2 Variable::Magic - Associate user-defined magic to variables from Perl.
8 use Variable::Magic qw/wizard cast dispell/;
10 my $wiz = wizard set => sub { print STDERR "now set to ${$_[0]}!\n" };
13 $a = 2; # "now set to 2!"
18 Magic is Perl way of enhancing objects. This mechanism lets the user add
19 extra data to any variable and hook syntaxical operations (such as
20 access, assignment or destruction) that can be applied to it. With this
21 module, you can add your own magic to any variable without having to
22 write a single line of XS.
24 You'll realize that these magic variables look a lot like tied
25 variables. It's not surprising, as tied variables are implemented as a
26 special kind of magic, just like any 'irregular' Perl variable : scalars
27 like $!, $( or $^W, the %ENV and %SIG hashes, the @ISA array, "vec()"
28 and "substr()" lvalues, thread::shared variables... They all share the
29 same underlying C API, and this module gives you direct access to it.
31 Still, the magic made available by this module differs from tieing and
32 overloading in several ways :
34 * It isn't copied on assignment.
36 You attach it to variables, not values (as for blessed references).
38 * It doesn't replace the original semantics.
40 Magic callbacks trigger before the original action take place, and
41 can't prevent it to happen. This makes catching individual events
42 easier than with "tie", where you have to provide fallbacks methods
43 for all actions by usually inheriting from the correct "Tie::Std*"
44 class and overriding individual methods in your own class.
48 The same magic can be applied on scalars, arrays, hashes, subs or
49 globs. But the same hook (see below for a list) may trigger
50 differently depending on the the type of the variable.
52 * It's mostly invisible at the Perl level.
54 Magical and non-magical variables cannot be distinguished with
55 "ref", "tied" or another trick.
57 * It's notably faster.
59 Mainly because perl's way of handling magic is lighter by nature,
60 and because there's no need for any method resolution. Also, since
61 you don't have to reimplement all the variable semantics, you only
62 pay for what you actually use.
64 The operations that can be overloaded are :
68 This magic is invoked when the variable is evaluated (does not
69 include array/hash subscripts and slices).
73 This one is triggered each time the value of the variable changes
74 (includes array/hash subscripts and slices).
78 This magic is a little special : it is called when the 'size' or the
79 'length' of the variable has to be known by Perl. Typically, it's
80 the magic involved when an array is evaluated in scalar context, but
81 also on array assignment and loops ("for", "map" or "grep"). The
82 callback has then to return the length as an integer.
86 This magic is invoked when the variable is reset, such as when an
87 array is emptied. Please note that this is different from undefining
88 the variable, even though the magic is called when the clearing is a
89 result of the undefine (e.g. for an array, but actually a bug
90 prevent it to work before perl 5.9.5 - see the history).
94 This one can be considered as an object destructor. It happens when
95 the variable goes out of scope (with the exception of global scope),
96 but not when it is undefined.
100 This magic only applies to tied arrays and hashes. It fires when you
101 try to access or change their elements. It is available on your perl
102 iff "MGf_COPY" is true.
106 Invoked when the variable is cloned across threads. Currently not
111 When this magic is set on a variable, all subsequent localizations
112 of the variable will trigger the callback. It is available on your
113 perl iff "MGf_LOCAL" is true.
115 The following actions only apply to hashes and are available iff
116 "VMG_UVAR" is true. They are referred to as "uvar" magics.
120 This magic happens each time an element is fetched from the hash.
124 This one is called when an element is stored into the hash.
128 This magic fires when a key is tested for existence in the hash.
132 This last one triggers when a key is deleted in the hash, regardless
133 of whether the key actually exists in it.
135 You can refer to the tests to have more insight of where the different
138 To prevent any clash between different magics defined with this module,
139 an unique numerical signature is attached to each kind of magic (i.e.
140 each set of callbacks for magic operations).
146 get => sub { my ($ref, $data [, $op]) = @_; ... },
147 set => sub { my ($ref, $data [, $op]) = @_; ... },
148 len => sub { my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen; },
149 clear => sub { my ($ref, $data [, $op]) = @_; ... },
150 free => sub { my ($ref, $data [, $op]) = @_, ... },
151 copy => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
152 local => sub { my ($ref, $data [, $op]) = @_; ... },
153 fetch => sub { my ($ref, $data, $key [, $op]) = @_; ... },
154 store => sub { my ($ref, $data, $key [, $op]) = @_; ... },
155 exists => sub { my ($ref, $data, $key [, $op]) = @_; ... },
156 delete => sub { my ($ref, $data, $key [, $op]) = @_; ... },
158 op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]
160 This function creates a 'wizard', an opaque type that holds the magic
161 information. It takes a list of keys / values as argument, whose keys
166 The numerical signature. If not specified or undefined, a random
167 signature is generated. If the signature matches an already defined
168 magic, then the existant magic object is returned.
172 A code reference to a private data constructor. It is called each
173 time this magic is cast on a variable, and the scalar returned is
174 used as private data storage for it. $_[0] is a reference to the
175 magic object and @_[1 .. @_-1] are all extra arguments that were
178 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
179 "store", "exists" and "delete"
181 Code references to the corresponding magic callbacks. You don't have
182 to specify all of them : the magic associated with undefined entries
183 simply won't be hooked. In those callbacks, $_[0] is always a
184 reference to the magic object and $_[1] is always the private data
185 (or "undef" when no private data constructor was supplied).
187 Moreover, when you pass "op_info => $num" to "wizard", the last
188 element of @_ will be the current op name if "$num ==
189 VMG_OP_INFO_NAME" and a "B::OP" object representing the current op
190 if "$num == VMG_OP_INFO_OBJECT". Both have a performance hit, but
191 just getting the name is lighter than getting the op object.
193 Other arguments are specific to the magic hooked :
197 When the variable is an array or a scalar, $_[2] contains
198 the non-magical length. The callback can return the new
199 scalar or array length to use, or "undef" to default to the
204 $_[2] is a either a copy or an alias of the current key,
205 which means that it is useless to try to change or cast
206 magic on it. $_[3] is an alias to the current element (i.e.
209 * "fetch", "store", "exists" and "delete"
211 $_[2] is an alias to the current key. Nothing prevents you
212 from changing it, but be aware that there lurk dangerous
213 side effects. For example, it may righteously be readonly if
214 the key was a bareword. You can get a copy instead by
215 passing "copy_key => 1" to "wizard", which allows you to
216 safely assign to $_[2] in order to e.g. redirect the action
217 to another key. This however has a little performance
218 drawback because of the copy.
220 All the callbacks are expected to return an integer, which is passed
221 straight to the perl magic API. However, only the return value of
222 the "len" callback currently holds a meaning.
224 # A simple scalar tracer
225 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
226 set => sub { print STDERR "set to ${$_[0]}\n" },
227 free => sub { print STDERR "${$_[0]} was deleted\n" }
230 With this tool, you can manually generate random magic signature between
231 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
232 no signature is supplied.
234 # Generate a signature
240 This accessor returns the magic signature of this wizard.
243 my $sig = getsig $wiz;
246 cast [$@%&*]var, [$wiz|$sig], ...
248 This function associates $wiz magic to the variable supplied, without
249 overwriting any other kind of magic. You can also supply the numeric
250 signature $sig instead of $wiz. It returns true on success or when $wiz
251 magic is already present, 0 on error, and "undef" when no magic
252 corresponds to the given signature (in case $sig was supplied). All
253 extra arguments specified after $wiz are passed to the private data
254 constructor. If the variable isn't a hash, any "uvar" callback of the
255 wizard is safely ignored.
257 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
259 die 'error' unless cast $x, $wiz;
261 The "var" argument can be an array or hash value. Magic for those
262 behaves like for any other scalar, except that it is dispelled when the
263 entry is deleted from the container. For example, if you want to call
264 "POSIX::tzset" each time the 'TZ' environment variable is changed in
268 cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
270 If you want to overcome the possible deletion of the 'TZ' entry, you
271 have no choice but to rely on "store" uvar magic.
274 getdata [$@%&*]var, [$wiz|$sig]
276 This accessor fetches the private data associated with the magic $wiz
277 (or the signature $sig) in the variable. "undef" is returned when no
278 such magic or data is found, or when $sig does not represent a current
281 # Get the attached data.
282 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
285 dispell [$@%&*]variable, [$wiz|$sig]
287 The exact opposite of "cast" : it dissociates $wiz magic from the
288 variable. You can also pass the magic signature $sig as the second
289 argument. True is returned on success, 0 on error or when no magic
290 represented by $wiz could be found in the variable, and "undef" when no
291 magic corresponds to the given signature (in case $sig was supplied).
293 # Dispell now. If $wiz isn't a signature, undef can't be returned.
294 die 'no such magic or error' unless dispell $x, $wiz;
298 The minimum integer used as a signature for user-defined magic.
301 The maximum integer used as a signature for user-defined magic.
304 SIG_NBR = SIG_MAX - SIG_MIN + 1
307 Evaluates to true iff the 'copy' magic is available.
310 Evaluates to true iff the 'dup' magic is available.
313 Evaluates to true iff the 'local' magic is available.
316 When this constant is true, you can use the "fetch,store,exists,delete"
319 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
320 True for perls that don't call 'len' magic when you push an element in a
323 "VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
324 True for perls that don't call 'len' magic when you unshift in void
325 context an element in a magical array.
327 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
328 True for perls that call 'clear' magic when undefining magical arrays.
330 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
331 True for perls that don't call 'len' magic when taking the "length" of a
334 "VMG_PERL_PATCHLEVEL"
335 The perl patchlevel this module was built with, or 0 for non-debugging
339 True iff this module could have been built with thread-safety features
343 Value to pass with "op_info" to get the current op name in the magic
347 Value to pass with "op_info" to get a "B::OP" object representing the
348 current op in the magic callbacks.
351 The places where magic is invoked have changed a bit through perl
352 history. Here's a little list of the most recent ones.
356 *p14416* : 'copy' and 'dup' magic.
360 *p28160* : Integration of *p25854* (see below).
362 *p32542* : Integration of *p31473* (see below).
366 *p25854* : 'len' magic is no longer called when pushing an element
369 *p26569* : 'local' magic.
373 *p31064* : Meaningful 'uvar' magic.
375 *p31473* : 'clear' magic wasn't invoked when undefining an array.
376 The bug is fixed as of this version.
380 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
381 'copy' magic on hash stores for (non-tied) hashes that also have
386 *p32969* : 'len' magic is no longer invoked when calling "length"
387 with a magical scalar.
389 *p34908* : 'len' magic is no longer called when pushing / unshifting
390 an element into a magical array in void context. The "push" part was
391 already covered by *p25854*.
394 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
395 "dispell" are only exported on request. All of them are exported by the
396 tags ':funcs' and ':all'.
398 All the constants are also only exported on request, either individually
399 or by the tags ':consts' and ':all'.
402 If you store a magic object in the private data slot, the magic won't be
403 accessible by "getdata" since it's not copied by assignment. The only
404 way to address this would be to return a reference.
406 If you define a wizard with a "free" callback and cast it on itself,
407 this destructor won't be called because the wizard will be destroyed
413 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
415 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
418 Some uvar tests need Hash::Util::FieldHash (standard since perl
421 Glob tests need Symbol (standard since perl 5.002).
423 Threads tests need threads and threads::shared.
426 perlguts and perlapi for internal information about magic.
428 perltie and overload for other ways of enhancing objects.
431 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
433 You can contact me by mail or on "irc.perl.org" (vincent).
436 Please report any bugs or feature requests to "bug-variable-magic at
437 rt.cpan.org", or through the web interface at
438 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
439 be notified, and then you'll automatically be notified of progress on
440 your bug as I make changes.
443 You can find documentation for this module with the perldoc command.
445 perldoc Variable::Magic
447 Tests code coverage report is available at
448 <http://www.profvince.com/perl/cover/Variable-Magic>.
451 Copyright 2007-2009 Vincent Pit, all rights reserved.
453 This program is free software; you can redistribute it and/or modify it
454 under the same terms as Perl itself.