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Annotation of OpenXM_contrib2/asir2000/gc/weakpointer.h, Revision 1.1

1.1     ! noro        1: #ifndef        _weakpointer_h_
        !             2: #define        _weakpointer_h_
        !             3:
        !             4: /****************************************************************************
        !             5:
        !             6: WeakPointer and CleanUp
        !             7:
        !             8:     Copyright (c) 1991 by Xerox Corporation.  All rights reserved.
        !             9:
        !            10:     THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
        !            11:     OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
        !            12:
        !            13:     Permission is hereby granted to copy this code for any purpose,
        !            14:     provided the above notices are retained on all copies.
        !            15:
        !            16:     Last modified on Mon Jul 17 18:16:01 PDT 1995 by ellis
        !            17:
        !            18: ****************************************************************************/
        !            19:
        !            20: /****************************************************************************
        !            21:
        !            22: WeakPointer
        !            23:
        !            24: A weak pointer is a pointer to a heap-allocated object that doesn't
        !            25: prevent the object from being garbage collected. Weak pointers can be
        !            26: used to track which objects haven't yet been reclaimed by the
        !            27: collector. A weak pointer is deactivated when the collector discovers
        !            28: its referent object is unreachable by normal pointers (reachability
        !            29: and deactivation are defined more precisely below). A deactivated weak
        !            30: pointer remains deactivated forever.
        !            31:
        !            32: ****************************************************************************/
        !            33:
        !            34:
        !            35: template< class T > class WeakPointer {
        !            36: public:
        !            37:
        !            38: WeakPointer( T* t = 0 )
        !            39:     /* Constructs a weak pointer for *t. t may be null. It is an error
        !            40:        if t is non-null and *t is not a collected object. */
        !            41:     {impl = _WeakPointer_New( t );}
        !            42:
        !            43: T* Pointer()
        !            44:     /* wp.Pointer() returns a pointer to the referent object of wp or
        !            45:        null if wp has been deactivated (because its referent object
        !            46:        has been discovered unreachable by the collector). */
        !            47:     {return (T*) _WeakPointer_Pointer( this->impl );}
        !            48:
        !            49: int operator==( WeakPointer< T > wp2 )
        !            50:     /* Given weak pointers wp1 and wp2, if wp1 == wp2, then wp1 and
        !            51:        wp2 refer to the same object. If wp1 != wp2, then either wp1
        !            52:        and wp2 don't refer to the same object, or if they do, one or
        !            53:        both of them has been deactivated. (Note: If objects t1 and t2
        !            54:        are never made reachable by their clean-up functions, then
        !            55:        WeakPointer<T>(t1) == WeakPointer<T>(t2) if and only t1 == t2.) */
        !            56:     {return _WeakPointer_Equal( this->impl, wp2.impl );}
        !            57:
        !            58: int Hash()
        !            59:     /* Returns a hash code suitable for use by multiplicative- and
        !            60:        division-based hash tables. If wp1 == wp2, then wp1.Hash() ==
        !            61:        wp2.Hash(). */
        !            62:     {return _WeakPointer_Hash( this->impl );}
        !            63:
        !            64: private:
        !            65: void* impl;
        !            66: };
        !            67:
        !            68: /*****************************************************************************
        !            69:
        !            70: CleanUp
        !            71:
        !            72: A garbage-collected object can have an associated clean-up function
        !            73: that will be invoked some time after the collector discovers the
        !            74: object is unreachable via normal pointers. Clean-up functions can be
        !            75: used to release resources such as open-file handles or window handles
        !            76: when their containing objects become unreachable.  If a C++ object has
        !            77: a non-empty explicit destructor (i.e. it contains programmer-written
        !            78: code), the destructor will be automatically registered as the object's
        !            79: initial clean-up function.
        !            80:
        !            81: There is no guarantee that the collector will detect every unreachable
        !            82: object (though it will find almost all of them). Clients should not
        !            83: rely on clean-up to cause some action to occur immediately -- clean-up
        !            84: is only a mechanism for improving resource usage.
        !            85:
        !            86: Every object with a clean-up function also has a clean-up queue. When
        !            87: the collector finds the object is unreachable, it enqueues it on its
        !            88: queue. The clean-up function is applied when the object is removed
        !            89: from the queue. By default, objects are enqueued on the garbage
        !            90: collector's queue, and the collector removes all objects from its
        !            91: queue after each collection. If a client supplies another queue for
        !            92: objects, it is his responsibility to remove objects (and cause their
        !            93: functions to be called) by polling it periodically.
        !            94:
        !            95: Clean-up queues allow clean-up functions accessing global data to
        !            96: synchronize with the main program. Garbage collection can occur at any
        !            97: time, and clean-ups invoked by the collector might access data in an
        !            98: inconsistent state. A client can control this by defining an explicit
        !            99: queue for objects and polling it at safe points.
        !           100:
        !           101: The following definitions are used by the specification below:
        !           102:
        !           103: Given a pointer t to a collected object, the base object BO(t) is the
        !           104: value returned by new when it created the object. (Because of multiple
        !           105: inheritance, t and BO(t) may not be the same address.)
        !           106:
        !           107: A weak pointer wp references an object *t if BO(wp.Pointer()) ==
        !           108: BO(t).
        !           109:
        !           110: ***************************************************************************/
        !           111:
        !           112: template< class T, class Data > class CleanUp {
        !           113: public:
        !           114:
        !           115: static void Set( T* t, void c( Data* d, T* t ), Data* d = 0 )
        !           116:     /* Sets the clean-up function of object BO(t) to be <c, d>,
        !           117:        replacing any previously defined clean-up function for BO(t); c
        !           118:        and d can be null, but t cannot. Sets the clean-up queue for
        !           119:        BO(t) to be the collector's queue. When t is removed from its
        !           120:        clean-up queue, its clean-up will be applied by calling c(d,
        !           121:        t). It is an error if *t is not a collected object. */
        !           122:        {_CleanUp_Set( t, c, d );}
        !           123:
        !           124: static void Call( T* t )
        !           125:     /* Sets the new clean-up function for BO(t) to be null and, if the
        !           126:        old one is non-null, calls it immediately, even if BO(t) is
        !           127:        still reachable. Deactivates any weak pointers to BO(t). */
        !           128:        {_CleanUp_Call( t );}
        !           129:
        !           130: class Queue {public:
        !           131:     Queue()
        !           132:         /* Constructs a new queue. */
        !           133:             {this->head = _CleanUp_Queue_NewHead();}
        !           134:
        !           135:     void Set( T* t )
        !           136:         /* q.Set(t) sets the clean-up queue of BO(t) to be q. */
        !           137:             {_CleanUp_Queue_Set( this->head, t );}
        !           138:
        !           139:     int Call()
        !           140:         /* If q is non-empty, q.Call() removes the first object and
        !           141:            calls its clean-up function; does nothing if q is
        !           142:            empty. Returns true if there are more objects in the
        !           143:            queue. */
        !           144:            {return _CleanUp_Queue_Call( this->head );}
        !           145:
        !           146:     private:
        !           147:     void* head;
        !           148:     };
        !           149: };
        !           150:
        !           151: /**********************************************************************
        !           152:
        !           153: Reachability and Clean-up
        !           154:
        !           155: An object O is reachable if it can be reached via a non-empty path of
        !           156: normal pointers from the registers, stacks, global variables, or an
        !           157: object with a non-null clean-up function (including O itself),
        !           158: ignoring pointers from an object to itself.
        !           159:
        !           160: This definition of reachability ensures that if object B is accessible
        !           161: from object A (and not vice versa) and if both A and B have clean-up
        !           162: functions, then A will always be cleaned up before B. Note that as
        !           163: long as an object with a clean-up function is contained in a cycle of
        !           164: pointers, it will always be reachable and will never be cleaned up or
        !           165: collected.
        !           166:
        !           167: When the collector finds an unreachable object with a null clean-up
        !           168: function, it atomically deactivates all weak pointers referencing the
        !           169: object and recycles its storage. If object B is accessible from object
        !           170: A via a path of normal pointers, A will be discovered unreachable no
        !           171: later than B, and a weak pointer to A will be deactivated no later
        !           172: than a weak pointer to B.
        !           173:
        !           174: When the collector finds an unreachable object with a non-null
        !           175: clean-up function, the collector atomically deactivates all weak
        !           176: pointers referencing the object, redefines its clean-up function to be
        !           177: null, and enqueues it on its clean-up queue. The object then becomes
        !           178: reachable again and remains reachable at least until its clean-up
        !           179: function executes.
        !           180:
        !           181: The clean-up function is assured that its argument is the only
        !           182: accessible pointer to the object. Nothing prevents the function from
        !           183: redefining the object's clean-up function or making the object
        !           184: reachable again (for example, by storing the pointer in a global
        !           185: variable).
        !           186:
        !           187: If the clean-up function does not make its object reachable again and
        !           188: does not redefine its clean-up function, then the object will be
        !           189: collected by a subsequent collection (because the object remains
        !           190: unreachable and now has a null clean-up function). If the clean-up
        !           191: function does make its object reachable again and a clean-up function
        !           192: is subsequently redefined for the object, then the new clean-up
        !           193: function will be invoked the next time the collector finds the object
        !           194: unreachable.
        !           195:
        !           196: Note that a destructor for a collected object cannot safely redefine a
        !           197: clean-up function for its object, since after the destructor executes,
        !           198: the object has been destroyed into "raw memory". (In most
        !           199: implementations, destroying an object mutates its vtbl.)
        !           200:
        !           201: Finally, note that calling delete t on a collected object first
        !           202: deactivates any weak pointers to t and then invokes its clean-up
        !           203: function (destructor).
        !           204:
        !           205: **********************************************************************/
        !           206:
        !           207: extern "C" {
        !           208:     void* _WeakPointer_New( void* t );
        !           209:     void* _WeakPointer_Pointer( void* wp );
        !           210:     int _WeakPointer_Equal( void* wp1, void* wp2 );
        !           211:     int _WeakPointer_Hash( void* wp );
        !           212:     void _CleanUp_Set( void* t, void (*c)( void* d, void* t ), void* d );
        !           213:     void _CleanUp_Call( void* t );
        !           214:     void* _CleanUp_Queue_NewHead ();
        !           215:     void _CleanUp_Queue_Set( void* h, void* t );
        !           216:     int _CleanUp_Queue_Call( void* h );
        !           217: }
        !           218:
        !           219: #endif /* _weakpointer_h_ */
        !           220:
        !           221:

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