protected:
      // The atomic policy uses fully-fenced builtins, single doesn't care.
      enum { _S_need_barriers = 0 };
    };
 
  template<>
    class _Mutex_base<_S_mutex>
    : public __gnu_cxx::__mutex
    {
    protected:
      // This policy is used when atomic builtins are not available.
      // The replacement atomic operations might not have the necessary
      // memory barriers.
      enum { _S_need_barriers = 1 };
    };
 
  template<_Lock_policy _Lp = __default_lock_policy>
    class _Sp_counted_base
    : public _Mutex_base<_Lp>
    {
    public:
      _Sp_counted_base() noexcept
      : _M_use_count(1), _M_weak_count(1) { }
 
      virtual
      ~_Sp_counted_base() noexcept
      { }
 
      // Called when _M_use_count drops to zero, to release the resources
      // managed by *this.
      virtual void
      _M_dispose() noexcept = 0;
 
      // Called when _M_weak_count drops to zero.
      virtual void
      _M_destroy() noexcept
      { delete this; }
 
      virtual void*
      _M_get_deleter(const std::type_info&) noexcept = 0;
 
      // Increment the use count (used when the count is greater than zero).
      void
      _M_add_ref_copy()
      { _S_chk(__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1)); }
 
      // Increment the use count if it is non-zero, throw otherwise.
      void
      _M_add_ref_lock()
      {
        if (!_M_add_ref_lock_nothrow())
          __throw_bad_weak_ptr();
      }
 
      // Increment the use count if it is non-zero.
      bool
      _M_add_ref_lock_nothrow() noexcept;
 
      // Decrement the use count.
      void
      _M_release() noexcept;
 
      // Called by _M_release() when the use count reaches zero.
      void
      _M_release_last_use() noexcept
      {
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
        _M_dispose();
        // There must be a memory barrier between dispose() and destroy()
        // to ensure that the effects of dispose() are observed in the
        // thread that runs destroy().
        // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html
        if (_Mutex_base<_Lp>::_S_need_barriers)
          {
            __atomic_thread_fence (__ATOMIC_ACQ_REL);
          }
 
        // Be race-detector-friendly.  For more info see bits/c++config.
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
        if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count,
                                                   -1) == 1)
          {
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
            _M_destroy();
          }
      }
 
      // As above, but 'noinline' to reduce code size on the cold path.
      __attribute__((__noinline__))
      void
      _M_release_last_use_cold() noexcept
      { _M_release_last_use(); }
 
      // Increment the weak count.
      void
      _M_weak_add_ref() noexcept
      {
        // _M_weak_count can always use negative values because it cannot be
        // observed by users (unlike _M_use_count). See _S_chk for details.
        constexpr _Atomic_word __max = -1;
        if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, 1) == __max)
          [[__unlikely__]] __builtin_trap();
      }
 
      // Decrement the weak count.
      void
      _M_weak_release() noexcept
      {
        // Be race-detector-friendly. For more info see bits/c++config.
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
        if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
          {
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
            if (_Mutex_base<_Lp>::_S_need_barriers)
              {
                // See _M_release(),
                // destroy() must observe results of dispose()
                __atomic_thread_fence (__ATOMIC_ACQ_REL);
              }
            _M_destroy();
          }
      }
 
      long
      _M_get_use_count() const noexcept
      {
        // No memory barrier is used here so there is no synchronization
        // with other threads.
        auto __count = __atomic_load_n(&_M_use_count, __ATOMIC_RELAXED);
 
        // If long is wider than _Atomic_word then we can treat _Atomic_word
        // as unsigned, and so double its usable range. If the widths are the
        // same then casting to unsigned and then to long is a no-op.
        return static_cast<_Unsigned_count_type>(__count);
      }
 
    private:
      _Sp_counted_base(_Sp_counted_base const&) = delete;
      _Sp_counted_base& operator=(_Sp_counted_base const&) = delete;
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wignored-attributes"
      // This is only to be used for arithmetic, not for atomic ops.
      using _Unsigned_count_type = make_unsigned<_Atomic_word>::type;
#pragma GCC diagnostic pop
 
      // Called when incrementing _M_use_count to cause a trap on overflow.
      // This should be passed the value of the counter before the increment.
      static void
      _S_chk(_Atomic_word __count)
      {
        constexpr _Atomic_word __max_atomic_word = _Unsigned_count_type(-1)/2;
 
        // __max is the maximum allowed value for the shared reference count.
        // All valid reference count values need to fit into [0,LONG_MAX)
        // because users can observe the count via shared_ptr::use_count().
        //
        // When long is wider than _Atomic_word, _M_use_count can go negative
        // and the cast in _Sp_counted_base::use_count() will turn it into a
        // positive value suitable for returning to users. The implementation
        // only cares whether _M_use_count reaches zero after a decrement,
        // so negative values are not a problem internally.
        // So when possible, use -1 for __max (incrementing past that would
        // overflow _M_use_count to 0, which means an empty shared_ptr).
        //
        // When long is not wider than _Atomic_word, __max is just the type's
        // maximum positive value. We cannot use negative counts because they
        // would not fit in [0,LONG_MAX) after casting to an unsigned type,
        // which would cause use_count() to return bogus values.
        constexpr _Atomic_word __max
          = sizeof(long) > sizeof(_Atomic_word) ? -1 : __max_atomic_word;
 
        if (__count == __max) [[__unlikely__]]
          __builtin_trap();
      }
 
      _Atomic_word  _M_use_count;     // #shared
      _Atomic_word  _M_weak_count;    // #weak + (#shared != 0)
    };
 
  // We use __atomic_add_single and __exchange_and_add_single in the _S_single
  // member specializations because they use unsigned arithmetic and so avoid
  // undefined overflow.
  template<>
    inline void
    _Sp_counted_base<_S_single>::_M_add_ref_copy()
    {
      _S_chk(_M_use_count);
      __gnu_cxx::__atomic_add_single(&_M_use_count, 1);
    }
 
  template<>
    inline void
    _Sp_counted_base<_S_single>::_M_weak_release() noexcept
    {
      if (__gnu_cxx::__exchange_and_add_single(&_M_weak_count, -1) == 1)
        _M_destroy();
    }
 
  template<>
    inline long
    _Sp_counted_base<_S_single>::_M_get_use_count() const noexcept
    {
      return static_cast<_Unsigned_count_type>(_M_use_count);
    }
 
 
  template<>
    inline bool
    _Sp_counted_base<_S_single>::
    _M_add_ref_lock_nothrow() noexcept
    {
      if (_M_use_count == 0)
        return false;
      _M_add_ref_copy();
      return true;
    }
 
  template<>
    inline bool
    _Sp_counted_base<_S_mutex>::
    _M_add_ref_lock_nothrow() noexcept
    {
      __gnu_cxx::__scoped_lock sentry(*this);
      if (auto __c = __gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1))
        _S_chk(__c);
      else
        {
          // Count was zero, so we cannot lock it to get a shared_ptr.
          // Reset to zero. This isn't racy, because there are no shared_ptr
          // objects using this count and any other weak_ptr objects using it
          // must call this function to modify _M_use_count, so would be
          // synchronized by the mutex.
          _M_use_count = 0;
          return false;
        }
      return true;
    }
 
  template<>
    inline bool
    _Sp_counted_base<_S_atomic>::
    _M_add_ref_lock_nothrow() noexcept
    {
      // Perform lock-free add-if-not-zero operation.
      _Atomic_word __count = _M_get_use_count();
      do
        {
          if (__count == 0)
            return false;
          // Replace the current counter value with the old value + 1, as
          // long as it's not changed meanwhile.
        }
      while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
                                          true, __ATOMIC_ACQ_REL,
                                          __ATOMIC_RELAXED));
      _S_chk(__count);
      return true;
    }
 
  template<>
    inline void
    _Sp_counted_base<_S_single>::_M_release() noexcept
    {
      if (__gnu_cxx::__exchange_and_add_single(&_M_use_count, -1) == 1)
        {
          _M_dispose();
          _M_weak_release();
        }
    }
 
  template<>
    inline void
    _Sp_counted_base<_S_mutex>::_M_release() noexcept
    {
      // Be race-detector-friendly.  For more info see bits/c++config.
      _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
        {
          _M_release_last_use();
        }
    }
 
  template<>
    inline void
    _Sp_counted_base<_S_atomic>::_M_release() noexcept
    {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
      _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
#if ! _GLIBCXX_TSAN
      constexpr bool __lock_free
        = __atomic_always_lock_free(sizeof(long long), 0)
        && __atomic_always_lock_free(sizeof(_Atomic_word), 0);
      constexpr bool __double_word
        = sizeof(long long) == 2 * sizeof(_Atomic_word);
      // The ref-count members follow the vptr, so are aligned to
      // alignof(void*).
      constexpr bool __aligned = __alignof(long long) <= alignof(void*);
      if constexpr (__lock_free && __double_word && __aligned)
        {
          constexpr int __wordbits = __CHAR_BIT__ * sizeof(_Atomic_word);
          constexpr int __shiftbits = __double_word ? __wordbits : 0;
          constexpr long long __unique_ref = 1LL + (1LL << __shiftbits);
          auto __both_counts = reinterpret_cast<long long*>(&_M_use_count);
 
          _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
          if (__atomic_load_n(__both_counts, __ATOMIC_ACQUIRE) == __unique_ref)
            {
              // Both counts are 1, so there are no weak references and
              // we are releasing the last strong reference. No other
              // threads can observe the effects of this _M_release()
              // call (e.g. calling use_count()) without a data race.
              _M_weak_count = _M_use_count = 0;
              _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
              _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
              _M_dispose();
              _M_destroy();
              return;
            }
          if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
            [[__unlikely__]]
            {
              _M_release_last_use_cold();
              return;
            }
        }
      else
#endif
      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
        {
          _M_release_last_use();
        }
#pragma GCC diagnostic pop
    }
 
  // Forward declarations.
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __shared_ptr;
 
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __weak_ptr;
 
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __enable_shared_from_this;
 
  template<typename _Tp>
    class shared_ptr;
 
  template<typename _Tp>
    class weak_ptr;
 
  template<typename _Tp>
    struct owner_less;
 
  template<typename _Tp>
    class enable_shared_from_this;
 
  template<_Lock_policy _Lp = __default_lock_policy>
    class __weak_count;
 
  template<_Lock_policy _Lp = __default_lock_policy>
    class __shared_count;
 
#ifdef __glibcxx_atomic_shared_ptr
  template<typename>
    class _Sp_atomic;
#endif
 
  // Counted ptr with no deleter or allocator support
  template<typename _Ptr, _Lock_policy _Lp>
    class _Sp_counted_ptr final : public _Sp_counted_base<_Lp>
    {
    public:
      explicit
      _Sp_counted_ptr(_Ptr __p) noexcept
      : _M_ptr(__p) { }
 
      virtual void
      _M_dispose() noexcept
      { delete _M_ptr; }
 
      virtual void
      _M_destroy() noexcept
      { delete this; }
 
      virtual void*
      _M_get_deleter(const std::type_info&) noexcept
      { return nullptr; }
 
      _Sp_counted_ptr(const _Sp_counted_ptr&) = delete;
      _Sp_counted_ptr& operator=(const _Sp_counted_ptr&) = delete;
 
    private:
      _Ptr             _M_ptr;
    };
 
  template<>
    inline void
    _Sp_counted_ptr<nullptr_t, _S_single>::_M_dispose() noexcept { }
 
  template<>
    inline void
    _Sp_counted_ptr<nullptr_t, _S_mutex>::_M_dispose() noexcept { }
 
  template<>
    inline void
    _Sp_counted_ptr<nullptr_t, _S_atomic>::_M_dispose() noexcept { }
 
  // FIXME: once __has_cpp_attribute(__no_unique_address__)) is true for
  // all supported compilers we can greatly simplify _Sp_ebo_helper.
  // N.B. unconditionally applying the attribute could change layout for
  // final types, which currently cannot use EBO so have a unique address.
 
  template<int _Nm, typename _Tp,
           bool __use_ebo = !__is_final(_Tp) && __is_empty(_Tp)>
    struct _Sp_ebo_helper;
 
  /// Specialization using EBO.
  template<int _Nm, typename _Tp>
    struct _Sp_ebo_helper<_Nm, _Tp, true> : private _Tp
    {
      explicit _Sp_ebo_helper(const _Tp& __tp) : _Tp(__tp) { }
      explicit _Sp_ebo_helper(_Tp&& __tp) : _Tp(std::move(__tp)) { }
 
      static _Tp&
      _S_get(_Sp_ebo_helper& __eboh) { return static_cast<_Tp&>(__eboh); }
    };
 
  /// Specialization not using EBO.
  template<int _Nm, typename _Tp>
    struct _Sp_ebo_helper<_Nm, _Tp, false>
    {
      explicit _Sp_ebo_helper(const _Tp& __tp) : _M_tp(__tp) { }
      explicit _Sp_ebo_helper(_Tp&& __tp) : _M_tp(std::move(__tp)) { }
 
      static _Tp&
      _S_get(_Sp_ebo_helper& __eboh)
      { return __eboh._M_tp; }
 
    private:
      _Tp _M_tp;
    };
 
  // Support for custom deleter and/or allocator
  template<typename _Ptr, typename _Deleter, typename _Alloc, _Lock_policy _Lp>
    class _Sp_counted_deleter final : public _Sp_counted_base<_Lp>
    {
      class _Impl : _Sp_ebo_helper<0, _Deleter>, _Sp_ebo_helper<1, _Alloc>
      {
        typedef _Sp_ebo_helper<0, _Deleter>     _Del_base;
        typedef _Sp_ebo_helper<1, _Alloc>       _Alloc_base;
 
      public:
        _Impl(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept
        : _Del_base(std::move(__d)), _Alloc_base(__a), _M_ptr(__p)
        { }
 
        _Deleter& _M_del() noexcept { return _Del_base::_S_get(*this); }
        _Alloc& _M_alloc() noexcept { return _Alloc_base::_S_get(*this); }
 
        _Ptr _M_ptr;
      };
 
    public:
      using __allocator_type = __alloc_rebind<_Alloc, _Sp_counted_deleter>;
 
      // __d(__p) must not throw.
      _Sp_counted_deleter(_Ptr __p, _Deleter __d) noexcept
      : _M_impl(__p, std::move(__d), _Alloc()) { }
 
      // __d(__p) must not throw.
      _Sp_counted_deleter(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept
      : _M_impl(__p, std::move(__d), __a) { }
 
#pragma GCC diagnostic push // PR tree-optimization/122197
#pragma GCC diagnostic ignored "-Wfree-nonheap-object"
#pragma GCC diagnostic ignored "-Warray-bounds"
  template<typename> class auto_ptr;
      ~_Sp_counted_deleter() noexcept { }
 
      virtual void
      _M_dispose() noexcept
      { _M_impl._M_del()(_M_impl._M_ptr); }
#pragma GCC diagnostic pop
 
      virtual void
      _M_destroy() noexcept
      {
        __allocator_type __a(_M_impl._M_alloc());
        __allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
        this->~_Sp_counted_deleter();
      }
 
      virtual void*
      _M_get_deleter(const type_info& __ti [[__gnu__::__unused__]]) noexcept
      {
#if __cpp_rtti
        // _GLIBCXX_RESOLVE_LIB_DEFECTS
        // 2400. shared_ptr's get_deleter() should use addressof()
        return __ti == typeid(_Deleter)
          ? std::__addressof(_M_impl._M_del())
          : nullptr;
#else
        return nullptr;
#endif
      }
 
    private:
#ifdef __glibcxx_out_ptr
      template<typename, typename, typename...> friend class out_ptr_t;
#endif
      _Impl _M_impl;
    };
 
  // helpers for make_shared / allocate_shared
 
  struct _Sp_make_shared_tag
  {
  private:
    template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
      friend class _Sp_counted_ptr_inplace;
 
    static const type_info&
    _S_ti() noexcept _GLIBCXX_VISIBILITY(default)
    {
      alignas(type_info) static constexpr char __tag[sizeof(type_info)] = { };
      return reinterpret_cast<const type_info&>(__tag);
    }
 
    static bool _S_eq(const type_info&) noexcept;
  };
 
  template<typename _Alloc>
    struct _Sp_alloc_shared_tag
    {
      const _Alloc& _M_a;
    };
 
  template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
    class _Sp_counted_ptr_inplace final : public _Sp_counted_base<_Lp>
    {
      class _Impl : _Sp_ebo_helper<0, _Alloc>
      {
        typedef _Sp_ebo_helper<0, _Alloc>       _A_base;
 
      public:
        explicit _Impl(_Alloc __a) noexcept : _A_base(__a) { }
 
        _Alloc& _M_alloc() noexcept { return _A_base::_S_get(*this); }
 
        __gnu_cxx::__aligned_buffer<__remove_cv_t<_Tp>> _M_storage;
      };
 
    public:
      using __allocator_type = __alloc_rebind<_Alloc, _Sp_counted_ptr_inplace>;
 
      // Alloc parameter is not a reference so doesn't alias anything in __args
      template<typename... _Args>
        _Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
        : _M_impl(__a)
        {
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 2070.  allocate_shared should use allocator_traits<A>::construct
          allocator_traits<_Alloc>::construct(__a, _M_ptr(),
              std::forward<_Args>(__args)...); // might throw
        }
 
#pragma GCC diagnostic push // PR tree-optimization/122197
#pragma GCC diagnostic ignored "-Warray-bounds"
      ~_Sp_counted_ptr_inplace() noexcept { }
 
      virtual void
      _M_dispose() noexcept
      {
        allocator_traits<_Alloc>::destroy(_M_impl._M_alloc(), _M_ptr());
      }
#pragma GCC diagnostic pop
 
      // Override because the allocator needs to know the dynamic type
      virtual void
      _M_destroy() noexcept
      {
        __allocator_type __a(_M_impl._M_alloc());
        __allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
        this->~_Sp_counted_ptr_inplace();
      }
 
    private:
      friend class __shared_count<_Lp>; // To be able to call _M_ptr().
 
      // No longer used, but code compiled against old libstdc++ headers
      // might still call it from __shared_ptr ctor to get the pointer out.
      virtual void*
      _M_get_deleter(const std::type_info& __ti) noexcept override
      {
        // Check for the fake type_info first, so we don't try to access it
        // as a real type_info object. Otherwise, check if it's the real
        // type_info for this class. With RTTI enabled we can check directly,
        // or call a library function to do it.
        if (&__ti == &_Sp_make_shared_tag::_S_ti()
            ||
#if __cpp_rtti
            __ti == typeid(_Sp_make_shared_tag)
#else
            _Sp_make_shared_tag::_S_eq(__ti)
#endif
           )
          return _M_ptr();
        return nullptr;
      }
 
      __remove_cv_t<_Tp>*
      _M_ptr() noexcept { return _M_impl._M_storage._M_ptr(); }
 
      _Impl _M_impl;
    };
 
#ifdef __glibcxx_smart_ptr_for_overwrite // C++ >= 20 && HOSTED
  struct _Sp_overwrite_tag { };
 
  // Partial specialization used for make_shared_for_overwrite<non-array>().
  // This partial specialization is used when the allocator's value type
  // is the special _Sp_overwrite_tag type.
#if __cpp_concepts
  template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
    requires is_same_v<typename _Alloc::value_type, _Sp_overwrite_tag>
    class _Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp> final
#else
  template<typename _Tp, template<typename> class _Alloc, _Lock_policy _Lp>
    class _Sp_counted_ptr_inplace<_Tp, _Alloc<_Sp_overwrite_tag>, _Lp> final
#endif
    : public _Sp_counted_base<_Lp>
    {
      [[no_unique_address]] _Alloc _M_alloc;
 
      union {
        remove_cv_t<_Tp> _M_obj;
        char _M_unused;
      };
 
      friend class __shared_count<_Lp>; // To be able to call _M_ptr().
 
      auto _M_ptr() noexcept { return std::__addressof(_M_obj); }
 
    public:
      using __allocator_type = __alloc_rebind<_Alloc, _Sp_counted_ptr_inplace>;
 
      _Sp_counted_ptr_inplace(const _Alloc& __a)
      : _M_alloc(__a)
      {
        ::new((void*)_M_ptr()) _Tp; // default-initialized, for overwrite.
      }
 
      ~_Sp_counted_ptr_inplace() noexcept { }
 
      virtual void
      _M_dispose() noexcept
      {
        _M_obj.~_Tp();
      }
 
      // Override because the allocator needs to know the dynamic type
      virtual void
      _M_destroy() noexcept
      {
        using pointer = typename allocator_traits<__allocator_type>::pointer;
        __allocator_type __a(_M_alloc);
        auto __p = pointer_traits<pointer>::pointer_to(*this);
        __allocated_ptr<__allocator_type> __guard_ptr{ __a, __p };
        this->~_Sp_counted_ptr_inplace();
      }
 
      void*
      _M_get_deleter(const std::type_info&) noexcept override
      { return nullptr; }
    };
#endif // __glibcxx_smart_ptr_for_overwrite
 
#if __glibcxx_shared_ptr_arrays >= 201707L // C++ >= 20 && HOSTED
  struct _Sp_overwrite_tag;
 
  // For make_shared<T[]>, make_shared<T[N]>, allocate_shared<T[]> etc.
  template<typename _Alloc>
    struct _Sp_counted_array_base
    {
      [[no_unique_address]] _Alloc _M_alloc{};
      size_t _M_n = 0;
      bool _M_overwrite = false;
 
      typename allocator_traits<_Alloc>::pointer
      _M_alloc_array(size_t __tail)
      {
        return allocator_traits<_Alloc>::allocate(_M_alloc, _M_n + __tail);
      }
 
      void
      _M_dealloc_array(typename allocator_traits<_Alloc>::pointer __p,
                       size_t __tail)
      {
        allocator_traits<_Alloc>::deallocate(_M_alloc, __p, _M_n + __tail);
      }
 
      // Init the array elements
      template<typename _Init>
        void
        _M_init(typename allocator_traits<_Alloc>::value_type* __p,
                _Init __init)
        {
          using _Tp = remove_pointer_t<_Init>;
          using _Up = typename allocator_traits<_Alloc>::value_type;
 
          if constexpr (is_same_v<_Init, _Sp_overwrite_tag>)
            {
              std::uninitialized_default_construct_n(__p, _M_n);
              _M_overwrite = true;
            }
          else if (__init == nullptr)
            std::__uninitialized_default_n_a(__p, _M_n, _M_alloc);
          else if constexpr (!is_array_v<_Tp>)
            std::__uninitialized_fill_n_a(__p, _M_n, *__init, _M_alloc);
          else
            {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-local-typedefs"
              struct _Iter
              {
                using value_type = _Up;
                using difference_type = ptrdiff_t;
                using pointer = const _Up*;
                using reference = const _Up&;
                using iterator_category = forward_iterator_tag;
 
                const _Up* _M_p;
                size_t _M_len;
                size_t _M_pos;
 
                _Iter& operator++() { ++_M_pos; return *this; }
                _Iter operator++(int) { auto __i(*this); ++_M_pos; return __i; }
 
                reference operator*() const { return _M_p[_M_pos % _M_len]; }
                pointer operator->() const { return _M_p + (_M_pos % _M_len); }
 
                bool operator==(const _Iter& __i) const
                { return _M_pos == __i._M_pos; }
              };
#pragma GCC diagnostic pop
 
              _Iter __first{_S_first_elem(__init), sizeof(_Tp) / sizeof(_Up)};
              _Iter __last = __first;
              __last._M_pos = _M_n;
              std::__uninitialized_copy_a(__first, __last, __p, _M_alloc);
            }
        }
 
    protected:
      // Destroy the array elements
      void
      _M_dispose_array(typename allocator_traits<_Alloc>::value_type* __p)
      {
        if (_M_overwrite)
          std::destroy_n(__p, _M_n);
        else
          {
            size_t __n = _M_n;
            while (__n--)
              allocator_traits<_Alloc>::destroy(_M_alloc, __p + __n);
          }
      }
 
    private:
      template<typename _Tp>
        static _Tp*
        _S_first_elem(_Tp* __p) { return __p; }
 
      template<typename _Tp, size_t _Nm>
        static auto
        _S_first_elem(_Tp (*__p)[_Nm]) { return _S_first_elem(*__p); }
    };
 
  // Control block for make_shared<T[]>, make_shared<T[N]> etc. that will be
  // placed into unused memory at the end of the array.
  template<typename _Alloc, _Lock_policy _Lp>
    class _Sp_counted_array final
    : public _Sp_counted_base<_Lp>, _Sp_counted_array_base<_Alloc>
    {
      using pointer = typename allocator_traits<_Alloc>::pointer;
 
      pointer _M_alloc_ptr;
 
      auto _M_ptr() const noexcept { return std::to_address(_M_alloc_ptr); }
 
      friend class __shared_count<_Lp>; // To be able to call _M_ptr().
 
    public:
      _Sp_counted_array(const _Sp_counted_array_base<_Alloc>& __a,
                        pointer __p) noexcept
      : _Sp_counted_array_base<_Alloc>(__a), _M_alloc_ptr(__p)
      { }
 
      ~_Sp_counted_array() = default;
 
      virtual void
      _M_dispose() noexcept
      {
        if (this->_M_n)
          this->_M_dispose_array(_M_ptr());
      }
 
      // Override because the allocator needs to know the dynamic type
      virtual void
      _M_destroy() noexcept
      {
        _Sp_counted_array_base<_Alloc> __a = *this;
        pointer __p = _M_alloc_ptr;
        this->~_Sp_counted_array();
        __a._M_dealloc_array(__p, _S_tail());
      }
 
      // Returns the number of additional array elements that must be
      // allocated in order to store a _Sp_counted_array at the end.
      static constexpr size_t
      _S_tail()
      {
        // The array elemenent type.
        using _Tp = typename allocator_traits<_Alloc>::value_type;
 
        // The space needed to store a _Sp_counted_array object.
        size_t __bytes = sizeof(_Sp_counted_array);
 
        // Add any padding needed for manual alignment within the buffer.
        if constexpr (alignof(_Tp) < alignof(_Sp_counted_array))
          __bytes += alignof(_Sp_counted_array) - alignof(_Tp);
 
        return (__bytes + sizeof(_Tp) - 1) / sizeof(_Tp);
      }
 
      void*
      _M_get_deleter(const std::type_info&) noexcept override
      { return nullptr; }
    };
#endif // __glibcxx_shared_ptr_arrays >= 201707L
 
  // The default deleter for shared_ptr<T[]> and shared_ptr<T[N]>.
  struct __sp_array_delete
  {
    template<typename _Yp>
      void operator()(_Yp* __p) const { delete[] __p; }
  };
 
  template<_Lock_policy _Lp>
    class __shared_count
    {
      // Prevent _Sp_alloc_shared_tag from matching the shared_ptr(P, D) ctor.
      template<typename _Tp>
        struct __not_alloc_shared_tag { using type = void; };
 
      template<typename _Tp>
        struct __not_alloc_shared_tag<_Sp_alloc_shared_tag<_Tp>> { };
 
#if __glibcxx_shared_ptr_arrays >= 201707L // C++ >= 20 && HOSTED
      template<typename _Alloc>
        struct __not_alloc_shared_tag<_Sp_counted_array_base<_Alloc>> { };
#endif
 
    public:
      constexpr __shared_count() noexcept : _M_pi(0)
      { }
 
      template<typename _Ptr>
        explicit
        __shared_count(_Ptr __p) : _M_pi(0)
        {
          __try
            {
              _M_pi = new _Sp_counted_ptr<_Ptr, _Lp>(__p);
            }
          __catch(...)
            {
              delete __p;
              __throw_exception_again;
            }
        }
 
      template<typename _Ptr>
        __shared_count(_Ptr __p, /* is_array = */ false_type)
        : __shared_count(__p)
        { }
 
      template<typename _Ptr>
        __shared_count(_Ptr __p, /* is_array = */ true_type)
        : __shared_count(__p, __sp_array_delete{}, allocator<void>())
        { }
 
      template<typename _Ptr, typename _Deleter,
               typename = typename __not_alloc_shared_tag<_Deleter>::type>
        __shared_count(_Ptr __p, _Deleter __d)
        : __shared_count(__p, std::move(__d), allocator<void>())
        { }
 
      template<typename _Ptr, typename _Deleter, typename _Alloc,
               typename = typename __not_alloc_shared_tag<_Deleter>::type>
        __shared_count(_Ptr __p, _Deleter __d, _Alloc __a) : _M_pi(0)
        {
          typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
          __try
            {
              typename _Sp_cd_type::__allocator_type __a2(__a);
              auto __guard = std::__allocate_guarded(__a2);
              _Sp_cd_type* __mem = __guard.get();
              ::new (__mem) _Sp_cd_type(__p, std::move(__d), std::move(__a));
              _M_pi = __mem;
              __guard = nullptr;
            }
          __catch(...)
            {
              __d(__p); // Call _Deleter on __p.
              __throw_exception_again;
            }
        }
 
      template<typename _Tp, typename _Alloc, typename... _Args>
        __shared_count(_Tp*& __p, _Sp_alloc_shared_tag<_Alloc> __a,
                       _Args&&... __args)
        {
          using _Tp2 = __remove_cv_t<_Tp>;
          using _Sp_cp_type = _Sp_counted_ptr_inplace<_Tp2, _Alloc, _Lp>;
          typename _Sp_cp_type::__allocator_type __a2(__a._M_a);
          auto __guard = std::__allocate_guarded(__a2);
          _Sp_cp_type* __mem = __guard.get();
          auto __pi = ::new (__mem)
            _Sp_cp_type(__a._M_a, std::forward<_Args>(__args)...);
          __guard = nullptr;
          _M_pi = __pi;
          __p = __pi->_M_ptr();
        }
 
#if __glibcxx_shared_ptr_arrays >= 201707L // C++ >= 20 && HOSTED
      template<typename _Tp, typename _Alloc, typename _Init>
        __shared_count(_Tp*& __p, const _Sp_counted_array_base<_Alloc>& __a,
                       _Init __init)
        {
          using _Up = remove_all_extents_t<_Tp>;
          static_assert(is_same_v<_Up, typename _Alloc::value_type>);
 
          using _Sp_ca_type = _Sp_counted_array<_Alloc, _Lp>;
          const size_t __tail = _Sp_ca_type::_S_tail();
 
          struct _Guarded_ptr : _Sp_counted_array_base<_Alloc>
          {
            typename allocator_traits<_Alloc>::pointer _M_ptr;
 
            _Guarded_ptr(_Sp_counted_array_base<_Alloc> __a)
            : _Sp_counted_array_base<_Alloc>(__a),
              _M_ptr(this->_M_alloc_array(_Sp_ca_type::_S_tail()))
            { }
 
            ~_Guarded_ptr()
            {
              if (_M_ptr)
                this->_M_dealloc_array(_M_ptr, _Sp_ca_type::_S_tail());
            }
          };
 
          _Guarded_ptr __guard{__a};
          _Up* const __raw = std::to_address(__guard._M_ptr);
          __guard._M_init(__raw, __init); // might throw
 
          void* __c = __raw + __a._M_n;
          if constexpr (alignof(_Up) < alignof(_Sp_ca_type))
            {
              size_t __space = sizeof(_Up) * __tail;
              __c = std::align(alignof(_Sp_ca_type), sizeof(_Sp_ca_type),
                               __c, __space);
            }
          auto __pi = ::new(__c) _Sp_ca_type(__guard, __guard._M_ptr);
          __guard._M_ptr = nullptr;
          _M_pi = __pi;
          __p = reinterpret_cast<_Tp*>(__raw);
        }
#endif
 
#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      // Special case for auto_ptr<_Tp> to provide the strong guarantee.
      template<typename _Tp>
        explicit
        __shared_count(std::auto_ptr<_Tp>&& __r);
#pragma GCC diagnostic pop
#endif
 
      // Special case for unique_ptr<_Tp,_Del> to provide the strong guarantee.
      template<typename _Tp, typename _Del>
        explicit
        __shared_count(std::unique_ptr<_Tp, _Del>&& __r) : _M_pi(0)
        {
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 2415. Inconsistency between unique_ptr and shared_ptr
          if (__r.get() == nullptr)
            return;
 
          using _Ptr = typename unique_ptr<_Tp, _Del>::pointer;
          using _Del2 = __conditional_t<is_reference<_Del>::value,
              reference_wrapper<typename remove_reference<_Del>::type>,
              _Del>;
          using _Sp_cd_type
            = _Sp_counted_deleter<_Ptr, _Del2, allocator<void>, _Lp>;
          using _Alloc = allocator<_Sp_cd_type>;
          using _Alloc_traits = allocator_traits<_Alloc>;
          _Alloc __a;
          _Sp_cd_type* __mem = _Alloc_traits::allocate(__a, 1);
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 3548. shared_ptr construction from unique_ptr should move
          // (not copy) the deleter
          _Alloc_traits::construct(__a, __mem, __r.release(),
                                   std::forward<_Del>(__r.get_deleter()));
          _M_pi = __mem;
        }
 
      // Throw bad_weak_ptr when __r._M_get_use_count() == 0.
      explicit __shared_count(const __weak_count<_Lp>& __r);
 
      // Does not throw if __r._M_get_use_count() == 0, caller must check.
      explicit
      __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t) noexcept;
 
      ~__shared_count() noexcept
      {
        if (_M_pi != nullptr)
          _M_pi->_M_release();
      }
 
      __shared_count(const __shared_count& __r) noexcept
      : _M_pi(__r._M_pi)
      {
        if (_M_pi != nullptr)
          _M_pi->_M_add_ref_copy();
      }
 
      __shared_count&
      operator=(const __shared_count& __r) noexcept
      {
        _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
        if (__tmp != _M_pi)
          {
            if (__tmp != nullptr)
              __tmp->_M_add_ref_copy();
            if (_M_pi != nullptr)
              _M_pi->_M_release();
            _M_pi = __tmp;
          }
        return *this;
      }
 
      void
      _M_swap(__shared_count& __r) noexcept
      {
        _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
        __r._M_pi = _M_pi;
        _M_pi = __tmp;
      }
 
      long
      _M_get_use_count() const noexcept
      { return _M_pi ? _M_pi->_M_get_use_count() : 0; }
 
      bool
      _M_unique() const noexcept
      { return this->_M_get_use_count() == 1; }
 
      void*
      _M_get_deleter(const std::type_info& __ti) const noexcept
      { return _M_pi ? _M_pi->_M_get_deleter(__ti) : nullptr; }
 
      bool
      _M_less(const __shared_count& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 
      bool
      _M_less(const __weak_count<_Lp>& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 
#ifdef __glibcxx_smart_ptr_owner_equality // >= C++26
      size_t
      _M_owner_hash() const noexcept
      { return std::hash<_Sp_counted_base<_Lp>*>()(this->_M_pi); }
#endif
 
      // Friend function injected into enclosing namespace and found by ADL
      friend inline bool
      operator==(const __shared_count& __a, const __shared_count& __b) noexcept
      { return __a._M_pi == __b._M_pi; }
 
    private:
      friend class __weak_count<_Lp>;
#ifdef __glibcxx_atomic_shared_ptr
      template<typename> friend class _Sp_atomic;
#endif
#ifdef __glibcxx_out_ptr
      template<typename, typename, typename...> friend class out_ptr_t;
#endif
 
      _Sp_counted_base<_Lp>*  _M_pi;
    };
 
 
  template<_Lock_policy _Lp>
    class __weak_count
    {
    public:
      constexpr __weak_count() noexcept : _M_pi(nullptr)
      { }
 
      __weak_count(const __shared_count<_Lp>& __r) noexcept
      : _M_pi(__r._M_pi)
      {
        if (_M_pi != nullptr)
          _M_pi->_M_weak_add_ref();
      }
 
      __weak_count(const __weak_count& __r) noexcept
      : _M_pi(__r._M_pi)
      {
        if (_M_pi != nullptr)
          _M_pi->_M_weak_add_ref();
      }
 
      __weak_count(__weak_count&& __r) noexcept
      : _M_pi(__r._M_pi)
      { __r._M_pi = nullptr; }
 
      ~__weak_count() noexcept
      {
        if (_M_pi != nullptr)
          _M_pi->_M_weak_release();
      }
 
      __weak_count&
      operator=(const __shared_count<_Lp>& __r) noexcept
      {
        _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
        if (__tmp != nullptr)
          __tmp->_M_weak_add_ref();
        if (_M_pi != nullptr)
          _M_pi->_M_weak_release();
        _M_pi = __tmp;
        return *this;
      }
 
      __weak_count&
      operator=(const __weak_count& __r) noexcept
      {
        _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
        if (__tmp != nullptr)
          __tmp->_M_weak_add_ref();
        if (_M_pi != nullptr)
          _M_pi->_M_weak_release();
        _M_pi = __tmp;
        return *this;
      }
 
      __weak_count&
      operator=(__weak_count&& __r) noexcept
      {
        if (_M_pi != nullptr)
          _M_pi->_M_weak_release();
        _M_pi = __r._M_pi;
        __r._M_pi = nullptr;
        return *this;
      }
 
      void
      _M_swap(__weak_count& __r) noexcept
      {
        _Sp_counted_base<_Lp>* __tmp = __r._M_pi;
        __r._M_pi = _M_pi;
        _M_pi = __tmp;
      }
 
      long
      _M_get_use_count() const noexcept
      { return _M_pi != nullptr ? _M_pi->_M_get_use_count() : 0; }
 
      bool
      _M_less(const __weak_count& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 
      bool
      _M_less(const __shared_count<_Lp>& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
 
#ifdef __glibcxx_smart_ptr_owner_equality // >= C++26
      size_t
      _M_owner_hash() const noexcept
      { return std::hash<_Sp_counted_base<_Lp>*>()(this->_M_pi); }
#endif
 
      // Friend function injected into enclosing namespace and found by ADL
      friend inline bool
      operator==(const __weak_count& __a, const __weak_count& __b) noexcept
      { return __a._M_pi == __b._M_pi; }
 
    private:
      friend class __shared_count<_Lp>;
#ifdef __glibcxx_atomic_shared_ptr
      template<typename> friend class _Sp_atomic;
#endif
 
      _Sp_counted_base<_Lp>*  _M_pi;
    };
 
  // Now that __weak_count is defined we can define this constructor:
  template<_Lock_policy _Lp>
    inline
    __shared_count<_Lp>::__shared_count(const __weak_count<_Lp>& __r)
    : _M_pi(__r._M_pi)
    {
      if (_M_pi == nullptr || !_M_pi->_M_add_ref_lock_nothrow())
        __throw_bad_weak_ptr();
    }
 
  // Now that __weak_count is defined we can define this constructor:
  template<_Lock_policy _Lp>
    inline
    __shared_count<_Lp>::
    __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t) noexcept
    : _M_pi(__r._M_pi)
    {
      if (_M_pi && !_M_pi->_M_add_ref_lock_nothrow())
        _M_pi = nullptr;
    }
 
  // Helper traits for shared_ptr of array:
 
  // A pointer type Y* is said to be compatible with a pointer type T* when
  // either Y* is convertible to T* or Y is U[N] and T is U cv [].
  template<typename _Yp_ptr, typename _Tp_ptr>
    struct __sp_compatible_with
    : false_type
    { };
 
  template<typename _Yp, typename _Tp>
    struct __sp_compatible_with<_Yp*, _Tp*>
    : is_convertible<_Yp*, _Tp*>::type
    { };
 
  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], _Up(*)[]>
    : true_type
    { };
 
  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], const _Up(*)[]>
    : true_type
    { };
 
  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], volatile _Up(*)[]>
    : true_type
    { };
 
  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], const volatile _Up(*)[]>
    : true_type
    { };
 
  // Test conversion from Y(*)[N] to U(*)[N] without forming invalid type Y[N].
  template<typename _Up, size_t _Nm, typename _Yp, typename = void>
    struct __sp_is_constructible_arrN
    : false_type
    { };
 
  template<typename _Up, size_t _Nm, typename _Yp>
    struct __sp_is_constructible_arrN<_Up, _Nm, _Yp, __void_t<_Yp[_Nm]>>
    : is_convertible<_Yp(*)[_Nm], _Up(*)[_Nm]>::type
    { };
 
  // Test conversion from Y(*)[] to U(*)[] without forming invalid type Y[].
  template<typename _Up, typename _Yp, typename = void>
    struct __sp_is_constructible_arr
    : false_type
    { };
 
  template<typename _Up, typename _Yp>
    struct __sp_is_constructible_arr<_Up, _Yp, __void_t<_Yp[]>>
    : is_convertible<_Yp(*)[], _Up(*)[]>::type
    { };
 
  // Trait to check if shared_ptr<T> can be constructed from Y*.
  template<typename _Tp, typename _Yp>
    struct __sp_is_constructible;
 
  // When T is U[N], Y(*)[N] shall be convertible to T*;
  template<typename _Up, size_t _Nm, typename _Yp>
    struct __sp_is_constructible<_Up[_Nm], _Yp>
    : __sp_is_constructible_arrN<_Up, _Nm, _Yp>::type
    { };
 
  // when T is U[], Y(*)[] shall be convertible to T*;
  template<typename _Up, typename _Yp>
    struct __sp_is_constructible<_Up[], _Yp>
    : __sp_is_constructible_arr<_Up, _Yp>::type
    { };
 
  // otherwise, Y* shall be convertible to T*.
  template<typename _Tp, typename _Yp>
    struct __sp_is_constructible
    : is_convertible<_Yp*, _Tp*>::type
    { };
 
 
  template<typename _Tp>
    [[__gnu__::__always_inline__]]
    inline _Tp*
    __shared_ptr_deref(_Tp* __p)
    {
      __glibcxx_assert(__p != nullptr);
      return __p;
    }
 
  // Define operator* and operator-> for shared_ptr<T>.
  template<typename _Tp, _Lock_policy _Lp,
           bool = is_array<_Tp>::value, bool = is_void<_Tp>::value>
    class __shared_ptr_access
    {
    public:
      using element_type = _Tp;
 
      element_type&
      operator*() const noexcept
      { return *std::__shared_ptr_deref(_M_get()); }
 
      element_type*
      operator->() const noexcept
      {
        _GLIBCXX_DEBUG_PEDASSERT(_M_get() != nullptr);
        return _M_get();
      }
 
    private:
      element_type*
      _M_get() const noexcept
      { return static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get(); }
    };
 
  // Define operator-> for shared_ptr<cv void>.
  template<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr_access<_Tp, _Lp, false, true>
    {
    public:
      using element_type = _Tp;
 
      element_type*
      operator->() const noexcept
      {
        auto __ptr = static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get();
        _GLIBCXX_DEBUG_PEDASSERT(__ptr != nullptr);
        return __ptr;
      }
    };
 
  // Define operator[] for shared_ptr<T[]> and shared_ptr<T[N]>.
  template<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr_access<_Tp, _Lp, true, false>
    {
    public:
      using element_type = typename remove_extent<_Tp>::type;
 
#if __cplusplus <= 201402L
      [[__deprecated__("shared_ptr<T[]>::operator* is absent from C++17")]]
      element_type&
      operator*() const noexcept
      { return *std::__shared_ptr_deref(_M_get()); }
 
      [[__deprecated__("shared_ptr<T[]>::operator-> is absent from C++17")]]
      element_type*
      operator->() const noexcept
      {
        _GLIBCXX_DEBUG_PEDASSERT(_M_get() != nullptr);
        return _M_get();
      }
#endif
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions"
      element_type&
      operator[](ptrdiff_t __i) const noexcept
      {
        if constexpr (extent<_Tp>::value)
          __glibcxx_assert(__i < extent<_Tp>::value);
        return std::__shared_ptr_deref(_M_get())[__i];
      }
#pragma GCC diagnostic pop
 
    private:
      element_type*
      _M_get() const noexcept
      { return static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get(); }
    };
 
  template<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr
    : public __shared_ptr_access<_Tp, _Lp>
    {
    public:
      using element_type = typename remove_extent<_Tp>::type;
 
    private:
      // Constraint for taking ownership of a pointer of type _Yp*:
      template<typename _Yp>
        using _SafeConv
          = typename enable_if<__sp_is_constructible<_Tp, _Yp>::value>::type;
 
      // Constraint for construction from shared_ptr and weak_ptr:
      template<typename _Yp, typename _Res = void>
        using _Compatible = typename
          enable_if<__sp_compatible_with<_Yp*, _Tp*>::value, _Res>::type;
 
      // Constraint for assignment from shared_ptr and weak_ptr:
      template<typename _Yp>
        using _Assignable = _Compatible<_Yp, __shared_ptr&>;
 
      // Constraint for construction from unique_ptr:
      template<typename _Yp, typename _Del, typename _Res = void,
               typename _Ptr = typename unique_ptr<_Yp, _Del>::pointer>
        using _UniqCompatible = __enable_if_t<__and_<
          __sp_compatible_with<_Yp*, _Tp*>,
          is_convertible<_Ptr, element_type*>,
          is_move_constructible<_Del>
          >::value, _Res>;
 
      // Constraint for assignment from unique_ptr:
      template<typename _Yp, typename _Del>
        using _UniqAssignable = _UniqCompatible<_Yp, _Del, __shared_ptr&>;
 
    public:
 
#if __cplusplus > 201402L
      using weak_type = __weak_ptr<_Tp, _Lp>;
#endif
 
      constexpr __shared_ptr() noexcept
      : _M_ptr(0), _M_refcount()
      { }
 
      template<typename _Yp, typename = _SafeConv<_Yp>>
        explicit
        __shared_ptr(_Yp* __p)
        : _M_ptr(__p), _M_refcount(__p, typename is_array<_Tp>::type())
        {
          static_assert( !is_void<_Yp>::value, "incomplete type" );
          static_assert( sizeof(_Yp) > 0, "incomplete type" );
          _M_enable_shared_from_this_with(__p);
        }
 
      template<typename _Yp, typename _Deleter, typename = _SafeConv<_Yp>>
        __shared_ptr(_Yp* __p, _Deleter __d)
        : _M_ptr(__p), _M_refcount(__p, std::move(__d))
        {
          static_assert(__is_invocable<_Deleter&, _Yp*&>::value,
              "deleter expression d(p) is well-formed");
          _M_enable_shared_from_this_with(__p);
        }
 
      template<typename _Yp, typename _Deleter, typename _Alloc,
               typename = _SafeConv<_Yp>>
        __shared_ptr(_Yp* __p, _Deleter __d, _Alloc __a)
        : _M_ptr(__p), _M_refcount(__p, std::move(__d), std::move(__a))
        {
          static_assert(__is_invocable<_Deleter&, _Yp*&>::value,
              "deleter expression d(p) is well-formed");
          _M_enable_shared_from_this_with(__p);
        }
 
      template<typename _Deleter>
        __shared_ptr(nullptr_t __p, _Deleter __d)
        : _M_ptr(0), _M_refcount(__p, std::move(__d))
        { }
 
      template<typename _Deleter, typename _Alloc>
        __shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
        : _M_ptr(0), _M_refcount(__p, std::move(__d), std::move(__a))
        { }
 
      // Aliasing constructor
      template<typename _Yp>
        __shared_ptr(const __shared_ptr<_Yp, _Lp>& __r,
                     element_type* __p) noexcept
        : _M_ptr(__p), _M_refcount(__r._M_refcount) // never throws
        { }
 
      // Aliasing constructor
      template<typename _Yp>
        __shared_ptr(__shared_ptr<_Yp, _Lp>&& __r,
                     element_type* __p) noexcept
        : _M_ptr(__p), _M_refcount()
        {
          _M_refcount._M_swap(__r._M_refcount);
          __r._M_ptr = nullptr;
        }
 
      __shared_ptr(const __shared_ptr&) noexcept = default;
      __shared_ptr& operator=(const __shared_ptr&) noexcept = default;
      ~__shared_ptr() = default;
 
      template<typename _Yp, typename = _Compatible<_Yp>>
        __shared_ptr(const __shared_ptr<_Yp, _Lp>& __r) noexcept
        : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
        { }
 
      __shared_ptr(__shared_ptr&& __r) noexcept
      : _M_ptr(__r._M_ptr), _M_refcount()
      {
        _M_refcount._M_swap(__r._M_refcount);
        __r._M_ptr = nullptr;
      }
 
      template<typename _Yp, typename = _Compatible<_Yp>>
        __shared_ptr(__shared_ptr<_Yp, _Lp>&& __r) noexcept
        : _M_ptr(__r._M_ptr), _M_refcount()
        {
          _M_refcount._M_swap(__r._M_refcount);
          __r._M_ptr = nullptr;
        }
 
      template<typename _Yp, typename = _Compatible<_Yp>>
        explicit __shared_ptr(const __weak_ptr<_Yp, _Lp>& __r)
        : _M_refcount(__r._M_refcount) // may throw
        {
          // It is now safe to copy __r._M_ptr, as
          // _M_refcount(__r._M_refcount) did not throw.
          _M_ptr = __r._M_ptr;
        }
 
      // If an exception is thrown this constructor has no effect.
      template<typename _Yp, typename _Del,
               typename = _UniqCompatible<_Yp, _Del>>
        __shared_ptr(unique_ptr<_Yp, _Del>&& __r)
        : _M_ptr(__r.get()), _M_refcount()
        {
          auto __raw = std::__to_address(__r.get());
          _M_refcount = __shared_count<_Lp>(std::move(__r));
          _M_enable_shared_from_this_with(__raw);
        }
 
#if __cplusplus <= 201402L && _GLIBCXX_USE_DEPRECATED
    protected:
      // If an exception is thrown this constructor has no effect.
      template<typename _Tp1, typename _Del,
               typename enable_if<__and_<
                 __not_<is_array<_Tp>>, is_array<_Tp1>,
                 is_convertible<typename unique_ptr<_Tp1, _Del>::pointer, _Tp*>
               >::value, bool>::type = true>
        __shared_ptr(unique_ptr<_Tp1, _Del>&& __r, __sp_array_delete)
        : _M_ptr(__r.get()), _M_refcount()
        {
          auto __raw = std::__to_address(__r.get());
          _M_refcount = __shared_count<_Lp>(std::move(__r));
          _M_enable_shared_from_this_with(__raw);
        }
    public:
#endif
 
#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      // Postcondition: use_count() == 1 and __r.get() == 0
      template<typename _Yp, typename = _Compatible<_Yp>>
        __shared_ptr(auto_ptr<_Yp>&& __r);
#pragma GCC diagnostic pop
#endif
 
      constexpr __shared_ptr(nullptr_t) noexcept : __shared_ptr() { }
 
      template<typename _Yp>
        _Assignable<_Yp>
        operator=(const __shared_ptr<_Yp, _Lp>& __r) noexcept
        {
          _M_ptr = __r._M_ptr;
          _M_refcount = __r._M_refcount; // __shared_count::op= doesn't throw
          return *this;
        }
 
#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      template<typename _Yp>
        _Assignable<_Yp>
        operator=(auto_ptr<_Yp>&& __r)
        {
          __shared_ptr(std::move(__r)).swap(*this);
          return *this;
        }
#pragma GCC diagnostic pop
#endif
 
      __shared_ptr&
      operator=(__shared_ptr&& __r) noexcept
      {
        __shared_ptr(std::move(__r)).swap(*this);
        return *this;
      }
 
      template<class _Yp>
        _Assignable<_Yp>
        operator=(__shared_ptr<_Yp, _Lp>&& __r) noexcept
        {
          __shared_ptr(std::move(__r)).swap(*this);
          return *this;
        }
 
      template<typename _Yp, typename _Del>
        _UniqAssignable<_Yp, _Del>
        operator=(unique_ptr<_Yp, _Del>&& __r)
        {
          __shared_ptr(std::move(__r)).swap(*this);
          return *this;
        }
 
      void
      reset() noexcept
      { __shared_ptr().swap(*this); }
 
      template<typename _Yp>
        _SafeConv<_Yp>
        reset(_Yp* __p) // _Yp must be complete.
        {
          // Catch self-reset errors.
          __glibcxx_assert(__p == nullptr || __p != _M_ptr);
          __shared_ptr(__p).swap(*this);
        }
 
      template<typename _Yp, typename _Deleter>
        _SafeConv<_Yp>
        reset(_Yp* __p, _Deleter __d)
        { __shared_ptr(__p, std::move(__d)).swap(*this); }
 
      template<typename _Yp, typename _Deleter, typename _Alloc>
        _SafeConv<_Yp>
        reset(_Yp* __p, _Deleter __d, _Alloc __a)
        { __shared_ptr(__p, std::move(__d), std::move(__a)).swap(*this); }
 
      /// Return the stored pointer.
      element_type*
      get() const noexcept
      { return _M_ptr; }
 
      /// Return true if the stored pointer is not null.
      explicit operator bool() const noexcept
      { return _M_ptr != nullptr; }
 
      /// Return true if use_count() == 1.
      bool
      unique() const noexcept
      { return _M_refcount._M_unique(); }
 
      /// If *this owns a pointer, return the number of owners, otherwise zero.
      long
      use_count() const noexcept
      { return _M_refcount._M_get_use_count(); }
 
      /// Exchange both the owned pointer and the stored pointer.
      void
      swap(__shared_ptr<_Tp, _Lp>& __other) noexcept
      {
        std::swap(_M_ptr, __other._M_ptr);
        _M_refcount._M_swap(__other._M_refcount);
      }
 
      /** @brief Define an ordering based on ownership.
       *
       * This function defines a strict weak ordering between two shared_ptr
       * or weak_ptr objects, such that one object is less than the other
       * unless they share ownership of the same pointer, or are both empty.
       * @{
      */
      template<typename _Tp1>
        bool
        owner_before(__shared_ptr<_Tp1, _Lp> const& __rhs) const noexcept
        { return _M_refcount._M_less(__rhs._M_refcount); }
 
      template<typename _Tp1>
        bool
        owner_before(__weak_ptr<_Tp1, _Lp> const& __rhs) const noexcept
        { return _M_refcount._M_less(__rhs._M_refcount); }
      /// @}
 
#ifdef __glibcxx_smart_ptr_owner_equality // >= C++26
      size_t owner_hash() const noexcept { return _M_refcount._M_owner_hash(); }
 
      template<typename _Tp1>
        bool
        owner_equal(__shared_ptr<_Tp1, _Lp> const& __rhs) const noexcept
        { return _M_refcount == __rhs._M_refcount; }
 
      template<typename _Tp1>
        bool
        owner_equal(__weak_ptr<_Tp1, _Lp> const& __rhs) const noexcept
        { return _M_refcount == __rhs._M_refcount; }
#endif
 
    protected:
      // This constructor is non-standard, it is used by allocate_shared.
      template<typename _Alloc, typename... _Args>
        __shared_ptr(_Sp_alloc_shared_tag<_Alloc> __tag, _Args&&... __args)
        : _M_ptr(), _M_refcount(_M_ptr, __tag, std::forward<_Args>(__args)...)
        { _M_enable_shared_from_this_with(_M_ptr); }
 
      template<typename _Tp1, _Lock_policy _Lp1, typename _Alloc,
               typename... _Args>
        friend __shared_ptr<_Tp1, _Lp1>
        __allocate_shared(const _Alloc& __a, _Args&&... __args);
 
#if __glibcxx_shared_ptr_arrays >= 201707L // C++ >= 20 && HOSTED
      // This constructor is non-standard, it is used by allocate_shared<T[]>.
      template<typename _Alloc, typename _Init = const remove_extent_t<_Tp>*>
        __shared_ptr(const _Sp_counted_array_base<_Alloc>& __a,
                     _Init __init = nullptr)
        : _M_ptr(), _M_refcount(_M_ptr, __a, __init)
        { }
#endif
 
      // This constructor is used by __weak_ptr::lock() and
      // shared_ptr::shared_ptr(const weak_ptr&, std::nothrow_t).
      __shared_ptr(const __weak_ptr<_Tp, _Lp>& __r, std::nothrow_t) noexcept
      : _M_refcount(__r._M_refcount, std::nothrow)
      {
        _M_ptr = _M_refcount._M_get_use_count() ? __r._M_ptr : nullptr;
      }
 
      friend class __weak_ptr<_Tp, _Lp>;
 
    private:
 
      template<typename _Yp>
        using __esft_base_t = decltype(__enable_shared_from_this_base(
              std::declval<const __shared_count<_Lp>&>(),
              std::declval<_Yp*>()));
 
      // Detect an accessible and unambiguous enable_shared_from_this base.
      template<typename _Yp, typename = void>
        struct __has_esft_base
        : false_type { };
 
      template<typename _Yp>
        struct __has_esft_base<_Yp, __void_t<__esft_base_t<_Yp>>>
        : __not_<is_array<_Tp>> { }; // No enable shared_from_this for arrays
 
      template<typename _Yp, typename _Yp2 = typename remove_cv<_Yp>::type>
        typename enable_if<__has_esft_base<_Yp2>::value>::type
        _M_enable_shared_from_this_with(_Yp* __p) noexcept
        {
          if (auto __base = __enable_shared_from_this_base(_M_refcount, __p))
            __base->_M_weak_assign(const_cast<_Yp2*>(__p), _M_refcount);
        }
 
      template<typename _Yp, typename _Yp2 = typename remove_cv<_Yp>::type>
        typename enable_if<!__has_esft_base<_Yp2>::value>::type
        _M_enable_shared_from_this_with(_Yp*) noexcept
        { }
 
      void*
      _M_get_deleter(const std::type_info& __ti) const noexcept
      { return _M_refcount._M_get_deleter(__ti); }
 
      template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
      template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
 
      template<typename _Del, typename _Tp1, _Lock_policy _Lp1>
        friend _Del* get_deleter(const __shared_ptr<_Tp1, _Lp1>&) noexcept;
 
      template<typename _Del, typename _Tp1>
        friend _Del* get_deleter(const shared_ptr<_Tp1>&) noexcept;
 
#ifdef __glibcxx_atomic_shared_ptr
      friend _Sp_atomic<shared_ptr<_Tp>>;
#endif
#ifdef __glibcxx_out_ptr
      template<typename, typename, typename...> friend class out_ptr_t;
#endif
 
      element_type*        _M_ptr;         // Contained pointer.
      __shared_count<_Lp>  _M_refcount;    // Reference counter.
    };
 
 
  // 20.7.2.2.7 shared_ptr comparisons
  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator==(const __shared_ptr<_Tp1, _Lp>& __a,
               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return __a.get() == __b.get(); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator==(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return !__a; }
 
#ifdef __cpp_lib_three_way_comparison
  template<typename _Tp, typename _Up, _Lock_policy _Lp>
    inline strong_ordering
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a,
                const __shared_ptr<_Up, _Lp>& __b) noexcept
    { return compare_three_way()(__a.get(), __b.get()); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline strong_ordering
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    {
      using pointer = typename __shared_ptr<_Tp, _Lp>::element_type*;
      return compare_three_way()(__a.get(), static_cast<pointer>(nullptr));
    }
#else
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator==(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return !__a; }
 
  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator!=(const __shared_ptr<_Tp1, _Lp>& __a,
               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return __a.get() != __b.get(); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator!=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return (bool)__a; }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator!=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return (bool)__a; }
 
  template<typename _Tp, typename _Up, _Lock_policy _Lp>
    inline bool
    operator<(const __shared_ptr<_Tp, _Lp>& __a,
              const __shared_ptr<_Up, _Lp>& __b) noexcept
    {
      using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
      using _Up_elt = typename __shared_ptr<_Up, _Lp>::element_type;
      using _Vp = typename common_type<_Tp_elt*, _Up_elt*>::type;
      return less<_Vp>()(__a.get(), __b.get());
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    {
      using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
      return less<_Tp_elt*>()(__a.get(), nullptr);
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    {
      using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
      return less<_Tp_elt*>()(nullptr, __a.get());
    }
 
  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator<=(const __shared_ptr<_Tp1, _Lp>& __a,
               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return !(__b < __a); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return !(nullptr < __a); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return !(__a < nullptr); }
 
  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator>(const __shared_ptr<_Tp1, _Lp>& __a,
              const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return (__b < __a); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return nullptr < __a; }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return __a < nullptr; }
 
  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator>=(const __shared_ptr<_Tp1, _Lp>& __a,
               const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return !(__a < __b); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return !(__a < nullptr); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return !(nullptr < __a); }
#endif // three-way comparison
 
  // 20.7.2.2.8 shared_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b) noexcept
    { __a.swap(__b); }
 
  // 20.7.2.2.9 shared_ptr casts
 
  // The seemingly equivalent code:
  // shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get()))
  // will eventually result in undefined behaviour, attempting to
  // delete the same object twice.
  /// static_pointer_cast
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      return _Sp(__r, static_cast<typename _Sp::element_type*>(__r.get()));
    }
 
  // The seemingly equivalent code:
  // shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get()))
  // will eventually result in undefined behaviour, attempting to
  // delete the same object twice.
  /// const_pointer_cast
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      return _Sp(__r, const_cast<typename _Sp::element_type*>(__r.get()));
    }
 
  // The seemingly equivalent code:
  // shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get()))
  // will eventually result in undefined behaviour, attempting to
  // delete the same object twice.
  /// dynamic_pointer_cast
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      if (auto* __p = dynamic_cast<typename _Sp::element_type*>(__r.get()))
        return _Sp(__r, __p);
      return _Sp();
    }
 
#if __cplusplus > 201402L
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    reinterpret_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      return _Sp(__r, reinterpret_cast<typename _Sp::element_type*>(__r.get()));
    }
#endif
 
  template<typename _Tp, _Lock_policy _Lp>
    class __weak_ptr
    {
    public:
      using element_type = typename remove_extent<_Tp>::type;
 
    private:
      template<typename _Yp, typename _Res = void>
        using _Compatible = typename
          enable_if<__sp_compatible_with<_Yp*, _Tp*>::value, _Res>::type;
 
      // Constraint for assignment from shared_ptr and weak_ptr:
      template<typename _Yp>
        using _Assignable = _Compatible<_Yp, __weak_ptr&>;
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
      // Helper for construction/assignment:
      template<typename _Yp>
        static element_type*
        _S_safe_upcast(const __weak_ptr<_Yp, _Lp>& __r)
        {
          // We know that _Yp and _Tp are compatible, that is, either
          // _Yp* is convertible to _Tp* or _Yp is U[N] and _Tp is U cv [].
 
          // If _Yp is the same as _Tp after removing extents and cv
          // qualifications, there's no pointer adjustments to do. This
          // also allows us to support incomplete types.
          using _At = typename remove_cv<typename remove_extent<_Tp>::type>::type;
          using _Bt = typename remove_cv<typename remove_extent<_Yp>::type>::type;
          if constexpr (is_same<_At, _Bt>::value)
            return __r._M_ptr;
          // If they're not the same type, but they're both scalars,
          // we again don't need any adjustment. This allows us to support e.g.
          // pointers to a differently cv qualified type X.
          else if constexpr (__and_<is_scalar<_At>, is_scalar<_Bt>>::value)
            return __r._M_ptr;
#if _GLIBCXX_USE_BUILTIN_TRAIT(__builtin_is_virtual_base_of)
          // If _Tp is not a virtual base class of _Yp, the pointer
          // conversion does not require dereferencing __r._M_ptr; just
          // rely on the implicit conversion.
          else if constexpr (!__builtin_is_virtual_base_of(_Tp, _Yp))
            return __r._M_ptr;
#endif
          // Expensive path; must lock() and do the pointer conversion while
          // a shared_ptr keeps the pointee alive (because we may need
          // to dereference).
          else
            return __r.lock().get();
        }
#pragma GCC diagnostic pop
 
    public:
      constexpr __weak_ptr() noexcept
      : _M_ptr(nullptr), _M_refcount()
      { }
 
      __weak_ptr(const __weak_ptr&) noexcept = default;
 
      ~__weak_ptr() = default;
 
      // The "obvious" converting constructor implementation:
      //
      //  template<typename _Tp1>
      //    __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
      //    : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
      //    { }
      //
      // has a serious problem.
      //
      //  __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr)
      //  conversion may require access to *__r._M_ptr (virtual inheritance).
      //
      // It is not possible to avoid spurious access violations since
      // in multithreaded programs __r._M_ptr may be invalidated at any point.
      template<typename _Yp, typename = _Compatible<_Yp>>
        __weak_ptr(const __weak_ptr<_Yp, _Lp>& __r) noexcept
        : _M_ptr(_S_safe_upcast(__r)), _M_refcount(__r._M_refcount)
        { }
 
      template<typename _Yp, typename = _Compatible<_Yp>>
        __weak_ptr(const __shared_ptr<_Yp, _Lp>& __r) noexcept
        : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
        { }
 
      __weak_ptr(__weak_ptr&& __r) noexcept
      : _M_ptr(__r._M_ptr), _M_refcount(std::move(__r._M_refcount))
      { __r._M_ptr = nullptr; }
 
      template<typename _Yp, typename = _Compatible<_Yp>>
        __weak_ptr(__weak_ptr<_Yp, _Lp>&& __r) noexcept
        : _M_ptr(_S_safe_upcast(__r)), _M_refcount(std::move(__r._M_refcount))
        { __r._M_ptr = nullptr; }
 
      __weak_ptr&
      operator=(const __weak_ptr& __r) noexcept = default;
 
      template<typename _Yp>
        _Assignable<_Yp>
        operator=(const __weak_ptr<_Yp, _Lp>& __r) noexcept
        {
          _M_ptr = _S_safe_upcast(__r);
          _M_refcount = __r._M_refcount;
          return *this;
        }
 
      template<typename _Yp>
        _Assignable<_Yp>
        operator=(const __shared_ptr<_Yp, _Lp>& __r) noexcept
        {
          _M_ptr = __r._M_ptr;
          _M_refcount = __r._M_refcount;
          return *this;
        }
 
      __weak_ptr&
      operator=(__weak_ptr&& __r) noexcept
      {
        __weak_ptr(std::move(__r)).swap(*this);
        return *this;
      }
 
      template<typename _Yp>
        _Assignable<_Yp>
        operator=(__weak_ptr<_Yp, _Lp>&& __r) noexcept
        {
          _M_ptr = _S_safe_upcast(__r);
          _M_refcount = std::move(__r._M_refcount);
          __r._M_ptr = nullptr;
          return *this;
        }
 
      __shared_ptr<_Tp, _Lp>
      lock() const noexcept
      { return __shared_ptr<_Tp, _Lp>(*this, std::nothrow); }
 
      long
      use_count() const noexcept
      { return _M_refcount._M_get_use_count(); }
 
      bool
      expired() const noexcept
      { return _M_refcount._M_get_use_count() == 0; }
 
      template<typename _Tp1>
        bool
        owner_before(const __shared_ptr<_Tp1, _Lp>& __rhs) const noexcept
        { return _M_refcount._M_less(__rhs._M_refcount); }
 
      template<typename _Tp1>
        bool
        owner_before(const __weak_ptr<_Tp1, _Lp>& __rhs) const noexcept
        { return _M_refcount._M_less(__rhs._M_refcount); }
 
#ifdef __glibcxx_smart_ptr_owner_equality // >= C++26
      size_t owner_hash() const noexcept { return _M_refcount._M_owner_hash(); }
 
      template<typename _Tp1>
      bool
      owner_equal(const __shared_ptr<_Tp1, _Lp> & __rhs) const noexcept
      { return _M_refcount == __rhs._M_refcount; }
 
      template<typename _Tp1>
      bool
      owner_equal(const __weak_ptr<_Tp1, _Lp> & __rhs) const noexcept
      { return _M_refcount == __rhs._M_refcount; }
#endif
 
      void
      reset() noexcept
      { __weak_ptr().swap(*this); }
 
      void
      swap(__weak_ptr& __s) noexcept
      {
        std::swap(_M_ptr, __s._M_ptr);
        _M_refcount._M_swap(__s._M_refcount);
      }
 
    private:
      // Used by __enable_shared_from_this.
      void
      _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
      {
        if (use_count() == 0)
          {
            _M_ptr = __ptr;
            _M_refcount = __refcount;
          }
      }
 
      template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
      template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
      friend class __enable_shared_from_this<_Tp, _Lp>;
      friend class enable_shared_from_this<_Tp>;
#ifdef __glibcxx_atomic_shared_ptr
      friend _Sp_atomic<weak_ptr<_Tp>>;
#endif
 
      element_type*      _M_ptr;         // Contained pointer.
      __weak_count<_Lp>  _M_refcount;    // Reference counter.
    };
 
  // 20.7.2.3.6 weak_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b) noexcept
    { __a.swap(__b); }
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
  template<typename _Tp, typename _Tp1>
    struct _Sp_owner_less : public binary_function<_Tp, _Tp, bool>
    {
      bool
      operator()(const _Tp& __lhs, const _Tp& __rhs) const noexcept
      { return __lhs.owner_before(__rhs); }
 
      bool
      operator()(const _Tp& __lhs, const _Tp1& __rhs) const noexcept
      { return __lhs.owner_before(__rhs); }
 
      bool
      operator()(const _Tp1& __lhs, const _Tp& __rhs) const noexcept
      { return __lhs.owner_before(__rhs); }
    };
#pragma GCC diagnostic pop
 
  template<>
    struct _Sp_owner_less<void, void>
    {
      template<typename _Tp, typename _Up>
        auto
        operator()(const _Tp& __lhs, const _Up& __rhs) const noexcept
        -> decltype(__lhs.owner_before(__rhs))
        { return __lhs.owner_before(__rhs); }
 
      using is_transparent = void;
    };
 
  template<typename _Tp, _Lock_policy _Lp>
    struct owner_less<__shared_ptr<_Tp, _Lp>>
    : public _Sp_owner_less<__shared_ptr<_Tp, _Lp>, __weak_ptr<_Tp, _Lp>>
    { };
 
  template<typename _Tp, _Lock_policy _Lp>
    struct owner_less<__weak_ptr<_Tp, _Lp>>
    : public _Sp_owner_less<__weak_ptr<_Tp, _Lp>, __shared_ptr<_Tp, _Lp>>
    { };
 
 
  template<typename _Tp, _Lock_policy _Lp>
    class __enable_shared_from_this
    {
    protected:
      constexpr __enable_shared_from_this() noexcept { }
 
      __enable_shared_from_this(const __enable_shared_from_this&) noexcept { }
 
      __enable_shared_from_this&
      operator=(const __enable_shared_from_this&) noexcept
      { return *this; }
 
      ~__enable_shared_from_this() { }
 
    public:
      __shared_ptr<_Tp, _Lp>
      shared_from_this()
      { return __shared_ptr<_Tp, _Lp>(this->_M_weak_this); }
 
      __shared_ptr<const _Tp, _Lp>
      shared_from_this() const
      { return __shared_ptr<const _Tp, _Lp>(this->_M_weak_this); }
 
#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
      __weak_ptr<_Tp, _Lp>
      weak_from_this() noexcept
      { return this->_M_weak_this; }
 
      __weak_ptr<const _Tp, _Lp>
      weak_from_this() const noexcept
      { return this->_M_weak_this; }
#endif
 
    private:
      template<typename _Tp1>
        void
        _M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const noexcept
        { _M_weak_this._M_assign(__p, __n); }
 
      friend const __enable_shared_from_this*
      __enable_shared_from_this_base(const __shared_count<_Lp>&,
                                     const __enable_shared_from_this* __p)
      { return __p; }
 
      template<typename, _Lock_policy>
        friend class __shared_ptr;
 
      mutable __weak_ptr<_Tp, _Lp>  _M_weak_this;
    };
 
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy,
           typename _Alloc, typename... _Args>
    inline __shared_ptr<_Tp, _Lp>
    __allocate_shared(const _Alloc& __a, _Args&&... __args)
    {
      static_assert(!is_array<_Tp>::value, "make_shared<T[]> not supported");
 
      return __shared_ptr<_Tp, _Lp>(_Sp_alloc_shared_tag<_Alloc>{__a},
                                    std::forward<_Args>(__args)...);
    }
 
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy,
           typename... _Args>
    inline __shared_ptr<_Tp, _Lp>
    __make_shared(_Args&&... __args)
    {
      typedef typename std::remove_const<_Tp>::type _Tp_nc;
      return std::__allocate_shared<_Tp, _Lp>(std::allocator<_Tp_nc>(),
                                              std::forward<_Args>(__args)...);
    }
 
  /// std::hash specialization for __shared_ptr.
  template<typename _Tp, _Lock_policy _Lp>
    struct hash<__shared_ptr<_Tp, _Lp>>
    : public __hash_base<size_t, __shared_ptr<_Tp, _Lp>>
    {
      size_t
      operator()(const __shared_ptr<_Tp, _Lp>& __s) const noexcept
      {
        return hash<typename __shared_ptr<_Tp, _Lp>::element_type*>()(
            __s.get());
      }
    };
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
 
#endif // _SHARED_PTR_BASE_H