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.
164 get => sub { my ($ref, $data [, $op]) = @_; ... },
165 set => sub { my ($ref, $data [, $op]) = @_; ... },
166 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
167 clear => sub { my ($ref, $data [, $op]) = @_; ... },
168 free => sub { my ($ref, $data [, $op]) = @_, ... },
169 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
170 local => sub { my ($ref, $data [, $op]) = @_; ... },
171 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
172 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
173 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
174 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
176 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
178 This function creates a 'wizard', an opaque type that holds the magic
179 information. It takes a list of keys / values as argument, whose keys
184 The numerical signature. If not specified or undefined, a random
185 signature is generated. If the signature matches an already defined
186 magic, then the existant magic object is returned.
190 A code reference to a private data constructor. It is called each
191 time this magic is cast on a variable, and the scalar returned is
192 used as private data storage for it. $_[0] is a reference to the
193 magic object and @_[1 .. @_-1] are all extra arguments that were
196 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
197 "store", "exists" and "delete"
199 Code references to the corresponding magic callbacks. You don't have
200 to specify all of them : the magic associated with undefined entries
201 simply won't be hooked. In those callbacks, $_[0] is always a
202 reference to the magic object and $_[1] is always the private data
203 (or "undef" when no private data constructor was supplied).
205 Moreover, when you pass "op_info => $num" to "wizard", the last
206 element of @_ will be the current op name if "$num ==
207 VMG_OP_INFO_NAME" and a "B::OP" object representing the current op
208 if "$num == VMG_OP_INFO_OBJECT". Both have a performance hit, but
209 just getting the name is lighter than getting the op object.
211 Other arguments are specific to the magic hooked :
215 When the variable is an array or a scalar, $_[2] contains
216 the non-magical length. The callback can return the new
217 scalar or array length to use, or "undef" to default to the
222 $_[2] is a either a copy or an alias of the current key,
223 which means that it is useless to try to change or cast
224 magic on it. $_[3] is an alias to the current element (i.e.
227 * "fetch", "store", "exists" and "delete"
229 $_[2] is an alias to the current key. Nothing prevents you
230 from changing it, but be aware that there lurk dangerous
231 side effects. For example, it may rightfully be readonly if
232 the key was a bareword. You can get a copy instead by
233 passing "copy_key => 1" to "wizard", which allows you to
234 safely assign to $_[2] in order to e.g. redirect the action
235 to another key. This however has a little performance
236 drawback because of the copy.
238 All the callbacks are expected to return an integer, which is passed
239 straight to the perl magic API. However, only the return value of
240 the "len" callback currently holds a meaning.
242 # A simple scalar tracer
243 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
244 set => sub { print STDERR "set to ${$_[0]}\n" },
245 free => sub { print STDERR "${$_[0]} was deleted\n" }
247 Note that "free" callbacks are *never* called during global destruction,
248 as there's no way to ensure that the wizard and the "free" callback
249 weren't destroyed before the variable.
252 With this tool, you can manually generate random magic signature between
253 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
254 no signature is supplied.
256 # Generate a signature
262 This accessor returns the magic signature of this wizard.
265 my $sig = getsig $wiz;
268 cast [$@%&*]var, [$wiz|$sig], ...
270 This function associates $wiz magic to the variable supplied, without
271 overwriting any other kind of magic. You can also supply the numeric
272 signature $sig instead of $wiz. It returns true on success or when $wiz
273 magic is already present, and croaks on error or when no magic
274 corresponds to the given signature (in case a $sig was supplied). All
275 extra arguments specified after $wiz are passed to the private data
276 constructor in @_[1 .. @_-1]. If the variable isn't a hash, any "uvar"
277 callback of the wizard is safely ignored.
279 # Casts $wiz onto $x, and pass '1' to the data constructor.
283 The "var" argument can be an array or hash value. Magic for those
284 behaves like for any other scalar, except that it is dispelled when the
285 entry is deleted from the container. For example, if you want to call
286 "POSIX::tzset" each time the 'TZ' environment variable is changed in
290 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
292 If you want to overcome the possible deletion of the 'TZ' entry, you
293 have no choice but to rely on "store" uvar magic.
295 "cast" can be called from any magical callback, and in particular from
296 "data". This allows you to recursively cast magic on datastructures :
301 my ($var, $depth) = @_;
305 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
306 } elsif ($r eq 'HASH') {
307 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
312 my ($var, $depth) = @_;
314 print "free $r at depth $depth\n";
326 When %h goes out of scope, this will print something among the lines of
331 free SCALAR at depth 2
332 free ARRAY at depth 1
333 free SCALAR at depth 3
334 free SCALAR at depth 3
336 Of course, this example does nothing with the values that are added
340 getdata [$@%&*]var, [$wiz|$sig]
342 This accessor fetches the private data associated with the magic $wiz
343 (or the signature $sig) in the variable. It croaks when $wiz or $sig do
344 not represent a valid magic object, and returns an empty list if no such
345 magic is attached to the variable or when the wizard has no data
348 # Get the attached data, or undef if the wizard does not attach any.
349 my $data = getdata $x, $wiz;
352 dispell [$@%&*]variable, [$wiz|$sig]
354 The exact opposite of "cast" : it dissociates $wiz magic from the
355 variable. You can also pass the magic signature $sig as the second
356 argument. This function returns true on success, 0 when no magic
357 represented by $wiz or $sig could be found in the variable, and croaks
358 if the supplied wizard or signature is invalid.
361 die 'no such magic in $x' unless dispell $x, $wiz;
365 The minimum integer used as a signature for user-defined magic.
368 The maximum integer used as a signature for user-defined magic.
371 SIG_NBR = SIG_MAX - SIG_MIN + 1
374 Evaluates to true iff the 'copy' magic is available.
377 Evaluates to true iff the 'dup' magic is available.
380 Evaluates to true iff the 'local' magic is available.
383 When this constant is true, you can use the "fetch,store,exists,delete"
386 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
387 True for perls that don't call 'len' magic when you push an element in a
390 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
391 True for perls that don't call 'len' magic when you unshift in void
392 context an element in a magical array.
394 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
395 True for perls that call 'clear' magic when undefining magical arrays.
397 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
398 True for perls that don't call 'len' magic when taking the "length" of a
401 "VMG_PERL_PATCHLEVEL"
402 The perl patchlevel this module was built with, or 0 for non-debugging
406 True iff this module could have been built with thread-safety features
410 True iff this module could have been built with fork-safety features
411 enabled. This will always be true except on Windows where it's false for
412 perl 5.10.0 and below .
415 Value to pass with "op_info" to get the current op name in the magic
419 Value to pass with "op_info" to get a "B::OP" object representing the
420 current op in the magic callbacks.
423 The places where magic is invoked have changed a bit through perl
424 history. Here's a little list of the most recent ones.
428 *p14416* : 'copy' and 'dup' magic.
432 *p28160* : Integration of *p25854* (see below).
434 *p32542* : Integration of *p31473* (see below).
438 *p25854* : 'len' magic is no longer called when pushing an element
441 *p26569* : 'local' magic.
445 *p31064* : Meaningful 'uvar' magic.
447 *p31473* : 'clear' magic wasn't invoked when undefining an array.
448 The bug is fixed as of this version.
452 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
453 'copy' magic on hash stores for (non-tied) hashes that also have
458 *p32969* : 'len' magic is no longer invoked when calling "length"
459 with a magical scalar.
461 *p34908* : 'len' magic is no longer called when pushing / unshifting
462 an element into a magical array in void context. The "push" part was
463 already covered by *p25854*.
466 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
467 "dispell" are only exported on request. All of them are exported by the
468 tags ':funcs' and ':all'.
470 All the constants are also only exported on request, either individually
471 or by the tags ':consts' and ':all'.
474 If you store a magic object in the private data slot, the magic won't be
475 accessible by "getdata" since it's not copied by assignment. The only
476 way to address this would be to return a reference.
478 If you define a wizard with a "free" callback and cast it on itself,
479 this destructor won't be called because the wizard will be destroyed
485 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
487 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
490 Some uvar tests need Hash::Util::FieldHash (standard since perl
493 Glob tests need Symbol (standard since perl 5.002).
495 Threads tests need threads and threads::shared.
498 perlguts and perlapi for internal information about magic.
500 perltie and overload for other ways of enhancing objects.
503 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
505 You can contact me by mail or on "irc.perl.org" (vincent).
508 Please report any bugs or feature requests to "bug-variable-magic at
509 rt.cpan.org", or through the web interface at
510 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
511 be notified, and then you'll automatically be notified of progress on
512 your bug as I make changes.
515 You can find documentation for this module with the perldoc command.
517 perldoc Variable::Magic
519 Tests code coverage report is available at
520 <http://www.profvince.com/perl/cover/Variable-Magic>.
523 Copyright 2007-2009 Vincent Pit, all rights reserved.
525 This program is free software; you can redistribute it and/or modify it
526 under the same terms as Perl itself.