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