Commit | Line | Data |
---|---|---|
62f9852b MW |
1 | %%% -*-latex-*- |
2 | %%% | |
3 | %%% In-depth exploration of the generated structures | |
4 | %%% | |
5 | %%% (c) 2015 Straylight/Edgeware | |
6 | %%% | |
7 | ||
8 | %%%----- Licensing notice --------------------------------------------------- | |
9 | %%% | |
10 | %%% This file is part of the Simple Object Definition system. | |
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 | \chapter{Object structures} \label{ch:structures} | |
27 | ||
28 | This chapter describes the structure and layout of standard Sod objects, | |
29 | classes and associated metadata. Note that Sod's object system is very | |
30 | flexible and it's possible for an extension to define a new root class which | |
31 | works very differently from the standard @|SodObject| described here. | |
32 | ||
33 | The concrete types described in \xref{sec:structures.common} and | |
34 | \ref{sec:structures.root} are declared by the header file @|<sod/sod.h>|. | |
43073476 MW |
35 | The definitions described in \xref{sec:structures.layout} are defined in the |
36 | header file generated by the containing module. | |
62f9852b MW |
37 | |
38 | %%%-------------------------------------------------------------------------- | |
39 | \section{Common instance structure} \label{sec:structures.common} | |
40 | ||
41 | As described below, a pointer to an instance actually points to an | |
42 | \emph{instance chain} structure within the instances overall layout | |
43 | structure. | |
44 | ||
45 | Instance chains contain slots and vtable pointers, as described below. All | |
9caad6bd MW |
46 | instances have the basic structure of a @|struct sod_instance|. |
47 | ||
48 | \begin{describe}[struct sod_instance]{type} | |
49 | {struct sod_instance \{ \\ \ind | |
50 | const struct sod_vtable *_vt; \- \\ | |
51 | \};} | |
52 | ||
53 | The basic structure of all instances. Members are as follows. | |
54 | \begin{description} \let\makelabel\code | |
55 | \item[_vt] A pointer to a \emph{vtable}, which has the basic structure of a | |
56 | @|struct sod_vtable|, described below. | |
57 | \end{description} | |
58 | \end{describe} | |
59 | ||
60 | \begin{describe}[struct sod_vtable]{type} | |
61 | {struct sod_vtable \{ \\ \ind | |
62 | const SodClass *_class; \\ | |
63 | size_t _base; \- \\ | |
64 | \};} | |
65 | ||
66 | A vtable contains static metadata needed for efficient conversions and | |
67 | message dispatch, and pointers to the instance's class. Each chain points | |
68 | to a different vtable. All vtables have the basic structure of a @|struct | |
69 | sod_vtable|, which has the following members. | |
70 | \begin{description} \let\makelabel\code | |
71 | \item[_class] A pointer to the instance's class object. | |
72 | \item[_base] The offset of this chain structure above the start of the | |
73 | overall instance layout, in bytes. Subtracting @|_base| from the | |
74 | instance chain pointer finds the layout base address. | |
75 | \end{description} | |
76 | \end{describe} | |
62f9852b MW |
77 | |
78 | %%%-------------------------------------------------------------------------- | |
79 | \section{Built-in root objects} \label{sec:structures.root} | |
80 | ||
81 | This section describes the built-in classes @|SodObject| and @|SodClass|, | |
82 | which are the standard roots of the inheritance and metaclass graphs | |
83 | respectively. Specifically, @|SodObject| has no direct superclasses, and | |
84 | @|SodClass| is its own metaclass. It is not possible to define root classes | |
85 | in module files because of circularities: @|SodObject| has @|SodClass| as its | |
86 | metaclass, and @|SodClass| is a subclass of @|SodObject|. Extensions can | |
87 | define additional root classes, but this is tricky, and not really to be | |
88 | recommended. | |
89 | ||
0a2d4b68 | 90 | |
62f9852b MW |
91 | \subsection{The SodObject class} \label{sec:structures.root.sodobject} |
92 | ||
9caad6bd MW |
93 | \begin{figure}[tbp] |
94 | \begin{tabular}{p{10pt}p{10pt}} | |
4effe575 | 95 | \begin{nprog} |
9caad6bd MW |
96 | struct SodObject__ilayout \{ \\ \ind |
97 | union \{ \\ \ind | |
98 | struct SodObject__ichain_obj \{ \\ \ind | |
99 | const struct SodObject__vt_obj *_vt; \- \\ | |
e160ec73 | 100 | \} obj; \- \\ |
9caad6bd MW |
101 | \} obj; \- \\ |
102 | \}; | |
4effe575 | 103 | \end{nprog} |
9caad6bd | 104 | & |
4effe575 | 105 | \begin{nprog} |
9caad6bd MW |
106 | struct SodObject__vt_obj \{ \\ \ind |
107 | const SodClass *_class; \\ | |
108 | size_t _base; \- \\ | |
109 | \}; | |
4effe575 | 110 | \end{nprog} \\ |
9caad6bd MW |
111 | \end{tabular} |
112 | \caption{Instance and vtable layout of @|SodObject|} | |
113 | \label{fig:structures.root.sodobject} | |
114 | \end{figure} | |
115 | ||
116 | \begin{describe}[SodObject]{cls} | |
bd441d33 | 117 | {[nick = obj, metaclass = SodClass, lisp_metaclass = sod_class] \\ |
9caad6bd MW |
118 | class SodObject \{ \}} |
119 | ||
120 | The @|SodObject| class defines no slots or messages. Because @|SodObject| | |
121 | has no direct superclasses, there is only one chain, and no inherited | |
122 | slots or messages, so the single chain contains only a vtable pointer. | |
123 | ||
124 | Since there are no messages, and @|SodClass| also has only one chain, the | |
125 | vtable contains only the standard class pointer and offset-to-base members. | |
126 | In a direct instance of @|SodObject| (why would you want one?) the class | |
127 | pointer contains the address of @|SodObject__class| and the offset is zero. | |
128 | ||
129 | The instance and vtable layout of @|SodObject| is shown in | |
130 | \xref{fig:structures.root.sodobject}. | |
131 | \end{describe} | |
62f9852b | 132 | |
0a2d4b68 | 133 | |
62f9852b MW |
134 | \subsection{The SodClass class} \label{sec:structures.root.sodclass} |
135 | ||
9caad6bd | 136 | \begin{describe}[SodClass]{cls} |
bd441d33 MW |
137 | {[nick = cls, link = SodObject] \\ |
138 | class SodClass : SodObject \{ \\ \ind | |
9caad6bd MW |
139 | const char *name; \\ |
140 | const char *nick; \\ | |
141 | size_t initsz; \\ | |
142 | void *(*imprint)(void *@<p>); \\ | |
143 | void *(*init)(void *@<p>); \\ | |
144 | size_t n_supers; \\ | |
145 | const SodClass *const *supers; \\ | |
146 | size_t n_cpl; \\ | |
147 | const SodClass *const *cpl; \\ | |
148 | const SodClass *link; \\ | |
149 | const SodClass *head; \\ | |
150 | size_t level; \\ | |
151 | size_t n_chains; \\ | |
152 | const struct sod_chain *chains; \\ | |
153 | size_t off_islots; \\ | |
154 | size_t islotsz; \- \\ | |
155 | \}} | |
156 | ||
157 | The @|SodClass| class defines no messages, but there are a number of slots. | |
158 | Its only direct superclass is @|SodObject| and so (like its superclass) its | |
159 | vtable is trivial. | |
160 | ||
161 | The slots defined are as follows. | |
162 | \begin{description} \let\makelabel\code | |
163 | ||
164 | \item[name] A pointer to the class's name. | |
165 | ||
166 | \item[nick] A pointer to the class's nickname. | |
167 | ||
168 | \item[initsz] The size in bytes required to store an instance of the class. | |
169 | ||
170 | \item[imprint] A pointer to a function: given a pointer @<p> to at least | |
171 | @<initsz> bytes of appropriately aligned memory, `imprint' this memory it | |
172 | so that it becomes a minimally functional instance of the class: all of | |
173 | the vtable and class pointers are properly initialized, but the slots are | |
174 | left untouched. The function returns its argument @<p>. | |
175 | ||
54fa3df9 MW |
176 | \item[init] A pointer to a function: given a pointer @<p> to an imprinted |
177 | instance, initialize all of its slots for which initializers are defined. | |
178 | Other slots are left untouched. The function returns its argument @<p>. | |
9caad6bd MW |
179 | |
180 | \item[n_supers] The number of direct superclasses. (This is zero exactly | |
181 | in the case of @|SodObject|.) | |
182 | ||
183 | \item[supers] A pointer to an array of @<n_supers> pointers to class | |
184 | objects listing the class's direct superclasses, in the order in which | |
185 | they were listed in the class definition. If @<n_supers> is zero, then | |
186 | this pointer is null. | |
187 | ||
188 | \item[n_cpl] The number of superclasses in the class's class precedence | |
189 | list. | |
190 | ||
191 | \item[cpl] A pointer to an array of pointers to class objects listing all | |
192 | of the class's superclasses, from most- to least-specific, starting with | |
ac8ddb83 | 193 | the class itself, so $@|$c$@->cls.cpl[0]| = c$ for all class objects |
9caad6bd MW |
194 | $c$. |
195 | ||
196 | \item[link] If the class is a chain head, then this is a null pointer; | |
197 | otherwise it points to the class's distinguished link superclass (which | |
198 | might or might not be a direct superclass). | |
199 | ||
200 | \item[head] A pointer to the least-specific class in this class's chain; so | |
ac8ddb83 MW |
201 | @|$c$@->cls.head@->cls.link| is always null, and either @|$c$@->cls.link| |
202 | is null (in which case $@|$c$@->cls.head| = c$) or $@|$c$@->cls.head| = | |
203 | @|$c$@->cls.link@->cls.head|$. | |
9caad6bd MW |
204 | |
205 | \item[level] The number of less specific superclasses in this class's | |
ac8ddb83 MW |
206 | chain. If @|$c$@->cls.link| is null then @|$c$@->cls.level| is zero; |
207 | otherwise $@|$c$@->cls.level| = @|$c$@->cls.link@->cls.level| + 1$. | |
9caad6bd MW |
208 | |
209 | \item[n_chains] The number of chains formed by the class's superclasses. | |
210 | ||
211 | \item[chains] A pointer to an array of @|struct sod_chain| structures (see | |
212 | below) describing the class's superclass chains, in decreasing order of | |
213 | specificity of their most specific classes. It is always the case that | |
ac8ddb83 | 214 | $@|$c$@->cls.chains[0].classes[$c$@->cls.level]| = c$. |
9caad6bd MW |
215 | |
216 | \item[off_islots] The offset of the class's @|islots| structure relative to | |
217 | its containing @|ichain| structure. The class doesn't define any slots | |
218 | if and only if this is zero. (The offset can't be zero because the | |
219 | vtable pointer is at offset zero.) | |
220 | ||
221 | \item[islotsz] The size required to store the class's direct slots, i.e., | |
222 | the size of its @|islots| structure. The class doesn't define any slots | |
223 | if and only if this is zero. | |
224 | ||
225 | \end{description} | |
226 | \end{describe} | |
227 | ||
228 | \begin{describe}[struct sod_chain]{type} | |
229 | {struct sod_chain \{ \\ \ind | |
230 | size_t n_classes; \\ | |
231 | const SodClass *const *classes; \\ | |
232 | size_t off_ichain; \\ | |
233 | const struct sod_vtable *vt; \\ | |
234 | size_t ichainsz; \- \\ | |
235 | \};} | |
236 | ||
b5229c16 MW |
237 | The @|struct sod_chain| structure describes an individual chain of |
238 | superclasses. It has the following members. | |
239 | \begin{description} \let\makelabel\code | |
9caad6bd | 240 | |
b5229c16 MW |
241 | \item[n_classes] The number of classes in the chain. This is always at |
242 | least one. | |
9caad6bd | 243 | |
b5229c16 MW |
244 | \item[classes] A pointer to an array of class pointers listing the classes |
245 | in the chain from least- to most-specific. So | |
ac8ddb83 MW |
246 | $@|@<classes>[$i$]@->cls.head| = @|@<classes>[0]|$ for all $0 \le i < |
247 | @<n_classes>$, @|@<classes>[0]@->cls.link| is always null, and | |
248 | $@|@<classes>[$i$]@->cls.link| = @|@<classes>[$i - 1$]|$ if $1 \le i < | |
b5229c16 | 249 | @<n_classes>$. |
9caad6bd | 250 | |
b5229c16 | 251 | \item[off_ichain] The size of the @|ichain| structure for this chain. |
9caad6bd | 252 | |
b5229c16 MW |
253 | \item[vt] The vtable for this chain. (It is possible, therefore, to |
254 | partially duplicate the behaviour of the @<imprint> function by walking | |
255 | the chain structure.\footnote{% | |
256 | There isn't enough information readily available to fill in the class | |
257 | pointers correctly.} % | |
258 | The @<imprint> function is much faster, though.) | |
9caad6bd | 259 | |
b5229c16 | 260 | \item[ichainsz] The size of the @|ichain| structure for this chain. |
9caad6bd | 261 | |
b5229c16 MW |
262 | \end{description} |
263 | \end{describe} | |
62f9852b MW |
264 | |
265 | %%%-------------------------------------------------------------------------- | |
266 | \section{Class and vtable layout} \label{sec:structures.layout} | |
267 | ||
268 | The layout algorithms for Sod instances and vtables are nontrivial. They are | |
269 | defined here in full detail, since they're effectively fixed by Sod's ABI | |
270 | compatibility guarantees, so they might as well be documented for the sake of | |
271 | interoperating programs. | |
272 | ||
273 | Unfortunately, the descriptions are rather complicated, and, for the most | |
274 | part not necessary to a working understanding of Sod. The skeleton structure | |
275 | definitions shown should be more than enough for readers attempting to make | |
276 | sense of the generated headers and tables. | |
277 | ||
278 | In the description that follows, uppercase letters vary over class names, | |
279 | while the corresponding lowercase letters indicate the class nicknames. | |
280 | Throughout, we consider a class $C$ (therefore with nickname $c$). | |
281 | ||
0a2d4b68 | 282 | |
62f9852b MW |
283 | \subsection{Generic instance structure} |
284 | \label{sec:structures.layout.instance} | |
285 | ||
286 | The entire state of an instance of $C$ is contained in a single structure of | |
287 | type @|struct $C$__ilayout|. | |
288 | ||
289 | \begin{prog} | |
290 | struct $C$__ilayout \{ \\ \ind | |
291 | union $C$__ichainu_$h$ \{ \\ \ind | |
292 | struct $C$__ichain_$h$ \{ \\ \ind | |
293 | const struct $C$__vt_$h$ *_vt; \\ | |
294 | struct $H$__islots $h$; \\ | |
295 | \quad$\vdots$ \\ | |
296 | struct $C$__islots \{ \\ \ind | |
297 | @<type>_1 @<slot>_1; \\ | |
298 | \quad$\vdots$ \\ | |
299 | @<type>_n @<slot>_n; \- \\ | |
300 | \} $c$; \- \\ | |
301 | \} $c$; \\ | |
302 | struct $H$__ichain_$h$ $h$; \\ | |
303 | \quad$\vdots$ \- \\ | |
304 | \} $h$; \\ | |
305 | union $B$__ichainu_$i$ $i$; \\ | |
306 | \quad$\vdots$ \- \\ | |
307 | \}; | |
ebf5ae2e | 308 | \\+ |
62f9852b MW |
309 | typedef struct $C$__ichain_$h$ $C$; |
310 | \end{prog} | |
311 | ||
312 | The set of superclasses of $C$, including itself, can be partitioned into | |
313 | chains by following their distinguished superclass links. (Formally, the | |
314 | chains are the equivalence classes determined by the reflexive, symmetric, | |
315 | transitive closure of the `links to' relation.) Chains are identified by | |
316 | naming their least specific classes; the least specific class in a chain is | |
317 | called the \emph{chain head}. Suppose that the chain head of the chain | |
318 | containing $C$ itself is named $H$ (though keep in mind that it's possible | |
319 | that .$H$ is in fact $C$ itself.) | |
320 | ||
321 | \subsubsection{The ilayout structure} | |
322 | The @|ilayout| structure contains one member for each of $C$'s superclass | |
323 | chains. The first such member is | |
324 | \begin{prog} | |
325 | union $C$__ichainu_$h$ $h$; | |
326 | \end{prog} | |
327 | described below; this is followed by members | |
328 | \begin{prog} | |
329 | union $B$__ichainu_$i$ $i$; | |
330 | \end{prog} | |
331 | for each other chain, where $I$ is the head and $B$ the tail (most-specific) | |
332 | class of the chain. The members are in decreasing order of the specificity | |
333 | of the chains' most-specific classes. (Note that all but the first of these | |
334 | unions has already been defined as part of the definition of the | |
335 | corresponding $B$.) | |
336 | ||
337 | \subsubsection{The ichainu union} | |
338 | The @|ichainu| union contains a member for each class in the chain. The | |
339 | first is | |
340 | \begin{prog} | |
341 | struct $C$__ichain_$h$ $c$; | |
342 | \end{prog} | |
343 | and this is followed by corresponding members | |
344 | \begin{prog} | |
345 | struct $A$__ichain_$h$ $a$; | |
346 | \end{prog} | |
347 | for each of $C$'s superclasses $A$ in the same chain in some (unimportant) | |
348 | order. | |
349 | ||
350 | \subsubsection{The ichain structure} | |
351 | The | |
352 | @|ichain| | |
353 | structure contains (in order), a pointer | |
354 | \begin{prog} | |
355 | const struct $C$__vt_$h$ *_vt; | |
356 | \end{prog} | |
357 | followed by a structure | |
358 | \begin{prog} | |
359 | struct $A$__islots $a$; | |
360 | \end{prog} | |
361 | for each superclass $A$ of $C$ in the same chain which defines slots, from | |
362 | least- to most-specific; if $C$ defines any slots, then the last member is | |
363 | \begin{prog} | |
364 | struct $C$__islots $c$; | |
365 | \end{prog} | |
366 | A `pointer to $C$' is always assumed (and, indeed, defined in C's | |
367 | type system) to be a pointer to the @|struct $C$__ichain_$h$|. | |
368 | ||
369 | \subsubsection{The islots structure} | |
370 | Finally, the @|islots| structure simply contains one member for each slot | |
371 | defined by $C$ in the order they appear in the class definition. | |
372 | ||
0a2d4b68 | 373 | |
62f9852b MW |
374 | \subsection{Generic vtable structure} \label{sec:structures.layout.vtable} |
375 | ||
376 | As described above, each @|ichain| structure of an instance's storage has a | |
377 | vtable pointer | |
378 | \begin{prog} | |
379 | const struct $C$__vt_$h$ *_vt; | |
380 | \end{prog} | |
381 | In general, the vtables for the different chains will have \emph{different} | |
382 | structures. | |
383 | ||
384 | The instance layout split neatly into disjoint chains. This is necessary | |
385 | because each @|ichain| must have as a prefix the @|ichain| for each | |
386 | superclass in the same chain, and each slot must be stored in exactly one | |
387 | place. The layout of vtables doesn't have this second requirement: it | |
388 | doesn't matter that there are multiple method entry pointers for the same | |
389 | effective method as long as they all work correctly. Indeed, it's essential | |
390 | that they do, because each chain's method entry function will need to apply a | |
391 | different offset to the receiver pointer before invoking the effective | |
392 | method. | |
393 | ||
394 | A vtable for a class $C$ with chain head $H$ has the following general | |
395 | structure. | |
396 | \begin{prog} | |
397 | union $C$__vtu_$h$ \{ \\ \ind | |
398 | struct $C$__vt_$h$ \{ \\ \ind | |
399 | const $P$ *_class; \\ | |
400 | size_t _base; \\ | |
401 | \quad$\vdots$ \\ | |
402 | const $Q$ *_cls_$j$; \\ | |
403 | \quad$\vdots$ \\ | |
404 | ptrdiff_t _off_$i$; \\ | |
405 | \quad$\vdots$ \\ | |
406 | struct $C$__vtmsgs_$a$ \{ \\ \ind | |
407 | @<type> (*@<msg>)($C$ *, $\dots$); \\ | |
408 | \quad$\vdots$ \- \\ | |
409 | \} $a$; \\ | |
410 | \quad$\vdots$ \- \\ | |
411 | \} $c$; \- \\ | |
412 | \}; | |
ebf5ae2e | 413 | \\+ |
62f9852b MW |
414 | extern const union $C$__vtu_$h$ $C$__vtable_$h$; |
415 | \end{prog} | |
416 | ||
417 | \subsubsection{The vtu union} | |
418 | The outer layer is a @|union $C$__vtu_$h$| containing a member | |
419 | \begin{prog} | |
420 | struct $A$__vt_$h$ $a$; | |
421 | \end{prog} | |
422 | for each of $C$'s superclasses $A$ in the same chain, with $C$ itself listed | |
423 | first. | |
424 | ||
425 | This is mostly an irrelevant detail, | |
426 | whose purpose is to defend against malicious compilers: | |
427 | pointers are always to one of the inner | |
428 | @|vt| | |
429 | structures. | |
430 | It's important only because it's the outer | |
431 | @|vtu| | |
432 | union which is exported by name. | |
433 | Specifically, for each chain of | |
434 | $C$'s | |
435 | superclasses | |
436 | there is an external object | |
437 | \begin{prog} | |
438 | const union $A$__vtu_$i$ $C$__vtable_$i$; | |
439 | \end{prog} | |
440 | where $A$ and $I$ are respectively the most and least specific classes in the | |
441 | chain. | |
442 | ||
443 | \subsubsection{The vt structure} | |
444 | The first member in the @|vt| structure is the \emph{root class pointer} | |
445 | \begin{prog} | |
446 | const $P$ *_class; | |
447 | \end{prog} | |
448 | Among the superclasses of $C$ there must be exactly one class $O$ which | |
449 | itself has no direct superclasses; this is the \emph{root superclass} of $C$. | |
450 | (This is a rule enforced by the Sod translator.) The metaclass $R$ of $O$ is | |
451 | then the \emph{root metaclass} of $C$. The @|_class| member points to the | |
452 | @|ichain| structure of most specific superclass $P$ of $M$ in the same chain | |
453 | as $R$. | |
454 | ||
455 | This is followed by the \emph{base offset} | |
456 | \begin{prog} | |
457 | size_t _base; | |
458 | \end{prog} | |
459 | which is simply the offset of the @|ichain| structure from the instance base. | |
460 | ||
461 | The rest of the vtable structure is populated by walking the superclass chain | |
462 | containing $C$ as follows. For each such superclass $B$, in increasing order | |
463 | of specificity, walk the class precedence list of $B$, again starting with | |
464 | its least-specific superclass. (This complex procedure guarantees that the | |
465 | vtable structure for a class is a prefix of the vtable structure for any of | |
466 | its subclasses in the same chain.) | |
467 | ||
468 | So, let $A$ be some superclass of $C$ which has been encountered during this | |
469 | traversal. | |
470 | ||
471 | \begin{itemize} | |
472 | ||
473 | \item Let $N$ be the metaclass of $A$. Examine the superclass chains of $N$ | |
474 | in order of decreasing specificity of their most-specific classes. Let $J$ | |
475 | be the chain head of such a chain, and let $Q$ be the most specific | |
476 | superclass of $M$ in the same chain as $J$. Then, if there is currently no | |
477 | class pointer of type $Q$, then add a member | |
478 | \begin{prog} | |
479 | const $Q$ *_cls_$j$; | |
480 | \end{prog} | |
481 | to the vtable pointing to the appropriate @|islots| structure within $M$'s | |
482 | class object. | |
483 | ||
484 | \item Examine the superclass chains of $A$ in order of decreasing specificity | |
485 | of their most-specific classes. Let $I$ be the chain head of such a chain. | |
486 | If there is currently no member @|_off_$i$| then add a member | |
487 | \begin{prog} | |
488 | ptrdiff_t _off_$i$; | |
489 | \end{prog} | |
490 | to the vtable, containing the (signed) offset from the @|ichain| structure | |
491 | of the chain headed by $h$ to that of the chain headed by $i$ within the | |
492 | instance's layout. | |
493 | ||
494 | \item If class $A$ defines any messages, and there is currently no member | |
495 | $a$, then add a member | |
496 | \begin{prog} | |
497 | struct $C$__vtmsgs_$a$ $a$; | |
498 | \end{prog} | |
499 | to the vtable. See below. | |
500 | ||
501 | \end{itemize} | |
502 | ||
503 | \subsubsection{The vtmsgs structure} | |
504 | Finally, the @|vtmsgs| structures contain pointers to the effective method | |
505 | entry functions for the messages defined by a superclass. There may be more | |
506 | than one method entry for a message, but all of the entry pointers for a | |
507 | message appear together, and entry pointers for separate messages appear in | |
508 | the order in which the messages are defined. If the receiver class has no | |
509 | applicable primary method for a message then it's usual for the method entry | |
510 | pointer to be null (though, as with a lot of things in Sod, extensions may do | |
511 | something different). | |
512 | ||
513 | For a standard message which takes a fixed number of arguments, defined as | |
514 | \begin{prog} | |
515 | @<type>_0 $m$(@<type>_1 @<arg>_1, $\ldots$, @<type>_n @<arg>_n); | |
516 | \end{prog} | |
517 | there is always a `main' entry point, | |
518 | \begin{prog} | |
519 | @<type>_0 $m$($C$ *me, @<type>_1 @<arg>_1, $\ldots$, @<type>_n @<arg>_n); | |
520 | \end{prog} | |
521 | ||
522 | For a standard message which takes a variable number of arguments, | |
523 | defined as | |
524 | \begin{prog} | |
525 | @<type>_0 $m$(@<type>_1 @<arg>_1, $\ldots$, @<type>_n @<arg>_n, \dots); | |
526 | \end{prog} | |
43073476 MW |
527 | or a standard message which takes keyword arguments, defined as |
528 | \begin{prog} | |
529 | @<type>_0 $m$(\=@<type>_1 @<arg>_1, $\ldots$, @<type>_n @<arg>_n? \+ \\ | |
530 | @<type>_{n+1} @<kw>_{n+1} @[= @<dflt>_{n+1}@], $\ldots$, | |
531 | @<type>_m @<kw>_m @[= @<dflt>_m@]); | |
532 | \end{prog} | |
62f9852b MW |
533 | two entry points are defined: the usual `main' entry point which accepts a |
534 | variable number of arguments, and a `valist' entry point which accepts an | |
535 | argument of type @|va_list| in place of the variable portion of the argument | |
43073476 | 536 | list or keywords. |
62f9852b MW |
537 | \begin{prog} |
538 | @<type>_0 $m$($C$ *me, @<type>_1 @<arg>_1, $\ldots$, | |
539 | @<type>_n @<arg>_n, \dots); \\ | |
540 | @<type>_0 $m$__v($C$ *me, @<type>_1 @<arg>_1, $\ldots$, | |
541 | @<type>_n @<arg>_n, va_list sod__ap); | |
542 | \end{prog} | |
543 | ||
0a2d4b68 | 544 | |
b8101b23 | 545 | \subsection{Additional definitions} \label{sec:structures.layout.additional} |
62f9852b MW |
546 | |
547 | In addition to the instance and vtable structures described above, the | |
548 | following definitions are made for each class $C$. | |
549 | ||
550 | For each message $m$ directly defined by $C$ there is a macro definition | |
551 | \begin{prog} | |
552 | \#define $C$_$m$(@<me>, $\ldots$) @<me>@->_vt@->$c$.$m$(@<me>, $\ldots$) | |
553 | \end{prog} | |
554 | which makes sending the message $m$ to an instance of (any subclass of) $C$ | |
555 | somewhat less ugly. | |
556 | ||
43073476 MW |
557 | If $m$ takes a variable number of arguments, or keyword arguments, the macro |
558 | is more complicated and is only available in compilers advertising C99 | |
559 | support, but the effect is the same. For each variable-argument message, | |
560 | there is also an additional macro for calling the `valist' entry point. | |
62f9852b MW |
561 | \begin{prog} |
562 | \#define $C$_$m$__v(@<me>, $\ldots$, @<sod__ap>) | |
563 | @<me>@->_vt@->$c$.$m$__v(@<me>, $\ldots$, @<sod__ap>) | |
564 | \end{prog} | |
565 | ||
566 | For each proper superclass $A$ of $C$, there is a macro defined | |
567 | \begin{prog} | |
568 | $A$ *$C$__CONV_$a$($C$ *_obj); | |
569 | \end{prog} | |
570 | (named in \emph{upper case}) which converts a (static-type) pointer to $C$ to | |
571 | a pointer to the same actual instance, but statically typed as a pointer to | |
572 | $A$. This is most useful when $A$ is not in the same chain as $C$ since | |
573 | in-chain upcasts are both trivial and rarely needed, but the full set is | |
574 | defined for the sake of completeness. | |
575 | ||
576 | Finally, the class object is defined as | |
577 | \begin{prog} | |
578 | extern const struct $R$__ilayout $C$__classobj; \\ | |
579 | \#define $C$__class (\&$C$__classobj.$j$.$r$) | |
580 | \end{prog} | |
581 | The exported symbol @|$C$__classobj| contains the entire class instance. | |
582 | This is usually rather unwieldy. The macro @|$C$__class| is usable as a | |
583 | pointer of type @|const $R$~*|, where $R$ is the root metaclass of $C$, i.e., | |
584 | the metaclass of the least specific superclass of $C$; usually this is | |
585 | @|const SodClass~*|. | |
586 | ||
587 | %%%----- That's all, folks -------------------------------------------------- | |
588 | ||
589 | %%% Local variables: | |
590 | %%% mode: LaTeX | |
591 | %%% TeX-master: "sod.tex" | |
592 | %%% TeX-PDF-mode: t | |
593 | %%% End: |