#ifndef BOOST_WEAK_PTR_HPP_INCLUDED

 #define BOOST_WEAK_PTR_HPP_INCLUDED

 //

 //  weak_ptr.hpp

 //

 //  Copyright (c) 2001, 2002, 2003 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)

 //

 //  See http://www.boost.org/libs/smart_ptr/weak_ptr.htm for documentation.

 //

 #include <memory> // boost.TR1 include order fix

 #include <boost/detail/shared_count.hpp>

 #include <boost/shared_ptr.hpp>

 #ifdef BOOST_MSVC  // moved here to work around VC++ compiler crash

 # pragma warning(push)

 # pragma warning(disable:4284) // odd return type for operator->

 #endif

 namespace boost

 {

 template<class T> class weak_ptr

 {

 private:

     // Borland 5.5.1 specific workarounds

     typedef weak_ptr<T> this_type;

 public:

     typedef T element_type;

     weak_ptr(): px(0), pn() // never throws in 1.30+

     {

     }

 //  generated copy constructor, assignment, destructor are fine

 //

 //  The "obvious" converting constructor implementation:

 //

 //  template<class Y>

 //  weak_ptr(weak_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws

 //  {

 //  }

 //

 //  has a serious problem.

 //

 //  r.px may already have been invalidated. The px(r.px)

 //  conversion may require access to *r.px (virtual inheritance).

 //

 //  It is not possible to avoid spurious access violations since

 //  in multithreaded programs r.px may be invalidated at any point.

 //

     template<class Y>

     weak_ptr(weak_ptr<Y> const & r): pn(r.pn) // never throws

     {

         px = r.lock().get();

     }

     template<class Y>

     weak_ptr(shared_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws

     {

     }

 #if !defined(BOOST_MSVC) || (BOOST_MSVC >= 1300)

     template<class Y>

     weak_ptr & operator=(weak_ptr<Y> const & r) // never throws

     {

         px = r.lock().get();

         pn = r.pn;

         return *this;

     }

     template<class Y>

     weak_ptr & operator=(shared_ptr<Y> const & r) // never throws

     {

         px = r.px;

         pn = r.pn;

         return *this;

     }

 #endif

     shared_ptr<T> lock() const // never throws

     {

 #if defined(BOOST_HAS_THREADS)

         // optimization: avoid throw overhead

         if(expired())

         {

             return shared_ptr<element_type>();

         }

         try

         {

             return shared_ptr<element_type>(*this);

         }

         catch(bad_weak_ptr const &)

         {

             // Q: how can we get here?

             // A: another thread may have invalidated r after the use_count test above.

             return shared_ptr<element_type>();

         }

 #else

         // optimization: avoid try/catch overhead when single threaded

         return expired()? shared_ptr<element_type>(): shared_ptr<element_type>(*this);

 #endif

     }

     long use_count() const // never throws

     {

         return pn.use_count();

     }

     bool expired() const // never throws

     {

         return pn.use_count() == 0;

     }

     void reset() // never throws in 1.30+

     {

         this_type().swap(*this);

     }

     void swap(this_type & other) // never throws

     {

         std::swap(px, other.px);

         pn.swap(other.pn);

     }

     void _internal_assign(T * px2, boost::detail::shared_count const & pn2)

     {

         px = px2;

         pn = pn2;

     }

     template<class Y> bool _internal_less(weak_ptr<Y> const & rhs) const

     {

         return pn < rhs.pn;

     }

 // Tasteless as this may seem, making all members public allows member templates

 // to work in the absence of member template friends. (Matthew Langston)

 #ifndef BOOST_NO_MEMBER_TEMPLATE_FRIENDS

 private:

     template<class Y> friend class weak_ptr;

     template<class Y> friend class shared_ptr;

 #endif

     T * px;                       // contained pointer

     boost::detail::weak_count pn; // reference counter

 };  // weak_ptr

 template<class T, class U> inline bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b)

 {

     return a._internal_less(b);

 }

 template<class T> void swap(weak_ptr<T> & a, weak_ptr<T> & b)

 {

     a.swap(b);

 }

 // deprecated, provided for backward compatibility

 template<class T> shared_ptr<T> make_shared(weak_ptr<T> const & r)

 {

     return r.lock();

 }

 } // namespace boost

 #ifdef BOOST_MSVC

 # pragma warning(pop)

 #endif    

 #endif  // #ifndef BOOST_WEAK_PTR_HPP_INCLUDED