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 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 scalars and arrays, and is triggered when
104 the 'size' or the 'length' of the variable has to be known by Perl.
105 This is typically the magic involved when an array is evaluated in
106 scalar context, but also on array assignment and loops ("for", "map"
107 or "grep"). The length is returned from the callback as an integer.
111 This magic is invoked when the variable is reset, such as when an
112 array is emptied. Please note that this is different from undefining
113 the variable, even though the magic is called when the clearing is a
114 result of the undefine (e.g. for an array, but actually a bug
115 prevent it to work before perl 5.9.5 - see the history).
119 This magic is called when a variable is destroyed as the result of
120 going out of scope (but not when it is undefined). It behaves
121 roughly like Perl object destructors (i.e. "DESTROY" methods),
122 except that exceptions thrown from inside a *free* callback will
123 always be propagated to the surrounding code.
127 This magic only applies to tied arrays and hashes, and fires when
128 you try to access or change their elements.
132 This magic is invoked when the variable is cloned across threads. It
133 is currently not available.
137 When this magic is set on a variable, all subsequent localizations
138 of the variable will trigger the callback. It is available on your
139 perl if and only if "MGf_LOCAL" is true.
141 The following actions only apply to hashes and are available if and only
142 if "VMG_UVAR" is true. They are referred to as *uvar* magics.
146 This magic is invoked each time an element is fetched from the hash.
150 This one is called when an element is stored into the hash.
154 This magic fires when a key is tested for existence in the hash.
158 This magic is triggered when a key is deleted in the hash,
159 regardless of whether the key actually exists in it.
161 You can refer to the tests to have more insight of where the different
168 get => sub { my ($ref, $data [, $op]) = @_; ... },
169 set => sub { my ($ref, $data [, $op]) = @_; ... },
171 my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
173 clear => sub { my ($ref, $data [, $op]) = @_; ... },
174 free => sub { my ($ref, $data [, $op]) = @_, ... },
175 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
176 local => sub { my ($ref, $data [, $op]) = @_; ... },
177 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
178 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
179 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
180 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
182 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
185 This function creates a 'wizard', an opaque object that holds the magic
186 information. It takes a list of keys / values as argument, whose keys
191 A code (or string) reference to a private data constructor. It is
192 called in scalar context each time the magic is cast onto a
193 variable, with $_[0] being a reference to this variable and @_[1 ..
194 @_-1] being all extra arguments that were passed to "cast". The
195 scalar returned from this call is then attached to the variable and
196 can be retrieved later with "getdata".
198 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
199 "store", "exists" and "delete"
201 Code (or string) references to the respective magic callbacks. You
202 don't have to specify all of them : the magic corresponding to
203 undefined entries will simply not be hooked.
205 When those callbacks are executed, $_[0] is a reference to the magic
206 variable and $_[1] is the associated private data (or "undef" when
207 no private data constructor is supplied with the wizard). Other
208 arguments depend on which kind of magic is involved :
212 $_[2] contains the natural, non-magical length of the
213 variable (which can only be a scalar or an array as *len*
214 magic is only relevant for these types). The callback is
215 expected to return the new scalar or array length to use, or
216 "undef" to default to the normal length.
220 $_[2] is a either an alias or a copy of the current key, and
221 $_[3] is an alias to the current element (i.e. the value).
222 Because $_[2] might be a copy, it is useless to try to
223 change it or cast magic on it.
225 * *fetch*, *store*, *exists* and *delete*
227 $_[2] is an alias to the current key. Note that $_[2] may
228 rightfully be readonly if the key comes from a bareword, and
229 as such it is unsafe to assign to it. You can ask for a copy
230 instead by passing "copy_key => 1" to "wizard" which, at the
231 price of a small performance hit, allows you to safely
232 assign to $_[2] in order to e.g. redirect the action to
235 Finally, if "op_info => $num" is also passed to "wizard", then one
236 extra element is appended to @_. Its nature depends on the value of
241 $_[-1] is the current op name.
243 * "VMG_OP_INFO_OBJECT"
245 $_[-1] is the "B::OP" object for the current op.
247 Both result in a small performance hit, but just getting the name is
248 lighter than getting the op object.
250 These callbacks are executed in scalar context and are expected to
251 return an integer, which is then passed straight to the perl magic
252 API. However, only the return value of the *len* magic callback
253 currently holds a meaning.
255 Each callback can be specified as :
257 * a code reference, which will be called as a subroutine.
259 * a string reference, where the string denotes which subroutine is to
260 be called when magic is triggered. If the subroutine name is not
261 fully qualified, then the current package at the time the magic is
262 invoked will be used instead.
264 * a reference to "undef", in which case a no-op magic callback is
265 installed instead of the default one. This may especially be helpful
266 for *local* magic, where an empty callback prevents magic from being
267 copied during localization.
269 Note that *free* magic is never called during global destruction, as
270 there is no way to ensure that the wizard object and the callback were
271 not destroyed before the variable.
273 Here is a simple usage example :
275 # A simple scalar tracer
277 get => sub { print STDERR "got ${$_[0]}\n" },
278 set => sub { print STDERR "set to ${$_[0]}\n" },
279 free => sub { print STDERR "${$_[0]} was deleted\n" },
283 cast [$@%&*]var, $wiz, @args
285 This function associates $wiz magic to the supplied variable, without
286 overwriting any other kind of magic. It returns true on success or when
287 $wiz magic is already attached, and croaks on error. When $wiz provides
288 a data constructor, it is called just before magic is cast onto the
289 variable, and it receives a reference to the target variable in $_[0]
290 and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.
292 # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
296 The "var" argument can be an array or hash value. Magic for these
297 scalars behaves like for any other, except that it is dispelled when the
298 entry is deleted from the container. For example, if you want to call
299 "POSIX::tzset" each time the 'TZ' environment variable is changed in
303 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
305 If you want to handle the possible deletion of the 'TZ' entry, you must
306 also specify *store* magic.
309 getdata [$@%&*]var, $wiz
311 This accessor fetches the private data associated with the magic $wiz in
312 the variable. It croaks when $wiz does not represent a valid magic
313 object, and returns an empty list if no such magic is attached to the
314 variable or when the wizard has no data constructor.
316 # Get the data attached to $wiz in $x, or undef if $wiz
317 # did not attach any.
318 my $data = getdata $x, $wiz;
321 dispell [$@%&*]variable, $wiz
323 The exact opposite of "cast" : it dissociates $wiz magic from the
324 variable. This function returns true on success, 0 when no magic
325 represented by $wiz could be found in the variable, and croaks if the
326 supplied wizard is invalid.
329 die 'no such magic in $x' unless dispell $x, $wiz;
333 Evaluates to true if and only if the *copy* magic is available. This is
334 the case for perl 5.7.3 and greater, which is ensured by the
335 requirements of this module.
338 Evaluates to true if and only if the *dup* magic is available. This is
339 the case for perl 5.7.3 and greater, which is ensured by the
340 requirements of this module.
343 Evaluates to true if and only if the *local* magic is available. This is
344 the case for perl 5.9.3 and greater.
347 When this constant is true, you can use the *fetch*, *store*, *exists*
348 and *delete* magics on hashes. Initial "VMG_UVAR" capability was
349 introduced in perl 5.9.5, with a fully functional implementation shipped
352 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
353 True for perls that don't call *len* magic when taking the "length" of a
356 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
357 True for perls that don't call *len* magic when you push an element in a
358 magical array. Starting from perl 5.11.0, this only refers to pushes in
359 non-void context and hence is false.
361 "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
362 True for perls that don't call *len* magic when you push in void context
363 an element in a magical array.
365 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
366 True for perls that don't call *len* magic when you unshift in void
367 context an element in a magical array.
369 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
370 True for perls that call *clear* magic when undefining magical arrays.
372 "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
373 True for perls that don't call *delete* magic when you delete an element
374 from a hash in void context.
376 "VMG_COMPAT_GLOB_GET"
377 True for perls that call *get* magic for operations on globs.
379 "VMG_PERL_PATCHLEVEL"
380 The perl patchlevel this module was built with, or 0 for non-debugging
384 True if and only if this module could have been built with thread-safety
388 True if and only if this module could have been built with fork-safety
389 features enabled. This is always true except on Windows where it is
390 false for perl 5.10.0 and below.
393 Value to pass with "op_info" to get the current op name in the magic
397 Value to pass with "op_info" to get a "B::OP" object representing the
398 current op in the magic callbacks.
401 Associate an object to any perl variable
402 This technique can be useful for passing user data through limited APIs.
403 It is similar to using inside-out objects, but without the drawback of
404 having to implement a complex destructor.
407 package Magical::UserData;
409 use Variable::Magic qw<wizard cast getdata>;
411 my $wiz = wizard data => sub { \$_[1] };
413 sub ud (\[$@%*&]) : lvalue {
415 my $data = &getdata($var, $wiz);
416 unless (defined $data) {
418 &cast($var, $wiz, $slot)
419 or die "Couldn't cast UserData magic onto the variable";
426 BEGIN { *ud = \&Magical::UserData::ud }
429 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
432 $cb->(); # Hello, world!
435 Recursively cast magic on datastructures
436 "cast" can be called from any magical callback, and in particular from
437 "data". This allows you to recursively cast magic on datastructures :
440 $wiz = wizard data => sub {
441 my ($var, $depth) = @_;
445 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
446 } elsif ($r eq 'HASH') {
447 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
452 my ($var, $depth) = @_;
454 print "free $r at depth $depth\n";
466 When %h goes out of scope, this prints something among the lines of :
470 free SCALAR at depth 2
471 free ARRAY at depth 1
472 free SCALAR at depth 3
473 free SCALAR at depth 3
475 Of course, this example does nothing with the values that are added
479 The places where magic is invoked have changed a bit through perl
480 history. Here is a little list of the most recent ones.
484 *p14416* : *copy* and *dup* magic.
488 *p28160* : Integration of *p25854* (see below).
490 *p32542* : Integration of *p31473* (see below).
494 *p25854* : *len* magic is no longer called when pushing an element
497 *p26569* : *local* magic.
501 *p31064* : Meaningful *uvar* magic.
503 *p31473* : *clear* magic was not invoked when undefining an array.
504 The bug is fixed as of this version.
508 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
509 *copy* magic on hash stores for (non-tied) hashes that also have
514 *p32969* : *len* magic is no longer invoked when calling "length"
515 with a magical scalar.
517 *p34908* : *len* magic is no longer called when pushing / unshifting
518 an element into a magical array in void context. The "push" part was
519 already covered by *p25854*.
521 *g9cdcb38b* : *len* magic is called again when pushing into a
522 magical array in non-void context.
525 The functions "wizard", "cast", "getdata" and "dispell" are only
526 exported on request. All of them are exported by the tags ':funcs' and
529 All the constants are also only exported on request, either individually
530 or by the tags ':consts' and ':all'.
533 In order to hook hash operations with magic, you need at least perl
534 5.10.0 (see "VMG_UVAR").
536 If you want to store a magic object in the private data slot, you will
537 not be able to recover the magic with "getdata", since magic is not
538 copied by assignment. You can work around this gotcha by storing a
539 reference to the magic object instead.
541 If you define a wizard with *free* magic and cast it on itself, it
542 results in a memory cycle, so this destructor will not be called when
548 A C compiler. This module may happen to build with a C++ compiler as
549 well, but don't rely on it, as no guarantee is made in this regard.
551 Carp (core since perl 5), XSLoader (since 5.006).
553 Copy tests need Tie::Array (core since perl 5.005) and Tie::Hash (since
554 5.002). Some uvar tests need Hash::Util::FieldHash (since 5.009004).
555 Glob tests need Symbol (since 5.002). Threads tests need threads and
556 threads::shared (both since 5.007003).
559 perlguts and perlapi for internal information about magic.
561 perltie and overload for other ways of enhancing objects.
564 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
566 You can contact me by mail or on "irc.perl.org" (vincent).
569 Please report any bugs or feature requests to "bug-variable-magic at
570 rt.cpan.org", or through the web interface at
571 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
572 be notified, and then you'll automatically be notified of progress on
573 your bug as I make changes.
576 You can find documentation for this module with the perldoc command.
578 perldoc Variable::Magic
580 Tests code coverage report is available at
581 <http://www.profvince.com/perl/cover/Variable-Magic>.
584 Copyright 2007,2008,2009,2010,2011,2012 Vincent Pit, all rights
587 This program is free software; you can redistribute it and/or modify it
588 under the same terms as Perl itself.