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diff --git a/gc/include/weakpointer.h b/gc/include/weakpointer.h new file mode 100644 index 0000000..84906b0 --- /dev/null +++ b/gc/include/weakpointer.h @@ -0,0 +1,221 @@ +#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_ */ + + |