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+#ifndef	_weakpointer_h_
+#define	_weakpointer_h_
+
+/****************************************************************************
+
+WeakPointer and CleanUp
+
+    Copyright (c) 1991 by Xerox Corporation.  All rights reserved.
+
+    THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
+    OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
+
+    Permission is hereby granted to copy this code for any purpose,
+    provided the above notices are retained on all copies.
+
+    Last modified on Mon Jul 17 18:16:01 PDT 1995 by ellis
+
+****************************************************************************/
+
+/****************************************************************************
+
+WeakPointer
+
+A weak pointer is a pointer to a heap-allocated object that doesn't
+prevent the object from being garbage collected. Weak pointers can be
+used to track which objects haven't yet been reclaimed by the
+collector. A weak pointer is deactivated when the collector discovers
+its referent object is unreachable by normal pointers (reachability
+and deactivation are defined more precisely below). A deactivated weak
+pointer remains deactivated forever.
+
+****************************************************************************/
+
+
+template< class T > class WeakPointer {
+public:
+
+WeakPointer( T* t = 0 )
+    /* Constructs a weak pointer for *t. t may be null. It is an error
+       if t is non-null and *t is not a collected object. */
+    {impl = _WeakPointer_New( t );}
+
+T* Pointer()
+    /* wp.Pointer() returns a pointer to the referent object of wp or
+       null if wp has been deactivated (because its referent object
+       has been discovered unreachable by the collector). */
+    {return (T*) _WeakPointer_Pointer( this->impl );}
+
+int operator==( WeakPointer< T > wp2 )
+    /* Given weak pointers wp1 and wp2, if wp1 == wp2, then wp1 and
+       wp2 refer to the same object. If wp1 != wp2, then either wp1
+       and wp2 don't refer to the same object, or if they do, one or
+       both of them has been deactivated. (Note: If objects t1 and t2
+       are never made reachable by their clean-up functions, then
+       WeakPointer<T>(t1) == WeakPointer<T>(t2) if and only t1 == t2.) */
+    {return _WeakPointer_Equal( this->impl, wp2.impl );}
+
+int Hash()
+    /* Returns a hash code suitable for use by multiplicative- and
+       division-based hash tables. If wp1 == wp2, then wp1.Hash() ==
+       wp2.Hash(). */
+    {return _WeakPointer_Hash( this->impl );}
+
+private:
+void* impl;
+};
+
+/*****************************************************************************
+
+CleanUp
+
+A garbage-collected object can have an associated clean-up function
+that will be invoked some time after the collector discovers the
+object is unreachable via normal pointers. Clean-up functions can be
+used to release resources such as open-file handles or window handles
+when their containing objects become unreachable.  If a C++ object has
+a non-empty explicit destructor (i.e. it contains programmer-written
+code), the destructor will be automatically registered as the object's
+initial clean-up function.
+
+There is no guarantee that the collector will detect every unreachable
+object (though it will find almost all of them). Clients should not
+rely on clean-up to cause some action to occur immediately -- clean-up
+is only a mechanism for improving resource usage.
+
+Every object with a clean-up function also has a clean-up queue. When
+the collector finds the object is unreachable, it enqueues it on its
+queue. The clean-up function is applied when the object is removed
+from the queue. By default, objects are enqueued on the garbage
+collector's queue, and the collector removes all objects from its
+queue after each collection. If a client supplies another queue for
+objects, it is his responsibility to remove objects (and cause their
+functions to be called) by polling it periodically.
+
+Clean-up queues allow clean-up functions accessing global data to
+synchronize with the main program. Garbage collection can occur at any
+time, and clean-ups invoked by the collector might access data in an
+inconsistent state. A client can control this by defining an explicit
+queue for objects and polling it at safe points.
+
+The following definitions are used by the specification below:
+
+Given a pointer t to a collected object, the base object BO(t) is the
+value returned by new when it created the object. (Because of multiple
+inheritance, t and BO(t) may not be the same address.)
+
+A weak pointer wp references an object *t if BO(wp.Pointer()) ==
+BO(t).
+
+***************************************************************************/
+
+template< class T, class Data > class CleanUp {
+public:
+
+static void Set( T* t, void c( Data* d, T* t ), Data* d = 0 )
+    /* Sets the clean-up function of object BO(t) to be <c, d>,
+       replacing any previously defined clean-up function for BO(t); c
+       and d can be null, but t cannot. Sets the clean-up queue for
+       BO(t) to be the collector's queue. When t is removed from its
+       clean-up queue, its clean-up will be applied by calling c(d,
+       t). It is an error if *t is not a collected object. */ 
+       {_CleanUp_Set( t, c, d );}
+
+static void Call( T* t )
+    /* Sets the new clean-up function for BO(t) to be null and, if the
+       old one is non-null, calls it immediately, even if BO(t) is
+       still reachable. Deactivates any weak pointers to BO(t). */
+       {_CleanUp_Call( t );}
+
+class Queue {public:
+    Queue()
+        /* Constructs a new queue. */
+            {this->head = _CleanUp_Queue_NewHead();}
+
+    void Set( T* t )
+        /* q.Set(t) sets the clean-up queue of BO(t) to be q. */
+            {_CleanUp_Queue_Set( this->head, t );}
+
+    int Call()
+        /* If q is non-empty, q.Call() removes the first object and
+           calls its clean-up function; does nothing if q is
+           empty. Returns true if there are more objects in the
+           queue. */
+           {return _CleanUp_Queue_Call( this->head );}
+
+    private:
+    void* head;
+    };
+};
+
+/**********************************************************************
+
+Reachability and Clean-up
+
+An object O is reachable if it can be reached via a non-empty path of
+normal pointers from the registers, stacks, global variables, or an
+object with a non-null clean-up function (including O itself),
+ignoring pointers from an object to itself.
+
+This definition of reachability ensures that if object B is accessible
+from object A (and not vice versa) and if both A and B have clean-up
+functions, then A will always be cleaned up before B. Note that as
+long as an object with a clean-up function is contained in a cycle of
+pointers, it will always be reachable and will never be cleaned up or
+collected.
+
+When the collector finds an unreachable object with a null clean-up
+function, it atomically deactivates all weak pointers referencing the
+object and recycles its storage. If object B is accessible from object
+A via a path of normal pointers, A will be discovered unreachable no
+later than B, and a weak pointer to A will be deactivated no later
+than a weak pointer to B.
+
+When the collector finds an unreachable object with a non-null
+clean-up function, the collector atomically deactivates all weak
+pointers referencing the object, redefines its clean-up function to be
+null, and enqueues it on its clean-up queue. The object then becomes
+reachable again and remains reachable at least until its clean-up
+function executes.
+
+The clean-up function is assured that its argument is the only
+accessible pointer to the object. Nothing prevents the function from
+redefining the object's clean-up function or making the object
+reachable again (for example, by storing the pointer in a global
+variable).
+
+If the clean-up function does not make its object reachable again and
+does not redefine its clean-up function, then the object will be
+collected by a subsequent collection (because the object remains
+unreachable and now has a null clean-up function). If the clean-up
+function does make its object reachable again and a clean-up function
+is subsequently redefined for the object, then the new clean-up
+function will be invoked the next time the collector finds the object
+unreachable.
+
+Note that a destructor for a collected object cannot safely redefine a
+clean-up function for its object, since after the destructor executes,
+the object has been destroyed into "raw memory". (In most
+implementations, destroying an object mutates its vtbl.)
+
+Finally, note that calling delete t on a collected object first
+deactivates any weak pointers to t and then invokes its clean-up
+function (destructor).
+
+**********************************************************************/
+
+extern "C" {
+    void* _WeakPointer_New( void* t );
+    void* _WeakPointer_Pointer( void* wp );
+    int _WeakPointer_Equal( void* wp1, void* wp2 );
+    int _WeakPointer_Hash( void* wp );
+    void _CleanUp_Set( void* t, void (*c)( void* d, void* t ), void* d );
+    void _CleanUp_Call( void* t );
+    void* _CleanUp_Queue_NewHead ();
+    void _CleanUp_Queue_Set( void* h, void* t );
+    int _CleanUp_Queue_Call( void* h );
+}
+
+#endif /* _weakpointer_h_ */
+
+