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1 | %%% -*-latex-*- |
2 | %%% | |
3 | %%% Conceptual background | |
4 | %%% | |
5 | %%% (c) 2015 Straylight/Edgeware | |
6 | %%% | |
7 | ||
8 | %%%----- Licensing notice --------------------------------------------------- | |
9 | %%% | |
e0808c47 | 10 | %%% This file is part of the Sensible Object Design, an object system for C. |
1f7d590d MW |
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 | ||
3cc520db | 26 | \chapter{Concepts} \label{ch:concepts} |
1f7d590d | 27 | |
3cc520db | 28 | %%%-------------------------------------------------------------------------- |
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29 | \section{Modules} \label{sec:concepts.modules} |
30 | ||
31 | A \emph{module} is the top-level syntactic unit of input to the Sod | |
32 | translator. As described above, given an input module, the translator | |
33 | generates C source and header files. | |
34 | ||
35 | A module can \emph{import} other modules. This makes the type names and | |
36 | classes defined in those other modules available to class definitions in the | |
37 | importing module. Sod's module system is intentionally very simple. There | |
38 | are no private declarations or attempts to hide things. | |
39 | ||
40 | As well as importing existing modules, a module can include a number of | |
41 | different kinds of \emph{items}: | |
42 | \begin{itemize} | |
43 | \item \emph{class definitions} describe new classes, possibly in terms of | |
44 | existing classes; | |
45 | \item \emph{type name declarations} introduce new type names to Sod's | |
46 | parser;\footnote{% | |
47 | This is unfortunately necessary because C syntax, upon which Sod's input | |
48 | language is based for obvious reasons, needs to treat type names | |
49 | differently from other kinds of identifiers.} % | |
50 | and | |
51 | \item \emph{code fragments} contain literal C code to be dropped into an | |
52 | appropriate place in an output file. | |
53 | \end{itemize} | |
54 | Each kind of item, and, indeed, a module as a whole, can have a collection of | |
55 | \emph{properties} associated with it. A property has a \emph{name} and a | |
56 | \emph{value}. Properties are an open-ended way of attaching additional | |
57 | information to module items, so extensions can make use of them without | |
58 | having to implement additional syntax. | |
59 | ||
60 | %%%-------------------------------------------------------------------------- | |
61 | \section{Classes, instances, and slots} \label{sec:concepts.classes} | |
62 | ||
63 | For the most part, Sod takes a fairly traditional view of what it means to be | |
64 | an object system. | |
65 | ||
66 | An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}. An | |
67 | object's state is maintained in named \emph{slots}, each of which can store a | |
68 | C value of an appropriate (scalar or aggregate) type. An object's behaviour | |
69 | is stimulated by sending it \emph{messages}. A message has a name, and may | |
70 | carry a number of arguments, which are C values; sending a message may result | |
71 | in the state of receiving object (or other objects) being changed, and a C | |
72 | value being returned to the sender. | |
73 | ||
74 | Every object is a (direct) instance of some \emph{class}. The class | |
75 | determines which slots its instances have, which messages its instances can | |
76 | be sent, and which methods are invoked when those messages are received. The | |
77 | Sod translator's main job is to read class definitions and convert them into | |
78 | appropriate C declarations, tables, and functions. An object cannot | |
79 | (usually) change its direct class, and the direct class of an object is not | |
80 | affected by, for example, the static type of a pointer to it. | |
81 | ||
0a2d4b68 | 82 | |
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83 | \subsection{Superclasses and inheritance} |
84 | \label{sec:concepts.classes.inherit} | |
85 | ||
86 | \subsubsection{Class relationships} | |
87 | Each class has zero or more \emph{direct superclasses}. | |
88 | ||
89 | A class with no direct superclasses is called a \emph{root class}. The Sod | |
90 | runtime library includes a root class named @|SodObject|; making new root | |
91 | classes is somewhat tricky, and won't be discussed further here. | |
92 | ||
93 | Classes can have more than one direct superclass, i.e., Sod supports | |
94 | \emph{multiple inheritance}. A Sod class definition for a class~$C$ lists | |
95 | the direct superclasses of $C$ in a particular order. This order is called | |
96 | the \emph{local precedence order} of $C$, and the list which consists of $C$ | |
97 | follows by $C$'s direct superclasses in local precedence order is called the | |
98 | $C$'s \emph{local precedence list}. | |
99 | ||
100 | The multiple inheritance in Sod works similarly to multiple inheritance in | |
101 | Lisp-like languages, such as Common Lisp, EuLisp, Dylan, and Python, which is | |
102 | very different from how multiple inheritance works in \Cplusplus.\footnote{% | |
103 | The latter can be summarized as `badly'. By default in \Cplusplus, an | |
104 | instance receives an additional copy of superclass's state for each path | |
105 | through the class graph from the instance's direct class to that | |
106 | superclass, though this behaviour can be overridden by declaring | |
107 | superclasses to be @|virtual|. Also, \Cplusplus\ offers only trivial | |
108 | method combination (\xref{sec:concepts.methods}), leaving programmers to | |
109 | deal with delegation manually and (usually) statically.} % | |
110 | ||
111 | If $C$ is a class, then the \emph{superclasses} of $C$ are | |
112 | \begin{itemize} | |
113 | \item $C$ itself, and | |
114 | \item the superclasses of each of $C$'s direct superclasses. | |
115 | \end{itemize} | |
116 | The \emph{proper superclasses} of a class $C$ are the superclasses of $C$ | |
117 | except for $C$ itself. If a class $B$ is a (direct, proper) superclass of | |
118 | $C$, then $C$ is a \emph{(direct, proper) subclass} of $B$. If $C$ is a root | |
119 | class then the only superclass of $C$ is $C$ itself, and $C$ has no proper | |
120 | superclasses. | |
121 | ||
122 | If an object is a direct instance of class~$C$ then the object is also an | |
123 | (indirect) instance of every superclass of $C$. | |
124 | ||
054e8f8f | 125 | If $C$ has a proper superclass $B$, then $B$ must not have $C$ as a direct |
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126 | superclass. In different terms, if we construct a directed graph, whose |
127 | nodes are classes, and draw an arc from each class to each of its direct | |
128 | superclasses, then this graph must be acyclic. In yet other terms, the `is a | |
129 | superclass of' relation is a partial order on classes. | |
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130 | |
131 | \subsubsection{The class precedence list} | |
132 | This partial order is not quite sufficient for our purposes. For each class | |
133 | $C$, we shall need to extend it into a total order on $C$'s superclasses. | |
134 | This calculation is called \emph{superclass linearization}, and the result is | |
135 | a \emph{class precedence list}, which lists each of $C$'s superclasses | |
136 | exactly once. If a superclass $B$ precedes (resp.\ follows) some other | |
137 | superclass $A$ in $C$'s class precedence list, then we say that $B$ is a more | |
138 | (resp.\ less) \emph{specific} superclass of $C$ than $A$ is. | |
139 | ||
140 | The superclass linearization algorithm isn't fixed, and extensions to the | |
141 | translator can introduce new linearizations for special effects, but the | |
142 | following properties are expected to hold. | |
143 | \begin{itemize} | |
144 | \item The first class in $C$'s class precedence list is $C$ itself; i.e., | |
145 | $C$ is always its own most specific superclass. | |
146 | \item If $A$ and $B$ are both superclasses of $C$, and $A$ is a proper | |
147 | superclass of $B$ then $A$ appears after $B$ in $C$'s class precedence | |
148 | list, i.e., $B$ is a more specific superclass of $C$ than $A$ is. | |
149 | \end{itemize} | |
150 | The default linearization algorithm used in Sod is the \emph{C3} algorithm, | |
9cd5cf15 | 151 | which has a number of good properties described in~\cite{Barrett:1996:MSL}. |
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152 | It works as follows. |
153 | \begin{itemize} | |
154 | \item A \emph{merge} of some number of input lists is a single list | |
155 | containing each item that is in any of the input lists exactly once, and no | |
156 | other items; if an item $x$ appears before an item $y$ in any input list, | |
157 | then $x$ also appears before $y$ in the merge. If a collection of lists | |
158 | have no merge then they are said to be \emph{inconsistent}. | |
159 | \item The class precedence list of a class $C$ is a merge of the local | |
160 | precedence list of $C$ together with the class precedence lists of each of | |
161 | $C$'s direct superclasses. | |
162 | \item If there are no such merges, then the definition of $C$ is invalid. | |
163 | \item Suppose that there are multiple candidate merges. Consider the | |
164 | earliest position in these candidate merges at which they disagree. The | |
165 | \emph{candidate classes} at this position are the classes appearing at this | |
166 | position in the candidate merges. Each candidate class must be a | |
781a8fbd | 167 | superclass of distinct direct superclasses of $C$, since otherwise the |
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168 | candidates would be ordered by their common subclass's class precedence |
169 | list. The class precedence list contains, at this position, that candidate | |
170 | class whose subclass appears earliest in $C$'s local precedence order. | |
171 | \end{itemize} | |
172 | ||
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173 | \begin{figure} |
174 | \centering | |
175 | \begin{tikzpicture}[x=7.5mm, y=-14mm, baseline=(current bounding box.east)] | |
176 | \node[lit] at ( 0, 0) (R) {SodObject}; | |
177 | \node[lit] at (-3, +1) (A) {A}; \draw[->] (A) -- (R); | |
178 | \node[lit] at (-1, +1) (B) {B}; \draw[->] (B) -- (R); | |
179 | \node[lit] at (+1, +1) (C) {C}; \draw[->] (C) -- (R); | |
180 | \node[lit] at (+3, +1) (D) {D}; \draw[->] (D) -- (R); | |
181 | \node[lit] at (-2, +2) (E) {E}; \draw[->] (E) -- (A); | |
182 | \draw[->] (E) -- (B); | |
183 | \node[lit] at (+2, +2) (F) {F}; \draw[->] (F) -- (A); | |
184 | \draw[->] (F) -- (D); | |
185 | \node[lit] at (-1, +3) (G) {G}; \draw[->] (G) -- (E); | |
186 | \draw[->] (G) -- (C); | |
187 | \node[lit] at (+1, +3) (H) {H}; \draw[->] (H) -- (F); | |
188 | \node[lit] at ( 0, +4) (I) {I}; \draw[->] (I) -- (G); | |
189 | \draw[->] (I) -- (H); | |
190 | \end{tikzpicture} | |
191 | \quad | |
192 | \vrule | |
193 | \quad | |
194 | \begin{minipage}[c]{0.45\hsize} | |
195 | \begin{nprog} | |
196 | class A: SodObject \{ \}\quad\=@/* @|A|, @|SodObject| */ \\ | |
197 | class B: SodObject \{ \}\>@/* @|B|, @|SodObject| */ \\ | |
198 | class C: SodObject \{ \}\>@/* @|B|, @|SodObject| */ \\ | |
199 | class D: SodObject \{ \}\>@/* @|B|, @|SodObject| */ \\+ | |
200 | class E: A, B \{ \}\quad\=@/* @|E|, @|A|, @|B|, \dots */ \\ | |
201 | class F: A, D \{ \}\>@/* @|F|, @|A|, @|D|, \dots */ \\+ | |
202 | class G: E, C \{ \}\>@/* @|G|, @|E|, @|A|, | |
203 | @|B|, @|C|, \dots */ \\ | |
204 | class H: F \{ \}\>@/* @|H|, @|F|, @|A|, @|D|, \dots */ \\+ | |
205 | class I: G, H \{ \}\>@/* @|I|, @|G|, @|E|, @|H|, @|F|, | |
206 | @|A|, @|B|, @|C|, @|D|, \dots */ | |
207 | \end{nprog} | |
208 | \end{minipage} | |
209 | ||
210 | \caption{An example class graph and class precedence lists} | |
211 | \label{fig:concepts.classes.cpl-example} | |
212 | \end{figure} | |
213 | ||
214 | \begin{example} | |
215 | Consider the class relationships shown in | |
216 | \xref{fig:concepts.classes.cpl-example}. | |
217 | ||
218 | \begin{itemize} | |
219 | ||
220 | \item @|SodObject| has no proper superclasses. Its class precedence list | |
221 | is therefore simply $\langle @|SodObject| \rangle$. | |
222 | ||
223 | \item In general, if $X$ is a direct subclass only of $Y$, and $Y$'s class | |
224 | precedence list is $\langle Y, \ldots \rangle$, then $X$'s class | |
225 | precedence list is $\langle X, Y, \ldots \rangle$. This explains $A$, | |
226 | $B$, $C$, $D$, and $H$. | |
227 | ||
228 | \item $E$'s list is found by merging its local precedence list $\langle E, | |
229 | A, B \rangle$ with the class precedence lists of its direct superclasses, | |
230 | which are $\langle A, @|SodObject| \rangle$ and $\langle B, @|SodObject| | |
231 | \rangle$. Clearly, @|SodObject| must be last, and $E$'s local precedence | |
232 | list orders the rest, giving $\langle E, A, B, @|SodObject|, \rangle$. | |
233 | $F$ is similar. | |
234 | ||
235 | \item We determine $G$'s class precedence list by merging the three lists | |
236 | $\langle G, E, C \rangle$, $\langle E, A, B, @|SodObject| \rangle$, and | |
237 | $\langle C, @|SodObject| \rangle$. The class precedence list begins | |
238 | $\langle G, E, \ldots \rangle$, but the individual lists don't order $A$ | |
239 | and $C$. Comparing these to $G$'s direct superclasses, we see that $A$ | |
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240 | is a superclass of $E$, while $C$ is a superclass of -- indeed equal to |
241 | -- $C$; so $A$ must precede $C$, as must $B$, and the final list is | |
242 | $\langle G, E, A, B, C, @|SodObject| \rangle$. | |
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243 | |
244 | \item Finally, we determine $I$'s class precedence list by merging $\langle | |
245 | I, G, H \rangle$, $\langle G, E, A, B, C, @|SodObject| \rangle$, and | |
246 | $\langle H, F, A, D, @|SodObject| \rangle$. The list begins $\langle I, | |
247 | G, \ldots \rangle$, and then we must break a tie between $E$ and $H$; but | |
54cf3a30 | 248 | $E$ is a superclass of $G$, so $E$ wins. Next, $H$ and $F$ must precede |
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249 | $A$, since these are ordered by $H$'s class precedence list. Then $B$ |
250 | and $C$ precede $D$, since the former are superclasses of $G$, and the | |
251 | final list is $\langle I, G, E, H, F, A, B, C, D, @|SodObject| \rangle$. | |
252 | ||
253 | \end{itemize} | |
254 | ||
255 | (This example combines elements from \cite{Barrett:1996:MSL} and | |
256 | \cite{Ducournau:1994:PMM}.) | |
257 | \end{example} | |
258 | ||
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259 | \subsubsection{Class links and chains} |
260 | The definition for a class $C$ may distinguish one of its proper superclasses | |
261 | as being the \emph{link superclass} for class $C$. Not every class need have | |
262 | a link superclass, and the link superclass of a class $C$, if it exists, need | |
263 | not be a direct superclass of $C$. | |
264 | ||
265 | Superclass links must obey the following rule: if $C$ is a class, then there | |
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266 | must be no three distinct superclasses $X$, $Y$ and~$Z$ of $C$ such that $Z$ |
267 | is the link superclass of both $X$ and $Y$. As a consequence of this rule, | |
268 | the superclasses of $C$ can be partitioned into linear \emph{chains}, such | |
269 | that superclasses $A$ and $B$ are in the same chain if and only if one can | |
270 | trace a path from $A$ to $B$ by following superclass links, or \emph{vice | |
271 | versa}. | |
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272 | |
273 | Since a class links only to one of its proper superclasses, the classes in a | |
274 | chain are naturally ordered from most- to least-specific. The least specific | |
275 | class in a chain is called the \emph{chain head}; the most specific class is | |
276 | the \emph{chain tail}. Chains are often named after their chain head | |
277 | classes. | |
278 | ||
c6c9615b | 279 | |
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280 | \subsection{Names} |
281 | \label{sec:concepts.classes.names} | |
282 | ||
283 | Classes have a number of other attributes: | |
284 | \begin{itemize} | |
285 | \item A \emph{name}, which is a C identifier. Class names must be globally | |
286 | unique. The class name is used in the names of a number of associated | |
287 | definitions, to be described later. | |
288 | \item A \emph{nickname}, which is also a C identifier. Unlike names, | |
289 | nicknames are not required to be globally unique. If $C$ is any class, | |
290 | then all the superclasses of $C$ must have distinct nicknames. | |
291 | \end{itemize} | |
292 | ||
0a2d4b68 | 293 | |
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294 | \subsection{Slots} \label{sec:concepts.classes.slots} |
295 | ||
296 | Each class defines a number of \emph{slots}. Much like a structure member, a | |
297 | slot has a \emph{name}, which is a C identifier, and a \emph{type}. Unlike | |
298 | many other object systems, different superclasses of a class $C$ can define | |
299 | slots with the same name without ambiguity, since slot references are always | |
300 | qualified by the defining class's nickname. | |
301 | ||
302 | \subsubsection{Slot initializers} | |
303 | As well as defining slot names and types, a class can also associate an | |
304 | \emph{initial value} with each slot defined by itself or one of its | |
98da9322 | 305 | subclasses. A class $C$ provides an \emph{initialization message} (see |
d24d47f5 | 306 | \xref{sec:concepts.lifecycle.birth}, and \xref{sec:structures.root.sodclass}) |
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307 | whose methods set the slots of a \emph{direct} instance of the class to the |
308 | correct initial values. If several of $C$'s superclasses define initializers | |
309 | for the same slot then the initializer from the most specific such class is | |
310 | used. If none of $C$'s superclasses define an initializer for some slot then | |
311 | that slot will be left uninitialized. | |
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312 | |
313 | The initializer for a slot with scalar type may be any C expression. The | |
314 | initializer for a slot with aggregate type must contain only constant | |
315 | expressions if the generated code is expected to be processed by a | |
316 | implementation of C89. Initializers will be evaluated once each time an | |
317 | instance is initialized. | |
318 | ||
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319 | Slots are initialized in reverse-precedence order of their defining classes; |
320 | i.e., slots defined by a less specific superclass are initialized earlier | |
321 | than slots defined by a more specific superclass. Slots defined by the same | |
322 | class are initialized in the order in which they appear in the class | |
323 | definition. | |
324 | ||
325 | The initializer for a slot may refer to other slots in the same object, via | |
326 | the @|me| pointer: in an initializer for a slot defined by a class $C$, @|me| | |
327 | has type `pointer to $C$'. (Note that the type of @|me| depends only on the | |
328 | class which defined the slot, not the class which defined the initializer.) | |
329 | ||
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330 | A class can also define \emph{class slot initializers}, which provide values |
331 | for a slot defined by its metaclass; see \xref{sec:concepts.metaclasses} for | |
332 | details. | |
333 | ||
0a2d4b68 | 334 | |
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335 | \subsection{C language integration} \label{sec:concepts.classes.c} |
336 | ||
337 | For each class~$C$, the Sod translator defines a C type, the \emph{class | |
338 | type}, with the same name. This is the usual type used when considering an | |
339 | object as an instance of class~$C$. No entire object will normally have a | |
340 | class type,\footnote{% | |
341 | In general, a class type only captures the structure of one of the | |
342 | superclass chains of an instance. A full instance layout contains multiple | |
343 | chains. See \xref{sec:structures.layout} for the full details.} % | |
344 | so access to instances is almost always via pointers. | |
345 | ||
346 | \subsubsection{Access to slots} | |
347 | The class type for a class~$C$ is actually a structure. It contains one | |
348 | member for each class in $C$'s superclass chain, named with that class's | |
349 | nickname. Each of these members is also a structure, containing the | |
350 | corresponding class's slots, one member per slot. There's nothing special | |
351 | about these slot members: C code can access them in the usual way. | |
352 | ||
f2309139 MW |
353 | For example, given the definition |
354 | \begin{prog} | |
355 | [nick = mine] \\ | |
356 | class MyClass: SodObject \{ \\ \ind | |
357 | int x; \-\\ | |
358 | \} | |
359 | \end{prog} | |
360 | the simple function | |
3cc520db | 361 | \begin{prog} |
c18d6aba | 362 | int get_x(MyClass *m) \{ return (m@->mine.x); \} |
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363 | \end{prog} |
364 | will extract the value of @|x| from an instance of @|MyClass|. | |
365 | ||
366 | All of this means that there's no such thing as `private' or `protected' | |
367 | slots. If you want to hide implementation details, the best approach is to | |
368 | stash them in a dynamically allocated private structure, and leave a pointer | |
369 | to it in a slot. (This will also help preserve binary compatibility, because | |
370 | the private structure can grow more members as needed. See | |
021d9f84 | 371 | \xref{sec:concepts.compatibility} for more details.) |
3cc520db | 372 | |
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373 | Slots defined by $C$'s link superclass, or any other superclass in the same |
374 | chain, can be accessed in the same way. Slots defined by other superclasses | |
375 | can't be accessed directly: the instance pointer must be \emph{converted} to | |
376 | point to a different chain. See the subsection `Conversions' below. | |
377 | ||
ff06eeb1 | 378 | |
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379 | \subsubsection{Sending messages} |
380 | Sod defines a macro for each message. If a class $C$ defines a message $m$, | |
381 | then the macro is called @|$C$_$m$|. The macro takes a pointer to the | |
382 | receiving object as its first argument, followed by the message arguments, if | |
383 | any, and returns the value returned by the object's effective method for the | |
384 | message (if any). If you have a pointer to an instance of any of $C$'s | |
385 | subclasses, then you can send it the message; it doesn't matter whether the | |
386 | subclass is on the same chain. Note that the receiver argument is evaluated | |
387 | twice, so it's not safe to write a receiver expression which has | |
388 | side-effects. | |
389 | ||
390 | For example, suppose we defined | |
391 | \begin{prog} | |
392 | [nick = soupy] \\ | |
393 | class Super: SodObject \{ \\ \ind | |
394 | void msg(const char *m); \-\\ | |
395 | \} \\+ | |
396 | class Sub: Super \{ \\ \ind | |
397 | void soupy.msg(const char *m) | |
398 | \{ printf("sub sent `\%s'@\\n", m); \} \-\\ | |
399 | \} | |
400 | \end{prog} | |
401 | then we can send the message like this: | |
402 | \begin{prog} | |
403 | Sub *sub = /* \dots\ */; \\ | |
404 | Super_msg(sub, "hello"); | |
405 | \end{prog} | |
406 | ||
407 | What happens under the covers is as follows. The structure pointed to by the | |
408 | instance pointer has a member named @|_vt|, which points to a structure | |
409 | called a `virtual table', or \emph{vtable}, which contains various pieces of | |
410 | information about the object's direct class and layout, and holds pointers to | |
411 | method entries for the messages which the object can receive. The | |
412 | message-sending macro in the example above expands to something similar to | |
413 | \begin{prog} | |
414 | sub@->_vt.sub.msg(sub, "Hello"); | |
415 | \end{prog} | |
416 | ||
417 | The vtable contains other useful information, such as a pointer to the | |
418 | instance's direct class's \emph{class object} (described below). The full | |
419 | details of the contents and layout of vtables are given in | |
420 | \xref{sec:structures.layout.vtable}. | |
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421 | |
422 | ||
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423 | \subsubsection{Class objects} |
424 | In Sod's object system, classes are objects too. Therefore classes are | |
425 | themselves instances; the class of a class is called a \emph{metaclass}. The | |
426 | consequences of this are explored in \xref{sec:concepts.metaclasses}. The | |
427 | \emph{class object} has the same name as the class, suffixed with | |
428 | `@|__class|'\footnote{% | |
429 | This is not quite true. @|$C$__class| is actually a macro. See | |
430 | \xref{sec:structures.layout.additional} for the gory details.} % | |
431 | and its type is usually @|SodClass|; @|SodClass|'s nickname is @|cls|. | |
432 | ||
433 | A class object's slots contain or point to useful information, tables and | |
434 | functions for working with that class's instances. (The @|SodClass| class | |
054e8f8f MW |
435 | doesn't define any messages, so it doesn't have any methods other than for |
436 | the @|SodObject| lifecycle messages @|init| and @|teardown|; see | |
437 | \xref{sec:concepts.lifecycle}. In Sod, a class slot containing a function | |
438 | pointer is not at all the same thing as a method.) | |
3cc520db | 439 | |
3cc520db | 440 | \subsubsection{Conversions} |
e4ea29d8 MW |
441 | Suppose one has a value of type pointer-to-class-type for some class~$C$, and |
442 | wants to convert it to a pointer-to-class-type for some other class~$B$. | |
3cc520db MW |
443 | There are three main cases to distinguish. |
444 | \begin{itemize} | |
445 | \item If $B$ is a superclass of~$C$, in the same chain, then the conversion | |
446 | is an \emph{in-chain upcast}. The conversion can be performed using the | |
447 | appropriate generated upcast macro (see below), or by simply casting the | |
448 | pointer, using C's usual cast operator (or the \Cplusplus\ @|static_cast<>| | |
449 | operator). | |
450 | \item If $B$ is a superclass of~$C$, in a different chain, then the | |
451 | conversion is a \emph{cross-chain upcast}. The conversion is more than a | |
452 | simple type change: the pointer value must be adjusted. If the direct | |
453 | class of the instance in question is not known, the conversion will require | |
454 | a lookup at runtime to find the appropriate offset by which to adjust the | |
455 | pointer. The conversion can be performed using the appropriate generated | |
456 | upcast macro (see below); the general case is handled by the macro | |
58f9b400 | 457 | \descref{SOD_XCHAIN}{mac}. |
e4ea29d8 | 458 | \item If $B$ is a subclass of~$C$ then the conversion is a \emph{downcast}; |
3cc520db MW |
459 | otherwise the conversion is a~\emph{cross-cast}. In either case, the |
460 | conversion can fail: the object in question might not be an instance of~$B$ | |
e4ea29d8 | 461 | after all. The macro \descref{SOD_CONVERT}{mac} and the function |
58f9b400 | 462 | \descref{sod_convert}{fun} perform general conversions. They return a null |
054e8f8f | 463 | pointer if the conversion fails. (These are therefore your analogue to the |
e4ea29d8 | 464 | \Cplusplus\ @|dynamic_cast<>| operator.) |
3cc520db MW |
465 | \end{itemize} |
466 | The Sod translator generates macros for performing both in-chain and | |
467 | cross-chain upcasts. For each class~$C$, and each proper superclass~$B$ | |
468 | of~$C$, a macro is defined: given an argument of type pointer to class type | |
469 | of~$C$, it returns a pointer to the same instance, only with type pointer to | |
470 | class type of~$B$, adjusted as necessary in the case of a cross-chain | |
471 | conversion. The macro is named by concatenating | |
472 | \begin{itemize} | |
473 | \item the name of class~$C$, in upper case, | |
474 | \item the characters `@|__CONV_|', and | |
475 | \item the nickname of class~$B$, in upper case; | |
476 | \end{itemize} | |
477 | e.g., if $C$ is named @|MyClass|, and $B$'s name is @|SuperClass| with | |
478 | nickname @|super|, then the macro @|MYCLASS__CONV_SUPER| converts a | |
479 | @|MyClass~*| to a @|SuperClass~*|. See | |
480 | \xref{sec:structures.layout.additional} for the formal description. | |
481 | ||
482 | %%%-------------------------------------------------------------------------- | |
9e91c8e7 MW |
483 | \section{Keyword arguments} \label{sec:concepts.keywords} |
484 | ||
485 | In standard C, the actual arguments provided to a function are matched up | |
486 | with the formal arguments given in the function definition according to their | |
487 | ordering in a list. Unless the (rather cumbersome) machinery for dealing | |
488 | with variable-length argument tails (@|<stdarg.h>|) is used, exactly the | |
489 | correct number of arguments must be supplied, and in the correct order. | |
490 | ||
491 | A \emph{keyword argument} is matched by its distinctive \emph{name}, rather | |
492 | than by its position in a list. Keyword arguments may be \emph{omitted}, | |
493 | causing some default behaviour by the function. A function can detect | |
494 | whether a particular keyword argument was supplied: so the default behaviour | |
495 | need not be the same as that caused by any specific value of the argument. | |
496 | ||
497 | Keyword arguments can be provided in three ways. | |
498 | \begin{enumerate} | |
499 | \item Directly, as a variable-length argument tail, consisting (for the most | |
500 | part) of alternating keyword names, as pointers to null-terminated strings, | |
501 | and argument values, and terminated by a null pointer. This is somewhat | |
502 | error-prone, and the support library defines some macros which help ensure | |
503 | that keyword argument lists are well formed. | |
504 | \item Indirectly, through a @|va_list| object capturing a variable-length | |
505 | argument tail passed to some other function. Such indirect argument tails | |
506 | have the same structure as the direct argument tails described above. | |
507 | Because @|va_list| objects are hard to copy, the keyword-argument support | |
508 | library consistently passes @|va_list| objects \emph{by reference} | |
509 | throughout its programming interface. | |
510 | \item Indirectly, through a vector of @|struct kwval| objects, each of which | |
511 | contains a keyword name, as a pointer to a null-terminated string, and the | |
512 | \emph{address} of a corresponding argument value. (This indirection is | |
513 | necessary so that the items in the vector can be of uniform size.) | |
514 | Argument vectors are rather inconvenient to use, but are the only practical | |
515 | way in which a caller can decide at runtime which arguments to include in a | |
516 | call, which is useful when writing wrapper functions. | |
517 | \end{enumerate} | |
518 | ||
519 | Keyword arguments are provided as a general feature for C functions. | |
43073476 | 520 | However, Sod has special support for messages which accept keyword arguments |
8ec911fa | 521 | (\xref{sec:concepts.methods.keywords}); and they play an essential rôle in |
a142609c | 522 | the instance construction protocol (\xref{sec:concepts.lifecycle.birth}). |
9e91c8e7 MW |
523 | |
524 | %%%-------------------------------------------------------------------------- | |
3cc520db MW |
525 | \section{Messages and methods} \label{sec:concepts.methods} |
526 | ||
527 | Objects can be sent \emph{messages}. A message has a \emph{name}, and | |
528 | carries a number of \emph{arguments}. When an object is sent a message, a | |
529 | function, determined by the receiving object's class, is invoked, passing it | |
530 | the receiver and the message arguments. This function is called the | |
531 | class's \emph{effective method} for the message. The effective method can do | |
532 | anything a C function can do, including reading or updating program state or | |
533 | object slots, sending more messages, calling other functions, issuing system | |
534 | calls, or performing I/O; if it finishes, it may return a value, which is | |
535 | returned in turn to the message sender. | |
536 | ||
537 | The set of messages an object can receive, characterized by their names, | |
538 | argument types, and return type, is determined by the object's class. Each | |
539 | class can define new messages, which can be received by any instance of that | |
540 | class. The messages defined by a single class must have distinct names: | |
541 | there is no `function overloading'. As with slots | |
542 | (\xref{sec:concepts.classes.slots}), messages defined by distinct classes are | |
543 | always distinct, even if they have the same names: references to messages are | |
544 | always qualified by the defining class's name or nickname. | |
545 | ||
546 | Messages may take any number of arguments, of any non-array value type. | |
547 | Since message sends are effectively function calls, arguments of array type | |
548 | are implicitly converted to values of the corresponding pointer type. While | |
549 | message definitions may ascribe an array type to an argument, the formal | |
550 | argument will have pointer type, as is usual for C functions. A message may | |
551 | accept a variable-length argument suffix, denoted @|\dots|. | |
552 | ||
553 | A class definition may include \emph{direct methods} for messages defined by | |
554 | it or any of its superclasses. | |
555 | ||
556 | Like messages, direct methods define argument lists and return types, but | |
8ec911fa | 557 | they may also have a \emph{body}, and a \emph{rôle}. |
3cc520db MW |
558 | |
559 | A direct method need not have the same argument list or return type as its | |
560 | message. The acceptable argument lists and return types for a method depend | |
561 | on the message, in particular its method combination | |
8ec911fa | 562 | (\xref{sec:concepts.methods.combination}), and the method's rôle. |
3cc520db MW |
563 | |
564 | A direct method body is a block of C code, and the Sod translator usually | |
565 | defines, for each direct method, a function with external linkage, whose body | |
566 | contains a copy of the direct method body. Within the body of a direct | |
567 | method defined for a class $C$, the variable @|me|, of type pointer to class | |
568 | type of $C$, refers to the receiving object. | |
569 | ||
0a2d4b68 | 570 | |
3cc520db MW |
571 | \subsection{Effective methods and method combinations} |
572 | \label{sec:concepts.methods.combination} | |
573 | ||
574 | For each message a direct instance of a class might receive, there is a set | |
575 | of \emph{applicable methods}, which are exactly the direct methods defined on | |
576 | the object's class and its superclasses. These direct methods are combined | |
577 | together to form the \emph{effective method} for that particular class and | |
578 | message. Direct methods can be combined into an effective method in | |
579 | different ways, according to the \emph{method combination} specified by the | |
8ec911fa MW |
580 | message. The method combination determines which direct method rôles are |
581 | acceptable, and, for each rôle, the appropriate argument lists and return | |
3cc520db MW |
582 | types. |
583 | ||
584 | One direct method, $M$, is said to be more (resp.\ less) \emph{specific} than | |
585 | another, $N$, with respect to a receiving class~$C$, if the class defining | |
586 | $M$ is a more (resp.\ less) specific superclass of~$C$ than the class | |
587 | defining $N$. | |
588 | ||
43073476 | 589 | \subsubsection{The standard method combination} |
3cc520db MW |
590 | The default method combination is called the \emph{standard method |
591 | combination}; other method combinations are useful occasionally for special | |
8ec911fa | 592 | effects. The standard method combination accepts four direct method rôles, |
9761db0d | 593 | called `primary' (the default), @|before|, @|after|, and @|around|. |
3cc520db MW |
594 | |
595 | All direct methods subject to the standard method combination must have | |
596 | argument lists which \emph{match} the message's argument list: | |
597 | \begin{itemize} | |
598 | \item the method's arguments must have the same types as the message, though | |
599 | the arguments may have different names; and | |
600 | \item if the message accepts a variable-length argument suffix then the | |
601 | direct method must instead have a final argument of type @|va_list|. | |
602 | \end{itemize} | |
b1254eb6 MW |
603 | Primary and @|around| methods must have the same return type as the message; |
604 | @|before| and @|after| methods must return @|void| regardless of the | |
605 | message's return type. | |
3cc520db MW |
606 | |
607 | If there are no applicable primary methods then no effective method is | |
608 | constructed: the vtables contain null pointers in place of pointers to method | |
609 | entry functions. | |
610 | ||
f1aa19a8 | 611 | \begin{figure} |
d82d5db5 | 612 | \hbox to\hsize{\hss\hbox{\begin{tikzpicture} |
a4094071 | 613 | [order/.append style={color=green!70!black}, |
f1aa19a8 MW |
614 | code/.append style={font=\sffamily}, |
615 | action/.append style={font=\itshape}, | |
616 | method/.append style={rectangle, draw=black, thin, fill=blue!30, | |
617 | text height=\ht\strutbox, text depth=\dp\strutbox, | |
618 | minimum width=40mm}] | |
619 | ||
620 | \def\delgstack#1#2#3{ | |
621 | \node (#10) [method, #2] {#3}; | |
622 | \node (#11) [method, above=6mm of #10] {#3}; | |
623 | \draw [->] ($(#10.north)!.5!(#10.north west) + (0mm, 1mm)$) -- | |
624 | ++(0mm, 4mm) | |
625 | node [code, left=4pt, midway] {next_method}; | |
626 | \draw [<-] ($(#10.north)!.5!(#10.north east) + (0mm, 1mm)$) -- | |
627 | ++(0mm, 4mm) | |
628 | node [action, right=4pt, midway] {return}; | |
629 | \draw [->] ($(#11.north)!.5!(#11.north west) + (0mm, 1mm)$) -- | |
630 | ++(0mm, 4mm) | |
631 | node [code, left=4pt, midway] {next_method} | |
632 | node (ld) [above] {$\smash\vdots\mathstrut$}; | |
633 | \draw [<-] ($(#11.north)!.5!(#11.north east) + (0mm, 1mm)$) -- | |
634 | ++(0mm, 4mm) | |
635 | node [action, right=4pt, midway] {return} | |
636 | node (rd) [above] {$\smash\vdots\mathstrut$}; | |
637 | \draw [->] ($(ld.north) + (0mm, 1mm)$) -- ++(0mm, 4mm) | |
638 | node [code, left=4pt, midway] {next_method}; | |
639 | \draw [<-] ($(rd.north) + (0mm, 1mm)$) -- ++(0mm, 4mm) | |
640 | node [action, right=4pt, midway] {return}; | |
641 | \node (p) at ($(ld.north)!.5!(rd.north)$) {}; | |
642 | \node (#1n) [method, above=5mm of p] {#3}; | |
643 | \draw [->, order] ($(#10.south east) + (4mm, 1mm)$) -- | |
644 | ($(#1n.north east) + (4mm, -1mm)$) | |
645 | node [midway, right, align=left] | |
646 | {Most to \\ least \\ specific};} | |
647 | ||
dc20d91f | 648 | \delgstack{a}{}{@|around| method} |
f1aa19a8 MW |
649 | \draw [<-] ($(a0.south)!.5!(a0.south west) - (0mm, 1mm)$) -- |
650 | ++(0mm, -4mm); | |
651 | \draw [->] ($(a0.south)!.5!(a0.south east) - (0mm, 1mm)$) -- | |
652 | ++(0mm, -4mm) | |
653 | node [action, right=4pt, midway] {return}; | |
654 | ||
655 | \draw [->] ($(an.north)!.6!(an.north west) + (0mm, 1mm)$) -- | |
656 | ++(-8mm, 8mm) | |
657 | node [code, midway, left=3mm] {next_method} | |
658 | node (b0) [method, above left = 1mm + 4mm and -6mm - 4mm] {}; | |
659 | \node (b1) [method] at ($(b0) - (2mm, 2mm)$) {}; | |
dc20d91f | 660 | \node (bn) [method] at ($(b1) - (2mm, 2mm)$) {@|before| method}; |
f1aa19a8 MW |
661 | \draw [->, order] ($(bn.west) - (6mm, 0mm)$) -- ++(12mm, 12mm) |
662 | node [midway, above left, align=center] {Most to \\ least \\ specific}; | |
663 | \draw [->] ($(b0.north east) + (-10mm, 1mm)$) -- ++(8mm, 8mm) | |
664 | node (p) {}; | |
665 | ||
666 | \delgstack{m}{above right=1mm and 0mm of an.west |- p}{Primary method} | |
667 | \draw [->] ($(mn.north)!.5!(mn.north west) + (0mm, 1mm)$) -- ++(0mm, 4mm) | |
668 | node [code, left=4pt, midway] {next_method} | |
669 | node [above right = 0mm and -8mm] | |
670 | {$\vcenter{\hbox{\Huge\textcolor{red}{!}}} | |
671 | \vcenter{\hbox{\begin{tabular}[c]{l} | |
672 | \textsf{next_method} \\ | |
673 | pointer is null | |
674 | \end{tabular}}}$}; | |
675 | ||
676 | \draw [->, color=blue, dotted] | |
677 | ($(m0.south)!.2!(m0.south east) - (0mm, 1mm)$) -- | |
678 | ($(an.north)!.2!(an.north east) + (0mm, 1mm)$) | |
679 | node [midway, sloped, below] {Return value}; | |
680 | ||
681 | \draw [<-] ($(an.north)!.6!(an.north east) + (0mm, 1mm)$) -- | |
682 | ++(8mm, 8mm) | |
683 | node [action, midway, right=3mm] {return} | |
684 | node (f0) [method, above right = 1mm and -6mm] {}; | |
685 | \node (f1) [method] at ($(f0) + (-2mm, 2mm)$) {}; | |
dc20d91f | 686 | \node (fn) [method] at ($(f1) + (-2mm, 2mm)$) {@|after| method}; |
f1aa19a8 MW |
687 | \draw [<-, order] ($(f0.east) + (6mm, 0mm)$) -- ++(-12mm, 12mm) |
688 | node [midway, above right, align=center] | |
689 | {Least to \\ most \\ specific}; | |
690 | \draw [<-] ($(fn.north west) + (6mm, 1mm)$) -- ++(-8mm, 8mm); | |
691 | ||
d82d5db5 | 692 | \end{tikzpicture}}\hss} |
f1aa19a8 MW |
693 | |
694 | \caption{The standard method combination} | |
695 | \label{fig:concepts.methods.stdmeth} | |
696 | \end{figure} | |
697 | ||
3cc520db | 698 | The effective method for a message with standard method combination works as |
f1aa19a8 | 699 | follows (see also~\xref{fig:concepts.methods.stdmeth}). |
3cc520db MW |
700 | \begin{enumerate} |
701 | ||
8ec911fa | 702 | \item If any applicable methods have the @|around| rôle, then the most |
3cc520db MW |
703 | specific such method, with respect to the class of the receiving object, is |
704 | invoked. | |
705 | ||
b1254eb6 | 706 | Within the body of an @|around| method, the variable @|next_method| is |
3cc520db MW |
707 | defined, having pointer-to-function type. The method may call this |
708 | function, as described below, any number of times. | |
709 | ||
b1254eb6 MW |
710 | If there any remaining @|around| methods, then @|next_method| invokes the |
711 | next most specific such method, returning whichever value that method | |
dc20d91f MW |
712 | returns; otherwise the behaviour of @|next_method| is to invoke the |
713 | @|before| methods (if any), followed by the most specific primary method, | |
b0563651 | 714 | followed by the @|after| methods (if any), and to return whichever value |
dc20d91f MW |
715 | was returned by the most specific primary method, as described in the |
716 | following items. That is, the behaviour of the least specific @|around| | |
717 | method's @|next_method| function is exactly the behaviour that the | |
718 | effective method would have if there were no @|around| methods. Note that | |
719 | if the least-specific @|around| method calls its @|next_method| more than | |
720 | once then the whole sequence of @|before|, primary, and @|after| methods | |
721 | occurs multiple times. | |
3cc520db | 722 | |
b1254eb6 MW |
723 | The value returned by the most specific @|around| method is the value |
724 | returned by the effective method. | |
3cc520db | 725 | |
8ec911fa | 726 | \item If any applicable methods have the @|before| rôle, then they are all |
3cc520db MW |
727 | invoked, starting with the most specific. |
728 | ||
729 | \item The most specific applicable primary method is invoked. | |
730 | ||
731 | Within the body of a primary method, the variable @|next_method| is | |
732 | defined, having pointer-to-function type. If there are no remaining less | |
733 | specific primary methods, then @|next_method| is a null pointer. | |
734 | Otherwise, the method may call the @|next_method| function any number of | |
735 | times. | |
736 | ||
737 | The behaviour of the @|next_method| function, if it is not null, is to | |
738 | invoke the next most specific applicable primary method, and to return | |
739 | whichever value that method returns. | |
740 | ||
b1254eb6 MW |
741 | If there are no applicable @|around| methods, then the value returned by |
742 | the most specific primary method is the value returned by the effective | |
743 | method; otherwise the value returned by the most specific primary method is | |
744 | returned to the least specific @|around| method, which called it via its | |
745 | own @|next_method| function. | |
3cc520db | 746 | |
8ec911fa | 747 | \item If any applicable methods have the @|after| rôle, then they are all |
3cc520db | 748 | invoked, starting with the \emph{least} specific. (Hence, the most |
b1254eb6 | 749 | specific @|after| method is invoked with the most `afterness'.) |
3cc520db MW |
750 | |
751 | \end{enumerate} | |
752 | ||
b1254eb6 MW |
753 | A typical use for @|around| methods is to allow a base class to set up the |
754 | dynamic environment appropriately for the primary methods of its subclasses, | |
756e9293 | 755 | e.g., by claiming a lock, and releasing it afterwards. |
3cc520db | 756 | |
9761db0d | 757 | The @|next_method| function provided to methods with the primary and |
8ec911fa | 758 | @|around| rôles accepts the same arguments, and returns the same type, as the |
3cc520db MW |
759 | message, except that one or two additional arguments are inserted at the |
760 | front of the argument list. The first additional argument is always the | |
761 | receiving object, @|me|. If the message accepts a variable argument suffix, | |
762 | then the second addition argument is a @|va_list|; otherwise there is no | |
763 | second additional argument; otherwise, In the former case, a variable | |
764 | @|sod__master_ap| of type @|va_list| is defined, containing a separate copy | |
765 | of the argument pointer (so the method body can process the variable argument | |
766 | suffix itself, and still pass a fresh copy on to the next method). | |
767 | ||
8ec911fa | 768 | A method with the primary or @|around| rôle may use the convenience macro |
3cc520db MW |
769 | @|CALL_NEXT_METHOD|, which takes no arguments itself, and simply calls |
770 | @|next_method| with appropriate arguments: the receiver @|me| pointer, the | |
771 | argument pointer @|sod__master_ap| (if applicable), and the method's | |
772 | arguments. If the method body has overwritten its formal arguments, then | |
773 | @|CALL_NEXT_METHOD| will pass along the updated values, rather than the | |
774 | original ones. | |
775 | ||
781a8fbd MW |
776 | A primary or @|around| method which invokes its @|next_method| function is |
777 | said to \emph{extend} the message behaviour; a method which does not invoke | |
778 | its @|next_method| is said to \emph{override} the behaviour. Note that a | |
779 | method may make a decision to override or extend at runtime. | |
780 | ||
43073476 | 781 | \subsubsection{Aggregating method combinations} |
3cc520db MW |
782 | A number of other method combinations are provided. They are called |
783 | `aggregating' method combinations because, instead of invoking just the most | |
784 | specific primary method, as the standard method combination does, they invoke | |
785 | the applicable primary methods in turn and aggregate the return values from | |
786 | each. | |
787 | ||
8ec911fa | 788 | The aggregating method combinations accept the same four rôles as the |
b1254eb6 MW |
789 | standard method combination, and @|around|, @|before|, and @|after| methods |
790 | work in the same way. | |
3cc520db MW |
791 | |
792 | The aggregating method combinations provided are as follows. | |
793 | \begin{description} \let\makelabel\code | |
794 | \item[progn] The message must return @|void|. The applicable primary methods | |
795 | are simply invoked in turn, most specific first. | |
796 | \item[sum] The message must return a numeric type.\footnote{% | |
797 | The Sod translator does not check this, since it doesn't have enough | |
798 | insight into @|typedef| names.} % | |
799 | The applicable primary methods are invoked in turn, and their return values | |
800 | added up. The final result is the sum of the individual values. | |
801 | \item[product] The message must return a numeric type. The applicable | |
802 | primary methods are invoked in turn, and their return values multiplied | |
803 | together. The final result is the product of the individual values. | |
804 | \item[min] The message must return a scalar type. The applicable primary | |
805 | methods are invoked in turn. The final result is the smallest of the | |
806 | individual values. | |
807 | \item[max] The message must return a scalar type. The applicable primary | |
808 | methods are invoked in turn. The final result is the largest of the | |
809 | individual values. | |
665a0455 MW |
810 | \item[and] The message must return a scalar type. The applicable primary |
811 | methods are invoked in turn. If any method returns zero then the final | |
812 | result is zero and no further methods are invoked. If all of the | |
813 | applicable primary methods return nonzero, then the final result is the | |
814 | result of the last primary method. | |
815 | \item[or] The message must return a scalar type. The applicable primary | |
816 | methods are invoked in turn. If any method returns nonzero then the final | |
817 | result is that nonzero value and no further methods are invoked. If all of | |
818 | the applicable primary methods return zero, then the final result is zero. | |
3cc520db MW |
819 | \end{description} |
820 | ||
821 | There is also a @|custom| aggregating method combination, which is described | |
822 | in \xref{sec:fixme.custom-aggregating-method-combination}. | |
823 | ||
43073476 | 824 | |
f4e44f7f | 825 | \subsection{Method entries} \label{sec:concepts.methods.entry} |
caa6f4b9 | 826 | |
caa6f4b9 MW |
827 | The effective methods for each class are determined at translation time, by |
828 | the Sod translator. For each effective method, one or more \emph{method | |
829 | entry functions} are constructed. A method entry function has three | |
830 | responsibilities. | |
831 | \begin{itemize} | |
832 | \item It converts the receiver pointer to the correct type. Method entry | |
833 | functions can perform these conversions extremely efficiently: there are | |
834 | separate method entries for each chain of each class which can receive a | |
835 | message, so method entry functions are in the privileged situation of | |
836 | knowing the \emph{exact} class of the receiving object. | |
837 | \item If the message accepts a variable-length argument tail, then two method | |
838 | entry functions are created for each chain of each class: one receives a | |
839 | variable-length argument tail, as intended, and captures it in a @|va_list| | |
840 | object; the other accepts an argument of type @|va_list| in place of the | |
841 | variable-length tail and arranges for it to be passed along to the direct | |
842 | methods. | |
843 | \item It invokes the effective method with the appropriate arguments. There | |
844 | might or might not be an actual function corresponding to the effective | |
845 | method itself: the translator may instead open-code the effective method's | |
846 | behaviour into each method entry function; and the machinery for handling | |
847 | `delegation chains', such as is used for @|around| methods and primary | |
848 | methods in the standard method combination, is necessarily scattered among | |
849 | a number of small functions. | |
850 | \end{itemize} | |
851 | ||
852 | ||
43073476 MW |
853 | \subsection{Messages with keyword arguments} |
854 | \label{sec:concepts.methods.keywords} | |
855 | ||
856 | A message or a direct method may declare that it accepts keyword arguments. | |
857 | A message which accepts keyword arguments is called a \emph{keyword message}; | |
858 | a direct method which accepts keyword arguments is called a \emph{keyword | |
859 | method}. | |
860 | ||
861 | While method combinations may set their own rules, usually keyword methods | |
862 | can only be defined on keyword messages, and all methods defined on a keyword | |
863 | message must be keyword methods. The direct methods defined on a keyword | |
864 | message may differ in the keywords they accept, both from each other, and | |
bf2e7452 MW |
865 | from the message. If two applicable methods on the same message both accept |
866 | a keyword argument with the same name, then these two keyword arguments must | |
867 | also have the same type. Different applicable methods may declare keyword | |
868 | arguments with the same name but different defaults; see below. | |
43073476 MW |
869 | |
870 | The keyword arguments acceptable in a message sent to an object are the | |
871 | keywords listed in the message definition, together with all of the keywords | |
872 | accepted by any applicable method. There is no easy way to determine at | |
873 | runtime whether a particular keyword is acceptable in a message to a given | |
874 | instance. | |
875 | ||
876 | At runtime, a direct method which accepts one or more keyword arguments | |
877 | receives an additional argument named @|suppliedp|. This argument is a small | |
878 | structure. For each keyword argument named $k$ accepted by the direct | |
879 | method, @|suppliedp| contains a one-bit-wide bitfield member of type | |
880 | @|unsigned|, also named $k$. If a keyword argument named $k$ was passed in | |
881 | the message, then @|suppliedp.$k$| is one, and $k$ contains the argument | |
882 | value; otherwise @|suppliedp.$k$| is zero, and $k$ contains the default value | |
883 | from the direct method definition if there was one, or an unspecified value | |
884 | otherwise. | |
885 | ||
3cc520db | 886 | %%%-------------------------------------------------------------------------- |
d24d47f5 MW |
887 | \section{The object lifecycle} \label{sec:concepts.lifecycle} |
888 | ||
889 | \subsection{Creation} \label{sec:concepts.lifecycle.birth} | |
890 | ||
891 | Construction of a new instance of a class involves three steps. | |
892 | \begin{enumerate} | |
893 | \item \emph{Allocation} arranges for there to be storage space for the | |
894 | instance's slots and associated metadata. | |
895 | \item \emph{Imprinting} fills in the instance's metadata, associating the | |
896 | instance with its class. | |
897 | \item \emph{Initialization} stores appropriate initial values in the | |
898 | instance's slots, and maybe links it into any external data structures as | |
899 | necessary. | |
900 | \end{enumerate} | |
901 | The \descref{SOD_DECL}[macro]{mac} handles constructing instances with | |
a42893dd MW |
902 | automatic storage duration (`on the stack'). Similarly, the |
903 | \descref{SOD_MAKE}[macro]{mac} and the \descref{sod_make}{fun} and | |
904 | \descref{sod_makev}{fun} functions construct instances allocated from the | |
905 | standard @|malloc| heap. Programmers can add support for other allocation | |
906 | strategies by using the \descref{SOD_INIT}[macro]{mac} and the | |
907 | \descref{sod_init}{fun} and \descref{sod_initv}{fun} functions, which package | |
908 | up imprinting and initialization. | |
d24d47f5 MW |
909 | |
910 | \subsubsection{Allocation} | |
911 | Instances of most classes (specifically including those classes defined by | |
912 | Sod itself) can be held in any storage of sufficient size. The in-memory | |
913 | layout of an instance of some class~$C$ is described by the type @|struct | |
914 | $C$__ilayout|, and if the relevant class is known at compile time then the | |
915 | best way to discover the layout size is with the @|sizeof| operator. Failing | |
916 | that, the size required to hold an instance of $C$ is available in a slot in | |
917 | $C$'s class object, as @|$C$__class@->cls.initsz|. | |
918 | ||
919 | It is not in general sufficient to declare, or otherwise allocate, an object | |
920 | of the class type $C$. The class type only describes a single chain of the | |
921 | object's layout. It is nearly always an error to use the class type as if it | |
922 | is a \emph{complete type}, e.g., to declare objects or arrays of the class | |
923 | type, or to enquire about its size or alignment requirements. | |
924 | ||
925 | Instance layouts may be declared as objects with automatic storage duration | |
926 | (colloquially, `allocated on the stack') or allocated dynamically, e.g., | |
927 | using @|malloc|. They may be included as members of structures or unions, or | |
928 | elements of arrays. Sod's runtime system doesn't retain addresses of | |
929 | instances, so, for example, Sod doesn't make using fancy allocators which | |
930 | sometimes move objects around in memory any more difficult than it needs to | |
931 | be. | |
932 | ||
933 | There isn't any way to discover the alignment required for a particular | |
934 | class's instances at runtime; it's best to be conservative and assume that | |
935 | the platform's strictest alignment requirement applies. | |
936 | ||
937 | The following simple function correctly allocates and returns space for an | |
938 | instance of a class given a pointer to its class object @<cls>. | |
939 | \begin{prog} | |
020b9e2b | 940 | void *allocate_instance(const SodClass *cls) \\ \ind |
d24d47f5 MW |
941 | \{ return malloc(cls@->cls.initsz); \} |
942 | \end{prog} | |
943 | ||
944 | \subsubsection{Imprinting} | |
945 | Once storage has been allocated, it must be \emph{imprinted} before it can be | |
946 | used as an instance of a class, e.g., before any messages can be sent to it. | |
947 | ||
948 | Imprinting an instance stores some metadata about its direct class in the | |
949 | instance structure, so that the rest of the program (and Sod's runtime | |
950 | library) can tell what sort of object it is, and how to use it.\footnote{% | |
951 | Specifically, imprinting an instance's storage involves storing the | |
952 | appropriate vtable pointers in the right places in it.} % | |
953 | A class object's @|imprint| slot points to a function which will correctly | |
954 | imprint storage for one of that class's instances. | |
955 | ||
956 | Once an instance's storage has been imprinted, it is technically possible to | |
957 | send messages to the instance; however the instance's slots are still | |
756e9293 MW |
958 | uninitialized at this point, so the applicable methods are unlikely to do |
959 | much of any use unless they've been written specifically for the purpose. | |
d24d47f5 MW |
960 | |
961 | The following simple function imprints storage at address @<p> as an instance | |
962 | of a class, given a pointer to its class object @<cls>. | |
963 | \begin{prog} | |
020b9e2b | 964 | void imprint_instance(const SodClass *cls, void *p) \\ \ind |
d24d47f5 MW |
965 | \{ cls@->cls.imprint(p); \} |
966 | \end{prog} | |
967 | ||
968 | \subsubsection{Initialization} | |
969 | The final step for constructing a new instance is to \emph{initialize} it, to | |
970 | establish the necessary invariants for the instance itself and the | |
971 | environment in which it operates. | |
972 | ||
973 | Details of initialization are necessarily class-specific, but typically it | |
974 | involves setting the instance's slots to appropriate values, and possibly | |
d1b394fa MW |
975 | linking it into some larger data structure to keep track of it. It is |
976 | possible for initialization methods to attempt to allocate resources, but | |
977 | this must be done carefully: there is currently no way to report an error | |
978 | from object initialization, so the object must be marked as incompletely | |
979 | initialized, and left in a state where it will be safe to tear down later. | |
d24d47f5 | 980 | |
a142609c MW |
981 | Initialization is performed by sending the imprinted instance an @|init| |
982 | message, defined by the @|SodObject| class. This message uses a nonstandard | |
983 | method combination which works like the standard combination, except that the | |
984 | \emph{default behaviour}, if there is no overriding method, is to initialize | |
b2983f35 MW |
985 | the instance's slots, as described below, and to invoke each superclass's |
986 | initialization fragments. This default behaviour may be invoked multiple | |
987 | times if some method calls on its @|next_method| more than once, unless some | |
988 | other method takes steps to prevent this. | |
a142609c | 989 | |
27ec3825 MW |
990 | Slots are initialized in a well-defined order. |
991 | \begin{itemize} | |
054e8f8f MW |
992 | \item Slots defined by a more specific superclass are initialized after slots |
993 | defined by a less specific superclass. | |
27ec3825 MW |
994 | \item Slots defined by the same class are initialized in the order in which |
995 | their definitions appear. | |
996 | \end{itemize} | |
997 | ||
a42893dd MW |
998 | A class can define \emph{initialization fragments}: pieces of literal code to |
999 | be executed to set up a new instance. Each superclass's initialization | |
1000 | fragments are executed with @|me| bound to an instance pointer of the | |
1001 | appropriate superclass type, immediately after that superclass's slots (if | |
1002 | any) have been initialized; therefore, fragments defined by a more specific | |
13cb243a | 1003 | superclass are executed after fragments defined by a less specific |
a42893dd MW |
1004 | superclass. A class may define more than one initialization fragment: the |
1005 | fragments are executed in the order in which they appear in the class | |
1006 | definition. It is possible for an initialization fragment to use @|return| | |
1007 | or @|goto| for special control-flow effects, but this is not likely to be a | |
1008 | good idea. | |
1009 | ||
b2983f35 MW |
1010 | The @|init| message accepts keyword arguments |
1011 | (\xref{sec:concepts.methods.keywords}). The set of acceptable keywords is | |
1012 | determined by the applicable methods as usual, but also by the | |
1013 | \emph{initargs} defined by the receiving instance's class and its | |
1014 | superclasses, which are made available to slot initializers and | |
1015 | initialization fragments. | |
1016 | ||
1017 | There are two kinds of initarg definitions. \emph{User initargs} are defined | |
1018 | by an explicit @|initarg| item appearing in a class definition: the item | |
1019 | defines a name, type, and (optionally) a default value for the initarg. | |
1020 | \emph{Slot initargs} are defined by attaching an @|initarg| property to a | |
756e9293 MW |
1021 | slot or slot initializer item: the property's value determines the initarg's |
1022 | name, while the type is taken from the underlying slot type; slot initargs do | |
1023 | not have default values. Both kinds define a \emph{direct initarg} for the | |
b2983f35 MW |
1024 | containing class. |
1025 | ||
1026 | Initargs are inherited. The \emph{applicable} direct initargs for an @|init| | |
1027 | effective method are those defined by the receiving object's class, and all | |
1028 | of its superclasses. Applicable direct initargs with the same name are | |
1029 | merged to form \emph{effective initargs}. An error is reported if two | |
1030 | applicable direct initargs have the same name but different types. The | |
1031 | default value of an effective initarg is taken from the most specific | |
1032 | applicable direct initarg which specifies a defalt value; if no applicable | |
1033 | direct initarg specifies a default value then the effective initarg has no | |
1034 | default. | |
1035 | ||
1036 | All initarg values are made available at runtime to user code -- | |
1037 | initialization fragments and slot initializer expressions -- through local | |
1038 | variables and a @|suppliedp| structure, as in a direct method | |
1039 | (\xref{sec:concepts.methods.keywords}). Furthermore, slot initarg | |
1040 | definitions influence the initialization of slots. | |
1041 | ||
1042 | The process for deciding how to initialize a particular slot works as | |
1043 | follows. | |
1044 | \begin{enumerate} | |
1045 | \item If there are any slot initargs defined on the slot, or any of its slot | |
1046 | initializers, \emph{and} the sender supplied a value for one or more of the | |
1047 | corresponding effective initargs, then the value of the most specific slot | |
1048 | initarg is stored in the slot. | |
1049 | \item Otherwise, if there are any slot initializers defined which include an | |
1050 | initializer expression, then the initializer expression from the most | |
1051 | specific such slot initializer is evaluated and its value stored in the | |
1052 | slot. | |
1053 | \item Otherwise, the slot is left uninitialized. | |
1054 | \end{enumerate} | |
1055 | Note that the default values (if any) of effective initargs do \emph{not} | |
1056 | affect this procedure. | |
d24d47f5 | 1057 | |
d24d47f5 MW |
1058 | |
1059 | \subsection{Destruction} | |
1060 | \label{sec:concepts.lifecycle.death} | |
1061 | ||
1062 | Destruction of an instance, when it is no longer required, consists of two | |
1063 | steps. | |
1064 | \begin{enumerate} | |
1065 | \item \emph{Teardown} releases any resources held by the instance and | |
1066 | disentangles it from any external data structures. | |
1067 | \item \emph{Deallocation} releases the memory used to store the instance so | |
1068 | that it can be reused. | |
1069 | \end{enumerate} | |
a42893dd MW |
1070 | Teardown alone, for objects which require special deallocation, or for which |
1071 | deallocation occurs automatically (e.g., instances with automatic storage | |
1072 | duration, or instances whose storage will be garbage-collected), is performed | |
1073 | using the \descref{sod_teardown}[function]{fun}. Destruction of instances | |
1074 | allocated from the standard @|malloc| heap is done using the | |
1075 | \descref{sod_destroy}[function]{fun}. | |
d24d47f5 MW |
1076 | |
1077 | \subsubsection{Teardown} | |
7646dc4c MW |
1078 | Details of teardown are necessarily class-specific, but typically it |
1079 | involves releasing resources held by the instance, and disentangling it from | |
1080 | any data structures it might be linked into. | |
a42893dd MW |
1081 | |
1082 | Teardown is performed by sending the instance the @|teardown| message, | |
1083 | defined by the @|SodObject| class. The message returns an integer, used as a | |
1084 | boolean flag. If the message returns zero, then the instance's storage | |
1085 | should be deallocated. If the message returns nonzero, then it is safe for | |
1086 | the caller to forget about instance, but should not deallocate its storage. | |
1087 | This is \emph{not} an error return: if some teardown method fails then the | |
1088 | program may be in an inconsistent state and should not continue. | |
d24d47f5 | 1089 | |
a42893dd MW |
1090 | This simple protocol can be used, for example, to implement a reference |
1091 | counting system, as follows. | |
d24d47f5 | 1092 | \begin{prog} |
020b9e2b | 1093 | [nick = ref] \\ |
d7451ac3 | 1094 | class ReferenceCountedObject: SodObject \{ \\ \ind |
020b9e2b MW |
1095 | unsigned nref = 1; \\- |
1096 | void inc() \{ me@->ref.nref++; \} \\- | |
1097 | [role = around] \\ | |
1098 | int obj.teardown() \\ | |
1099 | \{ \\ \ind | |
1100 | if (--\,--me@->ref.nref) return (1); \\ | |
1101 | else return (CALL_NEXT_METHOD); \-\\ | |
1102 | \} \-\\ | |
d24d47f5 MW |
1103 | \} |
1104 | \end{prog} | |
1105 | ||
fa7e2d72 MW |
1106 | The @|teardown| message uses a nonstandard method combination which works |
1107 | like the standard combination, except that the \emph{default behaviour}, if | |
1108 | there is no overriding method, is to execute the superclass's teardown | |
1109 | fragments, and to return zero. This default behaviour may be invoked | |
1110 | multiple times if some method calls on its @|next_method| more than once, | |
1111 | unless some other method takes steps to prevent this. | |
a42893dd MW |
1112 | |
1113 | A class can define \emph{teardown fragments}: pieces of literal code to be | |
1114 | executed to shut down an instance. Each superclass's teardown fragments are | |
1115 | executed with @|me| bound to an instance pointer of the appropriate | |
1116 | superclass type; fragments defined by a more specific superclass are executed | |
13cb243a | 1117 | before fragments defined by a less specific superclass. A class may define |
a42893dd MW |
1118 | more than one teardown fragment: the fragments are executed in the order in |
1119 | which they appear in the class definition. It is possible for an | |
1120 | initialization fragment to use @|return| or @|goto| for special control-flow | |
1121 | effects, but this is not likely to be a good idea. Similarly, it's probably | |
1122 | a better idea to use an @|around| method to influence the return value than | |
1123 | to write an explicit @|return| statement in a teardown fragment. | |
1124 | ||
d24d47f5 MW |
1125 | \subsubsection{Deallocation} |
1126 | The details of instance deallocation are obviously specific to the allocation | |
1127 | strategy used by the instance, and this is often orthogonal from the object's | |
1128 | class. | |
1129 | ||
1130 | The code which makes the decision to destroy an object may often not be aware | |
1131 | of the object's direct class. Low-level details of deallocation often | |
1132 | require the proper base address of the instance's storage, which can be | |
1133 | determined using the \descref{SOD_INSTBASE}[macro]{mac}. | |
1134 | ||
d24d47f5 | 1135 | %%%-------------------------------------------------------------------------- |
3cc520db | 1136 | \section{Metaclasses} \label{sec:concepts.metaclasses} |
1f7d590d | 1137 | |
71efc524 MW |
1138 | In Sod, every object is an instance of some class, and -- unlike, say, |
1139 | \Cplusplus\ -- classes are proper objects. It follows that, in Sod, every | |
1140 | class~$C$ is itself an instance of some class~$M$, which is called $C$'s | |
1141 | \emph{metaclass}. Metaclass instances are usually constructed statically, at | |
1142 | compile time, and marked read-only. | |
1143 | ||
1144 | As an added complication, Sod classes, and other metaobjects such as | |
1145 | messages, methods, slots and so on, also have classes \emph{at translation | |
1146 | time}. These translation-time metaclasses are not Sod classes; they are CLOS | |
1147 | classes, implemented in Common Lisp. | |
1148 | ||
1149 | ||
1150 | \subsection{Runtime metaclasses} | |
1151 | \label{sec:concepts.metaclasses.runtime} | |
1152 | ||
1153 | Like other classes, metaclasses can declare messages, and define slots and | |
1154 | methods. Slots defined by the metaclass are called \emph{class slots}, as | |
1155 | opposed to \emph{instance slots}. Similarly, messages and methods defined by | |
1156 | the metaclass are termed \emph{class messages} and \emph{class methods} | |
1157 | respectively, though these are used much less frequently. | |
1158 | ||
1159 | \subsubsection{The braid} | |
1160 | Every object is an instance of some class. There are only finitely many | |
1161 | classes. | |
1162 | ||
1163 | \begin{figure} | |
1164 | \centering | |
1165 | \begin{tikzpicture} | |
1166 | \node[lit] (obj) {SodObject}; | |
1167 | \node[lit] (cls) [right=10mm of obj] {SodClass}; | |
1168 | \draw [->, dashed] (obj) to[bend right] (cls); | |
1169 | \draw [->] (cls) to[bend right] (obj); | |
1170 | \draw [->, dashed] (cls) to[loop right] (cls); | |
1171 | \end{tikzpicture} | |
1172 | \qquad | |
1173 | \fbox{\ \begin{tikzpicture} | |
1174 | \node (subclass) {subclass of}; | |
1175 | \node (instance) [below=\jot of subclass] {instance of}; | |
1176 | \draw [->] ($(subclass.west) - (10mm, 0)$) -- ++(8mm, 0); | |
1177 | \draw [->, dashed] ($(instance.west) - (10mm, 0)$) -- ++(8mm, 0); | |
1178 | \end{tikzpicture}} | |
1179 | \caption{The Sod braid} \label{fig:concepts.metaclasses.braid} | |
1180 | \end{figure} | |
1181 | ||
1182 | Consider the directed graph whose nodes are classes, and where there is an | |
1183 | arc from $C$ to $D$ if and only if $C$ is an instance of $D$. There are only | |
1184 | finitely many nodes. Every node has an arc leaving it, because every object | |
1185 | -- and hence every class -- is an instance of some class. Therefore this | |
1186 | graph must contain at least one cycle. | |
1187 | ||
1188 | In Sod, this situation is resolved in the simplest manner possible: | |
1189 | @|SodClass| is the only predefined metaclass, and it is an instance of | |
1190 | itself. The only other predefined class is @|SodObject|, which is also an | |
1191 | instance of @|SodClass|. There is exactly one root class, namely | |
1192 | @|SodObject|; consequently, @|SodClass| is a direct subclass of @|SodObject|. | |
1193 | ||
1194 | \Xref{fig:concepts.metaclasses.braid} shows a diagram of this situation. | |
1195 | ||
1196 | \subsubsection{Class slots and initializers} | |
1197 | Instance initializers were described in \xref{sec:concepts.classes.slots}. A | |
1198 | class can also define \emph{class initializers}, which provide values for | |
1199 | slots defined by its metaclass. The initial value for a class slot is | |
1200 | determined as follows. | |
1201 | \begin{itemize} | |
1202 | \item Nonstandard slot classes may be initialized by custom Lisp code. For | |
1203 | example, all of the slots defined by @|SodClass| are of this kind. User | |
1204 | initializers are not permitted for such slots. | |
1205 | \item If the class or any of its superclasses defines a class initializer for | |
1206 | the slot, then the class initializer defined by the most specific such | |
1207 | superclass is used. | |
1208 | \item Otherwise, if the metaclass or one of its superclasses defines an | |
1209 | instance initializer, then the instance initializer defined by he most | |
1210 | specific such class is used. | |
1211 | \item Otherwise there is no initializer, and an error will be reported. | |
1212 | \end{itemize} | |
1213 | Initializers for class slots must be constant expressions (for scalar slots) | |
1214 | or aggregate initializers containing constant expressions. | |
1215 | ||
1216 | \subsubsection{Metaclass selection and consistency} | |
1217 | Sod enforces a \emph{metaclass consistency rule}: if $C$ has metaclass $M$, | |
1218 | then any subclass $C$ must have a metaclass which is a subclass of $M$. | |
1219 | ||
1220 | The definition of a new class can name the new class's metaclass explicitly, | |
1221 | by defining a @|metaclass| property; the Sod translator will verify that the | |
1222 | choice of metaclass is acceptable. | |
1223 | ||
1224 | If no @|metaclass| property is given, then the translator will select a | |
1225 | default metaclass as follows. Let $C_1$, $C_2$, \dots, $C_n$ be the direct | |
1226 | superclasses of the new class, and let $M_1$, $M_2$, \dots, $M_n$ be their | |
1227 | respective metaclasses (not necessarily distinct). If there exists exactly | |
1228 | one minimal metaclass $M_i$, i.e., there exists an $i$, with $1 \le i \le n$, | |
1229 | such that $M_i$ is a subclass of every $M_j$, for $1 \le j \le n$, then $M_i$ | |
1230 | is selected as the new class's metaclass. Otherwise the situation is | |
1231 | ambiguous and an error will be reported. Usually, the ambiguity can be | |
1232 | resolved satisfactorily by defining a new class $M^*$ as a direct subclass of | |
1233 | the minimal $M_j$. | |
1234 | ||
1235 | ||
1236 | \subsection{Translation-time metaobjects} | |
1237 | \label{sec:concepts.metaclasses.compile-time} | |
1238 | ||
1239 | ||
1240 | ||
1241 | \fixme{unwritten} | |
1242 | ||
caa6f4b9 MW |
1243 | %%%-------------------------------------------------------------------------- |
1244 | \section{Compatibility considerations} \label{sec:concepts.compatibility} | |
1245 | ||
1246 | Sod doesn't make source-level compatibility especially difficult. As long as | |
1247 | classes, slots, and messages don't change names or dissappear, and slots and | |
1248 | messages retain their approximate types, everything will be fine. | |
1249 | ||
1250 | Binary compatibility is much more difficult. Unfortunately, Sod classes have | |
1251 | rather fragile binary interfaces.\footnote{% | |
1252 | Research suggestion: investigate alternative instance and vtable layouts | |
1253 | which improve binary compatibility, probably at the expense of instance | |
1254 | compactness, and efficiency of slot access and message sending. There may | |
1255 | be interesting trade-offs to be made.} % | |
1256 | ||
6390b845 | 1257 | If instances are allocated \fixme{incomplete} |
caa6f4b9 | 1258 | |
1f7d590d MW |
1259 | %%%----- That's all, folks -------------------------------------------------- |
1260 | ||
1261 | %%% Local variables: | |
1262 | %%% mode: LaTeX | |
1263 | %%% TeX-master: "sod.tex" | |
1264 | %%% TeX-PDF-mode: t | |
1265 | %%% End: |