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1 | package Hash::Util::FieldHash; |
2 | |
3 | use 5.009004; |
4 | use strict; |
5 | use warnings; |
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6 | use Scalar::Util qw( reftype); |
7 | |
8 | require Exporter; |
9 | our @ISA = qw(Exporter); |
10 | our %EXPORT_TAGS = ( |
11 | 'all' => [ qw( |
12 | fieldhash |
13 | fieldhashes |
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14 | idhash |
15 | idhashes |
16 | id |
17 | id_2obj |
18 | register |
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19 | )], |
20 | ); |
21 | our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } ); |
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22 | |
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23 | our $VERSION = '1.01'; |
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24 | |
25 | { |
26 | require XSLoader; |
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27 | my %ob_reg; # private object registry |
28 | sub _ob_reg { \ %ob_reg } |
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29 | XSLoader::load('Hash::Util::FieldHash', $VERSION); |
30 | } |
31 | |
32 | sub fieldhash (\%) { |
33 | for ( shift ) { |
34 | return unless ref() && reftype( $_) eq 'HASH'; |
35 | return $_ if Hash::Util::FieldHash::_fieldhash( $_, 0); |
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36 | return $_ if Hash::Util::FieldHash::_fieldhash( $_, 2) == 2; |
37 | return; |
38 | } |
39 | } |
40 | |
41 | sub idhash (\%) { |
42 | for ( shift ) { |
43 | return unless ref() && reftype( $_) eq 'HASH'; |
44 | return $_ if Hash::Util::FieldHash::_fieldhash( $_, 0); |
45 | return $_ if Hash::Util::FieldHash::_fieldhash( $_, 1) == 1; |
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46 | return; |
47 | } |
48 | } |
49 | |
50 | sub fieldhashes { map &fieldhash( $_), @_ } |
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51 | sub idhashes { map &idhash( $_), @_ } |
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52 | |
53 | 1; |
54 | __END__ |
55 | |
56 | =head1 NAME |
57 | |
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58 | Hash::Util::FieldHash - Support for Inside-Out Classes |
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59 | |
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60 | =head1 Synopsis |
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61 | |
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62 | ### Create fieldhashes |
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63 | use Hash::Util qw(fieldhash fieldhashes); |
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64 | |
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65 | # Create a single field hash |
66 | fieldhash my %foo; |
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67 | |
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68 | # Create three at once... |
69 | fieldhashes \ my(%foo, %bar, %baz); |
70 | # ...or any number |
71 | fieldhashes @hashrefs; |
72 | |
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73 | ### Create an idhash and register it for garbage collection |
74 | use Hash::Util::FieldHash qw( idhash register); |
75 | idhash my %name; |
76 | my $obj = \ do { my $o }; |
77 | # register the idhash for garbage collection with $object |
78 | register( $obj, \ %name); |
79 | # the following entry wil be deleted when $obj goes out of scope |
80 | $name{ $obj} = 'John Doe'; |
81 | |
82 | ### Register an ordinary hash for garbage collection |
83 | use Hash::Util::FieldHash qw( id register); |
84 | my %name; |
85 | my $obj = \ do { my $o }; |
86 | # register the hash %name for garbage collection of $obj's id |
87 | register $obj, \ %name; |
88 | # the following entry wil be deleted when $obj goes out of scope |
89 | $name{id $obj} = 'John Doe'; |
90 | |
91 | |
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92 | =head1 Functions |
93 | |
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94 | C<Hash::Util::FieldHash> offers a number of functions in support of |
95 | L<the inside-out technique> of class construction. |
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96 | |
97 | =over |
98 | |
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99 | =item id |
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100 | |
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101 | id($obj) |
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102 | |
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103 | Returns the reference address of a reference $obj. If $obj is |
104 | not a reference, returns $obj. |
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105 | |
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106 | This function is a stand-in replacement for |
107 | L<Scalar::Util::refaddr|Scalar::Util/refaddr>, that is, it returns |
108 | the reference address of its argument as a numeric value. The only |
109 | difference is that C<refaddr()> returns C<undef> when given a |
110 | non-reference while C<id()> returns its argument unchanged. |
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111 | |
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112 | C<id()> also uses a caching technique that makes it faster when |
113 | the id of an object is requested often, but slower if it is needed |
114 | only once or twice. |
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115 | |
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116 | =item id_2obj |
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117 | |
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118 | $obj = id_2obj($id) |
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119 | |
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120 | If C<$id> is the id of a registered object (see L</register>), returns |
121 | the object, otherwise an undefined value. For registered objects this |
122 | is the inverse function of C<id()>. |
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123 | |
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124 | =item register |
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125 | |
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126 | register($obj) |
127 | register($obj, @hashrefs) |
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128 | |
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129 | In the first form, registers an object to work with for the function |
130 | C<id_2obj()>. In the second form, it additionally marks the given |
131 | hashrefs down for garbage collection. This means that when the object |
132 | goes out of scope, any entries in the given hashes under the key of |
133 | C<id($obj)> will be deleted from the hashes. |
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134 | |
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135 | It is a fatal error to register a non-reference $obj. Any non-hashrefs |
136 | among the following arguments are silently ignored. |
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137 | |
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138 | It is I<not> an error to register the same object multiple times with |
139 | varying sets of hashrefs. Any hashrefs that are not registered yet |
140 | will be added, others ignored. |
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141 | |
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142 | Registry also implies thread support. When a new thread is created, |
143 | all references are replaced with new ones, including all objects. |
144 | If a hash uses the reference address of an object as a key, that |
145 | connection would be broken. With a registered object, its id will |
146 | be updated in all hashes registered with it. |
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147 | |
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148 | =item idhash |
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149 | |
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150 | idhash my %hash |
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151 | |
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152 | Makes an idhash from the argument, which must be a hash. |
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153 | |
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154 | An I<idhash> works like a normal hash, except that it stringifies a |
155 | I<reference used as a key> differently. A reference is stringified |
156 | as if the C<id()> function had been invoked on it, that is, its |
157 | reference address in decimal is used as the key. |
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158 | |
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159 | =item idhashes |
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160 | |
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161 | idhashes \ my( %hash, %gnash, %trash) |
162 | idhashes \ @hashrefs |
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163 | |
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164 | Creates many idhashes from its hashref arguments. Returns those |
165 | arguments that could be converted or their number in scalar context. |
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166 | |
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167 | =item fieldhash |
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168 | |
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169 | fieldhash %hash; |
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170 | |
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171 | Creates a single fieldhash. The argument must be a hash. Returns |
172 | a reference to the given hash if successful, otherwise nothing. |
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173 | |
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174 | A I<fieldhash> is, in short, an idhash with auto-registry. When an |
175 | object (or, indeed, any reference) is used as a fieldhash key, the |
176 | fieldhash is automatically registered for garbage collection with |
177 | the object, as if C<register $obj, \ %fieldhash> had been called. |
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178 | |
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179 | =item fieldhashes |
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180 | |
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181 | fieldhashes @hashrefs; |
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182 | |
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183 | Creates any number of field hashes. Arguments must be hash references. |
184 | Returns the converted hashrefs in list context, their number in scalar |
185 | context. |
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186 | |
187 | =back |
188 | |
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189 | =head1 Description |
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190 | |
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191 | A word on terminology: I shall use the term I<field> for a scalar |
192 | piece of data that a class associates with an object. Other terms that |
193 | have been used for this concept are "object variable", "(object) property", |
194 | "(object) attribute" and more. Especally "attribute" has some currency |
195 | among Perl programmer, but that clashes with the C<attributes> pragma. The |
196 | term "field" also has some currency in this sense and doesn't seem |
197 | to conflict with other Perl terminology. |
198 | |
199 | In Perl, an object is a blessed reference. The standard way of associating |
200 | data with an object ist to store the data inside the object's body, that is, |
201 | the piece of data pointed to by the reference. |
202 | |
203 | In consequence, if two or more classes want to access an object they |
204 | I<must> agree on the type of refrerence and also on the organization of |
205 | data within the object body. Failure to agree on the type results in |
206 | immediate death when the wrong method tries to access an object. Failure |
207 | to agree on data organization may lead to one class trampling over the |
208 | data of another. |
209 | |
210 | This object model leads to a tight coupling between subclasses. |
211 | If one class wants to inherit from another (and both classes access |
212 | object data), the classes must agree about implementation details. |
213 | Inheritance can only be used among classes that are maintained together, |
214 | in a single source or not. |
215 | |
216 | In particular, it is not possible to write general-purpose classes |
217 | in this technique, classes that can advertise themselves as "Put me |
218 | on your @ISA list and use my methods". If the other class has different |
219 | ideas about how the object body is used, there is trouble. |
220 | |
221 | For reference L<Name_hash> in L<Example 1> shows the standard implementation of |
222 | a simple class C<Name> in the well-known hash based way. It also demonstrates |
223 | the predictable failure to construct a common subclass C<NamedFile> |
224 | of C<Name> and the class C<IO::File> (whose objects I<must> be globrefs). |
225 | |
226 | Thus, techniques are of interest that store object data I<not> in |
227 | the object body but some other place. |
228 | |
229 | =head2 The Inside-out Technique |
230 | |
231 | With I<inside-out> classes, each class declares a (typically lexical) |
232 | hash for each field it wants to use. The reference address of an |
233 | object is used as the hash key. By definition, the reference address |
234 | is unique to each object so this guarantees a place for each field that |
235 | is private to the class and unique to each object. See L<Name_id> in |
236 | L<Example 1> for a simple example. |
237 | |
238 | In comparison to the standard implementation where the object is a |
239 | hash and the fields correspond to hash keys, here the fields correspond |
240 | to hashes, and the object determines the hash key. Thus the hashes |
241 | appear to be turned I<inside out>. |
242 | |
243 | The body of an object is never examined by an inside-out class, only |
244 | its reference address is used. This allows for the body of an actual |
245 | object to be I<anything at all> while the object methods of the class |
246 | still work as designed. This is a key feature of inside-out classes. |
247 | |
248 | =head2 Problems of Inside-out |
249 | |
250 | Inside-out classes give us freedom of inheritance, but as usual there |
251 | is a price. |
252 | |
253 | Most obviously, there is the necessity of retrieving the reference |
254 | address of an object for each data access. It's a minor inconvenience, |
255 | but it does clutter the code. |
256 | |
257 | More important (and less obvious) is the necessity of garbage |
258 | collection. When a normal object dies, anything stored in the |
259 | object body is garbage-collected by perl. With inside-out objects, |
260 | Perl knows nothing about the data stored in field hashes by a class, |
261 | but these must be deleted when the object goes out of scope. Thus |
262 | the class must provide a C<DESTROY> method to take care of that. |
263 | |
264 | In the presence of multiple classes it can be non-trivial |
265 | to make sure that every relevant destructor is called for |
266 | every object. Perl calls the first one it finds on the |
267 | inheritance tree (if any) and that's it. |
268 | |
269 | A related issue is thread-safety. When a new thread is created, |
270 | the Perl interpreter is cloned, which implies that all reference |
271 | addresses in use will be replaced with new ones. Thus, if a class |
272 | tries to access a field of a cloned object its (cloned) data will |
273 | still be stored under the now invalid reference address of the |
274 | original in the parent thread. A general C<CLONE> method must |
275 | be provided to re-establish the association. |
276 | |
277 | =head2 Solutions |
278 | |
279 | C<Hash::Util::FieldHash> addresses these issues on several |
280 | levels. |
281 | |
282 | The C<id()> function is provided in addition to the |
283 | existing C<Scalar::Util::refaddr()>. Besides its short name |
284 | it can be a little faster under some circumstances (and a |
285 | bit slower under others). Benchmark if it matters. The |
286 | working of C<id()> also allows the use of the class name |
287 | as a I<generic object> as described L<further down|Generic Object>. |
288 | |
289 | The C<id()> function is incorporated in I<id hashes> in the sense |
290 | that it is called automatically on every key that is used with |
291 | the hash. No explicit call is necessary. |
292 | |
293 | The problems of garbage collection and thread safety are both |
294 | addressed by the function C<register()>. It registers an object |
295 | together with any number of hashes. Registry means that when the |
296 | object dies, an entry in any of the hashes under the reference |
297 | address of this object will be deleted. This guarantees garbage |
298 | collection in these hashes. It also means that on thread |
299 | cloning the object's entries in registered hashes will be |
300 | replaced with updated entries whose key is the cloned object's |
301 | reference address. Thus the object-data association becomes |
302 | thread-safe. |
303 | |
304 | Object registry is best done when the object is initialized |
305 | for use with a class. That way, garbage collection and thread |
306 | safety are established for every object and every field that is |
307 | initialized. |
308 | |
309 | Finally, I<field hashes> incorporate all these functions in one |
310 | package. Besides automatically calling the C<id()> function |
311 | on every object used as a key, the object is registered with |
312 | the field hash on first use. Classes based on field hashes |
313 | are fully garbage-collected and thread safe without further |
314 | measures. |
315 | |
316 | =head2 More Problems |
317 | |
318 | Another problem that occurs with inside-out classes is serialization. |
319 | Since the object data is not in its usual place, standard routines |
320 | like C<Storable::freeze()>, C<Storable::thaw()> and |
321 | C<Data::Dumper::Dumper()> can't deal with it on their own. Both |
322 | C<Data::Dumper> and C<Storable> provide the necessary hooks to |
323 | make things work, but the functions or methods used by the hooks |
324 | must be provided by each inside-out class. |
325 | |
326 | A general solution to the serialization problem would require another |
327 | level of registry, one that that associates I<classes> and fields. |
328 | So far, he functions of C<Hash::Util::Fieldhash> are unaware of |
329 | any classes, which I consider a feature. Therefore C<Hash::Util::FieldHash> |
330 | doesn't address the serialization problems. |
331 | |
332 | =head2 The Generic Object |
333 | |
334 | Classes based on the C<id()> function (and hence classes based on |
335 | C<idhash()> and C<fieldhash()>) show a peculiar behavior in that |
336 | the class name can be used like an object. Specifically, methods |
337 | that set or read data associated with an object continue to work as |
338 | class methods, just as if the class name were an object, distinct from |
339 | all other objects, with its own data. This object may be called |
340 | the I<generic object> of the class. |
341 | |
342 | This works because field hashes respond to keys that are not refrences |
343 | like a normal hash would and use the string offered as the hash key. |
344 | Thus, if a method is called as a class method, the field hash is presented |
345 | with the class name instead of an object and blithely uses it as a key. |
346 | Since the keys of real objects are decimal numbers, there is no |
347 | conflict and the slot in the field hash can be used like any other. |
348 | The C<id()> function behaves correspondingly with respect to non-reference |
349 | arguments. |
350 | |
351 | Two possible uses (besides ignoring the property) come to mind. |
352 | A singleton class could be implemented this using the generic object. |
353 | If necessary, an C<init()> method could die or ignore calls with |
354 | actual objects (references), so only the generic object will ever exist. |
355 | |
356 | Another use of the generic object would be as a template. It is |
357 | a convenient place to store class-specific defaults for various |
358 | fields to be used in actual object initialization. |
359 | |
360 | Usually, the feature can be entirely ignored. Calling I<object |
361 | methods> as I<class methods> normally leads to an error and isn't used |
362 | routinely anywhere. It may be a problem that this error isn't |
363 | indicated by a class with a generic object. |
364 | |
365 | =head2 How to use Field Hashes |
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366 | |
367 | Traditionally, the definition of an inside-out class contains a bare |
368 | block inside which a number of lexical hashes are declared and the |
369 | basic accessor methods defined, usually through C<Scalar::Util::refaddr>. |
370 | Further methods may be defined outside this block. There has to be |
371 | a DESTROY method and, for thread support, a CLONE method. |
372 | |
373 | When field hashes are used, the basic structure reamins the same. |
374 | Each lexical hash will be made a field hash. The call to C<refaddr> |
375 | can be omitted from the accessor methods. DESTROY and CLONE methods |
376 | are not necessary. |
377 | |
378 | If you have an existing inside-out class, simply making all hashes |
379 | field hashes with no other change should make no difference. Through |
380 | the calls to C<refaddr> or equivalent, the field hashes never get to |
381 | see a reference and work like normal hashes. Your DESTROY (and |
382 | CLONE) methods are still needed. |
383 | |
384 | To make the field hashes kick in, it is easiest to redefine C<refaddr> |
385 | as |
386 | |
387 | sub refaddr { shift } |
388 | |
389 | instead of importing it from C<Scalar::Util>. It should now be possible |
390 | to disable DESTROY and CLONE. Note that while it isn't disabled, |
391 | DESTROY will be called before the garbage collection of field hashes, |
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392 | so it will be invoked with a functional object and will continue to |
393 | function. |
394 | |
395 | It is not desirable to import the functions C<fieldhash> and/or |
396 | C<fieldhashes> into every class that is going to use them. They |
397 | are only used once to set up the class. When the class is up and running, |
398 | these functions serve no more purpose. |
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399 | |
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400 | If there are only a few field hashes to declare, it is simplest to |
401 | |
402 | use Hash::Util::FieldHash; |
403 | |
404 | early and call the functions qualified: |
405 | |
406 | Hash::Util::FieldHash::fieldhash my %foo; |
407 | |
408 | Otherwise, import the functions into a convenient package like |
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409 | C<HUF> or, more general, C<Aux> |
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410 | |
411 | { |
412 | package Aux; |
413 | use Hash::Util::FieldHash ':all'; |
414 | } |
415 | |
416 | and call |
417 | |
418 | Aux::fieldhash my %foo; |
419 | |
420 | as needed. |
421 | |
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422 | =head2 Garbage-Collected Hashes |
423 | |
424 | Garbage collection in a field hash means that entries will "spontaneously" |
425 | disappear when the object that created them disappears. That must be |
426 | borne in mind, especially when looping over a field hash. If anything |
427 | you do inside the loop could cause an object to go out of scope, a |
428 | random key may be deleted from the hash you are looping over. That |
429 | can throw the loop iterator, so it's best to cache a consistent snapshot |
430 | of the keys and/or values and loop over that. You will still have to |
431 | check that a cached entry still exists when you get to it. |
432 | |
433 | Garbage collection can be confusing when keys are created in a field hash |
434 | from normal scalars as well as references. Once a reference is I<used> with |
435 | a field hash, the entry will be collected, even if it was later overwritten |
436 | with a plain scalar key (every positive integer is a candidate). This |
437 | is true even if the original entry was deleted in the meantime. In fact, |
438 | deletion from a field hash, and also a test for existence constitute |
439 | I<use> in this sense and create a liability to delete the entry when |
440 | the reference goes out of scope. If you happen to create an entry |
441 | with an identical key from a string or integer, that will be collected |
442 | instead. Thus, mixed use of references and plain scalars as field hash |
443 | keys is not entirely supported. |
444 | |
445 | =head1 Examples |
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446 | |
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447 | The examples show a very simple class that implements a I<name>, consisting |
448 | of a first and last name (no middle initial). The name class has four |
449 | methods: |
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450 | |
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451 | =over |
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452 | |
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453 | =item * C<init()> |
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454 | |
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455 | An object method that initializes the first and last name to its |
456 | two arguments. If called as a class method, C<init()> creates an |
457 | object in the given class and initializes that. |
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458 | |
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459 | =item * C<first()> |
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460 | |
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461 | Retrieve the first name |
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462 | |
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463 | =item * C<last()> |
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464 | |
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465 | Retrieve the last name |
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466 | |
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467 | =item * C<name()> |
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468 | |
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469 | Retrieve the full name, the first and last name joined by a blank. |
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470 | |
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471 | =back |
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472 | |
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473 | The examples show this class implemented with different levels of |
474 | support by C<Hash::Util::FieldHash>. All supported combinations |
475 | are shown. The difference between implementations is often quite |
476 | small. The implementations are: |
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477 | |
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478 | =over |
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479 | |
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480 | =item * C<Name_hash> |
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481 | |
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482 | A conventional (not inside-out) implementation where an object is |
483 | a hash that stores the field values, without support by |
484 | C<Hash::Util::FieldHash>. This implementation doesn't allow |
485 | arbitrary inheritance. |
486 | |
487 | =item * C<Name_id> |
488 | |
489 | Inside-out implementation based on the C<id()> function. It needs |
490 | a C<DESTROY> method. For thread support a C<CLONE> method (not shown) |
491 | would also be needed. Instead of C<Hash::Util::FieldHash::id()> the |
492 | function C<Scalar::Util::refaddr> could be used with very little |
493 | functional difference. This is the basic pattern of an inside-out |
494 | class. |
495 | |
496 | =item * C<Name_idhash> |
497 | |
498 | Idhash-based inside-out implementation. Like L<Name_id> it needs |
499 | a C<DESTROY> method and would need C<CLONE> for thread support. |
500 | |
501 | =item * C<Name_id_reg> |
502 | |
503 | Inside-out implementation based on the C<id()> function with explicit |
504 | object registry. No destructor is needed and objects are thread safe. |
505 | |
506 | =item * C<Name_idhash_reg> |
507 | |
508 | Idhash-based inside-out implementation with explicit object registry. |
509 | No destructor is needed and objects are thread safe. |
510 | |
511 | =item * C<Name_fieldhash> |
512 | |
513 | Fieldhash-based inside-out implementation. Object registry happens |
514 | automatically. No destructor is needed and objects are thread safe. |
515 | |
516 | =back |
517 | |
518 | These examples are realized in the code below, which could be copied |
519 | to a file F<Example.pm>. |
520 | |
521 | =head2 Example 1 |
522 | |
523 | use strict; use warnings; |
524 | |
525 | { |
526 | package Name_hash; # standard implementation: the object is a hash |
527 | |
528 | sub init { |
529 | my $obj = shift; |
530 | my ( $first, $last) = @_; |
531 | # create an object if called as class method |
532 | $obj = bless {}, $obj unless ref $obj; |
533 | $obj->{ first} = $first; |
534 | $obj->{ last} = $last; |
535 | $obj; |
536 | } |
537 | |
538 | sub first { shift()->{ first} } |
539 | sub last { shift()->{ last} } |
540 | |
541 | sub name { |
542 | my $n = shift; |
543 | join ' ' => $n->first, $n->last; |
544 | } |
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545 | |
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546 | } |
547 | |
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548 | { |
549 | package Name_id; |
550 | use Hash::Util::FieldHash qw( id); |
551 | |
552 | my ( %first, %last); |
553 | |
554 | sub init { |
555 | my $obj = shift; |
556 | my ( $first, $last) = @_; |
557 | # create an object if called as class method |
558 | $obj = bless \ my $o, $obj unless ref $obj; |
559 | $first{ id $obj} = $first; |
560 | $last{ id $obj} = $last; |
561 | $obj; |
562 | } |
563 | |
564 | sub first { $first{ id shift()} } |
565 | sub last { $last{ id shift()} } |
566 | |
567 | sub name { |
568 | my $n = shift; |
569 | join ' ' => $n->first, $n->last; |
570 | } |
571 | |
572 | sub DESTROY { |
573 | my $id = id shift; |
574 | delete $first{ $id}; |
575 | delete $last{ $id}; |
576 | } |
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577 | |
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578 | } |
579 | |
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580 | { |
581 | package Name_idhash; |
582 | use Hash::Util::FieldHash; |
583 | |
584 | Hash::Util::FieldHash::idhashes( \ my ( %first, %last)); |
585 | |
586 | sub init { |
587 | my $obj = shift; |
588 | my ( $first, $last) = @_; |
589 | # create an object if called as class method |
590 | $obj = bless \ my $o, $obj unless ref $obj; |
591 | $first{ $obj} = $first; |
592 | $last{ $obj} = $last; |
593 | $obj; |
594 | } |
595 | |
596 | sub first { $first{ shift()} } |
597 | sub last { $last{ shift()} } |
598 | |
599 | sub name { |
600 | my $n = shift; |
601 | join ' ' => $n->first, $n->last; |
602 | } |
603 | |
604 | sub DESTROY { |
605 | my $n = shift; |
606 | delete $first{ $n}; |
607 | delete $last{ $n}; |
608 | } |
609 | |
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610 | } |
611 | |
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612 | { |
613 | package Name_id_reg; |
614 | use Hash::Util::FieldHash qw( id register); |
615 | |
616 | my ( %first, %last); |
617 | |
618 | sub init { |
619 | my $obj = shift; |
620 | my ( $first, $last) = @_; |
621 | # create an object if called as class method |
622 | $obj = bless \ my $o, $obj unless ref $obj; |
623 | register( $obj, \ ( %first, %last)); |
624 | $first{ id $obj} = $first; |
625 | $last{ id $obj} = $last; |
626 | $obj; |
627 | } |
628 | |
629 | sub first { $first{ id shift()} } |
630 | sub last { $last{ id shift()} } |
631 | |
632 | sub name { |
633 | my $n = shift; |
634 | join ' ' => $n->first, $n->last; |
635 | } |
636 | } |
637 | |
638 | { |
639 | package Name_idhash_reg; |
640 | use Hash::Util::FieldHash qw( register); |
641 | |
642 | Hash::Util::FieldHash::idhashes \ my ( %first, %last); |
643 | |
644 | sub init { |
645 | my $obj = shift; |
646 | my ( $first, $last) = @_; |
647 | # create an object if called as class method |
648 | $obj = bless \ my $o, $obj unless ref $obj; |
649 | register( $obj, \ ( %first, %last)); |
650 | $first{ $obj} = $first; |
651 | $last{ $obj} = $last; |
652 | $obj; |
653 | } |
654 | |
655 | sub first { $first{ shift()} } |
656 | sub last { $last{ shift()} } |
657 | |
658 | sub name { |
659 | my $n = shift; |
660 | join ' ' => $n->first, $n->last; |
661 | } |
662 | } |
663 | |
664 | { |
665 | package Name_fieldhash; |
666 | use Hash::Util::FieldHash; |
667 | |
668 | Hash::Util::FieldHash::fieldhashes \ my ( %first, %last); |
669 | |
670 | sub init { |
671 | my $obj = shift; |
672 | my ( $first, $last) = @_; |
673 | # create an object if called as class method |
674 | $obj = bless \ my $o, $obj unless ref $obj; |
675 | $first{ $obj} = $first; |
676 | $last{ $obj} = $last; |
677 | $obj; |
678 | } |
679 | |
680 | sub first { $first{ shift()} } |
681 | sub last { $last{ shift()} } |
682 | |
683 | sub name { |
684 | my $n = shift; |
685 | join ' ' => $n->first, $n->last; |
686 | } |
687 | } |
688 | |
689 | 1; |
690 | |
691 | To exercise the various implementations the script L<below|Example 2> can |
692 | be used. |
693 | |
694 | It sets up a class C<Name> that is a mirror of one of the implementation |
695 | classes C<Name_hash>, C<Name_id>, ..., C<Name_fieldhash>. That determines |
696 | which implementation is run. |
697 | |
698 | The script first verifies the function of the C<Name> class. |
699 | |
700 | In the second step, the free inheritablility of the implementation |
701 | (or lack thereof) is demonstrated. For this purpose it constructs |
702 | a class called C<NamedFile> which is a common subclass of C<Name> and |
703 | the standard class C<IO::File>. This puts inheritability to the test |
704 | because objects of C<IO::File> I<must> be globrefs. Objects of C<NamedFile> |
705 | should behave like a file opened for reading and also support the C<name()> |
706 | method. This class juncture works with exception of the C<Name_hash> |
707 | implementation, where object initialization fails because of the |
708 | incompatibility of object bodies. |
709 | |
710 | =head2 Example 2 |
711 | |
712 | use strict; use warnings; $| = 1; |
713 | |
714 | use Example; |
715 | |
716 | { |
717 | package Name; |
718 | use base 'Name_id'; # define here which implementation to run |
719 | } |
720 | |
721 | |
722 | # Verify that the base package works |
723 | my $n = Name->init( qw( Albert Einstein)); |
724 | print $n->name, "\n"; |
725 | print "\n"; |
726 | |
727 | # Create a named file handle (See definition below) |
728 | my $nf = NamedFile->init( qw( /tmp/x Filomena File)); |
729 | # use as a file handle... |
730 | for ( 1 .. 3 ) { |
731 | my $l = <$nf>; |
732 | print "line $_: $l"; |
733 | } |
734 | # ...and as a Name object |
735 | print "...brought to you by ", $nf->name, "\n"; |
736 | exit; |
737 | |
738 | |
739 | # Definition of NamedFile |
740 | package NamedFile; |
741 | use base 'Name'; |
742 | use base 'IO::File'; |
743 | |
744 | sub init { |
745 | my $obj = shift; |
746 | my ( $file, $first, $last) = @_; |
747 | $obj = $obj->IO::File::new() unless ref $obj; |
748 | $obj->open( $file) or die "Can't read '$file': $!"; |
749 | $obj->Name::init( $first, $last); |
750 | } |
751 | __END__ |
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752 | |
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753 | |
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754 | |
755 | =head1 Guts |
756 | |
757 | To make C<Hash::Util::FieldHash> work, there were two changes to |
758 | F<perl> itself. C<PERL_MAGIC_uvar> was made avaliable for hashes, |
759 | and weak references now call uvar C<get> magic after a weakref has been |
760 | cleared. The first feature is used to make field hashes intercept |
761 | their keys upon access. The second one triggers garbage collection. |
762 | |
763 | =head2 The C<PERL_MAGIC_uvar> interface for hashes |
764 | |
765 | C<PERL_MAGIC_uvar> I<get> magic is called from C<hv_fetch_common> and |
766 | C<hv_delete_common> through the function C<hv_magic_uvar_xkey>, which |
767 | defines the interface. The call happens for hashes with "uvar" magic |
768 | if the C<ufuncs> structure has equal values in the C<uf_val> and C<uf_set> |
769 | fields. Hashes are unaffected if (and as long as) these fields |
770 | hold different values. |
771 | |
772 | Upon the call, the C<mg_obj> field will hold the hash key to be accessed. |
773 | Upon return, the C<SV*> value in C<mg_obj> will be used in place of the |
774 | original key in the hash access. The integer index value in the first |
775 | parameter will be the C<action> value from C<hv_fetch_common>, or -1 |
776 | if the call is from C<hv_delete_common>. |
777 | |
778 | This is a template for a function suitable for the C<uf_val> field in |
779 | a C<ufuncs> structure for this call. The C<uf_set> and C<uf_index> |
780 | fields are irrelevant. |
781 | |
782 | IV watch_key(pTHX_ IV action, SV* field) { |
783 | MAGIC* mg = mg_find(field, PERL_MAGIC_uvar); |
784 | SV* keysv = mg->mg_obj; |
785 | /* Do whatever you need to. If you decide to |
786 | supply a different key newkey, return it like this |
787 | */ |
788 | sv_2mortal(newkey); |
789 | mg->mg_obj = newkey; |
790 | return 0; |
791 | } |
792 | |
793 | =head2 Weakrefs call uvar magic |
794 | |
795 | When a weak reference is stored in an C<SV> that has "uvar" magic, C<set> |
796 | magic is called after the reference has gone stale. This hook can be |
797 | used to trigger further garbage-collection activities associated with |
798 | the referenced object. |
799 | |
800 | =head2 How field hashes work |
801 | |
802 | The three features of key hashes, I<key replacement>, I<thread support>, |
803 | and I<garbage collection> are supported by a data structure called |
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804 | the I<object registry>. This is a private hash where every object |
805 | is stored. An "object" in this sense is any reference (blessed or |
806 | unblessed) that has been used as a field hash key. |
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807 | |
808 | The object registry keeps track of references that have been used as |
809 | field hash keys. The keys are generated from the reference address |
810 | like in a field hash (though the registry isn't a field hash). Each |
811 | value is a weak copy of the original reference, stored in an C<SV> that |
812 | is itself magical (C<PERL_MAGIC_uvar> again). The magical structure |
813 | holds a list (another hash, really) of field hashes that the reference |
814 | has been used with. When the weakref becomes stale, the magic is |
815 | activated and uses the list to delete the reference from all field |
816 | hashes it has been used with. After that, the entry is removed from |
817 | the object registry itself. Implicitly, that frees the magic structure |
818 | and the storage it has been using. |
819 | |
820 | Whenever a reference is used as a field hash key, the object registry |
821 | is checked and a new entry is made if necessary. The field hash is |
822 | then added to the list of fields this reference has used. |
823 | |
824 | The object registry is also used to repair a field hash after thread |
825 | cloning. Here, the entire object registry is processed. For every |
826 | reference found there, the field hashes it has used are visited and |
827 | the entry is updated. |
828 | |
829 | =head2 Internal function Hash::Util::FieldHash::_fieldhash |
830 | |
831 | # test if %hash is a field hash |
832 | my $result = _fieldhash \ %hash, 0; |
833 | |
834 | # make %hash a field hash |
835 | my $result = _fieldhash \ %hash, 1; |
836 | |
837 | C<_fieldhash> is the internal function used to create field hashes. |
838 | It takes two arguments, a hashref and a mode. If the mode is boolean |
839 | false, the hash is not changed but tested if it is a field hash. If |
840 | the hash isn't a field hash the return value is boolean false. If it |
841 | is, the return value indicates the mode of field hash. When called with |
842 | a boolean true mode, it turns the given hash into a field hash of this |
843 | mode, returning the mode of the created field hash. C<_fieldhash> |
844 | does not erase the given hash. |
845 | |
846 | Currently there is only one type of field hash, and only the boolean |
847 | value of the mode makes a difference, but that may change. |
848 | |
849 | =head1 AUTHOR |
850 | |
851 | Anno Siegel, E<lt>anno4000@zrz.tu-berlin.deE<gt> |
852 | |
853 | =head1 COPYRIGHT AND LICENSE |
854 | |
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855 | Copyright (C) 2006 by (Anno Siegel) |
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856 | |
857 | This library is free software; you can redistribute it and/or modify |
858 | it under the same terms as Perl itself, either Perl version 5.8.7 or, |
859 | at your option, any later version of Perl 5 you may have available. |
860 | |
861 | =cut |