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 trigger before the original action take place, and
56 can't prevent it to happen. This makes catching individual events
57 easier than with "tie", where you have to provide fallbacks methods
58 for all actions by usually inheriting from the correct "Tie::Std*"
59 class and overriding individual methods in your own class.
63 The same magic can be applied on scalars, arrays, hashes, subs or
64 globs. But the same hook (see below for a list) may trigger
65 differently depending on the the type of the variable.
67 * It's mostly invisible at the Perl level.
69 Magical and non-magical variables cannot be distinguished with
70 "ref", "tied" or another trick.
72 * It's notably faster.
74 Mainly because perl's way of handling magic is lighter by nature,
75 and because there's no need for any method resolution. Also, since
76 you don't have to reimplement all the variable semantics, you only
77 pay for what you actually use.
79 The operations that can be overloaded are :
83 This magic is invoked when the variable is evaluated (does not
84 include array/hash subscripts and slices).
88 This one is triggered each time the value of the variable changes
89 (includes array/hash subscripts and slices).
93 This magic is a little special : it is called when the 'size' or the
94 'length' of the variable has to be known by Perl. Typically, it's
95 the magic involved when an array is evaluated in scalar context, but
96 also on array assignment and loops ("for", "map" or "grep"). The
97 callback has then to return the length as an integer.
101 This magic is invoked when a container variable is reset, i.e. when
102 an array or a hash is emptied. Please note that this is different
103 from undefining the variable, even though the magic is called when
104 the clearing is a result of the undefine (e.g. for an array, but
105 actually a bug prevent it to work before perl 5.9.5 - see the
110 This one can be considered as an object destructor. It happens when
111 the variable goes out of scope (with the exception of global scope),
112 but not when it is undefined.
116 This magic only applies to tied arrays and hashes. It fires when you
117 try to access or change their elements. It is available on your perl
118 iff "MGf_COPY" is true.
122 Invoked when the variable is cloned across threads. Currently not
127 When this magic is set on a variable, all subsequent localizations
128 of the variable will trigger the callback. It is available on your
129 perl iff "MGf_LOCAL" is true.
131 The following actions only apply to hashes and are available iff
132 "VMG_UVAR" is true. They are referred to as "uvar" magics.
136 This magic happens each time an element is fetched from the hash.
140 This one is called when an element is stored into the hash.
144 This magic fires when a key is tested for existence in the hash.
148 This last one triggers when a key is deleted in the hash, regardless
149 of whether the key actually exists in it.
151 You can refer to the tests to have more insight of where the different
154 To prevent any clash between different magics defined with this module,
155 an unique numerical signature is attached to each kind of magic (i.e.
156 each set of callbacks for magic operations). At the C level, magic
157 tokens owned by magic created by this module have their "mg->mg_private"
158 field set to 0x3891 or 0x3892, so please don't use these magic (sic)
159 numbers in other extensions.
165 get => sub { my ($ref, $data [, $op]) = @_; ... },
166 set => sub { my ($ref, $data [, $op]) = @_; ... },
167 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
168 clear => sub { my ($ref, $data [, $op]) = @_; ... },
169 free => sub { my ($ref, $data [, $op]) = @_, ... },
170 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
171 local => sub { my ($ref, $data [, $op]) = @_; ... },
172 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
173 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
174 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
175 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
177 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
179 This function creates a 'wizard', an opaque type that holds the magic
180 information. It takes a list of keys / values as argument, whose keys
185 The numerical signature. If not specified or undefined, a random
186 signature is generated. If the signature matches an already defined
187 magic, then the existant magic object is returned.
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 righteously 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
263 This accessor returns the magic signature of this wizard.
266 my $sig = getsig $wiz;
269 cast [$@%&*]var, [$wiz|$sig], ...
271 This function associates $wiz magic to the variable supplied, without
272 overwriting any other kind of magic. You can also supply the numeric
273 signature $sig instead of $wiz. It returns true on success or when $wiz
274 magic is already present, 0 on error, and "undef" when no magic
275 corresponds to the given signature (in case $sig was supplied). All
276 extra arguments specified after $wiz are passed to the private data
277 constructor. If the variable isn't a hash, any "uvar" callback of the
278 wizard is safely ignored.
280 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
282 die 'error' unless cast $x, $wiz;
284 The "var" argument can be an array or hash value. Magic for those
285 behaves like for any other scalar, except that it is dispelled when the
286 entry is deleted from the container. For example, if you want to call
287 "POSIX::tzset" each time the 'TZ' environment variable is changed in
291 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
293 If you want to overcome the possible deletion of the 'TZ' entry, you
294 have no choice but to rely on "store" uvar magic.
296 "cast" can be called from any magical callback, and in particular from
297 "data". This allows you to recursively cast magic on datastructures :
302 my ($var, $depth) = @_;
306 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
307 } elsif ($r eq 'HASH') {
308 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
313 my ($var, $depth) = @_;
315 print "free $r at depth $depth\n";
327 When %h goes out of scope, this will print something among the lines of
332 free SCALAR at depth 2
333 free ARRAY at depth 1
334 free SCALAR at depth 3
335 free SCALAR at depth 3
337 Of course, this example does nothing with the values that are added
341 getdata [$@%&*]var, [$wiz|$sig]
343 This accessor fetches the private data associated with the magic $wiz
344 (or the signature $sig) in the variable. "undef" is returned when no
345 such magic or data is found, or when $sig does not represent a current
348 # Get the attached data.
349 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
352 dispell [$@%&*]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. True is returned on success, 0 on error or when no magic
357 represented by $wiz could be found in the variable, and "undef" when no
358 magic corresponds to the given signature (in case $sig was supplied).
360 # Dispell now. If $wiz isn't a signature, undef can't be returned.
361 die 'no such magic or error' 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 Value to pass with "op_info" to get the current op name in the magic
414 Value to pass with "op_info" to get a "B::OP" object representing the
415 current op in the magic callbacks.
418 The places where magic is invoked have changed a bit through perl
419 history. Here's a little list of the most recent ones.
423 *p14416* : 'copy' and 'dup' magic.
427 *p28160* : Integration of *p25854* (see below).
429 *p32542* : Integration of *p31473* (see below).
433 *p25854* : 'len' magic is no longer called when pushing an element
436 *p26569* : 'local' magic.
440 *p31064* : Meaningful 'uvar' magic.
442 *p31473* : 'clear' magic wasn't invoked when undefining an array.
443 The bug is fixed as of this version.
447 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
448 'copy' magic on hash stores for (non-tied) hashes that also have
453 *p32969* : 'len' magic is no longer invoked when calling "length"
454 with a magical scalar.
456 *p34908* : 'len' magic is no longer called when pushing / unshifting
457 an element into a magical array in void context. The "push" part was
458 already covered by *p25854*.
461 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
462 "dispell" are only exported on request. All of them are exported by the
463 tags ':funcs' and ':all'.
465 All the constants are also only exported on request, either individually
466 or by the tags ':consts' and ':all'.
469 If you store a magic object in the private data slot, the magic won't be
470 accessible by "getdata" since it's not copied by assignment. The only
471 way to address this would be to return a reference.
473 If you define a wizard with a "free" callback and cast it on itself,
474 this destructor won't be called because the wizard will be destroyed
480 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
482 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
485 Some uvar tests need Hash::Util::FieldHash (standard since perl
488 Glob tests need Symbol (standard since perl 5.002).
490 Threads tests need threads and threads::shared.
493 perlguts and perlapi for internal information about magic.
495 perltie and overload for other ways of enhancing objects.
498 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
500 You can contact me by mail or on "irc.perl.org" (vincent).
503 Please report any bugs or feature requests to "bug-variable-magic at
504 rt.cpan.org", or through the web interface at
505 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
506 be notified, and then you'll automatically be notified of progress on
507 your bug as I make changes.
510 You can find documentation for this module with the perldoc command.
512 perldoc Variable::Magic
514 Tests code coverage report is available at
515 <http://www.profvince.com/perl/cover/Variable-Magic>.
518 Copyright 2007-2009 Vincent Pit, all rights reserved.
520 This program is free software; you can redistribute it and/or modify it
521 under the same terms as Perl itself.