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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 | ||
125 | If $C$ has a proper superclass $B$, then $B$ is not allowed to have $C$ has a | |
126 | direct superclass. In different terms, if we construct a graph, whose | |
127 | vertices are classes, and draw an edge 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. | |
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, | |
151 | which has a number of good properties described in~\cite{FIXME:C3}. | |
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 | ||
173 | \subsubsection{Class links and chains} | |
174 | The definition for a class $C$ may distinguish one of its proper superclasses | |
175 | as being the \emph{link superclass} for class $C$. Not every class need have | |
176 | a link superclass, and the link superclass of a class $C$, if it exists, need | |
177 | not be a direct superclass of $C$. | |
178 | ||
179 | Superclass links must obey the following rule: if $C$ is a class, then there | |
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180 | must be no three superclasses $X$, $Y$ and~$Z$ of $C$ such that $Z$ is the |
181 | link superclass of both $X$ and $Y$. As a consequence of this rule, the | |
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182 | superclasses of $C$ can be partitioned into linear \emph{chains}, such that |
183 | superclasses $A$ and $B$ are in the same chain if and only if one can trace a | |
184 | path from $A$ to $B$ by following superclass links, or \emph{vice versa}. | |
185 | ||
186 | Since a class links only to one of its proper superclasses, the classes in a | |
187 | chain are naturally ordered from most- to least-specific. The least specific | |
188 | class in a chain is called the \emph{chain head}; the most specific class is | |
189 | the \emph{chain tail}. Chains are often named after their chain head | |
190 | classes. | |
191 | ||
192 | \subsection{Names} | |
193 | \label{sec:concepts.classes.names} | |
194 | ||
195 | Classes have a number of other attributes: | |
196 | \begin{itemize} | |
197 | \item A \emph{name}, which is a C identifier. Class names must be globally | |
198 | unique. The class name is used in the names of a number of associated | |
199 | definitions, to be described later. | |
200 | \item A \emph{nickname}, which is also a C identifier. Unlike names, | |
201 | nicknames are not required to be globally unique. If $C$ is any class, | |
202 | then all the superclasses of $C$ must have distinct nicknames. | |
203 | \end{itemize} | |
204 | ||
0a2d4b68 | 205 | |
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206 | \subsection{Slots} \label{sec:concepts.classes.slots} |
207 | ||
208 | Each class defines a number of \emph{slots}. Much like a structure member, a | |
209 | slot has a \emph{name}, which is a C identifier, and a \emph{type}. Unlike | |
210 | many other object systems, different superclasses of a class $C$ can define | |
211 | slots with the same name without ambiguity, since slot references are always | |
212 | qualified by the defining class's nickname. | |
213 | ||
214 | \subsubsection{Slot initializers} | |
215 | As well as defining slot names and types, a class can also associate an | |
216 | \emph{initial value} with each slot defined by itself or one of its | |
98da9322 | 217 | subclasses. A class $C$ provides an \emph{initialization message} (see |
d24d47f5 | 218 | \xref{sec:concepts.lifecycle.birth}, and \xref{sec:structures.root.sodclass}) |
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219 | whose methods set the slots of a \emph{direct} instance of the class to the |
220 | correct initial values. If several of $C$'s superclasses define initializers | |
221 | for the same slot then the initializer from the most specific such class is | |
222 | used. If none of $C$'s superclasses define an initializer for some slot then | |
223 | that slot will be left uninitialized. | |
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224 | |
225 | The initializer for a slot with scalar type may be any C expression. The | |
226 | initializer for a slot with aggregate type must contain only constant | |
227 | expressions if the generated code is expected to be processed by a | |
228 | implementation of C89. Initializers will be evaluated once each time an | |
229 | instance is initialized. | |
230 | ||
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231 | Slots are initialized in reverse-precedence order of their defining classes; |
232 | i.e., slots defined by a less specific superclass are initialized earlier | |
233 | than slots defined by a more specific superclass. Slots defined by the same | |
234 | class are initialized in the order in which they appear in the class | |
235 | definition. | |
236 | ||
237 | The initializer for a slot may refer to other slots in the same object, via | |
238 | the @|me| pointer: in an initializer for a slot defined by a class $C$, @|me| | |
239 | has type `pointer to $C$'. (Note that the type of @|me| depends only on the | |
240 | class which defined the slot, not the class which defined the initializer.) | |
241 | ||
0a2d4b68 | 242 | |
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243 | \subsection{C language integration} \label{sec:concepts.classes.c} |
244 | ||
245 | For each class~$C$, the Sod translator defines a C type, the \emph{class | |
246 | type}, with the same name. This is the usual type used when considering an | |
247 | object as an instance of class~$C$. No entire object will normally have a | |
248 | class type,\footnote{% | |
249 | In general, a class type only captures the structure of one of the | |
250 | superclass chains of an instance. A full instance layout contains multiple | |
251 | chains. See \xref{sec:structures.layout} for the full details.} % | |
252 | so access to instances is almost always via pointers. | |
253 | ||
254 | \subsubsection{Access to slots} | |
255 | The class type for a class~$C$ is actually a structure. It contains one | |
256 | member for each class in $C$'s superclass chain, named with that class's | |
257 | nickname. Each of these members is also a structure, containing the | |
258 | corresponding class's slots, one member per slot. There's nothing special | |
259 | about these slot members: C code can access them in the usual way. | |
260 | ||
261 | For example, if @|MyClass| has the nickname @|mine|, and defines a slot @|x| | |
262 | of type @|int|, then the simple function | |
263 | \begin{prog} | |
c18d6aba | 264 | int get_x(MyClass *m) \{ return (m@->mine.x); \} |
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265 | \end{prog} |
266 | will extract the value of @|x| from an instance of @|MyClass|. | |
267 | ||
268 | All of this means that there's no such thing as `private' or `protected' | |
269 | slots. If you want to hide implementation details, the best approach is to | |
270 | stash them in a dynamically allocated private structure, and leave a pointer | |
271 | to it in a slot. (This will also help preserve binary compatibility, because | |
272 | the private structure can grow more members as needed. See | |
e4ea29d8 | 273 | \xref{sec:fixme.compatibility} for more details.) |
3cc520db | 274 | |
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275 | \subsubsection{Vtables} |
276 | ||
277 | ||
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278 | \subsubsection{Class objects} |
279 | In Sod's object system, classes are objects too. Therefore classes are | |
280 | themselves instances; the class of a class is called a \emph{metaclass}. The | |
281 | consequences of this are explored in \xref{sec:concepts.metaclasses}. The | |
282 | \emph{class object} has the same name as the class, suffixed with | |
283 | `@|__class|'\footnote{% | |
284 | This is not quite true. @|$C$__class| is actually a macro. See | |
285 | \xref{sec:structures.layout.additional} for the gory details.} % | |
286 | and its type is usually @|SodClass|; @|SodClass|'s nickname is @|cls|. | |
287 | ||
288 | A class object's slots contain or point to useful information, tables and | |
289 | functions for working with that class's instances. (The @|SodClass| class | |
290 | doesn't define any messages, so it doesn't have any methods. In Sod, a class | |
291 | slot containing a function pointer is not at all the same thing as a method.) | |
292 | ||
3cc520db | 293 | \subsubsection{Conversions} |
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294 | Suppose one has a value of type pointer-to-class-type for some class~$C$, and |
295 | wants to convert it to a pointer-to-class-type for some other class~$B$. | |
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296 | There are three main cases to distinguish. |
297 | \begin{itemize} | |
298 | \item If $B$ is a superclass of~$C$, in the same chain, then the conversion | |
299 | is an \emph{in-chain upcast}. The conversion can be performed using the | |
300 | appropriate generated upcast macro (see below), or by simply casting the | |
301 | pointer, using C's usual cast operator (or the \Cplusplus\ @|static_cast<>| | |
302 | operator). | |
303 | \item If $B$ is a superclass of~$C$, in a different chain, then the | |
304 | conversion is a \emph{cross-chain upcast}. The conversion is more than a | |
305 | simple type change: the pointer value must be adjusted. If the direct | |
306 | class of the instance in question is not known, the conversion will require | |
307 | a lookup at runtime to find the appropriate offset by which to adjust the | |
308 | pointer. The conversion can be performed using the appropriate generated | |
309 | upcast macro (see below); the general case is handled by the macro | |
58f9b400 | 310 | \descref{SOD_XCHAIN}{mac}. |
e4ea29d8 | 311 | \item If $B$ is a subclass of~$C$ then the conversion is a \emph{downcast}; |
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312 | otherwise the conversion is a~\emph{cross-cast}. In either case, the |
313 | conversion can fail: the object in question might not be an instance of~$B$ | |
e4ea29d8 | 314 | after all. The macro \descref{SOD_CONVERT}{mac} and the function |
58f9b400 | 315 | \descref{sod_convert}{fun} perform general conversions. They return a null |
781a8fbd | 316 | pointer if the conversion fails. (There are therefore your analogue to the |
e4ea29d8 | 317 | \Cplusplus\ @|dynamic_cast<>| operator.) |
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318 | \end{itemize} |
319 | The Sod translator generates macros for performing both in-chain and | |
320 | cross-chain upcasts. For each class~$C$, and each proper superclass~$B$ | |
321 | of~$C$, a macro is defined: given an argument of type pointer to class type | |
322 | of~$C$, it returns a pointer to the same instance, only with type pointer to | |
323 | class type of~$B$, adjusted as necessary in the case of a cross-chain | |
324 | conversion. The macro is named by concatenating | |
325 | \begin{itemize} | |
326 | \item the name of class~$C$, in upper case, | |
327 | \item the characters `@|__CONV_|', and | |
328 | \item the nickname of class~$B$, in upper case; | |
329 | \end{itemize} | |
330 | e.g., if $C$ is named @|MyClass|, and $B$'s name is @|SuperClass| with | |
331 | nickname @|super|, then the macro @|MYCLASS__CONV_SUPER| converts a | |
332 | @|MyClass~*| to a @|SuperClass~*|. See | |
333 | \xref{sec:structures.layout.additional} for the formal description. | |
334 | ||
335 | %%%-------------------------------------------------------------------------- | |
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336 | \section{Keyword arguments} \label{sec:concepts.keywords} |
337 | ||
338 | In standard C, the actual arguments provided to a function are matched up | |
339 | with the formal arguments given in the function definition according to their | |
340 | ordering in a list. Unless the (rather cumbersome) machinery for dealing | |
341 | with variable-length argument tails (@|<stdarg.h>|) is used, exactly the | |
342 | correct number of arguments must be supplied, and in the correct order. | |
343 | ||
344 | A \emph{keyword argument} is matched by its distinctive \emph{name}, rather | |
345 | than by its position in a list. Keyword arguments may be \emph{omitted}, | |
346 | causing some default behaviour by the function. A function can detect | |
347 | whether a particular keyword argument was supplied: so the default behaviour | |
348 | need not be the same as that caused by any specific value of the argument. | |
349 | ||
350 | Keyword arguments can be provided in three ways. | |
351 | \begin{enumerate} | |
352 | \item Directly, as a variable-length argument tail, consisting (for the most | |
353 | part) of alternating keyword names, as pointers to null-terminated strings, | |
354 | and argument values, and terminated by a null pointer. This is somewhat | |
355 | error-prone, and the support library defines some macros which help ensure | |
356 | that keyword argument lists are well formed. | |
357 | \item Indirectly, through a @|va_list| object capturing a variable-length | |
358 | argument tail passed to some other function. Such indirect argument tails | |
359 | have the same structure as the direct argument tails described above. | |
360 | Because @|va_list| objects are hard to copy, the keyword-argument support | |
361 | library consistently passes @|va_list| objects \emph{by reference} | |
362 | throughout its programming interface. | |
363 | \item Indirectly, through a vector of @|struct kwval| objects, each of which | |
364 | contains a keyword name, as a pointer to a null-terminated string, and the | |
365 | \emph{address} of a corresponding argument value. (This indirection is | |
366 | necessary so that the items in the vector can be of uniform size.) | |
367 | Argument vectors are rather inconvenient to use, but are the only practical | |
368 | way in which a caller can decide at runtime which arguments to include in a | |
369 | call, which is useful when writing wrapper functions. | |
370 | \end{enumerate} | |
371 | ||
372 | Keyword arguments are provided as a general feature for C functions. | |
43073476 | 373 | However, Sod has special support for messages which accept keyword arguments |
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374 | (\xref{sec:concepts.methods.keywords}); and they play an essential role in |
375 | the instance construction protocol (\xref{sec:concepts.lifecycle.birth}). | |
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376 | |
377 | %%%-------------------------------------------------------------------------- | |
3cc520db MW |
378 | \section{Messages and methods} \label{sec:concepts.methods} |
379 | ||
380 | Objects can be sent \emph{messages}. A message has a \emph{name}, and | |
381 | carries a number of \emph{arguments}. When an object is sent a message, a | |
382 | function, determined by the receiving object's class, is invoked, passing it | |
383 | the receiver and the message arguments. This function is called the | |
384 | class's \emph{effective method} for the message. The effective method can do | |
385 | anything a C function can do, including reading or updating program state or | |
386 | object slots, sending more messages, calling other functions, issuing system | |
387 | calls, or performing I/O; if it finishes, it may return a value, which is | |
388 | returned in turn to the message sender. | |
389 | ||
390 | The set of messages an object can receive, characterized by their names, | |
391 | argument types, and return type, is determined by the object's class. Each | |
392 | class can define new messages, which can be received by any instance of that | |
393 | class. The messages defined by a single class must have distinct names: | |
394 | there is no `function overloading'. As with slots | |
395 | (\xref{sec:concepts.classes.slots}), messages defined by distinct classes are | |
396 | always distinct, even if they have the same names: references to messages are | |
397 | always qualified by the defining class's name or nickname. | |
398 | ||
399 | Messages may take any number of arguments, of any non-array value type. | |
400 | Since message sends are effectively function calls, arguments of array type | |
401 | are implicitly converted to values of the corresponding pointer type. While | |
402 | message definitions may ascribe an array type to an argument, the formal | |
403 | argument will have pointer type, as is usual for C functions. A message may | |
404 | accept a variable-length argument suffix, denoted @|\dots|. | |
405 | ||
406 | A class definition may include \emph{direct methods} for messages defined by | |
407 | it or any of its superclasses. | |
408 | ||
409 | Like messages, direct methods define argument lists and return types, but | |
410 | they may also have a \emph{body}, and a \emph{role}. | |
411 | ||
412 | A direct method need not have the same argument list or return type as its | |
413 | message. The acceptable argument lists and return types for a method depend | |
414 | on the message, in particular its method combination | |
415 | (\xref{sec:concepts.methods.combination}), and the method's role. | |
416 | ||
417 | A direct method body is a block of C code, and the Sod translator usually | |
418 | defines, for each direct method, a function with external linkage, whose body | |
419 | contains a copy of the direct method body. Within the body of a direct | |
420 | method defined for a class $C$, the variable @|me|, of type pointer to class | |
421 | type of $C$, refers to the receiving object. | |
422 | ||
0a2d4b68 | 423 | |
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424 | \subsection{Effective methods and method combinations} |
425 | \label{sec:concepts.methods.combination} | |
426 | ||
427 | For each message a direct instance of a class might receive, there is a set | |
428 | of \emph{applicable methods}, which are exactly the direct methods defined on | |
429 | the object's class and its superclasses. These direct methods are combined | |
430 | together to form the \emph{effective method} for that particular class and | |
431 | message. Direct methods can be combined into an effective method in | |
432 | different ways, according to the \emph{method combination} specified by the | |
433 | message. The method combination determines which direct method roles are | |
434 | acceptable, and, for each role, the appropriate argument lists and return | |
435 | types. | |
436 | ||
437 | One direct method, $M$, is said to be more (resp.\ less) \emph{specific} than | |
438 | another, $N$, with respect to a receiving class~$C$, if the class defining | |
439 | $M$ is a more (resp.\ less) specific superclass of~$C$ than the class | |
440 | defining $N$. | |
441 | ||
43073476 | 442 | \subsubsection{The standard method combination} |
3cc520db MW |
443 | The default method combination is called the \emph{standard method |
444 | combination}; other method combinations are useful occasionally for special | |
445 | effects. The standard method combination accepts four direct method roles, | |
9761db0d | 446 | called `primary' (the default), @|before|, @|after|, and @|around|. |
3cc520db MW |
447 | |
448 | All direct methods subject to the standard method combination must have | |
449 | argument lists which \emph{match} the message's argument list: | |
450 | \begin{itemize} | |
451 | \item the method's arguments must have the same types as the message, though | |
452 | the arguments may have different names; and | |
453 | \item if the message accepts a variable-length argument suffix then the | |
454 | direct method must instead have a final argument of type @|va_list|. | |
455 | \end{itemize} | |
b1254eb6 MW |
456 | Primary and @|around| methods must have the same return type as the message; |
457 | @|before| and @|after| methods must return @|void| regardless of the | |
458 | message's return type. | |
3cc520db MW |
459 | |
460 | If there are no applicable primary methods then no effective method is | |
461 | constructed: the vtables contain null pointers in place of pointers to method | |
462 | entry functions. | |
463 | ||
464 | The effective method for a message with standard method combination works as | |
465 | follows. | |
466 | \begin{enumerate} | |
467 | ||
468 | \item If any applicable methods have the @|around| role, then the most | |
469 | specific such method, with respect to the class of the receiving object, is | |
470 | invoked. | |
471 | ||
b1254eb6 | 472 | Within the body of an @|around| method, the variable @|next_method| is |
3cc520db MW |
473 | defined, having pointer-to-function type. The method may call this |
474 | function, as described below, any number of times. | |
475 | ||
b1254eb6 MW |
476 | If there any remaining @|around| methods, then @|next_method| invokes the |
477 | next most specific such method, returning whichever value that method | |
478 | returns; otherwise the behaviour of @|next_method| is to invoke the before | |
479 | methods (if any), followed by the most specific primary method, followed by | |
480 | the @|around| methods (if any), and to return whichever value was returned | |
781a8fbd MW |
481 | by the most specific primary method, as described in the following items. |
482 | That is, the behaviour of the least specific @|around| method's | |
483 | @|next_method| function is exactly the behaviour that the effective method | |
484 | would have if there were no @|around| methods. Note that if the | |
485 | least-specific @|around| method calls its @|next_method| more than once | |
486 | then the whole sequence of @|before|, primary, and @|after| methods occurs | |
487 | multiple times. | |
3cc520db | 488 | |
b1254eb6 MW |
489 | The value returned by the most specific @|around| method is the value |
490 | returned by the effective method. | |
3cc520db MW |
491 | |
492 | \item If any applicable methods have the @|before| role, then they are all | |
493 | invoked, starting with the most specific. | |
494 | ||
495 | \item The most specific applicable primary method is invoked. | |
496 | ||
497 | Within the body of a primary method, the variable @|next_method| is | |
498 | defined, having pointer-to-function type. If there are no remaining less | |
499 | specific primary methods, then @|next_method| is a null pointer. | |
500 | Otherwise, the method may call the @|next_method| function any number of | |
501 | times. | |
502 | ||
503 | The behaviour of the @|next_method| function, if it is not null, is to | |
504 | invoke the next most specific applicable primary method, and to return | |
505 | whichever value that method returns. | |
506 | ||
b1254eb6 MW |
507 | If there are no applicable @|around| methods, then the value returned by |
508 | the most specific primary method is the value returned by the effective | |
509 | method; otherwise the value returned by the most specific primary method is | |
510 | returned to the least specific @|around| method, which called it via its | |
511 | own @|next_method| function. | |
3cc520db MW |
512 | |
513 | \item If any applicable methods have the @|after| role, then they are all | |
514 | invoked, starting with the \emph{least} specific. (Hence, the most | |
b1254eb6 | 515 | specific @|after| method is invoked with the most `afterness'.) |
3cc520db MW |
516 | |
517 | \end{enumerate} | |
518 | ||
b1254eb6 MW |
519 | A typical use for @|around| methods is to allow a base class to set up the |
520 | dynamic environment appropriately for the primary methods of its subclasses, | |
521 | e.g., by claiming a lock, and restore it afterwards. | |
3cc520db | 522 | |
9761db0d | 523 | The @|next_method| function provided to methods with the primary and |
3cc520db MW |
524 | @|around| roles accepts the same arguments, and returns the same type, as the |
525 | message, except that one or two additional arguments are inserted at the | |
526 | front of the argument list. The first additional argument is always the | |
527 | receiving object, @|me|. If the message accepts a variable argument suffix, | |
528 | then the second addition argument is a @|va_list|; otherwise there is no | |
529 | second additional argument; otherwise, In the former case, a variable | |
530 | @|sod__master_ap| of type @|va_list| is defined, containing a separate copy | |
531 | of the argument pointer (so the method body can process the variable argument | |
532 | suffix itself, and still pass a fresh copy on to the next method). | |
533 | ||
9761db0d | 534 | A method with the primary or @|around| role may use the convenience macro |
3cc520db MW |
535 | @|CALL_NEXT_METHOD|, which takes no arguments itself, and simply calls |
536 | @|next_method| with appropriate arguments: the receiver @|me| pointer, the | |
537 | argument pointer @|sod__master_ap| (if applicable), and the method's | |
538 | arguments. If the method body has overwritten its formal arguments, then | |
539 | @|CALL_NEXT_METHOD| will pass along the updated values, rather than the | |
540 | original ones. | |
541 | ||
781a8fbd MW |
542 | A primary or @|around| method which invokes its @|next_method| function is |
543 | said to \emph{extend} the message behaviour; a method which does not invoke | |
544 | its @|next_method| is said to \emph{override} the behaviour. Note that a | |
545 | method may make a decision to override or extend at runtime. | |
546 | ||
43073476 | 547 | \subsubsection{Aggregating method combinations} |
3cc520db MW |
548 | A number of other method combinations are provided. They are called |
549 | `aggregating' method combinations because, instead of invoking just the most | |
550 | specific primary method, as the standard method combination does, they invoke | |
551 | the applicable primary methods in turn and aggregate the return values from | |
552 | each. | |
553 | ||
554 | The aggregating method combinations accept the same four roles as the | |
b1254eb6 MW |
555 | standard method combination, and @|around|, @|before|, and @|after| methods |
556 | work in the same way. | |
3cc520db MW |
557 | |
558 | The aggregating method combinations provided are as follows. | |
559 | \begin{description} \let\makelabel\code | |
560 | \item[progn] The message must return @|void|. The applicable primary methods | |
561 | are simply invoked in turn, most specific first. | |
562 | \item[sum] The message must return a numeric type.\footnote{% | |
563 | The Sod translator does not check this, since it doesn't have enough | |
564 | insight into @|typedef| names.} % | |
565 | The applicable primary methods are invoked in turn, and their return values | |
566 | added up. The final result is the sum of the individual values. | |
567 | \item[product] The message must return a numeric type. The applicable | |
568 | primary methods are invoked in turn, and their return values multiplied | |
569 | together. The final result is the product of the individual values. | |
570 | \item[min] The message must return a scalar type. The applicable primary | |
571 | methods are invoked in turn. The final result is the smallest of the | |
572 | individual values. | |
573 | \item[max] The message must return a scalar type. The applicable primary | |
574 | methods are invoked in turn. The final result is the largest of the | |
575 | individual values. | |
665a0455 MW |
576 | \item[and] The message must return a scalar type. The applicable primary |
577 | methods are invoked in turn. If any method returns zero then the final | |
578 | result is zero and no further methods are invoked. If all of the | |
579 | applicable primary methods return nonzero, then the final result is the | |
580 | result of the last primary method. | |
581 | \item[or] The message must return a scalar type. The applicable primary | |
582 | methods are invoked in turn. If any method returns nonzero then the final | |
583 | result is that nonzero value and no further methods are invoked. If all of | |
584 | the applicable primary methods return zero, then the final result is zero. | |
3cc520db MW |
585 | \end{description} |
586 | ||
587 | There is also a @|custom| aggregating method combination, which is described | |
588 | in \xref{sec:fixme.custom-aggregating-method-combination}. | |
589 | ||
43073476 | 590 | |
caa6f4b9 MW |
591 | \subsection{Sending messages in C} \label{sec:concepts.methods.c} |
592 | ||
593 | Each instance is associated with its direct class [FIXME] | |
594 | ||
595 | The effective methods for each class are determined at translation time, by | |
596 | the Sod translator. For each effective method, one or more \emph{method | |
597 | entry functions} are constructed. A method entry function has three | |
598 | responsibilities. | |
599 | \begin{itemize} | |
600 | \item It converts the receiver pointer to the correct type. Method entry | |
601 | functions can perform these conversions extremely efficiently: there are | |
602 | separate method entries for each chain of each class which can receive a | |
603 | message, so method entry functions are in the privileged situation of | |
604 | knowing the \emph{exact} class of the receiving object. | |
605 | \item If the message accepts a variable-length argument tail, then two method | |
606 | entry functions are created for each chain of each class: one receives a | |
607 | variable-length argument tail, as intended, and captures it in a @|va_list| | |
608 | object; the other accepts an argument of type @|va_list| in place of the | |
609 | variable-length tail and arranges for it to be passed along to the direct | |
610 | methods. | |
611 | \item It invokes the effective method with the appropriate arguments. There | |
612 | might or might not be an actual function corresponding to the effective | |
613 | method itself: the translator may instead open-code the effective method's | |
614 | behaviour into each method entry function; and the machinery for handling | |
615 | `delegation chains', such as is used for @|around| methods and primary | |
616 | methods in the standard method combination, is necessarily scattered among | |
617 | a number of small functions. | |
618 | \end{itemize} | |
619 | ||
620 | ||
43073476 MW |
621 | \subsection{Messages with keyword arguments} |
622 | \label{sec:concepts.methods.keywords} | |
623 | ||
624 | A message or a direct method may declare that it accepts keyword arguments. | |
625 | A message which accepts keyword arguments is called a \emph{keyword message}; | |
626 | a direct method which accepts keyword arguments is called a \emph{keyword | |
627 | method}. | |
628 | ||
629 | While method combinations may set their own rules, usually keyword methods | |
630 | can only be defined on keyword messages, and all methods defined on a keyword | |
631 | message must be keyword methods. The direct methods defined on a keyword | |
632 | message may differ in the keywords they accept, both from each other, and | |
633 | from the message. If two superclasses of some common class both define | |
634 | keyword methods on the same message, and the methods both accept a keyword | |
635 | argument with the same name, then these two keyword arguments must also have | |
636 | the same type. Different applicable methods may declare keyword arguments | |
637 | with the same name but different defaults; see below. | |
638 | ||
639 | The keyword arguments acceptable in a message sent to an object are the | |
640 | keywords listed in the message definition, together with all of the keywords | |
641 | accepted by any applicable method. There is no easy way to determine at | |
642 | runtime whether a particular keyword is acceptable in a message to a given | |
643 | instance. | |
644 | ||
645 | At runtime, a direct method which accepts one or more keyword arguments | |
646 | receives an additional argument named @|suppliedp|. This argument is a small | |
647 | structure. For each keyword argument named $k$ accepted by the direct | |
648 | method, @|suppliedp| contains a one-bit-wide bitfield member of type | |
649 | @|unsigned|, also named $k$. If a keyword argument named $k$ was passed in | |
650 | the message, then @|suppliedp.$k$| is one, and $k$ contains the argument | |
651 | value; otherwise @|suppliedp.$k$| is zero, and $k$ contains the default value | |
652 | from the direct method definition if there was one, or an unspecified value | |
653 | otherwise. | |
654 | ||
3cc520db | 655 | %%%-------------------------------------------------------------------------- |
d24d47f5 MW |
656 | \section{The object lifecycle} \label{sec:concepts.lifecycle} |
657 | ||
658 | \subsection{Creation} \label{sec:concepts.lifecycle.birth} | |
659 | ||
660 | Construction of a new instance of a class involves three steps. | |
661 | \begin{enumerate} | |
662 | \item \emph{Allocation} arranges for there to be storage space for the | |
663 | instance's slots and associated metadata. | |
664 | \item \emph{Imprinting} fills in the instance's metadata, associating the | |
665 | instance with its class. | |
666 | \item \emph{Initialization} stores appropriate initial values in the | |
667 | instance's slots, and maybe links it into any external data structures as | |
668 | necessary. | |
669 | \end{enumerate} | |
670 | The \descref{SOD_DECL}[macro]{mac} handles constructing instances with | |
a42893dd MW |
671 | automatic storage duration (`on the stack'). Similarly, the |
672 | \descref{SOD_MAKE}[macro]{mac} and the \descref{sod_make}{fun} and | |
673 | \descref{sod_makev}{fun} functions construct instances allocated from the | |
674 | standard @|malloc| heap. Programmers can add support for other allocation | |
675 | strategies by using the \descref{SOD_INIT}[macro]{mac} and the | |
676 | \descref{sod_init}{fun} and \descref{sod_initv}{fun} functions, which package | |
677 | up imprinting and initialization. | |
d24d47f5 MW |
678 | |
679 | \subsubsection{Allocation} | |
680 | Instances of most classes (specifically including those classes defined by | |
681 | Sod itself) can be held in any storage of sufficient size. The in-memory | |
682 | layout of an instance of some class~$C$ is described by the type @|struct | |
683 | $C$__ilayout|, and if the relevant class is known at compile time then the | |
684 | best way to discover the layout size is with the @|sizeof| operator. Failing | |
685 | that, the size required to hold an instance of $C$ is available in a slot in | |
686 | $C$'s class object, as @|$C$__class@->cls.initsz|. | |
687 | ||
688 | It is not in general sufficient to declare, or otherwise allocate, an object | |
689 | of the class type $C$. The class type only describes a single chain of the | |
690 | object's layout. It is nearly always an error to use the class type as if it | |
691 | is a \emph{complete type}, e.g., to declare objects or arrays of the class | |
692 | type, or to enquire about its size or alignment requirements. | |
693 | ||
694 | Instance layouts may be declared as objects with automatic storage duration | |
695 | (colloquially, `allocated on the stack') or allocated dynamically, e.g., | |
696 | using @|malloc|. They may be included as members of structures or unions, or | |
697 | elements of arrays. Sod's runtime system doesn't retain addresses of | |
698 | instances, so, for example, Sod doesn't make using fancy allocators which | |
699 | sometimes move objects around in memory any more difficult than it needs to | |
700 | be. | |
701 | ||
702 | There isn't any way to discover the alignment required for a particular | |
703 | class's instances at runtime; it's best to be conservative and assume that | |
704 | the platform's strictest alignment requirement applies. | |
705 | ||
706 | The following simple function correctly allocates and returns space for an | |
707 | instance of a class given a pointer to its class object @<cls>. | |
708 | \begin{prog} | |
020b9e2b | 709 | void *allocate_instance(const SodClass *cls) \\ \ind |
d24d47f5 MW |
710 | \{ return malloc(cls@->cls.initsz); \} |
711 | \end{prog} | |
712 | ||
713 | \subsubsection{Imprinting} | |
714 | Once storage has been allocated, it must be \emph{imprinted} before it can be | |
715 | used as an instance of a class, e.g., before any messages can be sent to it. | |
716 | ||
717 | Imprinting an instance stores some metadata about its direct class in the | |
718 | instance structure, so that the rest of the program (and Sod's runtime | |
719 | library) can tell what sort of object it is, and how to use it.\footnote{% | |
720 | Specifically, imprinting an instance's storage involves storing the | |
721 | appropriate vtable pointers in the right places in it.} % | |
722 | A class object's @|imprint| slot points to a function which will correctly | |
723 | imprint storage for one of that class's instances. | |
724 | ||
725 | Once an instance's storage has been imprinted, it is technically possible to | |
726 | send messages to the instance; however the instance's slots are still | |
727 | uninitialized at this point, the applicable methods are unlikely to do much | |
728 | of any use unless they've been written specifically for the purpose. | |
729 | ||
730 | The following simple function imprints storage at address @<p> as an instance | |
731 | of a class, given a pointer to its class object @<cls>. | |
732 | \begin{prog} | |
020b9e2b | 733 | void imprint_instance(const SodClass *cls, void *p) \\ \ind |
d24d47f5 MW |
734 | \{ cls@->cls.imprint(p); \} |
735 | \end{prog} | |
736 | ||
737 | \subsubsection{Initialization} | |
738 | The final step for constructing a new instance is to \emph{initialize} it, to | |
739 | establish the necessary invariants for the instance itself and the | |
740 | environment in which it operates. | |
741 | ||
742 | Details of initialization are necessarily class-specific, but typically it | |
743 | involves setting the instance's slots to appropriate values, and possibly | |
744 | linking it into some larger data structure to keep track of it. | |
745 | ||
a142609c MW |
746 | Initialization is performed by sending the imprinted instance an @|init| |
747 | message, defined by the @|SodObject| class. This message uses a nonstandard | |
748 | method combination which works like the standard combination, except that the | |
749 | \emph{default behaviour}, if there is no overriding method, is to initialize | |
b2983f35 MW |
750 | the instance's slots, as described below, and to invoke each superclass's |
751 | initialization fragments. This default behaviour may be invoked multiple | |
752 | times if some method calls on its @|next_method| more than once, unless some | |
753 | other method takes steps to prevent this. | |
a142609c | 754 | |
27ec3825 MW |
755 | Slots are initialized in a well-defined order. |
756 | \begin{itemize} | |
757 | \item Slots defined by a more specific superclasses are initialized after | |
758 | slots defined by a less specific superclass. | |
759 | \item Slots defined by the same class are initialized in the order in which | |
760 | their definitions appear. | |
761 | \end{itemize} | |
762 | ||
a42893dd MW |
763 | A class can define \emph{initialization fragments}: pieces of literal code to |
764 | be executed to set up a new instance. Each superclass's initialization | |
765 | fragments are executed with @|me| bound to an instance pointer of the | |
766 | appropriate superclass type, immediately after that superclass's slots (if | |
767 | any) have been initialized; therefore, fragments defined by a more specific | |
768 | superclass are executed after fragments defined by a more specific | |
769 | superclass. A class may define more than one initialization fragment: the | |
770 | fragments are executed in the order in which they appear in the class | |
771 | definition. It is possible for an initialization fragment to use @|return| | |
772 | or @|goto| for special control-flow effects, but this is not likely to be a | |
773 | good idea. | |
774 | ||
b2983f35 MW |
775 | The @|init| message accepts keyword arguments |
776 | (\xref{sec:concepts.methods.keywords}). The set of acceptable keywords is | |
777 | determined by the applicable methods as usual, but also by the | |
778 | \emph{initargs} defined by the receiving instance's class and its | |
779 | superclasses, which are made available to slot initializers and | |
780 | initialization fragments. | |
781 | ||
782 | There are two kinds of initarg definitions. \emph{User initargs} are defined | |
783 | by an explicit @|initarg| item appearing in a class definition: the item | |
784 | defines a name, type, and (optionally) a default value for the initarg. | |
785 | \emph{Slot initargs} are defined by attaching an @|initarg| property to a | |
786 | slot or slot initializer item: the property's determines the initarg's name, | |
787 | while the type is taken from the underlying slot type; slot initargs do not | |
788 | have default values. Both kinds define a \emph{direct initarg} for the | |
789 | containing class. | |
790 | ||
791 | Initargs are inherited. The \emph{applicable} direct initargs for an @|init| | |
792 | effective method are those defined by the receiving object's class, and all | |
793 | of its superclasses. Applicable direct initargs with the same name are | |
794 | merged to form \emph{effective initargs}. An error is reported if two | |
795 | applicable direct initargs have the same name but different types. The | |
796 | default value of an effective initarg is taken from the most specific | |
797 | applicable direct initarg which specifies a defalt value; if no applicable | |
798 | direct initarg specifies a default value then the effective initarg has no | |
799 | default. | |
800 | ||
801 | All initarg values are made available at runtime to user code -- | |
802 | initialization fragments and slot initializer expressions -- through local | |
803 | variables and a @|suppliedp| structure, as in a direct method | |
804 | (\xref{sec:concepts.methods.keywords}). Furthermore, slot initarg | |
805 | definitions influence the initialization of slots. | |
806 | ||
807 | The process for deciding how to initialize a particular slot works as | |
808 | follows. | |
809 | \begin{enumerate} | |
810 | \item If there are any slot initargs defined on the slot, or any of its slot | |
811 | initializers, \emph{and} the sender supplied a value for one or more of the | |
812 | corresponding effective initargs, then the value of the most specific slot | |
813 | initarg is stored in the slot. | |
814 | \item Otherwise, if there are any slot initializers defined which include an | |
815 | initializer expression, then the initializer expression from the most | |
816 | specific such slot initializer is evaluated and its value stored in the | |
817 | slot. | |
818 | \item Otherwise, the slot is left uninitialized. | |
819 | \end{enumerate} | |
820 | Note that the default values (if any) of effective initargs do \emph{not} | |
821 | affect this procedure. | |
d24d47f5 | 822 | |
d24d47f5 MW |
823 | |
824 | \subsection{Destruction} | |
825 | \label{sec:concepts.lifecycle.death} | |
826 | ||
827 | Destruction of an instance, when it is no longer required, consists of two | |
828 | steps. | |
829 | \begin{enumerate} | |
830 | \item \emph{Teardown} releases any resources held by the instance and | |
831 | disentangles it from any external data structures. | |
832 | \item \emph{Deallocation} releases the memory used to store the instance so | |
833 | that it can be reused. | |
834 | \end{enumerate} | |
a42893dd MW |
835 | Teardown alone, for objects which require special deallocation, or for which |
836 | deallocation occurs automatically (e.g., instances with automatic storage | |
837 | duration, or instances whose storage will be garbage-collected), is performed | |
838 | using the \descref{sod_teardown}[function]{fun}. Destruction of instances | |
839 | allocated from the standard @|malloc| heap is done using the | |
840 | \descref{sod_destroy}[function]{fun}. | |
d24d47f5 MW |
841 | |
842 | \subsubsection{Teardown} | |
a42893dd MW |
843 | Details of initialization are necessarily class-specific, but typically it |
844 | involves setting the instance's slots to appropriate values, and possibly | |
845 | linking it into some larger data structure to keep track of it. | |
846 | ||
847 | Teardown is performed by sending the instance the @|teardown| message, | |
848 | defined by the @|SodObject| class. The message returns an integer, used as a | |
849 | boolean flag. If the message returns zero, then the instance's storage | |
850 | should be deallocated. If the message returns nonzero, then it is safe for | |
851 | the caller to forget about instance, but should not deallocate its storage. | |
852 | This is \emph{not} an error return: if some teardown method fails then the | |
853 | program may be in an inconsistent state and should not continue. | |
d24d47f5 | 854 | |
a42893dd MW |
855 | This simple protocol can be used, for example, to implement a reference |
856 | counting system, as follows. | |
d24d47f5 | 857 | \begin{prog} |
020b9e2b MW |
858 | [nick = ref] \\ |
859 | class ReferenceCountedObject \{ \\ \ind | |
860 | unsigned nref = 1; \\- | |
861 | void inc() \{ me@->ref.nref++; \} \\- | |
862 | [role = around] \\ | |
863 | int obj.teardown() \\ | |
864 | \{ \\ \ind | |
865 | if (--\,--me@->ref.nref) return (1); \\ | |
866 | else return (CALL_NEXT_METHOD); \-\\ | |
867 | \} \-\\ | |
d24d47f5 MW |
868 | \} |
869 | \end{prog} | |
870 | ||
a42893dd MW |
871 | This message uses a nonstandard method combination which works like the |
872 | standard combination, except that the \emph{default behaviour}, if there is | |
873 | no overriding method, is to execute the superclass's teardown fragments, and | |
874 | to return zero. This default behaviour may be invoked multiple times if some | |
875 | method calls on its @|next_method| more than once, unless some other method | |
876 | takes steps to prevent this. | |
877 | ||
878 | A class can define \emph{teardown fragments}: pieces of literal code to be | |
879 | executed to shut down an instance. Each superclass's teardown fragments are | |
880 | executed with @|me| bound to an instance pointer of the appropriate | |
881 | superclass type; fragments defined by a more specific superclass are executed | |
882 | before fragments defined by a more specific superclass. A class may define | |
883 | more than one teardown fragment: the fragments are executed in the order in | |
884 | which they appear in the class definition. It is possible for an | |
885 | initialization fragment to use @|return| or @|goto| for special control-flow | |
886 | effects, but this is not likely to be a good idea. Similarly, it's probably | |
887 | a better idea to use an @|around| method to influence the return value than | |
888 | to write an explicit @|return| statement in a teardown fragment. | |
889 | ||
d24d47f5 MW |
890 | \subsubsection{Deallocation} |
891 | The details of instance deallocation are obviously specific to the allocation | |
892 | strategy used by the instance, and this is often orthogonal from the object's | |
893 | class. | |
894 | ||
895 | The code which makes the decision to destroy an object may often not be aware | |
896 | of the object's direct class. Low-level details of deallocation often | |
897 | require the proper base address of the instance's storage, which can be | |
898 | determined using the \descref{SOD_INSTBASE}[macro]{mac}. | |
899 | ||
d24d47f5 | 900 | %%%-------------------------------------------------------------------------- |
3cc520db | 901 | \section{Metaclasses} \label{sec:concepts.metaclasses} |
1f7d590d | 902 | |
caa6f4b9 MW |
903 | %%%-------------------------------------------------------------------------- |
904 | \section{Compatibility considerations} \label{sec:concepts.compatibility} | |
905 | ||
906 | Sod doesn't make source-level compatibility especially difficult. As long as | |
907 | classes, slots, and messages don't change names or dissappear, and slots and | |
908 | messages retain their approximate types, everything will be fine. | |
909 | ||
910 | Binary compatibility is much more difficult. Unfortunately, Sod classes have | |
911 | rather fragile binary interfaces.\footnote{% | |
912 | Research suggestion: investigate alternative instance and vtable layouts | |
913 | which improve binary compatibility, probably at the expense of instance | |
914 | compactness, and efficiency of slot access and message sending. There may | |
915 | be interesting trade-offs to be made.} % | |
916 | ||
917 | If instances are allocated [FIXME] | |
918 | ||
1f7d590d MW |
919 | %%%----- That's all, folks -------------------------------------------------- |
920 | ||
921 | %%% Local variables: | |
922 | %%% mode: LaTeX | |
923 | %%% TeX-master: "sod.tex" | |
924 | %%% TeX-PDF-mode: t | |
925 | %%% End: |