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 type-agnostic.
68 The same magic can be applied on scalars, arrays, hashes, subs or
69 globs. But the same hook (see below for a list) may trigger
70 differently depending on the the type of the variable.
72 * Magic is invisible at the Perl level.
74 Magical and non-magical variables cannot be distinguished with
75 "ref", "tied" or another trick.
77 * Magic is notably faster.
79 Mainly because perl's way of handling magic is lighter by nature,
80 and because there is no need for any method resolution. Also, since
81 you don't have to reimplement all the variable semantics, you only
82 pay for what you actually use.
84 The operations that can be overloaded are :
88 This magic is invoked when the variable is evaluated. It is never
89 called for arrays and hashes.
93 This magic is called each time the value of the variable changes. It
94 is called for array subscripts and slices, but never for hashes.
98 This magic only applies to scalars and arrays, and is triggered when
99 the 'size' or the 'length' of the variable has to be known by Perl.
100 This is typically the magic involved when an array is evaluated in
101 scalar context, but also on array assignment and loops ("for", "map"
102 or "grep"). The length is returned from the callback as an integer.
106 This magic is invoked when the variable is reset, such as when an
107 array is emptied. Please note that this is different from undefining
108 the variable, even though the magic is called when the clearing is a
109 result of the undefine (e.g. for an array, but actually a bug
110 prevent it to work before perl 5.9.5 - see the history).
114 This magic is called when an object is destroyed as the result of
115 the variable going out of scope (but not when the variable is
120 This magic only applies to tied arrays and hashes, and fires when
121 you try to access or change their elements.
125 This magic is invoked when the variable is cloned across threads. It
126 is currently not available.
130 When this magic is set on a variable, all subsequent localizations
131 of the variable will trigger the callback. It is available on your
132 perl if and only if "MGf_LOCAL" is true.
134 The following actions only apply to hashes and are available if and only
135 if "VMG_UVAR" is true. They are referred to as *uvar* magics.
139 This magic is invoked each time an element is fetched from the hash.
143 This one is called when an element is stored into the hash.
147 This magic fires when a key is tested for existence in the hash.
151 This magic is triggered when a key is deleted in the hash,
152 regardless of whether the key actually exists in it.
154 You can refer to the tests to have more insight of where the different
161 get => sub { my ($ref, $data [, $op]) = @_; ... },
162 set => sub { my ($ref, $data [, $op]) = @_; ... },
164 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
166 clear => sub { my ($ref, $data [, $op]) = @_; ... },
167 free => sub { my ($ref, $data [, $op]) = @_, ... },
168 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
169 local => sub { my ($ref, $data [, $op]) = @_; ... },
170 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
171 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
172 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
173 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
175 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
178 This function creates a 'wizard', an opaque object that holds the magic
179 information. It takes a list of keys / values as argument, whose keys
184 A code (or string) reference to a private data constructor. It is
185 called in scalar context each time the magic is cast onto a
186 variable, with $_[0] being a reference to this variable and @_[1 ..
187 @_-1] being all extra arguments that were passed to "cast". The
188 scalar returned from this call is then attached to the variable and
189 can be retrieved later with "getdata".
191 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
192 "store", "exists" and "delete"
194 Code (or string) references to the respective magic callbacks. You
195 don't have to specify all of them : the magic corresponding to
196 undefined entries will simply not be hooked.
198 When those callbacks are executed, $_[0] is a reference to the magic
199 variable and $_[1] is the associated private data (or "undef" when
200 no private data constructor is supplied with the wizard). Other
201 arguments depend on which kind of magic is involved :
205 $_[2] contains the natural, non-magical length of the
206 variable (which can only be a scalar or an array as *len*
207 magic is only relevant for these types). The callback is
208 expected to return the new scalar or array length to use, or
209 "undef" to default to the normal length.
213 $_[2] is a either an alias or a copy of the current key, and
214 $_[3] is an alias to the current element (i.e. the value).
215 Because $_[2] might be a copy, it is useless to try to
216 change it or cast magic on it.
218 * *fetch*, *store*, *exists* and *delete*
220 $_[2] is an alias to the current key. Note that $_[2] may
221 rightfully be readonly if the key comes from a bareword, and
222 as such it is unsafe to assign to it. You can ask for a copy
223 instead by passing "copy_key => 1" to "wizard" which, at the
224 price of a small performance hit, allows you to safely
225 assign to $_[2] in order to e.g. redirect the action to
228 Finally, if "op_info => $num" is also passed to "wizard", then one
229 extra element is appended to @_. Its nature depends on the value of
234 $_[-1] is the current op name.
236 * "VMG_OP_INFO_OBJECT"
238 $_[-1] is the "B::OP" object for the current op.
240 Both result in a small performance hit, but just getting the name is
241 lighter than getting the op object.
243 These callbacks are executed in scalar context and are expected to
244 return an integer, which is then passed straight to the perl magic
245 API. However, only the return value of the *len* magic callback
246 currently holds a meaning.
248 Each callback can be specified as :
250 * a code reference, which will be called as a subroutine.
252 * a string reference, where the string denotes which subroutine is to
253 be called when magic is triggered. If the subroutine name is not
254 fully qualified, then the current package at the time the magic is
255 invoked will be used instead.
257 * a reference to "undef", in which case a no-op magic callback is
258 installed instead of the default one. This may especially be helpful
259 for *local* magic, where an empty callback prevents magic from being
260 copied during localization.
262 Note that *free* magic is never called during global destruction, as
263 there is no way to ensure that the wizard object and the callback were
264 not destroyed before the variable.
266 Here is a simple usage example :
268 # A simple scalar tracer
270 get => sub { print STDERR "got ${$_[0]}\n" },
271 set => sub { print STDERR "set to ${$_[0]}\n" },
272 free => sub { print STDERR "${$_[0]} was deleted\n" },
276 cast [$@%&*]var, $wiz, @args
278 This function associates $wiz magic to the supplied variable, without
279 overwriting any other kind of magic. It returns true on success or when
280 $wiz magic is already attached, and croaks on error. When $wiz provides
281 a data constructor, it is called just before magic is cast onto the
282 variable, and it receives a reference to the target variable in $_[0]
283 and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.
285 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
289 The "var" argument can be an array or hash value. Magic for these
290 scalars behaves like for any other, except that it is dispelled when the
291 entry is deleted from the container. For example, if you want to call
292 "POSIX::tzset" each time the 'TZ' environment variable is changed in
296 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
298 If you want to handle the possible deletion of the 'TZ' entry, you must
299 also specify *store* magic.
302 getdata [$@%&*]var, $wiz
304 This accessor fetches the private data associated with the magic $wiz in
305 the variable. It croaks when $wiz does not represent a valid magic
306 object, and returns an empty list if no such magic is attached to the
307 variable or when the wizard has no data constructor.
309 # Get the data attached to $wiz in $x, or undef if $wiz
310 # did not attach any.
311 my $data = getdata $x, $wiz;
314 dispell [$@%&*]variable, $wiz
316 The exact opposite of "cast" : it dissociates $wiz magic from the
317 variable. This function returns true on success, 0 when no magic
318 represented by $wiz could be found in the variable, and croaks if the
319 supplied wizard is invalid.
322 die 'no such magic in $x' unless dispell $x, $wiz;
326 Evaluates to true if and only if the *copy* magic is available. This is
327 the case for perl 5.7.3 and greater, which is ensured by the
328 requirements of this module.
331 Evaluates to true if and only if the *dup* magic is available. This is
332 the case for perl 5.7.3 and greater, which is ensured by the
333 requirements of this module.
336 Evaluates to true if and only if the *local* magic is available. This is
337 the case for perl 5.9.3 and greater.
340 When this constant is true, you can use the *fetch*, *store*, *exists*
341 and *delete* magics on hashes. Initial "VMG_UVAR" capability was
342 introduced in perl 5.9.5, with a fully functional implementation shipped
345 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
346 True for perls that don't call *len* magic when taking the "length" of a
349 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
350 True for perls that don't call *len* magic when you push an element in a
351 magical array. Starting from perl 5.11.0, this only refers to pushes in
352 non-void context and hence is false.
354 "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
355 True for perls that don't call *len* magic when you push in void context
356 an element in a magical array.
358 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
359 True for perls that don't call *len* magic when you unshift in void
360 context an element in a magical array.
362 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
363 True for perls that call *clear* magic when undefining magical arrays.
365 "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
366 True for perls that don't call *delete* magic when you delete an element
367 from a hash in void context.
369 "VMG_COMPAT_GLOB_GET"
370 True for perls that call *get* magic for operations on globs.
372 "VMG_PERL_PATCHLEVEL"
373 The perl patchlevel this module was built with, or 0 for non-debugging
377 True if and only if this module could have been built with thread-safety
381 True if and only if this module could have been built with fork-safety
382 features enabled. This is always true except on Windows where it is
383 false for perl 5.10.0 and below.
386 Value to pass with "op_info" to get the current op name in the magic
390 Value to pass with "op_info" to get a "B::OP" object representing the
391 current op in the magic callbacks.
394 Associate an object to any perl variable
395 This technique can be useful for passing user data through limited APIs.
396 It is similar to using inside-out objects, but without the drawback of
397 having to implement a complex destructor.
400 package Magical::UserData;
402 use Variable::Magic qw<wizard cast getdata>;
404 my $wiz = wizard data => sub { \$_[1] };
406 sub ud (\[$@%*&]) : lvalue {
408 my $data = &getdata($var, $wiz);
409 unless (defined $data) {
411 &cast($var, $wiz, $slot)
412 or die "Couldn't cast UserData magic onto the variable";
419 BEGIN { *ud = \&Magical::UserData::ud }
422 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
425 $cb->(); # Hello, world!
428 Recursively cast magic on datastructures
429 "cast" can be called from any magical callback, and in particular from
430 "data". This allows you to recursively cast magic on datastructures :
433 $wiz = wizard data => sub {
434 my ($var, $depth) = @_;
438 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
439 } elsif ($r eq 'HASH') {
440 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
445 my ($var, $depth) = @_;
447 print "free $r at depth $depth\n";
459 When %h goes out of scope, this prints something among the lines of :
463 free SCALAR at depth 2
464 free ARRAY at depth 1
465 free SCALAR at depth 3
466 free SCALAR at depth 3
468 Of course, this example does nothing with the values that are added
472 The places where magic is invoked have changed a bit through perl
473 history. Here is a little list of the most recent ones.
477 *p14416* : *copy* and *dup* magic.
481 *p28160* : Integration of *p25854* (see below).
483 *p32542* : Integration of *p31473* (see below).
487 *p25854* : *len* magic is no longer called when pushing an element
490 *p26569* : *local* magic.
494 *p31064* : Meaningful *uvar* magic.
496 *p31473* : *clear* magic was not invoked when undefining an array.
497 The bug is fixed as of this version.
501 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
502 *copy* magic on hash stores for (non-tied) hashes that also have
507 *p32969* : *len* magic is no longer invoked when calling "length"
508 with a magical scalar.
510 *p34908* : *len* magic is no longer called when pushing / unshifting
511 an element into a magical array in void context. The "push" part was
512 already covered by *p25854*.
514 *g9cdcb38b* : *len* magic is called again when pushing into a
515 magical array in non-void context.
518 The functions "wizard", "cast", "getdata" and "dispell" are only
519 exported on request. All of them are exported by the tags ':funcs' and
522 All the constants are also only exported on request, either individually
523 or by the tags ':consts' and ':all'.
526 In order to hook hash operations with magic, you need at least perl
527 5.10.0 (see "VMG_UVAR").
529 If you want to store a magic object in the private data slot, you will
530 not be able to recover the magic with "getdata", since magic is not
531 copied by assignment. You can work around this gotcha by storing a
532 reference to the magic object instead.
534 If you define a wizard with *free* magic and cast it on itself, it
535 results in a memory cycle, so this destructor will not be called when
541 A C compiler. This module may happen to build with a C++ compiler as
542 well, but don't rely on it, as no guarantee is made in this regard.
544 Carp (core since perl 5), XSLoader (since 5.006).
546 Copy tests need Tie::Array (core since perl 5.005) and Tie::Hash (since
547 5.002). Some uvar tests need Hash::Util::FieldHash (since 5.009004).
548 Glob tests need Symbol (since 5.002). Threads tests need threads and
549 threads::shared (both since 5.007003).
552 perlguts and perlapi for internal information about magic.
554 perltie and overload for other ways of enhancing objects.
557 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
559 You can contact me by mail or on "irc.perl.org" (vincent).
562 Please report any bugs or feature requests to "bug-variable-magic at
563 rt.cpan.org", or through the web interface at
564 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
565 be notified, and then you'll automatically be notified of progress on
566 your bug as I make changes.
569 You can find documentation for this module with the perldoc command.
571 perldoc Variable::Magic
573 Tests code coverage report is available at
574 <http://www.profvince.com/perl/cover/Variable-Magic>.
577 Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights
580 This program is free software; you can redistribute it and/or modify it
581 under the same terms as Perl itself.