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refptr.h

// -*- c++ -*-
#ifndef _GLIBMM_REFPTR_H
#define _GLIBMM_REFPTR_H

/* $Id: refptr.h 4 2005-05-13 20:47:18Z taybin $ */

/* Copyright 2002 The gtkmm Development Team
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, 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
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the Free
 * Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */


namespace Glib
{

/** RefPtr<> is a reference-counting shared smartpointer.
 *
 * Some objects in gtkmm are obtained from a shared
 * store. Consequently you cannot instantiate them yourself. Instead they
 * return a RefPtr which behaves much like an ordinary pointer in that members
 * can be reached with the usual <code>object_ptr->member</code> notation.
 * Unlike most other smart pointers, RefPtr doesn't support dereferencing
 * through <code>*object_ptr</code>.
 *
 * Reference counting means that a shared reference count is incremented each
 * time a RefPtr is copied, and decremented each time a RefPtr is destroyed,
 * for instance when it leaves its scope. When the reference count reaches
 * zero, the contained object is deleted, meaning  you don't need to remember
 * to delete the object.
 *
 * RefPtr<> can store any class that has reference() and unreference() methods.
 * In gtkmm, that is anything derived from Glib::ObjectBase, such as
 * Gdk::Pixmap.
 *
 * See the "Memory Management" section in the "Programming with gtkmm"
 * book for further information.
 */
template <class T_CppObject>
00051 class RefPtr
{
public:
  /** Default constructor
   *
   * Afterwards it will be null and use of -> will cause a segmentation fault.
   */
  inline RefPtr();
  
  /// Destructor - decrements reference count.
  inline ~RefPtr();

  /// For use only by the ::create() methods.
  explicit inline RefPtr(T_CppObject* pCppObject);

  /** Copy constructor
   *
   * This increments the shared reference count.
   */
  inline RefPtr(const RefPtr<T_CppObject>& src);

  /** Copy constructor (from different, but castable type).
   *
   * Increments the reference count.
   */
  template <class T_CastFrom>
  inline RefPtr(const RefPtr<T_CastFrom>& src);

  /** Swap the contents of two RefPtr<>.
   * This method swaps the internal pointers to T_CppObject.  This can be
   * done safely without involving a reference/unreference cycle and is
   * therefore highly efficient.
   */
  inline void swap(RefPtr<T_CppObject>& other);

  /// Copy from another RefPtr:
  inline RefPtr<T_CppObject>& operator=(const RefPtr<T_CppObject>& src);

  /** Copy from different, but castable type).
   *
   * Increments the reference count.
   */
  template <class T_CastFrom>
  inline RefPtr<T_CppObject>& operator=(const RefPtr<T_CastFrom>& src);

  /// Tests whether the RefPtr<> point to the same underlying instance.
  inline bool operator==(const RefPtr<T_CppObject>& src) const;
  
  /// See operator==().
  inline bool operator!=(const RefPtr<T_CppObject>& src) const;

  /** Dereferencing.
   *
   * Use the methods of the underlying instance like so:
   * <code>refptr->memberfun()</code>.
   */
  inline T_CppObject* operator->() const;

  /** Test whether the RefPtr<> points to any underlying instance.
   *
   * Mimics usage of ordinary pointers:
   * @code
   *   if (ptr)
   *     do_something();
   * @endcode
   */
  inline operator bool() const;

  /// Set underlying instance to 0, decrementing reference count of existing instance appropriately.
  inline void clear();


  /** Dynamic cast to derived class.
   *
   * The RefPtr can't be cast with the usual notation so instead you can use
   * @code
   *   ptr_derived = RefPtr<Derived>::cast_dynamic(ptr_base);
   * @endcode
   */
  template <class T_CastFrom>
  static inline RefPtr<T_CppObject> cast_dynamic(const RefPtr<T_CastFrom>& src);

  /** Static cast to derived class.
   *
   * Like the dynamic cast; the notation is 
   * @code
   *   ptr_derived = RefPtr<Derived>::cast_static(ptr_base);
   * @endcode
   */
  template <class T_CastFrom>
  static inline RefPtr<T_CppObject> cast_static(const RefPtr<T_CastFrom>& src);

private:
  T_CppObject* pCppObject_;
};


#ifndef DOXYGEN_SHOULD_SKIP_THIS

// RefPtr<>::operator->() comes first here since it's used by other methods.
// If it would come after them it wouldn't be inlined.

template <class T_CppObject> inline
00154 T_CppObject* RefPtr<T_CppObject>::operator->() const
{
  return pCppObject_;
}

template <class T_CppObject> inline
00160 RefPtr<T_CppObject>::RefPtr()
:
  pCppObject_ (0)
{}

template <class T_CppObject> inline
00166 RefPtr<T_CppObject>::~RefPtr()
{
  if(pCppObject_)
    pCppObject_->unreference(); // This could cause pCppObject to be deleted.
}

template <class T_CppObject> inline
00173 RefPtr<T_CppObject>::RefPtr(T_CppObject* pCppObject)
:
  pCppObject_ (pCppObject)
{}

template <class T_CppObject> inline
00179 RefPtr<T_CppObject>::RefPtr(const RefPtr<T_CppObject>& src)
:
  pCppObject_ (src.pCppObject_)
{
  if(pCppObject_)
    pCppObject_->reference();
}

// The templated ctor allows copy construction from any object that's
// castable.  Thus, it does downcasts:
//   base_ref = derived_ref
template <class T_CppObject>
  template <class T_CastFrom>
inline
00193 RefPtr<T_CppObject>::RefPtr(const RefPtr<T_CastFrom>& src)
:
  // A different RefPtr<> will not allow us access to pCppObject_.  We need
  // to add a get_underlying() for this, but that would encourage incorrect
  // use, so we use the less well-known operator->() accessor:
  pCppObject_ (src.operator->())
{
  if(pCppObject_)
    pCppObject_->reference();
}

template <class T_CppObject> inline
00205 void RefPtr<T_CppObject>::swap(RefPtr<T_CppObject>& other)
{
  T_CppObject *const temp = pCppObject_;
  pCppObject_ = other.pCppObject_;
  other.pCppObject_ = temp;
}

template <class T_CppObject> inline
00213 RefPtr<T_CppObject>& RefPtr<T_CppObject>::operator=(const RefPtr<T_CppObject>& src)
{
  // In case you haven't seen the swap() technique to implement copy
  // assignment before, here's what it does:
  //
  // 1) Create a temporary RefPtr<> instance via the copy ctor, thereby
  //    increasing the reference count of the source object.
  //
  // 2) Swap the internal object pointers of *this and the temporary
  //    RefPtr<>.  After this step, *this already contains the new pointer,
  //    and the old pointer is now managed by temp.
  //
  // 3) The destructor of temp is executed, thereby unreferencing the
  //    old object pointer.
  //
  // This technique is described in Herb Sutter's "Exceptional C++", and
  // has a number of advantages over conventional approaches:
  //
  // - Code reuse by calling the copy ctor.
  // - Strong exception safety for free.
  // - Self assignment is handled implicitely.
  // - Simplicity.
  // - It just works and is hard to get wrong; i.e. you can use it without
  //   even thinking about it to implement copy assignment whereever the
  //   object data is managed indirectly via a pointer, which is very common.

  RefPtr<T_CppObject> temp (src);
  this->swap(temp);
  return *this;
}

template <class T_CppObject>
  template <class T_CastFrom>
inline
00247 RefPtr<T_CppObject>& RefPtr<T_CppObject>::operator=(const RefPtr<T_CastFrom>& src)
{
  RefPtr<T_CppObject> temp (src);
  this->swap(temp);
  return *this;
}

template <class T_CppObject> inline
00255 bool RefPtr<T_CppObject>::operator==(const RefPtr<T_CppObject>& src) const
{
  return (pCppObject_ == src.pCppObject_);
}

template <class T_CppObject> inline
00261 bool RefPtr<T_CppObject>::operator!=(const RefPtr<T_CppObject>& src) const
{
  return (pCppObject_ != src.pCppObject_);
}

template <class T_CppObject> inline
00267 RefPtr<T_CppObject>::operator bool() const
{
  return (pCppObject_ != 0);
}

template <class T_CppObject> inline
00273 void RefPtr<T_CppObject>::clear()
{
  RefPtr<T_CppObject> temp; // swap with an empty RefPtr<> to clear *this
  this->swap(temp);
}

template <class T_CppObject>
  template <class T_CastFrom>
inline
00282 RefPtr<T_CppObject> RefPtr<T_CppObject>::cast_dynamic(const RefPtr<T_CastFrom>& src)
{
  T_CppObject *const pCppObject = dynamic_cast<T_CppObject*>(src.operator->());

  if(pCppObject)
    pCppObject->reference();

  return RefPtr<T_CppObject>(pCppObject);
}

template <class T_CppObject>
  template <class T_CastFrom>
inline
00295 RefPtr<T_CppObject> RefPtr<T_CppObject>::cast_static(const RefPtr<T_CastFrom>& src)
{
  T_CppObject *const pCppObject = static_cast<T_CppObject*>(src.operator->());

  if(pCppObject)
    pCppObject->reference();

  return RefPtr<T_CppObject>(pCppObject);
}

#endif /* DOXYGEN_SHOULD_SKIP_THIS */

/** @relates Glib::RefPtr */
template <class T_CppObject> inline
00309 void swap(RefPtr<T_CppObject>& lhs, RefPtr<T_CppObject>& rhs)
{
  lhs.swap(rhs);
}

} // namespace Glib


#endif /* _GLIBMM_REFPTR_H */


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