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
208 Value to pass with "op_info" to get the current op name in the magic
212 Value to pass with "op_info" to get a "B::OP" object representing the
213 current op in the magic callbacks.
219 get => sub { my ($ref, $data [, $op]) = @_; ... },
220 set => sub { my ($ref, $data [, $op]) = @_; ... },
221 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
222 clear => sub { my ($ref, $data [, $op]) = @_; ... },
223 free => sub { my ($ref, $data [, $op]) = @_, ... },
224 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
225 local => sub { my ($ref, $data [, $op]) = @_; ... },
226 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
227 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
228 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
229 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
231 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
233 This function creates a 'wizard', an opaque type that holds the magic
234 information. It takes a list of keys / values as argument, whose keys
239 The numerical signature. If not specified or undefined, a random
240 signature is generated. If the signature matches an already defined
241 magic, then the existant magic object is returned.
245 A code reference to a private data constructor. It is called each
246 time this magic is cast on a variable, and the scalar returned is
247 used as private data storage for it. $_[0] is a reference to the
248 magic object and @_[1 .. @_-1] are all extra arguments that were
251 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
252 "store", "exists" and "delete"
254 Code references to the corresponding magic callbacks. You don't have
255 to specify all of them : the magic associated with undefined entries
256 simply won't be hooked. In those callbacks, $_[0] is always a
257 reference to the magic object and $_[1] is always the private data
258 (or "undef" when no private data constructor was supplied).
260 Moreover, when you pass "op_info => $num" to "wizard", the last
261 element of @_ will be the current op name if "$num ==
262 VMG_OP_INFO_NAME" and a "B::OP" object representing the current op
263 if "$num == VMG_OP_INFO_OBJECT". Both have a performance hit, but
264 just getting the name is lighter than getting the op object.
266 Other arguments are specific to the magic hooked :
270 When the variable is an array or a scalar, $_[2] contains
271 the non-magical length. The callback can return the new
272 scalar or array length to use, or "undef" to default to the
277 $_[2] is a either a copy or an alias of the current key,
278 which means that it is useless to try to change or cast
279 magic on it. $_[3] is an alias to the current element (i.e.
282 * "fetch", "store", "exists" and "delete"
284 $_[2] is an alias to the current key. Nothing prevents you
285 from changing it, but be aware that there lurk dangerous
286 side effects. For example, it may righteously be readonly if
287 the key was a bareword. You can get a copy instead by
288 passing "copy_key => 1" to "wizard", which allows you to
289 safely assign to $_[2] in order to e.g. redirect the action
290 to another key. This however has a little performance
291 drawback because of the copy.
293 All the callbacks are expected to return an integer, which is passed
294 straight to the perl magic API. However, only the return value of
295 the "len" callback currently holds a meaning.
297 # A simple scalar tracer
298 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
299 set => sub { print STDERR "set to ${$_[0]}\n" },
300 free => sub { print STDERR "${$_[0]} was deleted\n" }
303 With this tool, you can manually generate random magic signature between
304 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
305 no signature is supplied.
307 # Generate a signature
313 This accessor returns the magic signature of this wizard.
316 my $sig = getsig $wiz;
319 cast [$@%&*]var, [$wiz|$sig], ...
321 This function associates $wiz magic to the variable supplied, without
322 overwriting any other kind of magic. You can also supply the numeric
323 signature $sig instead of $wiz. It returns true on success or when $wiz
324 magic is already present, 0 on error, and "undef" when no magic
325 corresponds to the given signature (in case $sig was supplied). All
326 extra arguments specified after $wiz are passed to the private data
327 constructor. If the variable isn't a hash, any "uvar" callback of the
328 wizard is safely ignored.
330 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
332 die 'error' unless cast $x, $wiz;
334 The "var" argument can be an array or hash value. Magic for those
335 behaves like for any other scalar, except that it is dispelled when the
336 entry is deleted from the container. For example, if you want to call
337 "POSIX::tzset" each time the 'TZ' environment variable is changed in
341 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
343 If you want to overcome the possible deletion of the 'TZ' entry, you
344 have no choice but to rely on "store" uvar magic.
347 getdata [$@%&*]var, [$wiz|$sig]
349 This accessor fetches the private data associated with the magic $wiz
350 (or the signature $sig) in the variable. "undef" is returned when no
351 such magic or data is found, or when $sig does not represent a current
354 # Get the attached data.
355 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
358 dispell [$@%&*]variable, [$wiz|$sig]
360 The exact opposite of "cast" : it dissociates $wiz magic from the
361 variable. You can also pass the magic signature $sig as the second
362 argument. True is returned on success, 0 on error or when no magic
363 represented by $wiz could be found in the variable, and "undef" when no
364 magic corresponds to the given signature (in case $sig was supplied).
366 # Dispell now. If $wiz isn't a signature, undef can't be returned.
367 die 'no such magic or error' unless dispell $x, $wiz;
370 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
371 "dispell" are only exported on request. All of them are exported by the
372 tags ':funcs' and ':all'.
374 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
375 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
376 all exported by the tags ':consts' and ':all'.
379 If you store a magic object in the private data slot, the magic won't be
380 accessible by "getdata" since it's not copied by assignation. The only
381 way to address this would be to return a reference.
383 If you define a wizard with a "free" callback and cast it on itself,
384 this destructor won't be called because the wizard will be destroyed
390 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
392 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
395 Some uvar tests need Hash::Util::FieldHash (standard since perl
398 Glob tests need Symbol (standard since perl 5.002).
400 Threads tests need threads and threads::shared.
403 perlguts and perlapi for internal information about magic.
405 perltie and overload for other ways of enhancing objects.
408 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
410 You can contact me by mail or on "irc.perl.org" (vincent).
413 Please report any bugs or feature requests to "bug-variable-magic at
414 rt.cpan.org", or through the web interface at
415 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
416 be notified, and then you'll automatically be notified of progress on
417 your bug as I make changes.
420 You can find documentation for this module with the perldoc command.
422 perldoc Variable::Magic
424 Tests code coverage report is available at
425 <http://www.profvince.com/perl/cover/Variable-Magic>.
428 Copyright 2007-2009 Vincent Pit, all rights reserved.
430 This program is free software; you can redistribute it and/or modify it
431 under the same terms as Perl itself.