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 let the user add
19 extra data to any variable and overload syntaxical operations (such as
20 access, assignation or destruction) that can be applied to it. With this
21 module, you can add your own magic to any variable without the pain of
24 The operations that can be overloaded are :
28 This magic is invoked when the variable is evaluated (does not
29 include array/hash subscripts and slices).
33 This one is triggered each time the value of the variable changes
34 (includes array/hash subscripts and slices).
38 This magic is a little special : it is called when the 'size' or the
39 'length' of the variable has to be known by Perl. Typically, it's
40 the magic involved when an array is evaluated in scalar context, but
41 also on array assignation and loops ("for", "map" or "grep"). The
42 callback has then to return the length as an integer.
46 This magic is invoked when the variable is reset, such as when an
47 array is emptied. Please note that this is different from undefining
48 the variable, even though the magic is called when the clearing is a
49 result of the undefine (e.g. for an array, but actually a bug
50 prevent it to work before perl 5.9.5 - see the history).
54 This one can be considered as an object destructor. It happens when
55 the variable goes out of scope (with the exception of global scope),
56 but not when it is undefined.
60 This magic only applies to tied arrays and hashes. It fires when you
61 try to access or change their elements. It is available on your perl
62 iff "MGf_COPY" is true.
66 Invoked when the variable is cloned across threads. Currently not
71 When this magic is set on a variable, all subsequent localizations
72 of the variable will trigger the callback. It is available on your
73 perl iff "MGf_LOCAL" is true.
75 The following actions only apply to hashes and are available iff
76 "VMG_UVAR" is true. They are referred to as "uvar" magics.
80 This magic happens each time an element is fetched from the hash.
84 This one is called when an element is stored into the hash.
88 This magic fires when a key is tested for existence in the hash.
92 This last one triggers when a key is deleted in the hash, regardless
93 of whether the key actually exists in it.
95 You can refer to the tests to have more insight of where the different
98 To prevent any clash between different magics defined with this module,
99 an unique numerical signature is attached to each kind of magic (i.e.
100 each set of callbacks for magic operations).
103 The places where magic is invoked have changed a bit through perl
104 history. Here's a little list of the most recent ones.
108 *p14416* : 'copy' and 'dup' magic.
112 *p25854* : 'len' magic is no longer called when pushing an element
115 *p26569* : 'local' magic.
119 *p31064* : Meaningful 'uvar' magic.
121 *p31473* : 'clear' magic wasn't invoked when undefining an array.
122 The bug is fixed as of this version.
126 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
127 'copy' magic on hash stores for (non-tied) hashes that also have
132 *p32969* : 'len' magic is no longer invoked when calling "length"
133 with a magical scalar.
137 The minimum integer used as a signature for user-defined magic.
140 The maximum integer used as a signature for user-defined magic.
143 SIG_NBR = SIG_MAX - SIG_MIN + 1
146 Evaluates to true iff the 'copy' magic is available.
149 Evaluates to true iff the 'dup' magic is available.
152 Evaluates to true iff the 'local' magic is available.
155 When this constant is true, you can use the "fetch,store,exists,delete"
158 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
159 True for perls that don't call 'len' magic when you push an element in a
162 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
163 True for perls that call 'clear' magic when undefining magical arrays.
165 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
166 True for perls that don't call 'len' magic when taking the "length" of a
169 "VMG_PERL_PATCHLEVEL"
170 The perl patchlevel this module was built with, or 0 for non-debugging
174 True iff this module could have been built with thread-safety features
181 get => sub { my ($ref, $data) = @_; ... },
182 set => sub { my ($ref, $data) = @_; ... },
183 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
184 clear => sub { my ($ref, $data) = @_; ... },
185 free => sub { my ($ref, $data) = @_, ... },
186 copy => sub { my ($ref, $data, $key, $elt) = @_; ... },
187 local => sub { my ($ref, $data) = @_; ... },
188 fetch => sub { my ($ref, $data, $key) = @_; ... },
189 store => sub { my ($ref, $data, $key) = @_; ... },
190 exists => sub { my ($ref, $data, $key) = @_; ... },
191 delete => sub { my ($ref, $data, $key) = @_; ... }
193 This function creates a 'wizard', an opaque type that holds the magic
194 information. It takes a list of keys / values as argument, whose keys
199 The numerical signature. If not specified or undefined, a random
200 signature is generated. If the signature matches an already defined
201 magic, then the existant magic object is returned.
205 A code reference to a private data constructor. It is called each
206 time this magic is cast on a variable, and the scalar returned is
207 used as private data storage for it. $_[0] is a reference to the
208 magic object and @_[1 .. @_-1] are all extra arguments that were
211 * "get", "set", "len", "clear", "free", "copy", "local", "fetch",
212 "store", "exists" and "delete"
214 Code references to corresponding magic callbacks. You don't have to
215 specify all of them : the magic associated with undefined entries
216 simply won't be hooked. In those callbacks, $_[0] is always a
217 reference to the magic object and $_[1] is always the private data
218 (or "undef" when no private data constructor was supplied). In the
219 special case of "len" magic and when the variable is an array, $_[2]
220 contains its normal length. $_[2] is the current key in "copy",
221 "fetch", "store", "exists" and "delete" callbacks, although for
222 "copy" it may just be a copy of the actual key so it's useless to
223 (for example) cast magic on it. "copy" magic also receives the
224 current element (i.e. the value) in $_[3].
226 # A simple scalar tracer
227 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
228 set => sub { print STDERR "set to ${$_[0]}\n" },
229 free => sub { print STDERR "${$_[0]} was deleted\n" }
232 With this tool, you can manually generate random magic signature between
233 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
234 no signature is supplied.
236 # Generate a signature
242 This accessor returns the magic signature of this wizard.
245 my $sig = getsig $wiz;
248 cast [$@%&*]var, [$wiz|$sig], ...
250 This function associates $wiz magic to the variable supplied, without
251 overwriting any other kind of magic. You can also supply the numeric
252 signature $sig instead of $wiz. It returns true on success or when $wiz
253 magic is already present, 0 on error, and "undef" when no magic
254 corresponds to the given signature (in case $sig was supplied). All
255 extra arguments specified after $wiz are passed to the private data
256 constructor. If the variable isn't a hash, any "uvar" callback of the
257 wizard is safely ignored.
259 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
261 die 'error' unless cast $x, $wiz;
264 getdata [$@%&*]var, [$wiz|$sig]
266 This accessor fetches the private data associated with the magic $wiz
267 (or the signature $sig) in the variable. "undef" is returned when no
268 such magic or data is found, or when $sig does not represent a current
271 # Get the attached data.
272 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
275 dispell [$@%&*]variable, [$wiz|$sig]
277 The exact opposite of "cast" : it dissociates $wiz magic from the
278 variable. You can also pass the magic signature $sig as the second
279 argument. True is returned on success, 0 on error or when no magic
280 represented by $wiz could be found in the variable, and "undef" when no
281 magic corresponds to the given signature (in case $sig was supplied).
283 # Dispell now. If $wiz isn't a signature, undef can't be returned.
284 die 'no such magic or error' unless dispell $x, $wiz;
287 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
288 "dispell" are only exported on request. All of them are exported by the
289 tags ':funcs' and ':all'.
291 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
292 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
293 all exported by the tags ':consts' and ':all'.
296 If you store a magic object in the private data slot, the magic won't be
297 accessible by "getdata" since it's not copied by assignation. The only
298 way to address this would be to return a reference.
300 If you define a wizard with a "free" callback and cast it on itself,
301 this destructor won't be called because the wizard will be destroyed
307 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
309 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
312 Some uvar tests need Hash::Util::FieldHash (standard since perl
315 Glob tests need Symbol (standard since perl 5.002).
317 Threads tests need threads and threads::shared.
320 perlguts and perlapi for internal information about magic.
322 perltie and overload for other ways of enhancing objects.
325 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
327 You can contact me by mail or on #perl @ FreeNode (vincent or
331 Please report any bugs or feature requests to "bug-variable-magic at
332 rt.cpan.org", or through the web interface at
333 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
334 be notified, and then you'll automatically be notified of progress on
335 your bug as I make changes.
338 You can find documentation for this module with the perldoc command.
340 perldoc Variable::Magic
342 Tests code coverage report is available at
343 <http://www.profvince.com/perl/cover/Variable-Magic>.
346 Copyright 2007-2008 Vincent Pit, all rights reserved.
348 This program is free software; you can redistribute it and/or modify it
349 under the same terms as Perl itself.