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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_ */
-
-