boost_shared_ptr.h

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    /** @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( _M_refcount, __p, __p );
      }

    //
    // Requirements: D's 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 D is CopyConstructible and d(p) well-formed

        __enable_shared_from_this( _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>& __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>& __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(__r);

        __enable_shared_from_this( _M_refcount, __tmp, __tmp );
      }

    template<typename _Tp1>
      shared_ptr(const shared_ptr<_Tp1>& __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>& __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>& __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();
      }

    template<typename _Tp1>
      shared_ptr&
      operator=(const shared_ptr<_Tp1>& __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
      {
        _GLIBCXX_DEBUG_ASSERT(__p == 0 || __p != _M_ptr); // catch self-reset
	                                                  // errors
        shared_ptr(__p).swap(*this);
      }

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

    // error to instantiate if _Tp is [cv-qual] void
    _Reference
    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.unique(); }

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

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

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

    template<typename _Tp1>
      bool
      _M_less(const shared_ptr<_Tp1>& __rhs) const
      { return _M_refcount < __rhs._M_refcount; }

    template<typename _Tp1> friend class shared_ptr;
    template<typename _Tp1> friend class weak_ptr;

    template<typename _Del, typename _Tp1>
      friend _Del* get_deleter(const shared_ptr<_Tp1>&);

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

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

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

    _Tp*         _M_ptr;         // contained pointer
    shared_count _M_refcount;    // reference counter
  };  // shared_ptr

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

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

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

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

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

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


template<typename _Tp>
  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<class Y>
    //    weak_ptr(weak_ptr<Y> const & 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>& 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>& 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>& 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>& r) // never throws
      {
        _M_ptr = r._M_ptr;
        _M_refcount = r._M_refcount;
        return *this;
      }

    shared_ptr<_Tp>
    lock() const // never throws
    {
#ifdef __GTHREADS

      // optimization: avoid throw overhead
      if (expired())
	return shared_ptr<element_type>();
      
      try
	{
	  return shared_ptr<element_type>(*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>();
	}

#else

      // optimization: avoid try/catch overhead when single threaded
      return expired() ? shared_ptr<element_type>()
	               : shared_ptr<element_type>(*this);

#endif
    } // XXX MT

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

    bool
    expired() const // never throws
    { return _M_refcount.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.swap(__s._M_refcount);
    }

  private:

    template<typename _Tp1>
      bool
      _M_less(const weak_ptr<_Tp1>& __rhs) const
      { return _M_refcount < __rhs._M_refcount; }

    // used by __enable_shared_from_this
    void
    _M_assign(_Tp* __ptr, const shared_count& __refcount)
    {
      _M_ptr = __ptr;
      _M_refcount = __refcount;
    }

    // friend injected into namespace and found by ADL

    template<typename _Tp1>
      friend inline bool
      operator<(const weak_ptr& __lhs, const weak_ptr<_Tp1>& __rhs)
      { return __lhs._M_less(__rhs); }

    template<typename _Tp1> friend class weak_ptr;
    template<typename _Tp1> friend class shared_ptr;
    friend class enable_shared_from_this<_Tp>;

    _Tp*       _M_ptr;           // contained pointer
    weak_count _M_refcount;      // reference counter

  };  // weak_ptr

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


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

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

  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(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;
  };

} // namespace tr1
} // namespace std

#endif