2 Variable::Magic - Associate user-defined magic to variables from Perl.
8 use Variable::Magic qw/wizard cast VMG_OP_INFO_NAME/;
11 my $wiz = wizard set => sub { print "now set to ${$_[0]}!\n" },
12 free => sub { print "destroyed!\n" };
16 $a = 2; # "now set to 2!"
19 { # A hash with a default value
20 my $wiz = wizard data => sub { $_[1] },
21 fetch => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () },
22 store => sub { print "key $_[2] stored in $_[-1]\n" },
24 op_info => VMG_OP_INFO_NAME;
26 my %h = (_default => 0, apple => 2);
27 cast %h, $wiz, '_default';
28 print $h{banana}, "\n"; # "0", because the 'banana' key doesn't exist in %h
29 $h{pear} = 1; # "key pear stored in helem"
33 Magic is Perl way of enhancing objects. This mechanism lets the user add
34 extra data to any variable and hook syntaxical operations (such as
35 access, assignment or destruction) that can be applied to it. With this
36 module, you can add your own magic to any variable without having to
37 write a single line of XS.
39 You'll realize that these magic variables look a lot like tied
40 variables. It's not surprising, as tied variables are implemented as a
41 special kind of magic, just like any 'irregular' Perl variable : scalars
42 like $!, $( or $^W, the %ENV and %SIG hashes, the @ISA array, "vec()"
43 and "substr()" lvalues, threads::shared variables... They all share the
44 same underlying C API, and this module gives you direct access to it.
46 Still, the magic made available by this module differs from tieing and
47 overloading in several ways :
49 * It isn't copied on assignment.
51 You attach it to variables, not values (as for blessed references).
53 * It doesn't replace the original semantics.
55 Magic callbacks usually trigger before the original action take
56 place, and can't prevent it to happen. This also makes catching
57 individual events easier than with "tie", where you have to provide
58 fallbacks methods for all actions by usually inheriting from the
59 correct "Tie::Std*" class and overriding individual methods in your
64 The same magic can be applied on scalars, arrays, hashes, subs or
65 globs. But the same hook (see below for a list) may trigger
66 differently depending on the the type of the variable.
68 * It's mostly invisible at the Perl level.
70 Magical and non-magical variables cannot be distinguished with
71 "ref", "tied" or another trick.
73 * It's notably faster.
75 Mainly because perl's way of handling magic is lighter by nature,
76 and because there's no need for any method resolution. Also, since
77 you don't have to reimplement all the variable semantics, you only
78 pay for what you actually use.
80 The operations that can be overloaded are :
84 This magic is invoked when the variable is evaluated. It is never
85 called for arrays and hashes.
89 This one is triggered each time the value of the variable changes.
90 It is called for array subscripts and slices, but never for hashes.
94 This magic is a little special : it is called when the 'size' or the
95 'length' of the variable has to be known by Perl. Typically, it's
96 the magic involved when an array is evaluated in scalar context, but
97 also on array assignment and loops ("for", "map" or "grep"). The
98 callback has then to return the length as an integer.
102 This magic is invoked when the variable is reset, such as when an
103 array is emptied. Please note that this is different from undefining
104 the variable, even though the magic is called when the clearing is a
105 result of the undefine (e.g. for an array, but actually a bug
106 prevent it to work before perl 5.9.5 - see the history).
110 This one can be considered as an object destructor. It happens when
111 the variable goes out of scope, but not when it is undefined.
115 This magic only applies to tied arrays and hashes. It fires when you
116 try to access or change their elements. It is available on your perl
117 iff "MGf_COPY" is true.
121 Invoked when the variable is cloned across threads. Currently not
126 When this magic is set on a variable, all subsequent localizations
127 of the variable will trigger the callback. It is available on your
128 perl iff "MGf_LOCAL" is true.
130 The following actions only apply to hashes and are available iff
131 "VMG_UVAR" is true. They are referred to as "uvar" magics.
135 This magic happens each time an element is fetched from the hash.
139 This one is called when an element is stored into the hash.
143 This magic fires when a key is tested for existence in the hash.
147 This last one triggers when a key is deleted in the hash, regardless
148 of whether the key actually exists in it.
150 You can refer to the tests to have more insight of where the different
153 To prevent any clash between different magics defined with this module,
154 an unique numerical signature is attached to each kind of magic (i.e.
155 each set of callbacks for magic operations). At the C level, magic
156 tokens owned by magic created by this module have their "mg->mg_private"
157 field set to 0x3891 or 0x3892, so please don't use these magic (sic)
158 numbers in other extensions.
162 wizard data => sub { ... },
163 get => sub { my ($ref, $data [, $op]) = @_; ... },
164 set => sub { my ($ref, $data [, $op]) = @_; ... },
165 len => sub { my ($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 ]
177 This function creates a 'wizard', an opaque type that holds the magic
178 information. It takes a list of keys / values as argument, whose keys
183 The numerical signature. If not specified or undefined, a random
184 signature is generated. If the signature matches an already defined
185 magic, then the existant magic object is returned.
187 This option is deprecated and will be removed in december 2009.
191 A code reference to a private data constructor. It is called each
192 time this magic is cast on a variable, and the scalar returned is
193 used as private data storage for it. $_[0] is a reference to the
194 magic object and @_[1 .. @_-1] are all extra arguments that were
197 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
198 "store", "exists" and "delete"
200 Code references to the corresponding magic callbacks. You don't have
201 to specify all of them : the magic associated with undefined entries
202 simply won't be hooked. In those callbacks, $_[0] is always a
203 reference to the magic object and $_[1] is always the private data
204 (or "undef" when no private data constructor was supplied).
206 Moreover, when you pass "op_info => $num" to "wizard", the last
207 element of @_ will be the current op name if "$num ==
208 VMG_OP_INFO_NAME" and a "B::OP" object representing the current op
209 if "$num == VMG_OP_INFO_OBJECT". Both have a performance hit, but
210 just getting the name is lighter than getting the op object.
212 Other arguments are specific to the magic hooked :
216 When the variable is an array or a scalar, $_[2] contains
217 the non-magical length. The callback can return the new
218 scalar or array length to use, or "undef" to default to the
223 $_[2] is a either a copy or an alias of the current key,
224 which means that it is useless to try to change or cast
225 magic on it. $_[3] is an alias to the current element (i.e.
228 * "fetch", "store", "exists" and "delete"
230 $_[2] is an alias to the current key. Nothing prevents you
231 from changing it, but be aware that there lurk dangerous
232 side effects. For example, it may rightfully be readonly if
233 the key was a bareword. You can get a copy instead by
234 passing "copy_key => 1" to "wizard", which allows you to
235 safely assign to $_[2] in order to e.g. redirect the action
236 to another key. This however has a little performance
237 drawback because of the copy.
239 All the callbacks are expected to return an integer, which is passed
240 straight to the perl magic API. However, only the return value of
241 the "len" callback currently holds a meaning.
243 # A simple scalar tracer
244 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
245 set => sub { print STDERR "set to ${$_[0]}\n" },
246 free => sub { print STDERR "${$_[0]} was deleted\n" }
248 Note that "free" callbacks are *never* called during global destruction,
249 as there's no way to ensure that the wizard and the "free" callback
250 weren't destroyed before the variable.
253 With this tool, you can manually generate random magic signature between
254 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
255 no signature is supplied.
257 # Generate a signature
260 This function is deprecated and will be removed in december 2009.
265 This accessor returns the magic signature of this wizard.
268 my $sig = getsig $wiz;
270 This function is deprecated and will be removed in december 2009.
273 cast [$@%&*]var, $wiz, ...
275 This function associates $wiz magic to the variable supplied, without
276 overwriting any other kind of magic. It returns true on success or when
277 $wiz magic is already present, and croaks on error. All extra arguments
278 specified after $wiz are passed to the private data constructor in @_[1
279 .. @_-1]. If the variable isn't a hash, any "uvar" callback of the
280 wizard is safely ignored.
282 # Casts $wiz onto $x, and pass '1' to the data constructor.
286 The "var" argument can be an array or hash value. Magic for those
287 behaves like for any other scalar, except that it is dispelled when the
288 entry is deleted from the container. For example, if you want to call
289 "POSIX::tzset" each time the 'TZ' environment variable is changed in
293 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
295 If you want to overcome the possible deletion of the 'TZ' entry, you
296 have no choice but to rely on "store" uvar magic.
299 getdata [$@%&*]var, $wiz
301 This accessor fetches the private data associated with the magic $wiz in
302 the variable. It croaks when $wiz do not represent a valid magic object,
303 and returns an empty list if no such magic is attached to the variable
304 or when the wizard has no data constructor.
306 # Get the attached data, or undef if the wizard does not attach any.
307 my $data = getdata $x, $wiz;
310 dispell [$@%&*]variable, $wiz
312 The exact opposite of "cast" : it dissociates $wiz magic from the
313 variable. This function returns true on success, 0 when no magic
314 represented by $wiz could be found in the variable, and croaks if the
315 supplied wizard is invalid.
318 die 'no such magic in $x' unless dispell $x, $wiz;
322 The minimum integer used as a signature for user-defined magic.
324 This constant is deprecated and will be removed in december 2009.
327 The maximum integer used as a signature for user-defined magic.
329 This constant is deprecated and will be removed in december 2009.
332 SIG_NBR = SIG_MAX - SIG_MIN + 1
334 This constant is deprecated and will be removed in december 2009.
337 Evaluates to true iff the 'copy' magic is available.
340 Evaluates to true iff the 'dup' magic is available.
343 Evaluates to true iff the 'local' magic is available.
346 When this constant is true, you can use the "fetch,store,exists,delete"
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_SCALAR_LENGTH_NOLEN"
366 True for perls that don't call 'len' magic when taking the "length" of a
369 "VMG_PERL_PATCHLEVEL"
370 The perl patchlevel this module was built with, or 0 for non-debugging
374 True iff this module could have been built with thread-safety features
378 True iff this module could have been built with fork-safety features
379 enabled. This will always be true except on Windows where it's false for
380 perl 5.10.0 and below .
383 Value to pass with "op_info" to get the current op name in the magic
387 Value to pass with "op_info" to get a "B::OP" object representing the
388 current op in the magic callbacks.
391 Associate an object to any perl variable
392 This can be useful for passing user data through limited APIs.
395 package Magical::UserData;
397 use Variable::Magic qw/wizard cast getdata/;
399 my $wiz = wizard data => sub { \$_[1] };
401 sub ud (\[$@%*&]) : lvalue {
403 my $data = &getdata($var, $wiz);
404 unless (defined $data) {
406 $data = &getdata($var, $wiz);
407 die "Couldn't cast UserData magic onto the variable" unless defined $data;
414 BEGIN { *ud = \&Magical::UserData::ud }
417 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
420 $cb->(); # Hello, world!
423 Recursively cast magic on datastructures
424 "cast" can be called from any magical callback, and in particular from
425 "data". This allows you to recursively cast magic on datastructures :
428 $wiz = wizard data => sub {
429 my ($var, $depth) = @_;
433 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
434 } elsif ($r eq 'HASH') {
435 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
440 my ($var, $depth) = @_;
442 print "free $r at depth $depth\n";
454 When %h goes out of scope, this will print something among the lines of
459 free SCALAR at depth 2
460 free ARRAY at depth 1
461 free SCALAR at depth 3
462 free SCALAR at depth 3
464 Of course, this example does nothing with the values that are added
468 The places where magic is invoked have changed a bit through perl
469 history. Here's a little list of the most recent ones.
473 *p14416* : 'copy' and 'dup' magic.
477 *p28160* : Integration of *p25854* (see below).
479 *p32542* : Integration of *p31473* (see below).
483 *p25854* : 'len' magic is no longer called when pushing an element
486 *p26569* : 'local' magic.
490 *p31064* : Meaningful 'uvar' magic.
492 *p31473* : 'clear' magic wasn't invoked when undefining an array.
493 The bug is fixed as of this version.
497 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
498 'copy' magic on hash stores for (non-tied) hashes that also have
503 *p32969* : 'len' magic is no longer invoked when calling "length"
504 with a magical scalar.
506 *p34908* : 'len' magic is no longer called when pushing / unshifting
507 an element into a magical array in void context. The "push" part was
508 already covered by *p25854*.
510 *g9cdcb38b* : 'len' magic is called again when pushing into a
511 magical array in non-void context.
514 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
515 "dispell" are only exported on request. All of them are exported by the
516 tags ':funcs' and ':all'.
518 All the constants are also only exported on request, either individually
519 or by the tags ':consts' and ':all'.
522 If you store a magic object in the private data slot, the magic won't be
523 accessible by "getdata" since it's not copied by assignment. The only
524 way to address this would be to return a reference.
526 If you define a wizard with a "free" callback and cast it on itself,
527 this destructor won't be called because the wizard will be destroyed
533 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
535 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
538 Some uvar tests need Hash::Util::FieldHash (standard since perl
541 Glob tests need Symbol (standard since perl 5.002).
543 Threads tests need threads and threads::shared.
546 perlguts and perlapi for internal information about magic.
548 perltie and overload for other ways of enhancing objects.
551 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
553 You can contact me by mail or on "irc.perl.org" (vincent).
556 Please report any bugs or feature requests to "bug-variable-magic at
557 rt.cpan.org", or through the web interface at
558 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
559 be notified, and then you'll automatically be notified of progress on
560 your bug as I make changes.
563 You can find documentation for this module with the perldoc command.
565 perldoc Variable::Magic
567 Tests code coverage report is available at
568 <http://www.profvince.com/perl/cover/Variable-Magic>.
571 Copyright 2007-2009 Vincent Pit, all rights reserved.
573 This program is free software; you can redistribute it and/or modify it
574 under the same terms as Perl itself.