#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_ */