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 This magic only applies to tied arrays and hashes, and fires when
136 you try to access or change their elements.
140 This magic is invoked when the variable is cloned across threads. It
141 is currently not available.
145 When this magic is set on a variable, all subsequent localizations
146 of the variable will trigger the callback. It is available on your
147 perl if and only if "MGf_LOCAL" is true.
149 The following actions only apply to hashes and are available if and only
150 if "VMG_UVAR" is true. They are referred to as *uvar* magics.
154 This magic is invoked each time an element is fetched from the hash.
158 This one is called when an element is stored into the hash.
162 This magic fires when a key is tested for existence in the hash.
166 This magic is triggered when a key is deleted in the hash,
167 regardless of whether the key actually exists in it.
169 You can refer to the tests to have more insight of where the different
176 get => sub { my ($ref, $data [, $op]) = @_; ... },
177 set => sub { my ($ref, $data [, $op]) = @_; ... },
179 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
181 clear => sub { my ($ref, $data [, $op]) = @_; ... },
182 free => sub { my ($ref, $data [, $op]) = @_, ... },
183 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
184 local => sub { my ($ref, $data [, $op]) = @_; ... },
185 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
186 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
187 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
188 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
190 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
193 This function creates a 'wizard', an opaque object that holds the magic
194 information. It takes a list of keys / values as argument, whose keys
199 A code (or string) reference to a private data constructor. It is
200 called in scalar context each time the magic is cast onto a
201 variable, with $_[0] being a reference to this variable and @_[1 ..
202 @_-1] being all extra arguments that were passed to "cast". The
203 scalar returned from this call is then attached to the variable and
204 can be retrieved later with "getdata".
206 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
207 "store", "exists" and "delete"
209 Code (or string) references to the respective magic callbacks. You
210 don't have to specify all of them : the magic corresponding to
211 undefined entries will simply not be hooked.
213 When those callbacks are executed, $_[0] is a reference to the magic
214 variable and $_[1] is the associated private data (or "undef" when
215 no private data constructor is supplied with the wizard). Other
216 arguments depend on which kind of magic is involved :
220 $_[2] contains the natural, non-magical length of the
221 variable (which can only be a scalar or an array as *len*
222 magic is only relevant for these types). The callback is
223 expected to return the new scalar or array length to use, or
224 "undef" to default to the normal length.
228 $_[2] is a either an alias or a copy of the current key, and
229 $_[3] is an alias to the current element (i.e. the value).
230 Because $_[2] might be a copy, it is useless to try to
231 change it or cast magic on it.
233 * *fetch*, *store*, *exists* and *delete*
235 $_[2] is an alias to the current key. Note that $_[2] may
236 rightfully be readonly if the key comes from a bareword, and
237 as such it is unsafe to assign to it. You can ask for a copy
238 instead by passing "copy_key => 1" to "wizard" which, at the
239 price of a small performance hit, allows you to safely
240 assign to $_[2] in order to e.g. redirect the action to
243 Finally, if "op_info => $num" is also passed to "wizard", then one
244 extra element is appended to @_. Its nature depends on the value of
249 $_[-1] is the current op name.
251 * "VMG_OP_INFO_OBJECT"
253 $_[-1] is the "B::OP" object for the current op.
255 Both result in a small performance hit, but just getting the name is
256 lighter than getting the op object.
258 These callbacks are executed in scalar context and are expected to
259 return an integer, which is then passed straight to the perl magic
260 API. However, only the return value of the *len* magic callback
261 currently holds a meaning.
263 Each callback can be specified as :
265 * a code reference, which will be called as a subroutine.
267 * a string reference, where the string denotes which subroutine is to
268 be called when magic is triggered. If the subroutine name is not
269 fully qualified, then the current package at the time the magic is
270 invoked will be used instead.
272 * a reference to "undef", in which case a no-op magic callback is
273 installed instead of the default one. This may especially be helpful
274 for *local* magic, where an empty callback prevents magic from being
275 copied during localization.
277 Note that *free* magic is never called during global destruction, as
278 there is no way to ensure that the wizard object and the callback were
279 not destroyed before the variable.
281 Here is a simple usage example :
283 # A simple scalar tracer
285 get => sub { print STDERR "got ${$_[0]}\n" },
286 set => sub { print STDERR "set to ${$_[0]}\n" },
287 free => sub { print STDERR "${$_[0]} was deleted\n" },
291 cast [$@%&*]var, $wiz, @args
293 This function associates $wiz magic to the supplied variable, without
294 overwriting any other kind of magic. It returns true on success or when
295 $wiz magic is already attached, and croaks on error. When $wiz provides
296 a data constructor, it is called just before magic is cast onto the
297 variable, and it receives a reference to the target variable in $_[0]
298 and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.
300 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
304 The "var" argument can be an array or hash value. Magic for these
305 scalars behaves like for any other, except that it is dispelled when the
306 entry is deleted from the container. For example, if you want to call
307 "POSIX::tzset" each time the 'TZ' environment variable is changed in
311 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
313 If you want to handle the possible deletion of the 'TZ' entry, you must
314 also specify *store* magic.
317 getdata [$@%&*]var, $wiz
319 This accessor fetches the private data associated with the magic $wiz in
320 the variable. It croaks when $wiz does not represent a valid magic
321 object, and returns an empty list if no such magic is attached to the
322 variable or when the wizard has no data constructor.
324 # Get the data attached to $wiz in $x, or undef if $wiz
325 # did not attach any.
326 my $data = getdata $x, $wiz;
329 dispell [$@%&*]variable, $wiz
331 The exact opposite of "cast" : it dissociates $wiz magic from the
332 variable. This function returns true on success, 0 when no magic
333 represented by $wiz could be found in the variable, and croaks if the
334 supplied wizard is invalid.
337 die 'no such magic in $x' unless dispell $x, $wiz;
341 Evaluates to true if and only if the *copy* magic is available. This is
342 the case for perl 5.7.3 and greater, which is ensured by the
343 requirements of this module.
346 Evaluates to true if and only if the *dup* magic is available. This is
347 the case for perl 5.7.3 and greater, which is ensured by the
348 requirements of this module.
351 Evaluates to true if and only if the *local* magic is available. This is
352 the case for perl 5.9.3 and greater.
355 When this constant is true, you can use the *fetch*, *store*, *exists*
356 and *delete* magics on hashes. Initial "VMG_UVAR" capability was
357 introduced in perl 5.9.5, with a fully functional implementation shipped
360 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
361 True for perls that don't call *len* magic when taking the "length" of a
364 "VMG_COMPAT_SCALAR_NOLEN"
365 True for perls that don't call *len* magic on scalars. Implies
366 "VMG_COMPAT_SCALAR_LENGTH_NOLEN".
368 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
369 True for perls that don't call *len* magic when you push an element in a
370 magical array. Starting from perl 5.11.0, this only refers to pushes in
371 non-void context and hence is false.
373 "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
374 True for perls that don't call *len* magic when you push in void context
375 an element in a magical array.
377 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
378 True for perls that don't call *len* magic when you unshift in void
379 context an element in a magical array.
381 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
382 True for perls that call *clear* magic when undefining magical arrays.
384 "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
385 True for perls that don't call *delete* magic when you delete an element
386 from a hash in void context.
388 "VMG_COMPAT_GLOB_GET"
389 True for perls that call *get* magic for operations on globs.
391 "VMG_PERL_PATCHLEVEL"
392 The perl patchlevel this module was built with, or 0 for non-debugging
396 True if and only if this module could have been built with thread-safety
400 True if and only if this module could have been built with fork-safety
401 features enabled. This is always true except on Windows where it is
402 false for perl 5.10.0 and below.
405 Value to pass with "op_info" to get the current op name in the magic
409 Value to pass with "op_info" to get a "B::OP" object representing the
410 current op in the magic callbacks.
413 Associate an object to any perl variable
414 This technique can be useful for passing user data through limited APIs.
415 It is similar to using inside-out objects, but without the drawback of
416 having to implement a complex destructor.
419 package Magical::UserData;
421 use Variable::Magic qw<wizard cast getdata>;
423 my $wiz = wizard data => sub { \$_[1] };
425 sub ud (\[$@%*&]) : lvalue {
427 my $data = &getdata($var, $wiz);
428 unless (defined $data) {
430 &cast($var, $wiz, $slot)
431 or die "Couldn't cast UserData magic onto the variable";
438 BEGIN { *ud = \&Magical::UserData::ud }
441 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
444 $cb->(); # Hello, world!
447 Recursively cast magic on datastructures
448 "cast" can be called from any magical callback, and in particular from
449 "data". This allows you to recursively cast magic on datastructures :
452 $wiz = wizard data => sub {
453 my ($var, $depth) = @_;
457 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
458 } elsif ($r eq 'HASH') {
459 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
464 my ($var, $depth) = @_;
466 print "free $r at depth $depth\n";
478 When %h goes out of scope, this prints something among the lines of :
482 free SCALAR at depth 2
483 free ARRAY at depth 1
484 free SCALAR at depth 3
485 free SCALAR at depth 3
487 Of course, this example does nothing with the values that are added
491 The places where magic is invoked have changed a bit through perl
492 history. Here is a little list of the most recent ones.
496 *p14416* : *copy* and *dup* magic.
500 *p28160* : Integration of *p25854* (see below).
502 *p32542* : Integration of *p31473* (see below).
506 *p25854* : *len* magic is no longer called when pushing an element
509 *p26569* : *local* magic.
513 *p31064* : Meaningful *uvar* magic.
515 *p31473* : *clear* magic was not invoked when undefining an array.
516 The bug is fixed as of this version.
520 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
521 *copy* magic on hash stores for (non-tied) hashes that also have
526 *p32969* : *len* magic is no longer invoked when calling "length"
527 with a magical scalar.
529 *p34908* : *len* magic is no longer called when pushing / unshifting
530 an element into a magical array in void context. The "push" part was
531 already covered by *p25854*.
533 *g9cdcb38b* : *len* magic is called again when pushing into a
534 magical array in non-void context.
537 The functions "wizard", "cast", "getdata" and "dispell" are only
538 exported on request. All of them are exported by the tags ':funcs' and
541 All the constants are also only exported on request, either individually
542 or by the tags ':consts' and ':all'.
545 In order to hook hash operations with magic, you need at least perl
546 5.10.0 (see "VMG_UVAR").
548 If you want to store a magic object in the private data slot, you will
549 not be able to recover the magic with "getdata", since magic is not
550 copied by assignment. You can work around this gotcha by storing a
551 reference to the magic object instead.
553 If you define a wizard with *free* magic and cast it on itself, it
554 results in a memory cycle, so this destructor will not be called when
560 A C compiler. This module may happen to build with a C++ compiler as
561 well, but don't rely on it, as no guarantee is made in this regard.
563 Carp (core since perl 5), XSLoader (since 5.6.0).
565 Copy tests need Tie::Array (core since perl 5.005) and Tie::Hash (since
566 5.002). Some uvar tests need Hash::Util::FieldHash (since 5.9.4). Glob
567 tests need Symbol (since 5.002). Threads tests need threads and
568 threads::shared (both since 5.7.3).
571 perlguts and perlapi for internal information about magic.
573 perltie and overload for other ways of enhancing objects.
576 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
578 You can contact me by mail or on "irc.perl.org" (vincent).
581 Please report any bugs or feature requests to "bug-variable-magic at
582 rt.cpan.org", or through the web interface at
583 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
584 be notified, and then you'll automatically be notified of progress on
585 your bug as I make changes.
588 You can find documentation for this module with the perldoc command.
590 perldoc Variable::Magic
592 Tests code coverage report is available at
593 <http://www.profvince.com/perl/cover/Variable-Magic>.
596 Copyright 2007,2008,2009,2010,2011,2012,2013 Vincent Pit, all rights
599 This program is free software; you can redistribute it and/or modify it
600 under the same terms as Perl itself.