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 *p25854* : 'len' magic is no longer called when pushing an element
131 *p26569* : 'local' magic.
135 *p31064* : Meaningful 'uvar' magic.
137 *p31473* : 'clear' magic wasn't invoked when undefining an array.
138 The bug is fixed as of this version.
142 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
143 'copy' magic on hash stores for (non-tied) hashes that also have
148 *p32969* : 'len' magic is no longer invoked when calling "length"
149 with a magical scalar.
153 The minimum integer used as a signature for user-defined magic.
156 The maximum integer used as a signature for user-defined magic.
159 SIG_NBR = SIG_MAX - SIG_MIN + 1
162 Evaluates to true iff the 'copy' magic is available.
165 Evaluates to true iff the 'dup' magic is available.
168 Evaluates to true iff the 'local' magic is available.
171 When this constant is true, you can use the "fetch,store,exists,delete"
174 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
175 True for perls that don't call 'len' magic when you push an element in a
178 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
179 True for perls that call 'clear' magic when undefining magical arrays.
181 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
182 True for perls that don't call 'len' magic when taking the "length" of a
185 "VMG_PERL_PATCHLEVEL"
186 The perl patchlevel this module was built with, or 0 for non-debugging
190 True iff this module could have been built with thread-safety features
197 get => sub { my ($ref, $data) = @_; ... },
198 set => sub { my ($ref, $data) = @_; ... },
199 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
200 clear => sub { my ($ref, $data) = @_; ... },
201 free => sub { my ($ref, $data) = @_, ... },
202 copy => sub { my ($ref, $data, $key, $elt) = @_; ... },
203 local => sub { my ($ref, $data) = @_; ... },
204 fetch => sub { my ($ref, $data, $key) = @_; ... },
205 store => sub { my ($ref, $data, $key) = @_; ... },
206 exists => sub { my ($ref, $data, $key) = @_; ... },
207 delete => sub { my ($ref, $data, $key) = @_; ... }
209 This function creates a 'wizard', an opaque type that holds the magic
210 information. It takes a list of keys / values as argument, whose keys
215 The numerical signature. If not specified or undefined, a random
216 signature is generated. If the signature matches an already defined
217 magic, then the existant magic object is returned.
221 A code reference to a private data constructor. It is called each
222 time this magic is cast on a variable, and the scalar returned is
223 used as private data storage for it. $_[0] is a reference to the
224 magic object and @_[1 .. @_-1] are all extra arguments that were
227 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
228 "store", "exists" and "delete"
230 Code references to corresponding magic callbacks. You don't have to
231 specify all of them : the magic associated with undefined entries
232 simply won't be hooked. In those callbacks, $_[0] is always a
233 reference to the magic object and $_[1] is always the private data
234 (or "undef" when no private data constructor was supplied). In the
235 special case of "len" magic and when the variable is an array, $_[2]
236 contains its normal length. $_[2] is the current key in "copy",
237 "fetch", "store", "exists" and "delete" callbacks, although for
238 "copy" it may just be a copy of the actual key so it's useless to
239 (for example) cast magic on it. "copy" magic also receives the
240 current element (i.e. the value) in $_[3].
242 # A simple scalar tracer
243 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
244 set => sub { print STDERR "set to ${$_[0]}\n" },
245 free => sub { print STDERR "${$_[0]} was deleted\n" }
248 With this tool, you can manually generate random magic signature between
249 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
250 no signature is supplied.
252 # Generate a signature
258 This accessor returns the magic signature of this wizard.
261 my $sig = getsig $wiz;
264 cast [$@%&*]var, [$wiz|$sig], ...
266 This function associates $wiz magic to the variable supplied, without
267 overwriting any other kind of magic. You can also supply the numeric
268 signature $sig instead of $wiz. It returns true on success or when $wiz
269 magic is already present, 0 on error, and "undef" when no magic
270 corresponds to the given signature (in case $sig was supplied). All
271 extra arguments specified after $wiz are passed to the private data
272 constructor. If the variable isn't a hash, any "uvar" callback of the
273 wizard is safely ignored.
275 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
277 die 'error' unless cast $x, $wiz;
280 getdata [$@%&*]var, [$wiz|$sig]
282 This accessor fetches the private data associated with the magic $wiz
283 (or the signature $sig) in the variable. "undef" is returned when no
284 such magic or data is found, or when $sig does not represent a current
287 # Get the attached data.
288 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
291 dispell [$@%&*]variable, [$wiz|$sig]
293 The exact opposite of "cast" : it dissociates $wiz magic from the
294 variable. You can also pass the magic signature $sig as the second
295 argument. True is returned on success, 0 on error or when no magic
296 represented by $wiz could be found in the variable, and "undef" when no
297 magic corresponds to the given signature (in case $sig was supplied).
299 # Dispell now. If $wiz isn't a signature, undef can't be returned.
300 die 'no such magic or error' unless dispell $x, $wiz;
303 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
304 "dispell" are only exported on request. All of them are exported by the
305 tags ':funcs' and ':all'.
307 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
308 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
309 all exported by the tags ':consts' and ':all'.
312 If you store a magic object in the private data slot, the magic won't be
313 accessible by "getdata" since it's not copied by assignation. The only
314 way to address this would be to return a reference.
316 If you define a wizard with a "free" callback and cast it on itself,
317 this destructor won't be called because the wizard will be destroyed
323 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
325 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
328 Some uvar tests need Hash::Util::FieldHash (standard since perl
331 Glob tests need Symbol (standard since perl 5.002).
333 Threads tests need threads and threads::shared.
336 perlguts and perlapi for internal information about magic.
338 perltie and overload for other ways of enhancing objects.
341 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
343 You can contact me by mail or on "irc.perl.org" (vincent).
346 Please report any bugs or feature requests to "bug-variable-magic at
347 rt.cpan.org", or through the web interface at
348 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
349 be notified, and then you'll automatically be notified of progress on
350 your bug as I make changes.
353 You can find documentation for this module with the perldoc command.
355 perldoc Variable::Magic
357 Tests code coverage report is available at
358 <http://www.profvince.com/perl/cover/Variable-Magic>.
361 Copyright 2007-2008 Vincent Pit, all rights reserved.
363 This program is free software; you can redistribute it and/or modify it
364 under the same terms as Perl itself.