2 Variable::Magic - Associate user-defined magic to variables from Perl.
8 use Variable::Magic qw<wizard cast VMG_OP_INFO_NAME>;
12 set => sub { print "now set to ${$_[0]}!\n" },
13 free => sub { print "destroyed!\n" },
18 $a = 2; # "now set to 2!"
21 { # A hash with a default value
23 data => sub { $_[1] },
24 fetch => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () },
25 store => sub { print "key $_[2] stored in $_[-1]\n" },
27 op_info => VMG_OP_INFO_NAME,
30 my %h = (_default => 0, apple => 2);
31 cast %h, $wiz, '_default';
32 print $h{banana}, "\n"; # "0" (there is no 'banana' key in %h)
33 $h{pear} = 1; # "key pear stored in helem"
37 Magic is Perl's way of enhancing variables. This mechanism lets the user
38 add extra data to any variable and hook syntactical operations (such as
39 access, assignment or destruction) that can be applied to it. With this
40 module, you can add your own magic to any variable without having to
41 write a single line of XS.
43 You'll realize that these magic variables look a lot like tied
44 variables. It is not surprising, as tied variables are implemented as a
45 special kind of magic, just like any 'irregular' Perl variable : scalars
46 like $!, $( or $^W, the %ENV and %SIG hashes, the @ISA array, "vec()"
47 and "substr()" lvalues, threads::shared variables... They all share the
48 same underlying C API, and this module gives you direct access to it.
50 Still, the magic made available by this module differs from tieing and
51 overloading in several ways :
53 * Magic is not copied on assignment.
55 You attach it to variables, not values (as for blessed references).
57 * Magic does not replace the original semantics.
59 Magic callbacks usually get triggered before the original action
60 takes place, and cannot prevent it from happening. This also makes
61 catching individual events easier than with "tie", where you have to
62 provide fallbacks methods for all actions by usually inheriting from
63 the correct "Tie::Std*" class and overriding individual methods in
66 * Magic is multivalued.
68 You can safely apply different kinds of magics to the same variable,
69 and each of them will be invoked successively.
71 * Magic is type-agnostic.
73 The same magic can be applied on scalars, arrays, hashes, subs or
74 globs. But the same hook (see below for a list) may trigger
75 differently depending on the type of the variable.
77 * Magic is invisible at Perl level.
79 Magical and non-magical variables cannot be distinguished with
80 "ref", "tied" or another trick.
82 * Magic is notably faster.
84 Mainly because perl's way of handling magic is lighter by nature,
85 and because there is no need for any method resolution. Also, since
86 you don't have to reimplement all the variable semantics, you only
87 pay for what you actually use.
89 The operations that can be overloaded are :
93 This magic is invoked when the variable is evaluated. It is never
94 called for arrays and hashes.
98 This magic is called each time the value of the variable changes. It
99 is called for array subscripts and slices, but never for hashes.
103 This magic only applies to arrays (though it used to also apply to
104 scalars), and is triggered when the 'size' or the 'length' of the
105 variable has to be known by Perl. This is typically the magic
106 involved when an array is evaluated in scalar context, but also on
107 array assignment and loops ("for", "map" or "grep"). The length is
108 returned from the callback as an integer.
110 Starting from perl 5.12, this magic is no longer called by the
111 "length" keyword, and starting from perl 5.17.4 it is also no longer
112 called for scalars in any situation, making this magic only
113 meaningful on arrays. You can use the constants
114 "VMG_COMPAT_SCALAR_LENGTH_NOLEN" and "VMG_COMPAT_SCALAR_NOLEN" to
115 see if this magic is available for scalars or not.
119 This magic is invoked when the variable is reset, such as when an
120 array is emptied. Please note that this is different from undefining
121 the variable, even though the magic is called when the clearing is a
122 result of the undefine (e.g. for an array, but actually a bug
123 prevent it to work before perl 5.9.5 - see the history).
127 This magic is called when a variable is destroyed as the result of
128 going out of scope (but not when it is undefined). It behaves
129 roughly like Perl object destructors (i.e. "DESTROY" methods),
130 except that exceptions thrown from inside a *free* callback will
131 always be propagated to the surrounding code.
135 When applied to tied arrays and hashes, this magic fires when you
136 try to access or change their elements.
138 Starting from perl 5.17.0, it can also be applied to closure
139 prototypes, in which case the magic will be called when the
140 prototype is cloned. The "VMG_COMPAT_CODE_COPY_CLONE" constant is
141 true when your perl support this feature.
145 This magic is invoked when the variable is cloned across threads. It
146 is currently not available.
150 When this magic is set on a variable, all subsequent localizations
151 of the variable will trigger the callback. It is available on your
152 perl if and only if "MGf_LOCAL" is true.
154 The following actions only apply to hashes and are available if and only
155 if "VMG_UVAR" is true. They are referred to as *uvar* magics.
159 This magic is invoked each time an element is fetched from the hash.
163 This one is called when an element is stored into the hash.
167 This magic fires when a key is tested for existence in the hash.
171 This magic is triggered when a key is deleted in the hash,
172 regardless of whether the key actually exists in it.
174 You can refer to the tests to have more insight of where the different
181 get => sub { my ($ref, $data [, $op]) = @_; ... },
182 set => sub { my ($ref, $data [, $op]) = @_; ... },
184 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
186 clear => sub { my ($ref, $data [, $op]) = @_; ... },
187 free => sub { my ($ref, $data [, $op]) = @_, ... },
188 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
189 local => sub { my ($ref, $data [, $op]) = @_; ... },
190 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
191 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
192 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
193 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
195 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
198 This function creates a 'wizard', an opaque object that holds the magic
199 information. It takes a list of keys / values as argument, whose keys
204 A code (or string) reference to a private data constructor. It is
205 called in scalar context each time the magic is cast onto a
206 variable, with $_[0] being a reference to this variable and @_[1 ..
207 @_-1] being all extra arguments that were passed to "cast". The
208 scalar returned from this call is then attached to the variable and
209 can be retrieved later with "getdata".
211 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
212 "store", "exists" and "delete"
214 Code (or string) references to the respective magic callbacks. You
215 don't have to specify all of them : the magic corresponding to
216 undefined entries will simply not be hooked.
218 When those callbacks are executed, $_[0] is a reference to the magic
219 variable and $_[1] is the associated private data (or "undef" when
220 no private data constructor is supplied with the wizard). Other
221 arguments depend on which kind of magic is involved :
225 $_[2] contains the natural, non-magical length of the
226 variable (which can only be a scalar or an array as *len*
227 magic is only relevant for these types). The callback is
228 expected to return the new scalar or array length to use, or
229 "undef" to default to the normal length.
233 When the variable for which the magic is invoked is an array
234 or an hash, $_[2] is a either an alias or a copy of the
235 current key, and $_[3] is an alias to the current element
236 (i.e. the value). Since $_[2] might be a copy, it is useless
237 to try to change it or cast magic on it.
239 Starting from perl 5.17.0, this magic can also be called for
240 code references. In this case, $_[2] is always "undef" and
241 $_[3] is a reference to the cloned anonymous subroutine.
243 * *fetch*, *store*, *exists* and *delete*
245 $_[2] is an alias to the current key. Note that $_[2] may
246 rightfully be readonly if the key comes from a bareword, and
247 as such it is unsafe to assign to it. You can ask for a copy
248 instead by passing "copy_key => 1" to "wizard" which, at the
249 price of a small performance hit, allows you to safely
250 assign to $_[2] in order to e.g. redirect the action to
253 Finally, if "op_info => $num" is also passed to "wizard", then one
254 extra element is appended to @_. Its nature depends on the value of
259 $_[-1] is the current op name.
261 * "VMG_OP_INFO_OBJECT"
263 $_[-1] is the "B::OP" object for the current op.
265 Both result in a small performance hit, but just getting the name is
266 lighter than getting the op object.
268 These callbacks are always executed in scalar context. The returned
269 value is coerced into a signed integer, which is then passed
270 straight to the perl magic API. However, note that perl currently
271 only cares about the return value of the *len* magic callback and
272 ignores all the others. Starting with Variable::Magic 0.58, a
273 reference returned from a non-*len* magic callback will not be
274 destroyed immediately but will be allowed to survive until the end
275 of the statement that triggered the magic. This lets you use this
276 return value as a token for triggering a destructor after the
277 original magic action takes place. You can see an example of this
278 technique in the cookbook.
280 Each callback can be specified as :
282 * a code reference, which will be called as a subroutine.
284 * a string reference, where the string denotes which subroutine is to
285 be called when magic is triggered. If the subroutine name is not
286 fully qualified, then the current package at the time the magic is
287 invoked will be used instead.
289 * a reference to "undef", in which case a no-op magic callback is
290 installed instead of the default one. This may especially be helpful
291 for *local* magic, where an empty callback prevents magic from being
292 copied during localization.
294 Note that *free* magic is never called during global destruction, as
295 there is no way to ensure that the wizard object and the callback were
296 not destroyed before the variable.
298 Here is a simple usage example :
300 # A simple scalar tracer
302 get => sub { print STDERR "got ${$_[0]}\n" },
303 set => sub { print STDERR "set to ${$_[0]}\n" },
304 free => sub { print STDERR "${$_[0]} was deleted\n" },
308 cast [$@%&*]var, $wiz, @args
310 This function associates $wiz magic to the supplied variable, without
311 overwriting any other kind of magic. It returns true on success or when
312 $wiz magic is already attached, and croaks on error. When $wiz provides
313 a data constructor, it is called just before magic is cast onto the
314 variable, and it receives a reference to the target variable in $_[0]
315 and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.
317 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
321 The "var" argument can be an array or hash value. Magic for these
322 scalars behaves like for any other, except that it is dispelled when the
323 entry is deleted from the container. For example, if you want to call
324 "POSIX::tzset" each time the 'TZ' environment variable is changed in
328 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
330 If you want to handle the possible deletion of the 'TZ' entry, you must
331 also specify *store* magic.
334 getdata [$@%&*]var, $wiz
336 This accessor fetches the private data associated with the magic $wiz in
337 the variable. It croaks when $wiz does not represent a valid magic
338 object, and returns an empty list if no such magic is attached to the
339 variable or when the wizard has no data constructor.
341 # Get the data attached to $wiz in $x, or undef if $wiz
342 # did not attach any.
343 my $data = getdata $x, $wiz;
346 dispell [$@%&*]variable, $wiz
348 The exact opposite of "cast" : it dissociates $wiz magic from the
349 variable. This function returns true on success, 0 when no magic
350 represented by $wiz could be found in the variable, and croaks if the
351 supplied wizard is invalid.
354 die 'no such magic in $x' unless dispell $x, $wiz;
358 Evaluates to true if and only if the *copy* magic is available. This is
359 the case for perl 5.7.3 and greater, which is ensured by the
360 requirements of this module.
363 Evaluates to true if and only if the *dup* magic is available. This is
364 the case for perl 5.7.3 and greater, which is ensured by the
365 requirements of this module.
368 Evaluates to true if and only if the *local* magic is available. This is
369 the case for perl 5.9.3 and greater.
372 When this constant is true, you can use the *fetch*, *store*, *exists*
373 and *delete* magics on hashes. Initial "VMG_UVAR" capability was
374 introduced in perl 5.9.5, with a fully functional implementation shipped
377 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
378 True for perls that don't call *len* magic when taking the "length" of a
381 "VMG_COMPAT_SCALAR_NOLEN"
382 True for perls that don't call *len* magic on scalars. Implies
383 "VMG_COMPAT_SCALAR_LENGTH_NOLEN".
385 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
386 True for perls that don't call *len* magic when you push an element in a
387 magical array. Starting from perl 5.11.0, this only refers to pushes in
388 non-void context and hence is false.
390 "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
391 True for perls that don't call *len* magic when you push in void context
392 an element in a magical array.
394 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
395 True for perls that don't call *len* magic when you unshift in void
396 context an element in a magical array.
398 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
399 True for perls that call *clear* magic when undefining magical arrays.
401 "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
402 True for perls that don't call *delete* magic when you delete an element
403 from a hash in void context.
405 "VMG_COMPAT_CODE_COPY_CLONE"
406 True for perls that call *copy* magic when a magical closure prototype
409 "VMG_COMPAT_GLOB_GET"
410 True for perls that call *get* magic for operations on globs.
412 "VMG_PERL_PATCHLEVEL"
413 The perl patchlevel this module was built with, or 0 for non-debugging
417 True if and only if this module could have been built with thread-safety
421 True if and only if this module could have been built with fork-safety
422 features enabled. This is always true except on Windows where it is
423 false for perl 5.10.0 and below.
426 Value to pass with "op_info" to get the current op name in the magic
430 Value to pass with "op_info" to get a "B::OP" object representing the
431 current op in the magic callbacks.
434 Associate an object to any perl variable
435 This technique can be useful for passing user data through limited APIs.
436 It is similar to using inside-out objects, but without the drawback of
437 having to implement a complex destructor.
440 package Magical::UserData;
442 use Variable::Magic qw<wizard cast getdata>;
444 my $wiz = wizard data => sub { \$_[1] };
446 sub ud (\[$@%*&]) : lvalue {
448 my $data = &getdata($var, $wiz);
449 unless (defined $data) {
451 &cast($var, $wiz, $slot)
452 or die "Couldn't cast UserData magic onto the variable";
459 BEGIN { *ud = \&Magical::UserData::ud }
462 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
465 $cb->(); # Hello, world!
468 Recursively cast magic on datastructures
469 "cast" can be called from any magical callback, and in particular from
470 "data". This allows you to recursively cast magic on datastructures :
473 $wiz = wizard data => sub {
474 my ($var, $depth) = @_;
478 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
479 } elsif ($r eq 'HASH') {
480 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
485 my ($var, $depth) = @_;
487 print "free $r at depth $depth\n";
499 When %h goes out of scope, this prints something among the lines of :
503 free SCALAR at depth 2
504 free ARRAY at depth 1
505 free SCALAR at depth 3
506 free SCALAR at depth 3
508 Of course, this example does nothing with the values that are added
511 Delayed magic actions
512 Starting with Variable::Magic 0.58, the return value of the magic
513 callbacks can be used to delay the action until after the original
517 my $delayed_aux = wizard(
518 data => sub { $_[1] },
520 my ($target) = $_[1];
521 my $target_data = &getdata($target, $delayed);
522 local $target_data->{guard} = 1;
523 if (ref $target eq 'SCALAR') {
525 $$target = $target_data->{mangler}->($orig);
532 return +{ guard => 0, mangler => $_[1] };
535 return if $_[1]->{guard};
537 cast $token, $delayed_aux, $_[0];
542 cast $x, $delayed => sub { $_[0] * 2 };
545 # But note that the delayed action only takes place at the end of the
546 # current statement :
547 my @y = ($x = 5, $x);
548 # $x is now 10, but @y is (5, 5)
551 The places where magic is invoked have changed a bit through perl
552 history. Here is a little list of the most recent ones.
556 *p14416* : *copy* and *dup* magic.
560 *p28160* : Integration of *p25854* (see below).
562 *p32542* : Integration of *p31473* (see below).
566 *p25854* : *len* magic is no longer called when pushing an element
569 *p26569* : *local* magic.
573 *p31064* : Meaningful *uvar* magic.
575 *p31473* : *clear* magic was not invoked when undefining an array.
576 The bug is fixed as of this version.
580 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
581 *copy* magic on hash stores for (non-tied) hashes that also have
586 *p32969* : *len* magic is no longer invoked when calling "length"
587 with a magical scalar.
589 *p34908* : *len* magic is no longer called when pushing / unshifting
590 an element into a magical array in void context. The "push" part was
591 already covered by *p25854*.
593 *g9cdcb38b* : *len* magic is called again when pushing into a
594 magical array in non-void context.
597 The functions "wizard", "cast", "getdata" and "dispell" are only
598 exported on request. All of them are exported by the tags ':funcs' and
601 All the constants are also only exported on request, either individually
602 or by the tags ':consts' and ':all'.
605 In order to hook hash operations with magic, you need at least perl
606 5.10.0 (see "VMG_UVAR").
608 If you want to store a magic object in the private data slot, you will
609 not be able to recover the magic with "getdata", since magic is not
610 copied by assignment. You can work around this gotcha by storing a
611 reference to the magic object instead.
613 If you define a wizard with *free* magic and cast it on itself, it
614 results in a memory cycle, so this destructor will not be called when
620 A C compiler. This module may happen to build with a C++ compiler as
621 well, but don't rely on it, as no guarantee is made in this regard.
623 Carp (core since perl 5), XSLoader (since 5.6.0).
626 perlguts and perlapi for internal information about magic.
628 perltie and overload for other ways of enhancing objects.
631 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
633 You can contact me by mail or on "irc.perl.org" (vincent).
636 Please report any bugs or feature requests to "bug-variable-magic at
637 rt.cpan.org", or through the web interface at
638 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
639 be notified, and then you'll automatically be notified of progress on
640 your bug as I make changes.
643 You can find documentation for this module with the perldoc command.
645 perldoc Variable::Magic
648 Copyright 2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017 Vincent
649 Pit, all rights reserved.
651 This program is free software; you can redistribute it and/or modify it
652 under the same terms as Perl itself.