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 executed in scalar context and are expected to
269 return an integer, which is then passed straight to the perl magic
270 API. However, only the return value of the *len* magic callback
271 currently holds a meaning.
273 Each callback can be specified as :
275 * a code reference, which will be called as a subroutine.
277 * a string reference, where the string denotes which subroutine is to
278 be called when magic is triggered. If the subroutine name is not
279 fully qualified, then the current package at the time the magic is
280 invoked will be used instead.
282 * a reference to "undef", in which case a no-op magic callback is
283 installed instead of the default one. This may especially be helpful
284 for *local* magic, where an empty callback prevents magic from being
285 copied during localization.
287 Note that *free* magic is never called during global destruction, as
288 there is no way to ensure that the wizard object and the callback were
289 not destroyed before the variable.
291 Here is a simple usage example :
293 # A simple scalar tracer
295 get => sub { print STDERR "got ${$_[0]}\n" },
296 set => sub { print STDERR "set to ${$_[0]}\n" },
297 free => sub { print STDERR "${$_[0]} was deleted\n" },
301 cast [$@%&*]var, $wiz, @args
303 This function associates $wiz magic to the supplied variable, without
304 overwriting any other kind of magic. It returns true on success or when
305 $wiz magic is already attached, and croaks on error. When $wiz provides
306 a data constructor, it is called just before magic is cast onto the
307 variable, and it receives a reference to the target variable in $_[0]
308 and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.
310 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
314 The "var" argument can be an array or hash value. Magic for these
315 scalars behaves like for any other, except that it is dispelled when the
316 entry is deleted from the container. For example, if you want to call
317 "POSIX::tzset" each time the 'TZ' environment variable is changed in
321 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
323 If you want to handle the possible deletion of the 'TZ' entry, you must
324 also specify *store* magic.
327 getdata [$@%&*]var, $wiz
329 This accessor fetches the private data associated with the magic $wiz in
330 the variable. It croaks when $wiz does not represent a valid magic
331 object, and returns an empty list if no such magic is attached to the
332 variable or when the wizard has no data constructor.
334 # Get the data attached to $wiz in $x, or undef if $wiz
335 # did not attach any.
336 my $data = getdata $x, $wiz;
339 dispell [$@%&*]variable, $wiz
341 The exact opposite of "cast" : it dissociates $wiz magic from the
342 variable. This function returns true on success, 0 when no magic
343 represented by $wiz could be found in the variable, and croaks if the
344 supplied wizard is invalid.
347 die 'no such magic in $x' unless dispell $x, $wiz;
351 Evaluates to true if and only if the *copy* magic is available. This is
352 the case for perl 5.7.3 and greater, which is ensured by the
353 requirements of this module.
356 Evaluates to true if and only if the *dup* magic is available. This is
357 the case for perl 5.7.3 and greater, which is ensured by the
358 requirements of this module.
361 Evaluates to true if and only if the *local* magic is available. This is
362 the case for perl 5.9.3 and greater.
365 When this constant is true, you can use the *fetch*, *store*, *exists*
366 and *delete* magics on hashes. Initial "VMG_UVAR" capability was
367 introduced in perl 5.9.5, with a fully functional implementation shipped
370 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
371 True for perls that don't call *len* magic when taking the "length" of a
374 "VMG_COMPAT_SCALAR_NOLEN"
375 True for perls that don't call *len* magic on scalars. Implies
376 "VMG_COMPAT_SCALAR_LENGTH_NOLEN".
378 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
379 True for perls that don't call *len* magic when you push an element in a
380 magical array. Starting from perl 5.11.0, this only refers to pushes in
381 non-void context and hence is false.
383 "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
384 True for perls that don't call *len* magic when you push in void context
385 an element in a magical array.
387 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
388 True for perls that don't call *len* magic when you unshift in void
389 context an element in a magical array.
391 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
392 True for perls that call *clear* magic when undefining magical arrays.
394 "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
395 True for perls that don't call *delete* magic when you delete an element
396 from a hash in void context.
398 "VMG_COMPAT_CODE_COPY_CLONE"
399 True for perls that call *copy* magic when a magical closure prototype
402 "VMG_COMPAT_GLOB_GET"
403 True for perls that call *get* magic for operations on globs.
405 "VMG_PERL_PATCHLEVEL"
406 The perl patchlevel this module was built with, or 0 for non-debugging
410 True if and only if this module could have been built with thread-safety
414 True if and only if this module could have been built with fork-safety
415 features enabled. This is always true except on Windows where it is
416 false for perl 5.10.0 and below.
419 Value to pass with "op_info" to get the current op name in the magic
423 Value to pass with "op_info" to get a "B::OP" object representing the
424 current op in the magic callbacks.
427 Associate an object to any perl variable
428 This technique can be useful for passing user data through limited APIs.
429 It is similar to using inside-out objects, but without the drawback of
430 having to implement a complex destructor.
433 package Magical::UserData;
435 use Variable::Magic qw<wizard cast getdata>;
437 my $wiz = wizard data => sub { \$_[1] };
439 sub ud (\[$@%*&]) : lvalue {
441 my $data = &getdata($var, $wiz);
442 unless (defined $data) {
444 &cast($var, $wiz, $slot)
445 or die "Couldn't cast UserData magic onto the variable";
452 BEGIN { *ud = \&Magical::UserData::ud }
455 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
458 $cb->(); # Hello, world!
461 Recursively cast magic on datastructures
462 "cast" can be called from any magical callback, and in particular from
463 "data". This allows you to recursively cast magic on datastructures :
466 $wiz = wizard data => sub {
467 my ($var, $depth) = @_;
471 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
472 } elsif ($r eq 'HASH') {
473 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
478 my ($var, $depth) = @_;
480 print "free $r at depth $depth\n";
492 When %h goes out of scope, this prints something among the lines of :
496 free SCALAR at depth 2
497 free ARRAY at depth 1
498 free SCALAR at depth 3
499 free SCALAR at depth 3
501 Of course, this example does nothing with the values that are added
505 The places where magic is invoked have changed a bit through perl
506 history. Here is a little list of the most recent ones.
510 *p14416* : *copy* and *dup* magic.
514 *p28160* : Integration of *p25854* (see below).
516 *p32542* : Integration of *p31473* (see below).
520 *p25854* : *len* magic is no longer called when pushing an element
523 *p26569* : *local* magic.
527 *p31064* : Meaningful *uvar* magic.
529 *p31473* : *clear* magic was not invoked when undefining an array.
530 The bug is fixed as of this version.
534 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
535 *copy* magic on hash stores for (non-tied) hashes that also have
540 *p32969* : *len* magic is no longer invoked when calling "length"
541 with a magical scalar.
543 *p34908* : *len* magic is no longer called when pushing / unshifting
544 an element into a magical array in void context. The "push" part was
545 already covered by *p25854*.
547 *g9cdcb38b* : *len* magic is called again when pushing into a
548 magical array in non-void context.
551 The functions "wizard", "cast", "getdata" and "dispell" are only
552 exported on request. All of them are exported by the tags ':funcs' and
555 All the constants are also only exported on request, either individually
556 or by the tags ':consts' and ':all'.
559 In order to hook hash operations with magic, you need at least perl
560 5.10.0 (see "VMG_UVAR").
562 If you want to store a magic object in the private data slot, you will
563 not be able to recover the magic with "getdata", since magic is not
564 copied by assignment. You can work around this gotcha by storing a
565 reference to the magic object instead.
567 If you define a wizard with *free* magic and cast it on itself, it
568 results in a memory cycle, so this destructor will not be called when
574 A C compiler. This module may happen to build with a C++ compiler as
575 well, but don't rely on it, as no guarantee is made in this regard.
577 Carp (core since perl 5), XSLoader (since 5.6.0).
579 Copy tests need Tie::Array (core since perl 5.005) and Tie::Hash (since
580 5.002). Some uvar tests need Hash::Util::FieldHash (since 5.9.4). Glob
581 tests need Symbol (since 5.002). Threads tests need threads and
582 threads::shared (both since 5.7.3).
585 perlguts and perlapi for internal information about magic.
587 perltie and overload for other ways of enhancing objects.
590 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
592 You can contact me by mail or on "irc.perl.org" (vincent).
595 Please report any bugs or feature requests to "bug-variable-magic at
596 rt.cpan.org", or through the web interface at
597 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
598 be notified, and then you'll automatically be notified of progress on
599 your bug as I make changes.
602 You can find documentation for this module with the perldoc command.
604 perldoc Variable::Magic
606 Tests code coverage report is available at
607 <http://www.profvince.com/perl/cover/Variable-Magic>.
610 Copyright 2007,2008,2009,2010,2011,2012,2013,2014 Vincent Pit, all
613 This program is free software; you can redistribute it and/or modify it
614 under the same terms as Perl itself.