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's 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 * It isn't copied on assignment.
55 You attach it to variables, not values (as for blessed references).
57 * It doesn't replace the original semantics.
59 Magic callbacks usually get triggered before the original action
60 takes place, and can't 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
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 * It's mostly invisible at the Perl level.
74 Magical and non-magical variables cannot be distinguished with
75 "ref", "tied" or another trick.
77 * It's notably faster.
79 Mainly because perl's way of handling magic is lighter by nature,
80 and because there's 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 one is triggered each time the value of the variable changes.
94 It is called for array subscripts and slices, but never for hashes.
98 This magic is a little special : it is called when the 'size' or the
99 'length' of the variable has to be known by Perl. Typically, it's
100 the magic involved when an array is evaluated in scalar context, but
101 also on array assignment and loops ("for", "map" or "grep"). The
102 callback has then to return the length 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 one can be considered as an object destructor. It happens when
115 the variable goes out of scope, but not when it is undefined.
119 This magic only applies to tied arrays and hashes. It fires when you
120 try to access or change their elements. It is available on your perl
121 iff "MGf_COPY" is true.
125 Invoked when the variable is cloned across threads. Currently not
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 iff "MGf_LOCAL" is true.
134 The following actions only apply to hashes and are available iff
135 "VMG_UVAR" is true. They are referred to as "uvar" magics.
139 This magic happens 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 last one triggers when a key is deleted in the hash, regardless
152 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 type 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 each time this magic is cast on a variable, and the scalar
186 returned is used as private data storage for it. $_[0] is a
187 reference to the magic object and @_[1 .. @_-1] are all extra
188 arguments that were passed to "cast".
190 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
191 "store", "exists" and "delete"
193 Code (or string) references to the corresponding magic callbacks.
194 You don't have to specify all of them : the magic associated with
195 undefined entries simply won't be hooked. In those callbacks, $_[0]
196 is always a reference to the magic object and $_[1] is always the
197 private data (or "undef" when no private data constructor was
200 Moreover, when you pass "op_info => $num" to "wizard", the last
201 element of @_ will be the current op name if "$num ==
202 VMG_OP_INFO_NAME" and a "B::OP" object representing the current op
203 if "$num == VMG_OP_INFO_OBJECT". Both have a performance hit, but
204 just getting the name is lighter than getting the op object.
206 Other arguments are specific to the magic hooked :
210 When the variable is an array or a scalar, $_[2] contains
211 the non-magical length. The callback can return the new
212 scalar or array length to use, or "undef" to default to the
217 $_[2] is a either a copy or an alias of the current key,
218 which means that it is useless to try to change or cast
219 magic on it. $_[3] is an alias to the current element (i.e.
222 * "fetch", "store", "exists" and "delete"
224 $_[2] is an alias to the current key. Nothing prevents you
225 from changing it, but be aware that there lurk dangerous
226 side effects. For example, it may rightfully be readonly if
227 the key was a bareword. You can get a copy instead by
228 passing "copy_key => 1" to "wizard", which allows you to
229 safely assign to $_[2] in order to e.g. redirect the action
230 to another key. This however has a little performance
231 drawback because of the copy.
233 All the callbacks are expected to return an integer, which is passed
234 straight to the perl magic API. However, only the return value of
235 the "len" callback currently holds a meaning.
237 Each callback can be specified as a code or a string reference, in which
238 case the function denoted by the string will be used as the callback.
240 Note that "free" callbacks are *never* called during global destruction,
241 as there's no way to ensure that the wizard and the "free" callback
242 weren't destroyed before the variable.
244 Here's a simple usage example :
246 # A simple scalar tracer
248 get => sub { print STDERR "got ${$_[0]}\n" },
249 set => sub { print STDERR "set to ${$_[0]}\n" },
250 free => sub { print STDERR "${$_[0]} was deleted\n" },
254 cast [$@%&*]var, $wiz, ...
256 This function associates $wiz magic to the variable supplied, without
257 overwriting any other kind of magic. It returns true on success or when
258 $wiz magic is already present, and croaks on error. All extra arguments
259 specified after $wiz are passed to the private data constructor in @_[1
260 .. @_-1]. If the variable isn't a hash, any "uvar" callback of the
261 wizard is safely ignored.
263 # Casts $wiz onto $x, and pass '1' to the data constructor.
267 The "var" argument can be an array or hash value. Magic for those
268 behaves like for any other scalar, except that it is dispelled when the
269 entry is deleted from the container. For example, if you want to call
270 "POSIX::tzset" each time the 'TZ' environment variable is changed in
274 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
276 If you want to overcome the possible deletion of the 'TZ' entry, you
277 have no choice but to rely on "store" uvar magic.
280 getdata [$@%&*]var, $wiz
282 This accessor fetches the private data associated with the magic $wiz in
283 the variable. It croaks when $wiz do not represent a valid magic object,
284 and returns an empty list if no such magic is attached to the variable
285 or when the wizard has no data constructor.
287 # Get the attached data, or undef if the wizard does not attach any.
288 my $data = getdata $x, $wiz;
291 dispell [$@%&*]variable, $wiz
293 The exact opposite of "cast" : it dissociates $wiz magic from the
294 variable. This function returns true on success, 0 when no magic
295 represented by $wiz could be found in the variable, and croaks if the
296 supplied wizard is invalid.
299 die 'no such magic in $x' unless dispell $x, $wiz;
303 Evaluates to true iff the 'copy' magic is available.
306 Evaluates to true iff the 'dup' magic is available.
309 Evaluates to true iff the 'local' magic is available.
312 When this constant is true, you can use the "fetch,store,exists,delete"
313 callbacks on hashes. Initial VMG_UVAR capability was introduced in perl
314 5.9.5, with a fully functional implementation shipped with perl 5.10.0.
316 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
317 True for perls that don't call 'len' magic when taking the "length" of a
320 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
321 True for perls that don't call 'len' magic when you push an element in a
322 magical array. Starting from perl 5.11.0, this only refers to pushes in
323 non-void context and hence is false.
325 "VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
326 True for perls that don't call 'len' magic when you push in void context
327 an element in a magical array.
329 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
330 True for perls that don't call 'len' magic when you unshift in void
331 context an element in a magical array.
333 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
334 True for perls that call 'clear' magic when undefining magical arrays.
336 "VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
337 True for perls that don't call 'delete' uvar magic when you delete an
338 element from a hash in void context.
340 "VMG_COMPAT_GLOB_GET"
341 True for perls that call 'get' magic for operations on globs.
343 "VMG_PERL_PATCHLEVEL"
344 The perl patchlevel this module was built with, or 0 for non-debugging
348 True iff this module could have been built with thread-safety features
352 True iff this module could have been built with fork-safety features
353 enabled. This will always be true except on Windows where it's false for
354 perl 5.10.0 and below .
357 Value to pass with "op_info" to get the current op name in the magic
361 Value to pass with "op_info" to get a "B::OP" object representing the
362 current op in the magic callbacks.
365 Associate an object to any perl variable
366 This technique can be useful for passing user data through limited APIs.
367 It is similar to using inside-out objects, but without the drawback of
368 having to implement a complex destructor.
371 package Magical::UserData;
373 use Variable::Magic qw<wizard cast getdata>;
375 my $wiz = wizard data => sub { \$_[1] };
377 sub ud (\[$@%*&]) : lvalue {
379 my $data = &getdata($var, $wiz);
380 unless (defined $data) {
382 &cast($var, $wiz, $slot)
383 or die "Couldn't cast UserData magic onto the variable";
390 BEGIN { *ud = \&Magical::UserData::ud }
393 $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };
396 $cb->(); # Hello, world!
399 Recursively cast magic on datastructures
400 "cast" can be called from any magical callback, and in particular from
401 "data". This allows you to recursively cast magic on datastructures :
404 $wiz = wizard data => sub {
405 my ($var, $depth) = @_;
409 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
410 } elsif ($r eq 'HASH') {
411 &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
416 my ($var, $depth) = @_;
418 print "free $r at depth $depth\n";
430 When %h goes out of scope, this will print something among the lines of
435 free SCALAR at depth 2
436 free ARRAY at depth 1
437 free SCALAR at depth 3
438 free SCALAR at depth 3
440 Of course, this example does nothing with the values that are added
444 The places where magic is invoked have changed a bit through perl
445 history. Here's a little list of the most recent ones.
449 *p14416* : 'copy' and 'dup' magic.
453 *p28160* : Integration of *p25854* (see below).
455 *p32542* : Integration of *p31473* (see below).
459 *p25854* : 'len' magic is no longer called when pushing an element
462 *p26569* : 'local' magic.
466 *p31064* : Meaningful 'uvar' magic.
468 *p31473* : 'clear' magic wasn't invoked when undefining an array.
469 The bug is fixed as of this version.
473 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
474 'copy' magic on hash stores for (non-tied) hashes that also have
479 *p32969* : 'len' magic is no longer invoked when calling "length"
480 with a magical scalar.
482 *p34908* : 'len' magic is no longer called when pushing / unshifting
483 an element into a magical array in void context. The "push" part was
484 already covered by *p25854*.
486 *g9cdcb38b* : 'len' magic is called again when pushing into a
487 magical array in non-void context.
490 The functions "wizard", "cast", "getdata" and "dispell" are only
491 exported on request. All of them are exported by the tags ':funcs' and
494 All the constants are also only exported on request, either individually
495 or by the tags ':consts' and ':all'.
498 If you store a magic object in the private data slot, the magic won't be
499 accessible by "getdata" since it's not copied by assignment. The only
500 way to address this would be to return a reference.
502 If you define a wizard with a "free" callback and cast it on itself,
503 this destructor won't be called because the wizard will be destroyed
506 In order to define magic on hash members, you need at least perl 5.10.0
512 A C compiler. This module may happen to build with a C++ compiler as
513 well, but don't rely on it, as no guarantee is made in this regard.
515 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
517 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
520 Some uvar tests need Hash::Util::FieldHash (standard since perl
523 Glob tests need Symbol (standard since perl 5.002).
525 Threads tests need threads and threads::shared.
528 perlguts and perlapi for internal information about magic.
530 perltie and overload for other ways of enhancing objects.
533 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
535 You can contact me by mail or on "irc.perl.org" (vincent).
538 Please report any bugs or feature requests to "bug-variable-magic at
539 rt.cpan.org", or through the web interface at
540 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
541 be notified, and then you'll automatically be notified of progress on
542 your bug as I make changes.
545 You can find documentation for this module with the perldoc command.
547 perldoc Variable::Magic
549 Tests code coverage report is available at
550 <http://www.profvince.com/perl/cover/Variable-Magic>.
553 Copyright 2007,2008,2009,2010,2011 Vincent Pit, all rights reserved.
555 This program is free software; you can redistribute it and/or modify it
556 under the same terms as Perl itself.