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 :
27 This magic is invoked when the variable is evaluated (does not
28 include array/hash subscripts and slices).
31 This one is triggered each time the value of the variable changes
32 (includes array/hash subscripts and slices).
35 This magic is a little special : it is called when the 'size' or the
36 'length' of the variable has to be known by Perl. Typically, it's
37 the magic involved when an array is evaluated in scalar context, but
38 also on array assignation and loops ("for", "map" or "grep"). The
39 callback has then to return the length as an integer.
42 This magic is invoked when the variable is reset, such as when an
43 array is emptied. Please note that this is different from undefining
44 the variable, even though the magic is called when the clearing is a
45 result of the undefine (e.g. for an array, but actually a bug
46 prevent it to work before perl 5.9.5 - see the history).
49 This one can be considered as an object destructor. It happens when
50 the variable goes out of scope (with the exception of global scope),
51 but not when it is undefined.
54 This magic only applies to tied arrays and hashes. It fires when you
55 try to access or change their elements. It is available on your perl
56 iff "MGf_COPY" is true.
59 Invoked when the variable is cloned across threads. Currently not
63 When this magic is set on a variable, all subsequent localizations
64 of the variable will trigger the callback. It is available on your
65 perl iff "MGf_LOCAL" is true.
67 The following actions only apply to hashes and are available iff
68 "VMG_UVAR" is true. They are referred to as "uvar" magics.
71 This magic happens each time an element is fetched from the hash.
74 This one is called when an element is stored into the hash.
77 This magic fires when a key is tested for existence in the hash.
80 This last one triggers when a key is deleted in the hash, regardless
81 of whether the key actually exists in it.
83 You can refer to the tests to have more insight of where the different
86 To prevent any clash between different magics defined with this module,
87 an unique numerical signature is attached to each kind of magic (i.e.
88 each set of callbacks for magic operations).
91 The places where magic is invoked have changed a bit through perl
92 history. Here's a little list of the most recent ones.
95 *p14416* : 'copy' and 'dup' magic.
98 'len' magic is no longer called when pushing an element into a magic
100 *p26569* : 'local' magic.
103 *p31064* : Meaningful 'uvar' magic.
104 *p31473* : 'clear' magic wasn't invoked when undefining an array. The
105 bug is fixed as of this version.
109 The minimum integer used as a signature for user-defined magic.
112 The maximum integer used as a signature for user-defined magic.
115 SIG_NBR = SIG_MAX - SIG_MIN + 1
118 Evaluates to true iff the 'copy' magic is available.
121 Evaluates to true iff the 'dup' magic is available.
124 Evaluates to true iff the 'local' magic is available.
127 When this constant is true, you can use the "fetch,store,exists,delete"
134 get => sub { my ($ref, $data) = @_; ... },
135 set => sub { my ($ref, $data) = @_; ... },
136 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
137 clear => sub { my ($ref, $data) = @_; ... },
138 free => sub { my ($ref, $data) = @_, ... },
139 copy => sub { my ($ref, $data, $elt) = @_; ... },
140 local => sub { my ($ref, $data) = @_; ... },
141 fetch => sub { my ($ref, $data, $key) = @_; ... },
142 store => sub { my ($ref, $data, $key) = @_; ... },
143 exists => sub { my ($ref, $data, $key) = @_; ... },
144 delete => sub { my ($ref, $data, $key) = @_; ... }
146 This function creates a 'wizard', an opaque type that holds the magic
147 information. It takes a list of keys / values as argument, whose keys
151 The numerical signature. If not specified or undefined, a random
152 signature is generated. If the signature matches an already defined
153 magic, then the existant magic object is returned.
156 A code reference to a private data constructor. It is called each
157 time this magic is cast on a variable, and the scalar returned is
158 used as private data storage for it. $_[0] is a reference to the
159 magic object and @_[1 .. @_-1] are all extra arguments that were
162 "get", "set", "len", "clear", "free", "copy", "local", "fetch", "store",
163 "exists" and "delete"
164 Code references to corresponding magic callbacks. You don't have to
165 specify all of them : the magic associated with undefined entries
166 simply won't be hooked. In those callbacks, $_[0] is always a
167 reference to the magic object and $_[1] is always the private data
168 (or "undef" when no private data constructor was supplied). In the
169 special case of "len" magic and when the variable is an array, $_[2]
170 contains its normal length. "copy" magic receives the current
171 element (i.e. the value) in $_[2]. $_[2] is also the current key in
172 "fetch", "store", "exists" and "delete" callbacks.
174 # A simple scalar tracer
175 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
176 set => sub { print STDERR "set to ${$_[0]}\n" },
177 free => sub { print STDERR "${$_[0]} was deleted\n" }
180 With this tool, you can manually generate random magic signature between
181 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
182 no signature is supplied.
184 # Generate a signature
190 This accessor returns the magic signature of this wizard.
193 my $sig = getsig $wiz;
196 cast [$@%&*]var, [$wiz|$sig], ...
198 This function associates $wiz magic to the variable supplied, without
199 overwriting any other kind of magic. You can also supply the numeric
200 signature $sig instead of $wiz. It returns true on success or when $wiz
201 magic is already present, 0 on error, and "undef" when no magic
202 corresponds to the given signature (in case $sig was supplied). All
203 extra arguments specified after $wiz are passed to the private data
204 constructor. If the variable isn't a hash, any "uvar" callback of the
205 wizard is safely ignored.
207 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
209 die 'error' unless cast $x, $wiz;
212 getdata [$@%&*]var, [$wiz|$sig]
214 This accessor fetches the private data associated with the magic $wiz
215 (or the signature $sig) in the variable. "undef" is returned when no
216 such magic or data is found, or when $sig does not represent a current
219 # Get the attached data.
220 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
223 dispell [$@%&*]variable, [$wiz|$sig]
225 The exact opposite of "cast" : it dissociates $wiz magic from the
226 variable. You can also pass the magic signature $sig as the second
227 argument. True is returned on success, 0 on error or when no magic
228 represented by $wiz could be found in the variable, and "undef" when no
229 magic corresponds to the given signature (in case $sig was supplied).
231 # Dispell now. If $wiz isn't a signature, undef can't be returned.
232 die 'no such magic or error' unless dispell $x, $wiz;
235 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
236 "dispell" are only exported on request. All of them are exported by the
237 tags ':funcs' and ':all'.
239 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
240 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
241 all exported by the tags ':consts' and ':all'.
244 If you store a magic object in the private data slot, the magic won't be
245 accessible by "getdata" since it's not copied by assignation. The only
246 way to address this would be to return a reference.
248 If you define a wizard with a "free" callback and cast it on itself,
249 this destructor won't be called because the wizard will be destroyed
255 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
257 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
260 Some uvar tests need Hash::Util::FieldHash (standard since perl
263 Glob tests need Symbol (standard since perl 5.002).
266 perlguts and perlapi for internal information about magic.
268 perltie and overload for other ways of enhancing objects.
271 Vincent Pit, "<perl at profvince.com>"
273 You can contact me by mail or on #perl @ FreeNode (Prof_Vince).
276 Please report any bugs or feature requests to "bug-variable-magic at
277 rt.cpan.org", or through the web interface at
278 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
279 be notified, and then you'll automatically be notified of progress on
280 your bug as I make changes.
283 You can find documentation for this module with the perldoc command.
285 perldoc Variable::Magic
288 Copyright 2007-2008 Vincent Pit, all rights reserved.
290 This program is free software; you can redistribute it and/or modify it
291 under the same terms as Perl itself.