boehm-gc/include/weakpointer.h

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