xref: /freebsd-12.1/contrib/libstdc++/include/tr1/boost_shared_ptr.h (revision 05a636f0)

// <tr1/boost_shared_ptr.h> -*- C++ -*-

// Copyright (C) 2005, 2006 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 2, 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.

// You should have received a copy of the GNU General Public License along
// with this library; see the file COPYING.  If not, write to the Free
// Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
// USA.

// As a special exception, you may use this file as part of a free software
// library without restriction.  Specifically, if other files instantiate
// templates or use macros or inline functions from this file, or you compile
// this file and link it with other files to produce an executable, this
// file does not by itself cause the resulting executable to be covered by
// the GNU General Public License.  This exception does not however
// invalidate any other reasons why the executable file might be covered by
// the GNU General Public License.

//  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)

// GCC Note:  based on version 1.32.0 of the Boost library.

/** @file tr1/boost_shared_ptr.h
 *  This is an internal header file, included by other library headers.
 *  You should not attempt to use it directly.
 */

#ifndef _BOOST_SHARED_PTR_H
#define _BOOST_SHARED_PTR_H 1

namespace std
{
_GLIBCXX_BEGIN_NAMESPACE(tr1)

  class bad_weak_ptr : public std::exception
  {
  public:
    virtual char const*
    what() const throw()
    { return "tr1::bad_weak_ptr"; }
  };

  // Substitute for bad_weak_ptr object in the case of -fno-exceptions.
  inline void
  __throw_bad_weak_ptr()
  {
#if __EXCEPTIONS
    throw bad_weak_ptr();
#else
    std::abort();
#endif
  }

  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;

  template<typename _Tp>
    struct _Sp_deleter
    {
      typedef void result_type;
      typedef _Tp* argument_type;

      void
      operator()(_Tp* __p) const
      { delete __p; }
    };

  // Empty helper class except when the template argument is _S_mutex.
  template<_Lock_policy _Lp>
    class _Mutex_base
    { };

  template<>
    class _Mutex_base<_S_mutex>
    : public __gnu_cxx::__mutex
    { };

  template<_Lock_policy _Lp = __default_lock_policy>
    class _Sp_counted_base
    : public _Mutex_base<_Lp>
    {
    public:
      _Sp_counted_base()
      : _M_use_count(1), _M_weak_count(1) { }

      virtual
      ~_Sp_counted_base() // nothrow
      { }

      // Called when _M_use_count drops to zero, to release the resources
      // managed by *this.
      virtual void
      _M_dispose() = 0; // nothrow

      // Called when _M_weak_count drops to zero.
      virtual void
      _M_destroy() // nothrow
      { delete this; }

      virtual void*
      _M_get_deleter(const std::type_info&) = 0;

      void
      _M_add_ref_copy()
      { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }

      void
      _M_add_ref_lock();

      void
      _M_release() // nothrow
      {
	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count,
						   -1) == 1)
	  {
	    _M_dispose();
#ifdef __GTHREADS
	    _GLIBCXX_READ_MEM_BARRIER;
	    _GLIBCXX_WRITE_MEM_BARRIER;
#endif
	    if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count,
						       -1) == 1)
	      _M_destroy();
	  }
      }

      void
      _M_weak_add_ref() // nothrow
      { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }

      void
      _M_weak_release() // nothrow
      {
	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
	  {
#ifdef __GTHREADS
	    _GLIBCXX_READ_MEM_BARRIER;
	    _GLIBCXX_WRITE_MEM_BARRIER;
#endif
	    _M_destroy();
	  }
      }

      long
      _M_get_use_count() const // nothrow
      { return _M_use_count; }  // XXX is this MT safe?

    private:
      _Sp_counted_base(_Sp_counted_base const&);
      _Sp_counted_base& operator=(_Sp_counted_base const&);

      _Atomic_word  _M_use_count;     // #shared
      _Atomic_word  _M_weak_count;    // #weak + (#shared != 0)
    };

  template<>
    inline void
    _Sp_counted_base<_S_single>::
    _M_add_ref_lock()
    {
      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
	{
	  _M_use_count = 0;
	  __throw_bad_weak_ptr();
	}
    }

#ifdef __GTHREADS
  template<>
    inline void
    _Sp_counted_base<_S_mutex>::
    _M_add_ref_lock()
    {
      __gnu_cxx::__scoped_lock sentry(*this);
      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
	{
	  _M_use_count = 0;
	  __throw_bad_weak_ptr();
	}
    }
#endif

  template<>
    inline void
    _Sp_counted_base<_S_atomic>::
    _M_add_ref_lock()
    {
      // Perform lock-free add-if-not-zero operation.
      _Atomic_word __count;
      do
	{
	  __count = _M_use_count;
	  if (__count == 0)
	    __throw_bad_weak_ptr();

	  // Replace the current counter value with the old value + 1, as
	  // long as it's not changed meanwhile.
	}
      while (!__sync_bool_compare_and_swap(&_M_use_count, __count,
					   __count + 1));
    }

  template<typename _Ptr, typename _Deleter, _Lock_policy _Lp>
    class _Sp_counted_base_impl
    : public _Sp_counted_base<_Lp>
    {
    public:
      /**
       *  @brief
       *  @pre     __d(__p) must not throw.
       */
      _Sp_counted_base_impl(_Ptr __p, _Deleter __d)
      : _M_ptr(__p), _M_del(__d) { }

      virtual void
      _M_dispose() // nothrow
      { _M_del(_M_ptr); }

      virtual void*
      _M_get_deleter(const std::type_info& __ti)
      { return __ti == typeid(_Deleter) ? &_M_del : 0; }

    private:
      _Sp_counted_base_impl(const _Sp_counted_base_impl&);
      _Sp_counted_base_impl& operator=(const _Sp_counted_base_impl&);

      _Ptr      _M_ptr;  // copy constructor must not throw
      _Deleter  _M_del;  // copy constructor must not throw
    };

  template<_Lock_policy _Lp = __default_lock_policy>
    class __weak_count;

  template<_Lock_policy _Lp = __default_lock_policy>
    class __shared_count
    {
    public:
      __shared_count()
      : _M_pi(0) // nothrow
      { }

      template<typename _Ptr, typename _Deleter>
        __shared_count(_Ptr __p, _Deleter __d) : _M_pi(0)
        {
	  try
	    {
	      _M_pi = new _Sp_counted_base_impl<_Ptr, _Deleter, _Lp>(__p, __d);
	    }
	  catch(...)
	    {
	      __d(__p); // Call _Deleter on __p.
	      __throw_exception_again;
	    }
	}

      // Special case for auto_ptr<_Tp> to provide the strong guarantee.
      template<typename _Tp>
        explicit
        __shared_count(std::auto_ptr<_Tp>& __r)
	: _M_pi(new _Sp_counted_base_impl<_Tp*,
		_Sp_deleter<_Tp>, _Lp >(__r.get(), _Sp_deleter<_Tp>()))
        { __r.release(); }

      // Throw bad_weak_ptr when __r._M_get_use_count() == 0.
      explicit
      __shared_count(const __weak_count<_Lp>& __r);

      ~__shared_count() // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_release();
      }

      __shared_count(const __shared_count& __r)
      : _M_pi(__r._M_pi) // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_add_ref_copy();
      }

      __shared_count&
      operator=(const __shared_count& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != _M_pi)
	  {
	    if (__tmp != 0)
	      __tmp->_M_add_ref_copy();
	    if (_M_pi != 0)
	      _M_pi->_M_release();
	    _M_pi = __tmp;
	  }
	return *this;
      }

      void
      _M_swap(__shared_count& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	__r._M_pi = _M_pi;
	_M_pi = __tmp;
      }

      long
      _M_get_use_count() const // nothrow
      { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }

      bool
      _M_unique() const // nothrow
      { return this->_M_get_use_count() == 1; }

      friend inline bool
      operator==(const __shared_count& __a, const __shared_count& __b)
      { return __a._M_pi == __b._M_pi; }

      friend inline bool
      operator<(const __shared_count& __a, const __shared_count& __b)
      { return std::less<_Sp_counted_base<_Lp>*>()(__a._M_pi, __b._M_pi); }

      void*
      _M_get_deleter(const std::type_info& __ti) const
      { return _M_pi ? _M_pi->_M_get_deleter(__ti) : 0; }

    private:
      friend class __weak_count<_Lp>;

      _Sp_counted_base<_Lp>*  _M_pi;
    };

  template<_Lock_policy _Lp>
    class __weak_count
    {
    public:
      __weak_count()
      : _M_pi(0) // nothrow
      { }

      __weak_count(const __shared_count<_Lp>& __r)
      : _M_pi(__r._M_pi) // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_weak_add_ref();
      }

      __weak_count(const __weak_count<_Lp>& __r)
      : _M_pi(__r._M_pi) // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_weak_add_ref();
      }

      ~__weak_count() // nothrow
      {
	if (_M_pi != 0)
	  _M_pi->_M_weak_release();
      }

      __weak_count<_Lp>&
      operator=(const __shared_count<_Lp>& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != 0)
	  __tmp->_M_weak_add_ref();
	if (_M_pi != 0)
	  _M_pi->_M_weak_release();
	_M_pi = __tmp;
	return *this;
      }

      __weak_count<_Lp>&
      operator=(const __weak_count<_Lp>& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != 0)
	  __tmp->_M_weak_add_ref();
	if (_M_pi != 0)
	  _M_pi->_M_weak_release();
	_M_pi = __tmp;
	return *this;
      }

      void
      _M_swap(__weak_count<_Lp>& __r) // nothrow
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	__r._M_pi = _M_pi;
	_M_pi = __tmp;
      }

      long
      _M_get_use_count() const // nothrow
      { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; }

      friend inline bool
      operator==(const __weak_count<_Lp>& __a, const __weak_count<_Lp>& __b)
      { return __a._M_pi == __b._M_pi; }

      friend inline bool
      operator<(const __weak_count<_Lp>& __a, const __weak_count<_Lp>& __b)
      { return std::less<_Sp_counted_base<_Lp>*>()(__a._M_pi, __b._M_pi); }

    private:
      friend class __shared_count<_Lp>;

      _Sp_counted_base<_Lp>*  _M_pi;
    };

  template<_Lock_policy _Lp>
    inline
    __shared_count<_Lp>::
    __shared_count(const __weak_count<_Lp>& __r)
    : _M_pi(__r._M_pi)
    {
      if (_M_pi != 0)
	_M_pi->_M_add_ref_lock();
      else
	__throw_bad_weak_ptr();
    }


  // 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>
    class enable_shared_from_this;

  // Support for enable_shared_from_this.

  // Friend of __enable_shared_from_this.
  template<_Lock_policy _Lp, typename _Tp1, typename _Tp2>
    void
    __enable_shared_from_this_helper(const __shared_count<_Lp>&,
				     const __enable_shared_from_this<_Tp1,
				     _Lp>*, const _Tp2*);

  // Friend of enable_shared_from_this.
  template<typename _Tp1, typename _Tp2>
    void
    __enable_shared_from_this_helper(const __shared_count<>&,
				     const enable_shared_from_this<_Tp1>*,
				     const _Tp2*);

  template<_Lock_policy _Lp>
    inline void
    __enable_shared_from_this_helper(const __shared_count<_Lp>&, ...)
    { }


  struct __static_cast_tag { };
  struct __const_cast_tag { };
  struct __dynamic_cast_tag { };

  /**
   *  @class shared_ptr <tr1/memory>
   *
   *  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<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr
    {
    public:
      typedef _Tp   element_type;

      /** @brief  Construct an empty %__shared_ptr.
       *  @post   use_count()==0 && get()==0
       */
      __shared_ptr()
      : _M_ptr(0), _M_refcount() // never throws
      { }

      /** @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 && get() == __p
       *  @throw  std::bad_alloc, in which case @c delete @a __p is called.
       */
      template<typename _Tp1>
        explicit
        __shared_ptr(_Tp1* __p)
	: _M_ptr(__p), _M_refcount(__p, _Sp_deleter<_Tp1>())
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  // __glibcxx_function_requires(_CompleteConcept<_Tp1*>)
	  __enable_shared_from_this_helper(_M_refcount, __p, __p);
	}

      //
      // Requirements: _Deleter' copy constructor and destructor must not throw
      //
      // __shared_ptr will release __p by calling __d(__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 && get() == __p
       *  @throw  std::bad_alloc, in which case @a __d(__p) is called.
       */
      template<typename _Tp1, typename _Deleter>
        __shared_ptr(_Tp1* __p, _Deleter __d)
	: _M_ptr(__p), _M_refcount(__p, __d)
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  // TODO requires _Deleter CopyConstructible and __d(__p) well-formed
	  __enable_shared_from_this_helper(_M_refcount, __p, __p);
	}

      //  generated copy constructor, assignment, destructor are fine.

      /** @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() && use_count() == __r.use_count()
       *  @throw  std::bad_alloc, in which case
       */
      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r)
	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
        { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) }

      /** @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<typename _Tp1>
        explicit
        __shared_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
	: _M_refcount(__r._M_refcount) // may throw
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  // It is now safe to copy __r._M_ptr, as _M_refcount(__r._M_refcount)
	  // did not throw.
	  _M_ptr = __r._M_ptr;
	}

      /**
       * @post use_count() == 1 and __r.get() == 0
       */
      template<typename _Tp1>
        explicit
        __shared_ptr(std::auto_ptr<_Tp1>& __r)
	: _M_ptr(__r.get()), _M_refcount()
        {
	  // TODO requires __r.release() convertible to _Tp*, _Tp1 is complete,
	  // delete __r.release() well-formed
	  _Tp1* __tmp = __r.get();
	  _M_refcount = __shared_count<_Lp>(__r);
	  __enable_shared_from_this_helper(_M_refcount, __tmp, __tmp);
	}

      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __static_cast_tag)
	: _M_ptr(static_cast<element_type*>(__r._M_ptr)),
	  _M_refcount(__r._M_refcount)
        { }

      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __const_cast_tag)
	: _M_ptr(const_cast<element_type*>(__r._M_ptr)),
	  _M_refcount(__r._M_refcount)
        { }

      template<typename _Tp1>
        __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __dynamic_cast_tag)
	: _M_ptr(dynamic_cast<element_type*>(__r._M_ptr)),
	  _M_refcount(__r._M_refcount)
        {
	  if (_M_ptr == 0) // need to allocate new counter -- the cast failed
	    _M_refcount = __shared_count<_Lp>();
	}

      template<typename _Tp1>
        __shared_ptr&
        operator=(const __shared_ptr<_Tp1, _Lp>& __r) // never throws
        {
	  _M_ptr = __r._M_ptr;
	  _M_refcount = __r._M_refcount; // __shared_count::op= doesn't throw
	  return *this;
	}

      template<typename _Tp1>
        __shared_ptr&
        operator=(std::auto_ptr<_Tp1>& __r)
        {
	  __shared_ptr(__r).swap(*this);
	  return *this;
	}

      void
      reset() // never throws
      { __shared_ptr().swap(*this); }

      template<typename _Tp1>
        void
        reset(_Tp1* __p) // _Tp1 must be complete.
        {
	  // Catch self-reset errors.
	  _GLIBCXX_DEBUG_ASSERT(__p == 0 || __p != _M_ptr);
	  __shared_ptr(__p).swap(*this);
	}

      template<typename _Tp1, typename _Deleter>
        void
        reset(_Tp1* __p, _Deleter __d)
        { __shared_ptr(__p, __d).swap(*this); }

      // Allow class instantiation when _Tp is [cv-qual] void.
      typename add_reference<_Tp>::type
      operator*() const // never throws
      {
	_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
	return *_M_ptr;
      }

      _Tp*
      operator->() const // never throws
      {
	_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
	return _M_ptr;
      }

      _Tp*
      get() const // never throws
      { return _M_ptr; }

      // Implicit conversion to "bool"
    private:
      typedef _Tp* __shared_ptr::*__unspecified_bool_type;

    public:
      operator __unspecified_bool_type() const // never throws
      { return _M_ptr == 0 ? 0 : &__shared_ptr::_M_ptr; }

      bool
      unique() const // never throws
      { return _M_refcount._M_unique(); }

      long
      use_count() const // never throws
      { return _M_refcount._M_get_use_count(); }

      void
      swap(__shared_ptr<_Tp, _Lp>& __other) // never throws
      {
	std::swap(_M_ptr, __other._M_ptr);
	_M_refcount._M_swap(__other._M_refcount);
      }

    private:
      void*
      _M_get_deleter(const std::type_info& __ti) const
      { return _M_refcount._M_get_deleter(__ti); }

      template<typename _Tp1, _Lock_policy _Lp1>
        bool
        _M_less(const __shared_ptr<_Tp1, _Lp1>& __rhs) const
        { return _M_refcount < __rhs._M_refcount; }

      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>&);

      // Friends injected into enclosing namespace and found by ADL:
      template<typename _Tp1>
        friend inline bool
        operator==(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b)
        { return __a.get() == __b.get(); }

      template<typename _Tp1>
        friend inline bool
        operator!=(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b)
        { return __a.get() != __b.get(); }

      template<typename _Tp1>
        friend inline bool
        operator<(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b)
        { return __a._M_less(__b); }

      _Tp*         	   _M_ptr;         // Contained pointer.
      __shared_count<_Lp>  _M_refcount;    // Reference counter.
    };

  // 2.2.3.8 shared_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b)
    { __a.swap(__b); }

  // 2.2.3.9 shared_ptr casts
  /** @warning The seemingly equivalent
   *           <code>shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get()))</code>
   *           will eventually result in undefined behaviour,
   *           attempting to delete the same object twice.
   */
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    __shared_ptr<_Tp, _Lp>
    static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r)
    { return __shared_ptr<_Tp, _Lp>(__r, __static_cast_tag()); }

  /** @warning The seemingly equivalent
   *           <code>shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get()))</code>
   *           will eventually result in undefined behaviour,
   *           attempting to delete the same object twice.
   */
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    __shared_ptr<_Tp, _Lp>
    const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r)
    { return __shared_ptr<_Tp, _Lp>(__r, __const_cast_tag()); }

  /** @warning The seemingly equivalent
   *           <code>shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get()))</code>
   *           will eventually result in undefined behaviour,
   *           attempting to delete the same object twice.
   */
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    __shared_ptr<_Tp, _Lp>
    dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r)
    { return __shared_ptr<_Tp, _Lp>(__r, __dynamic_cast_tag()); }

  // 2.2.3.7 shared_ptr I/O
  template<typename _Ch, typename _Tr, typename _Tp, _Lock_policy _Lp>
    std::basic_ostream<_Ch, _Tr>&
    operator<<(std::basic_ostream<_Ch, _Tr>& __os,
	       const __shared_ptr<_Tp, _Lp>& __p)
    {
      __os << __p.get();
      return __os;
    }

  // 2.2.3.10 shared_ptr get_deleter (experimental)
  template<typename _Del, typename _Tp, _Lock_policy _Lp>
    inline _Del*
    get_deleter(const __shared_ptr<_Tp, _Lp>& __p)
    { return static_cast<_Del*>(__p._M_get_deleter(typeid(_Del))); }


  template<typename _Tp, _Lock_policy _Lp>
    class __weak_ptr
    {
    public:
      typedef _Tp element_type;

      __weak_ptr()
      : _M_ptr(0), _M_refcount() // never throws
      { }

      // Generated copy constructor, assignment, destructor are fine.

      // 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 _Tp1>
        __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
	: _M_refcount(__r._M_refcount) // never throws
        {
	  __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)
	  _M_ptr = __r.lock().get();
	}

      template<typename _Tp1>
        __weak_ptr(const __shared_ptr<_Tp1, _Lp>& __r)
	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
        { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) }

      template<typename _Tp1>
        __weak_ptr&
        operator=(const __weak_ptr<_Tp1, _Lp>& __r) // never throws
        {
	  _M_ptr = __r.lock().get();
	  _M_refcount = __r._M_refcount;
	  return *this;
	}

      template<typename _Tp1>
        __weak_ptr&
        operator=(const __shared_ptr<_Tp1, _Lp>& __r) // never throws
        {
	  _M_ptr = __r._M_ptr;
	  _M_refcount = __r._M_refcount;
	  return *this;
	}

      __shared_ptr<_Tp, _Lp>
      lock() const // never throws
      {
#ifdef __GTHREADS
	// Optimization: avoid throw overhead.
	if (expired())
	  return __shared_ptr<element_type, _Lp>();

	try
	  {
	    return __shared_ptr<element_type, _Lp>(*this);
	  }
	catch(const bad_weak_ptr&)
	  {
	    // Q: How can we get here?
	    // A: Another thread may have invalidated r after the
	    //    use_count test above.
	    return __shared_ptr<element_type, _Lp>();
	  }

#else
	// Optimization: avoid try/catch overhead when single threaded.
	return expired() ? __shared_ptr<element_type, _Lp>()
	                 : __shared_ptr<element_type, _Lp>(*this);

#endif
      } // XXX MT

      long
      use_count() const // never throws
      { return _M_refcount._M_get_use_count(); }

      bool
      expired() const // never throws
      { return _M_refcount._M_get_use_count() == 0; }

      void
      reset() // never throws
      { __weak_ptr().swap(*this); }

      void
      swap(__weak_ptr& __s) // never throws
      {
	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)
      {
	_M_ptr = __ptr;
	_M_refcount = __refcount;
      }

      template<typename _Tp1>
        bool
        _M_less(const __weak_ptr<_Tp1, _Lp>& __rhs) const
        { return _M_refcount < __rhs._M_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>;

      // Friend injected into namespace and found by ADL.
      template<typename _Tp1>
        friend inline bool
        operator<(const __weak_ptr& __lhs, const __weak_ptr<_Tp1, _Lp>& __rhs)
        { return __lhs._M_less(__rhs); }

      _Tp*       	 _M_ptr;         // Contained pointer.
      __weak_count<_Lp>  _M_refcount;    // Reference counter.
    };

  // 2.2.4.7 weak_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b)
    { __a.swap(__b); }


  template<typename _Tp, _Lock_policy _Lp>
    class __enable_shared_from_this
    {
    protected:
      __enable_shared_from_this() { }

      __enable_shared_from_this(const __enable_shared_from_this&) { }

      __enable_shared_from_this&
      operator=(const __enable_shared_from_this&)
      { 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); }

    private:
      template<typename _Tp1>
        void
        _M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const
        { _M_weak_this._M_assign(__p, __n); }

      template<typename _Tp1>
        friend void
        __enable_shared_from_this_helper(const __shared_count<_Lp>& __pn,
					 const __enable_shared_from_this* __pe,
					 const _Tp1* __px)
        {
	  if (__pe != 0)
	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
	}

      mutable __weak_ptr<_Tp, _Lp>  _M_weak_this;
    };


  // The actual TR1 shared_ptr, with forwarding constructors and
  // assignment operators.
  template<typename _Tp>
    class shared_ptr
    : public __shared_ptr<_Tp>
    {
    public:
      shared_ptr()
      : __shared_ptr<_Tp>() { }

      template<typename _Tp1>
        explicit
        shared_ptr(_Tp1* __p)
	: __shared_ptr<_Tp>(__p) { }

      template<typename _Tp1, typename _Deleter>
        shared_ptr(_Tp1* __p, _Deleter __d)
	: __shared_ptr<_Tp>(__p, __d) { }

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r)
	: __shared_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        explicit
        shared_ptr(const weak_ptr<_Tp1>& __r)
	: __shared_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        explicit
        shared_ptr(std::auto_ptr<_Tp1>& __r)
	: __shared_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r, __static_cast_tag)
	: __shared_ptr<_Tp>(__r, __static_cast_tag()) { }

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r, __const_cast_tag)
	: __shared_ptr<_Tp>(__r, __const_cast_tag()) { }

      template<typename _Tp1>
        shared_ptr(const shared_ptr<_Tp1>& __r, __dynamic_cast_tag)
	: __shared_ptr<_Tp>(__r, __dynamic_cast_tag()) { }

      template<typename _Tp1>
        shared_ptr&
        operator=(const shared_ptr<_Tp1>& __r) // never throws
        {
	  this->__shared_ptr<_Tp>::operator=(__r);
	  return *this;
	}

      template<typename _Tp1>
        shared_ptr&
        operator=(std::auto_ptr<_Tp1>& __r)
        {
	  this->__shared_ptr<_Tp>::operator=(__r);
	  return *this;
	}
    };

  template<typename _Tp, typename _Tp1>
    shared_ptr<_Tp>
    static_pointer_cast(const shared_ptr<_Tp1>& __r)
    { return shared_ptr<_Tp>(__r, __static_cast_tag()); }

  template<typename _Tp, typename _Tp1>
    shared_ptr<_Tp>
    const_pointer_cast(const shared_ptr<_Tp1>& __r)
    { return shared_ptr<_Tp>(__r, __const_cast_tag()); }

  template<typename _Tp, typename _Tp1>
    shared_ptr<_Tp>
    dynamic_pointer_cast(const shared_ptr<_Tp1>& __r)
    { return shared_ptr<_Tp>(__r, __dynamic_cast_tag()); }


  // The actual TR1 weak_ptr, with forwarding constructors and
  // assignment operators.
  template<typename _Tp>
    class weak_ptr
    : public __weak_ptr<_Tp>
    {
    public:
      weak_ptr()
      : __weak_ptr<_Tp>() { }

      template<typename _Tp1>
        weak_ptr(const weak_ptr<_Tp1>& __r)
	: __weak_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        weak_ptr(const shared_ptr<_Tp1>& __r)
	: __weak_ptr<_Tp>(__r) { }

      template<typename _Tp1>
        weak_ptr&
        operator=(const weak_ptr<_Tp1>& __r) // never throws
        {
	  this->__weak_ptr<_Tp>::operator=(__r);
	  return *this;
	}

      template<typename _Tp1>
        weak_ptr&
        operator=(const shared_ptr<_Tp1>& __r) // never throws
        {
	  this->__weak_ptr<_Tp>::operator=(__r);
	  return *this;
	}

      shared_ptr<_Tp>
      lock() const // never throws
      {
#ifdef __GTHREADS
	if (this->expired())
	  return shared_ptr<_Tp>();

	try
	  {
	    return shared_ptr<_Tp>(*this);
	  }
	catch(const bad_weak_ptr&)
	  {
	    return shared_ptr<_Tp>();
	  }
#else
	return this->expired() ? shared_ptr<_Tp>()
	                       : shared_ptr<_Tp>(*this);
#endif
      }
    };


  template<typename _Tp>
    class enable_shared_from_this
    {
    protected:
      enable_shared_from_this() { }

      enable_shared_from_this(const enable_shared_from_this&) { }

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

      ~enable_shared_from_this() { }

    public:
      shared_ptr<_Tp>
      shared_from_this()
      { return shared_ptr<_Tp>(this->_M_weak_this); }

      shared_ptr<const _Tp>
      shared_from_this() const
      { return shared_ptr<const _Tp>(this->_M_weak_this); }

    private:
      template<typename _Tp1>
        void
        _M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const
        { _M_weak_this._M_assign(__p, __n); }

      template<typename _Tp1>
        friend void
        __enable_shared_from_this_helper(const __shared_count<>& __pn,
					 const enable_shared_from_this* __pe,
					 const _Tp1* __px)
        {
	  if (__pe != 0)
	    __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
	}

      mutable weak_ptr<_Tp>  _M_weak_this;
    };

_GLIBCXX_END_NAMESPACE
} // namespace std

#endif