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 *p25854* : 'len' magic is no longer called when pushing an element into
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.
108 Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
109 'copy' magic on hash stores for (non-tied) hashes that also have 'uvar'
113 *p32969* : 'len' magic is no longer invoked when calling "length" with a
118 The minimum integer used as a signature for user-defined magic.
121 The maximum integer used as a signature for user-defined magic.
124 SIG_NBR = SIG_MAX - SIG_MIN + 1
127 Evaluates to true iff the 'copy' magic is available.
130 Evaluates to true iff the 'dup' magic is available.
133 Evaluates to true iff the 'local' magic is available.
136 When this constant is true, you can use the "fetch,store,exists,delete"
139 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
140 True for perls that don't call 'len' magic when you push an element in a
143 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
144 True for perls that call 'clear' magic when undefining magical arrays.
146 "VMG_COMPAT_SCALAR_LENGTH_NOLEN"
147 True for perls that don't call 'len' magic when taking the "length" of a
154 get => sub { my ($ref, $data) = @_; ... },
155 set => sub { my ($ref, $data) = @_; ... },
156 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
157 clear => sub { my ($ref, $data) = @_; ... },
158 free => sub { my ($ref, $data) = @_, ... },
159 copy => sub { my ($ref, $data, $key, $elt) = @_; ... },
160 local => sub { my ($ref, $data) = @_; ... },
161 fetch => sub { my ($ref, $data, $key) = @_; ... },
162 store => sub { my ($ref, $data, $key) = @_; ... },
163 exists => sub { my ($ref, $data, $key) = @_; ... },
164 delete => sub { my ($ref, $data, $key) = @_; ... }
166 This function creates a 'wizard', an opaque type that holds the magic
167 information. It takes a list of keys / values as argument, whose keys
171 The numerical signature. If not specified or undefined, a random
172 signature is generated. If the signature matches an already defined
173 magic, then the existant magic object is returned.
176 A code reference to a private data constructor. It is called each
177 time this magic is cast on a variable, and the scalar returned is
178 used as private data storage for it. $_[0] is a reference to the
179 magic object and @_[1 .. @_-1] are all extra arguments that were
182 "get", "set", "len", "clear", "free", "copy", "local", "fetch", "store",
183 "exists" and "delete"
184 Code references to corresponding magic callbacks. You don't have to
185 specify all of them : the magic associated with undefined entries
186 simply won't be hooked. In those callbacks, $_[0] is always a
187 reference to the magic object and $_[1] is always the private data
188 (or "undef" when no private data constructor was supplied). In the
189 special case of "len" magic and when the variable is an array, $_[2]
190 contains its normal length. $_[2] is the current key in "copy",
191 "fetch", "store", "exists" and "delete" callbacks, although for
192 "copy" it may just be a copy of the actual key so it's useless to
193 (for example) cast magic on it. "copy" magic also receives the
194 current element (i.e. the value) in $_[3].
196 # A simple scalar tracer
197 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
198 set => sub { print STDERR "set to ${$_[0]}\n" },
199 free => sub { print STDERR "${$_[0]} was deleted\n" }
202 With this tool, you can manually generate random magic signature between
203 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
204 no signature is supplied.
206 # Generate a signature
212 This accessor returns the magic signature of this wizard.
215 my $sig = getsig $wiz;
218 cast [$@%&*]var, [$wiz|$sig], ...
220 This function associates $wiz magic to the variable supplied, without
221 overwriting any other kind of magic. You can also supply the numeric
222 signature $sig instead of $wiz. It returns true on success or when $wiz
223 magic is already present, 0 on error, and "undef" when no magic
224 corresponds to the given signature (in case $sig was supplied). All
225 extra arguments specified after $wiz are passed to the private data
226 constructor. If the variable isn't a hash, any "uvar" callback of the
227 wizard is safely ignored.
229 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
231 die 'error' unless cast $x, $wiz;
234 getdata [$@%&*]var, [$wiz|$sig]
236 This accessor fetches the private data associated with the magic $wiz
237 (or the signature $sig) in the variable. "undef" is returned when no
238 such magic or data is found, or when $sig does not represent a current
241 # Get the attached data.
242 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
245 dispell [$@%&*]variable, [$wiz|$sig]
247 The exact opposite of "cast" : it dissociates $wiz magic from the
248 variable. You can also pass the magic signature $sig as the second
249 argument. True is returned on success, 0 on error or when no magic
250 represented by $wiz could be found in the variable, and "undef" when no
251 magic corresponds to the given signature (in case $sig was supplied).
253 # Dispell now. If $wiz isn't a signature, undef can't be returned.
254 die 'no such magic or error' unless dispell $x, $wiz;
257 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
258 "dispell" are only exported on request. All of them are exported by the
259 tags ':funcs' and ':all'.
261 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
262 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
263 all exported by the tags ':consts' and ':all'.
266 If you store a magic object in the private data slot, the magic won't be
267 accessible by "getdata" since it's not copied by assignation. The only
268 way to address this would be to return a reference.
270 If you define a wizard with a "free" callback and cast it on itself,
271 this destructor won't be called because the wizard will be destroyed
277 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
279 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
282 Some uvar tests need Hash::Util::FieldHash (standard since perl
285 Glob tests need Symbol (standard since perl 5.002).
288 perlguts and perlapi for internal information about magic.
290 perltie and overload for other ways of enhancing objects.
293 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
295 You can contact me by mail or on #perl @ FreeNode (vincent or
299 Please report any bugs or feature requests to "bug-variable-magic at
300 rt.cpan.org", or through the web interface at
301 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
302 be notified, and then you'll automatically be notified of progress on
303 your bug as I make changes.
306 You can find documentation for this module with the perldoc command.
308 perldoc Variable::Magic
310 Tests code coverage report is available at
311 <http://www.profvince.com/perl/cover/Variable-Magic>.
314 Copyright 2007-2008 Vincent Pit, all rights reserved.
316 This program is free software; you can redistribute it and/or modify it
317 under the same terms as Perl itself.