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1 | /* -*-c-*- |
2 | * | |
3 | * Runtime support for the Sensible Object Design | |
4 | * | |
5 | * (c) 2009 Straylight/Edgeware | |
6 | */ | |
7 | ||
8 | /*----- Licensing notice --------------------------------------------------* | |
9 | * | |
dea4d055 | 10 | * This file is part of the Sensble Object Design, an object system for C. |
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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 | ||
ddee4bb1 | 29 | #include "sod.h" |
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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 | */ | |
ddee4bb1 | 63 | for (chain = sub->cls.chains, limit = chain + sub->cls.n_chains; |
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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 | ||
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82 | int sod_subclassp(const SodClass *sub, const SodClass *super) |
83 | { return (!!find_chain(sub, super)); } | |
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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, void *p) | |
112 | { | |
113 | const struct sod_instance *inst = p; | |
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) | |
119 | return (0); | |
120 | return ((char *)p - vt->_base + chain->off_ichain); | |
121 | } | |
122 | ||
123 | /*----- That's all, folks -------------------------------------------------*/ |