2 Variable::Magic - Associate user-defined magic to variables from Perl.
8 use Variable::Magic qw/wizard cast dispell/;
10 my $wiz = wizard set => sub { print STDERR "now set to ${$_[0]}!\n" };
13 $a = 2; # "now set to 2!"
18 Magic is Perl way of enhancing objects. This mechanism let the user add
19 extra data to any variable and hook syntaxical operations (such as
20 access, assignation or destruction) that can be applied to it. With this
21 module, you can add your own magic to any variable without the pain of
24 Magic differs from tieing and overloading in several ways :
26 * Magic isn't copied on assignation (as for blessed references) : you
27 attach it to variables, not values.
29 * It doesn't replace the original semantics : magic callbacks trigger
30 before the original action take place, and can't prevent it to
33 * It's mostly invisible at the Perl level : magical and non-magical
34 variables cannot be distinguished with "ref", "reftype" or another
37 * It's notably faster, since perl's way of handling magic is lighter
38 by nature, and there's no need for any method resolution.
40 The operations that can be overloaded are :
44 This magic is invoked when the variable is evaluated (does not
45 include array/hash subscripts and slices).
49 This one is triggered each time the value of the variable changes
50 (includes array/hash subscripts and slices).
54 This magic is a little special : it is called when the 'size' or the
55 'length' of the variable has to be known by Perl. Typically, it's
56 the magic involved when an array is evaluated in scalar context, but
57 also on array assignation and loops ("for", "map" or "grep"). The
58 callback has then to return the length as an integer.
62 This magic is invoked when the variable is reset, such as when an
63 array is emptied. Please note that this is different from undefining
64 the variable, even though the magic is called when the clearing is a
65 result of the undefine (e.g. for an array, but actually a bug
66 prevent it to work before perl 5.9.5 - see the history).
70 This one can be considered as an object destructor. It happens when
71 the variable goes out of scope (with the exception of global scope),
72 but not when it is undefined.
76 This magic only applies to tied arrays and hashes. It fires when you
77 try to access or change their elements. It is available on your perl
78 iff "MGf_COPY" is true.
82 Invoked when the variable is cloned across threads. Currently not
87 When this magic is set on a variable, all subsequent localizations
88 of the variable will trigger the callback. It is available on your
89 perl iff "MGf_LOCAL" is true.
91 The following actions only apply to hashes and are available iff
92 "VMG_UVAR" is true. They are referred to as "uvar" magics.
96 This magic happens each time an element is fetched from the hash.
100 This one is called when an element is stored into the hash.
104 This magic fires when a key is tested for existence in the hash.
108 This last one triggers when a key is deleted in the hash, regardless
109 of whether the key actually exists in it.
111 You can refer to the tests to have more insight of where the different
114 To prevent any clash between different magics defined with this module,
115 an unique numerical signature is attached to each kind of magic (i.e.
116 each set of callbacks for magic operations).
119 The places where magic is invoked have changed a bit through perl
120 history. Here's a little list of the most recent ones.
124 *p14416* : 'copy' and 'dup' magic.
128 *p28160* : Integration of *p25854* (see below).
130 *p32542* : Integration of *p31473* (see below).
134 *p25854* : 'len' magic is no longer called when pushing an element
137 *p26569* : 'local' magic.
141 *p31064* : Meaningful 'uvar' magic.
143 *p31473* : 'clear' magic wasn't invoked when undefining an array.
144 The bug is fixed as of this version.
148 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
149 'copy' magic on hash stores for (non-tied) hashes that also have
154 *p32969* : 'len' magic is no longer invoked when calling "length"
155 with a magical scalar.
157 *p34908* : 'len' magic is no longer called when pushing / unshifting
158 an element into a magical array in void context. The "push" part was
159 already covered by *p25854*.
163 The minimum integer used as a signature for user-defined magic.
166 The maximum integer used as a signature for user-defined magic.
169 SIG_NBR = SIG_MAX - SIG_MIN + 1
172 Evaluates to true iff the 'copy' magic is available.
175 Evaluates to true iff the 'dup' magic is available.
178 Evaluates to true iff the 'local' magic is available.
181 When this constant is true, you can use the "fetch,store,exists,delete"
184 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
185 True for perls that don't call 'len' magic when you push an element in a
188 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
189 True for perls that don't call 'len' magic when you unshift in void
190 context an element in a magical array.
192 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
193 True for perls that call 'clear' magic when undefining magical arrays.
195 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
196 True for perls that don't call 'len' magic when taking the "length" of a
199 "VMG_PERL_PATCHLEVEL"
200 The perl patchlevel this module was built with, or 0 for non-debugging
204 True iff this module could have been built with thread-safety features
211 get => sub { my ($ref, $data) = @_; ... },
212 set => sub { my ($ref, $data) = @_; ... },
213 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
214 clear => sub { my ($ref, $data) = @_; ... },
215 free => sub { my ($ref, $data) = @_, ... },
216 copy => sub { my ($ref, $data, $key, $elt) = @_; ... },
217 local => sub { my ($ref, $data) = @_; ... },
218 fetch => sub { my ($ref, $data, $key) = @_; ... },
219 store => sub { my ($ref, $data, $key) = @_; ... },
220 exists => sub { my ($ref, $data, $key) = @_; ... },
221 delete => sub { my ($ref, $data, $key) = @_; ... }
223 This function creates a 'wizard', an opaque type that holds the magic
224 information. It takes a list of keys / values as argument, whose keys
229 The numerical signature. If not specified or undefined, a random
230 signature is generated. If the signature matches an already defined
231 magic, then the existant magic object is returned.
235 A code reference to a private data constructor. It is called each
236 time this magic is cast on a variable, and the scalar returned is
237 used as private data storage for it. $_[0] is a reference to the
238 magic object and @_[1 .. @_-1] are all extra arguments that were
241 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
242 "store", "exists" and "delete"
244 Code references to corresponding magic callbacks. You don't have to
245 specify all of them : the magic associated with undefined entries
246 simply won't be hooked. In those callbacks, $_[0] is always a
247 reference to the magic object and $_[1] is always the private data
248 (or "undef" when no private data constructor was supplied). In the
249 special case of "len" magic and when the variable is an array, $_[2]
250 contains its normal length. $_[2] is the current key in "copy",
251 "fetch", "store", "exists" and "delete" callbacks, although for
252 "copy" it may just be a copy of the actual key so it's useless to
253 (for example) cast magic on it. "copy" magic also receives the
254 current element (i.e. the value) in $_[3].
256 All the callbacks are expected to return an integer, which is passed
257 straight to the perl magic API. However, only the return value of
258 the "len" callback currently holds a meaning.
260 # A simple scalar tracer
261 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
262 set => sub { print STDERR "set to ${$_[0]}\n" },
263 free => sub { print STDERR "${$_[0]} was deleted\n" }
266 With this tool, you can manually generate random magic signature between
267 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
268 no signature is supplied.
270 # Generate a signature
276 This accessor returns the magic signature of this wizard.
279 my $sig = getsig $wiz;
282 cast [$@%&*]var, [$wiz|$sig], ...
284 This function associates $wiz magic to the variable supplied, without
285 overwriting any other kind of magic. You can also supply the numeric
286 signature $sig instead of $wiz. It returns true on success or when $wiz
287 magic is already present, 0 on error, and "undef" when no magic
288 corresponds to the given signature (in case $sig was supplied). All
289 extra arguments specified after $wiz are passed to the private data
290 constructor. If the variable isn't a hash, any "uvar" callback of the
291 wizard is safely ignored.
293 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
295 die 'error' unless cast $x, $wiz;
298 getdata [$@%&*]var, [$wiz|$sig]
300 This accessor fetches the private data associated with the magic $wiz
301 (or the signature $sig) in the variable. "undef" is returned when no
302 such magic or data is found, or when $sig does not represent a current
305 # Get the attached data.
306 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
309 dispell [$@%&*]variable, [$wiz|$sig]
311 The exact opposite of "cast" : it dissociates $wiz magic from the
312 variable. You can also pass the magic signature $sig as the second
313 argument. True is returned on success, 0 on error or when no magic
314 represented by $wiz could be found in the variable, and "undef" when no
315 magic corresponds to the given signature (in case $sig was supplied).
317 # Dispell now. If $wiz isn't a signature, undef can't be returned.
318 die 'no such magic or error' unless dispell $x, $wiz;
321 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
322 "dispell" are only exported on request. All of them are exported by the
323 tags ':funcs' and ':all'.
325 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
326 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
327 all exported by the tags ':consts' and ':all'.
330 If you store a magic object in the private data slot, the magic won't be
331 accessible by "getdata" since it's not copied by assignation. The only
332 way to address this would be to return a reference.
334 If you define a wizard with a "free" callback and cast it on itself,
335 this destructor won't be called because the wizard will be destroyed
338 Using "get" and "clear" magics on hashes may cause segfaults.
343 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
345 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
348 Some uvar tests need Hash::Util::FieldHash (standard since perl
351 Glob tests need Symbol (standard since perl 5.002).
353 Threads tests need threads and threads::shared.
356 perlguts and perlapi for internal information about magic.
358 perltie and overload for other ways of enhancing objects.
361 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
363 You can contact me by mail or on "irc.perl.org" (vincent).
366 Please report any bugs or feature requests to "bug-variable-magic at
367 rt.cpan.org", or through the web interface at
368 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
369 be notified, and then you'll automatically be notified of progress on
370 your bug as I make changes.
373 You can find documentation for this module with the perldoc command.
375 perldoc Variable::Magic
377 Tests code coverage report is available at
378 <http://www.profvince.com/perl/cover/Variable-Magic>.
381 Copyright 2007-2009 Vincent Pit, all rights reserved.
383 This program is free software; you can redistribute it and/or modify it
384 under the same terms as Perl itself.