mdw@git.distorted.org.uk Git - sod/blob - lib/sod.c

   1 /* -*-c-*-
   2  *
   3  * Runtime support for the Sensible Object Design
   4  *
   5  * (c) 2009 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of the Sensble Object Design, an object system for C.
  11  *
  12  * SOD is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU General Public License as published by
  14  * the Free Software Foundation; either version 2 of the License, or
  15  * (at your option) any later version.
  16  *
  17  * SOD is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU General Public License
  23  * along with SOD; if not, write to the Free Software Foundation,
  24  * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  25  */
  26
  27 /*----- Header files ------------------------------------------------------*/
  28
  29 #include "sod.h"
  30
  31 /*----- Main code ---------------------------------------------------------*/
  32
  33 /* --- @find_chain@ --- *
  34  *
  35  * Arguments:   @const SodClass *sub, *super@ = pointers to two classes
  36  *
  37  * Returns:     If @sub@ is a subclass of @super@, then a pointer to @sub@'s
  38  *              chain entry describing @super@'s chain; otherwise null.
  39  */
  40
  41 static const struct sod_chain *find_chain(const SodClass *sub,
  42                                           const SodClass *super)
  43 {
  44   const SodClass *head = super->cls.head;
  45   const struct sod_chain *chain, *limit;
  46
  47   /* Slightly fancy footwork.  Each class carries a table describing its
  48    * constituent chains, and each chain has a vector of the classes in that
  49    * chain, with the head (least specific) first.  Chains are always
  50    * non-empty.
  51    *
  52    * Another useful bit of information in the class is its level, i.e., its
  53    * index in its own chain vector.  This is invariant because the chains are
  54    * linear.
  55    *
  56    * This suggests the following algorithm.  Search @sub@'s chains for one
  57    * headed by @super@'s chain head.  If we find one, check that the chain's
  58    * class vector is long enough, and look at the entry corresponding to
  59    * @super@'s level.  If it matches @super@ then @sub@ is indeed a subclass
  60    * and we're done.  Otherwise it isn't, and we lose.  We also lose if no
  61    * matching chain is found.
  62    */
  63   for (chain = sub->cls.chains, limit = chain + sub->cls.n_chains;
  64        chain < limit; chain++) {
  65     if (chain->classes[0] != head)
  66       continue;
  67     if (super->cls.level < chain->n_classes &&
  68         chain->classes[super->cls.level] == super)
  69       return (chain);
  70     break;
  71   }
  72   return (0);
  73 }
  74
  75 /* --- @sod_subclassp@ --- *
  76  *
  77  * Arguments:   @const SodClass *sub, *super@ = pointers to two classes
  78  *
  79  * Returns:     Nonzero if @sub@ is a subclass of @super@.
  80  */
  81
  82 int sod_subclassp(const SodClass *sub, const SodClass *super)
  83   { return (!!find_chain(sub, super)); }
  84
  85 /* --- @sod_convert@ --- *
  86  *
  87  * Arguments:   @const SodClass *cls@ = desired class object
  88  *              @const void *obj@ = pointer to instance
  89  *
  90  * Returns:     Pointer to appropriate ichain of object, or null if the
  91  *              instance isn't of the specified class.
  92  *
  93  * Use:         General down/cross-casting function.
  94  *
  95  *              Upcasts can be performed efficiently using the automatically
  96  *              generated macros.  In particular, upcasts with a chain are
  97  *              trivial; cross-chain upcasts require information from vtables
  98  *              but are fairly fast.  This function is rather slower, but is
  99  *              much more general.
 100  *
 101  *              Suppose we have an instance of a class C, referred to by a
 102  *              pointer to an instance of one of C's superclasses S.  If S'
 103  *              is some other superclass of C then this function will return
 104  *              a pointer to C suitable for use as an instance of S'.  If S'
 105  *              is not a superclass of C, then the function returns null.
 106  *              (If the pointer doesn't point to an instance of some class
 107  *              then the behaviour is undefined.)  Note that you don't need
 108  *              to know what C or S actually are.
 109  */
 110
 111 void *sod_convert(const SodClass *cls, const void *obj)
 112 {
 113   const struct sod_instance *inst = obj;
 114   const struct sod_vtable *vt = inst->_vt;
 115   const SodClass *realcls = vt->_class;
 116   const struct sod_chain *chain = find_chain(realcls, cls);
 117
 118   if (!chain) return (0);
 119   return ((char *)obj - vt->_base + chain->off_ichain);
 120 }
 121
 122 /*----- That's all, folks -------------------------------------------------*/