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'
114 The minimum integer used as a signature for user-defined magic.
117 The maximum integer used as a signature for user-defined magic.
120 SIG_NBR = SIG_MAX - SIG_MIN + 1
123 Evaluates to true iff the 'copy' magic is available.
126 Evaluates to true iff the 'dup' magic is available.
129 Evaluates to true iff the 'local' magic is available.
132 When this constant is true, you can use the "fetch,store,exists,delete"
135 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
136 True for perls that don't call 'len' magic when you push an element in a
139 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
140 True for perls that call 'clear' magic when undefining magical arrays.
146 get => sub { my ($ref, $data) = @_; ... },
147 set => sub { my ($ref, $data) = @_; ... },
148 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
149 clear => sub { my ($ref, $data) = @_; ... },
150 free => sub { my ($ref, $data) = @_, ... },
151 copy => sub { my ($ref, $data, $key, $elt) = @_; ... },
152 local => sub { my ($ref, $data) = @_; ... },
153 fetch => sub { my ($ref, $data, $key) = @_; ... },
154 store => sub { my ($ref, $data, $key) = @_; ... },
155 exists => sub { my ($ref, $data, $key) = @_; ... },
156 delete => sub { my ($ref, $data, $key) = @_; ... }
158 This function creates a 'wizard', an opaque type that holds the magic
159 information. It takes a list of keys / values as argument, whose keys
163 The numerical signature. If not specified or undefined, a random
164 signature is generated. If the signature matches an already defined
165 magic, then the existant magic object is returned.
168 A code reference to a private data constructor. It is called each
169 time this magic is cast on a variable, and the scalar returned is
170 used as private data storage for it. $_[0] is a reference to the
171 magic object and @_[1 .. @_-1] are all extra arguments that were
174 "get", "set", "len", "clear", "free", "copy", "local", "fetch", "store",
175 "exists" and "delete"
176 Code references to corresponding magic callbacks. You don't have to
177 specify all of them : the magic associated with undefined entries
178 simply won't be hooked. In those callbacks, $_[0] is always a
179 reference to the magic object and $_[1] is always the private data
180 (or "undef" when no private data constructor was supplied). In the
181 special case of "len" magic and when the variable is an array, $_[2]
182 contains its normal length. $_[2] is the current key in "copy",
183 "fetch", "store", "exists" and "delete" callbacks, although for
184 "copy" it may just be a copy of the actual key so it's useless to
185 (for example) cast magic on it. "copy" magic also receives the
186 current element (i.e. the value) in $_[3].
188 # A simple scalar tracer
189 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
190 set => sub { print STDERR "set to ${$_[0]}\n" },
191 free => sub { print STDERR "${$_[0]} was deleted\n" }
194 With this tool, you can manually generate random magic signature between
195 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
196 no signature is supplied.
198 # Generate a signature
204 This accessor returns the magic signature of this wizard.
207 my $sig = getsig $wiz;
210 cast [$@%&*]var, [$wiz|$sig], ...
212 This function associates $wiz magic to the variable supplied, without
213 overwriting any other kind of magic. You can also supply the numeric
214 signature $sig instead of $wiz. It returns true on success or when $wiz
215 magic is already present, 0 on error, and "undef" when no magic
216 corresponds to the given signature (in case $sig was supplied). All
217 extra arguments specified after $wiz are passed to the private data
218 constructor. If the variable isn't a hash, any "uvar" callback of the
219 wizard is safely ignored.
221 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
223 die 'error' unless cast $x, $wiz;
226 getdata [$@%&*]var, [$wiz|$sig]
228 This accessor fetches the private data associated with the magic $wiz
229 (or the signature $sig) in the variable. "undef" is returned when no
230 such magic or data is found, or when $sig does not represent a current
233 # Get the attached data.
234 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
237 dispell [$@%&*]variable, [$wiz|$sig]
239 The exact opposite of "cast" : it dissociates $wiz magic from the
240 variable. You can also pass the magic signature $sig as the second
241 argument. True is returned on success, 0 on error or when no magic
242 represented by $wiz could be found in the variable, and "undef" when no
243 magic corresponds to the given signature (in case $sig was supplied).
245 # Dispell now. If $wiz isn't a signature, undef can't be returned.
246 die 'no such magic or error' unless dispell $x, $wiz;
249 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
250 "dispell" are only exported on request. All of them are exported by the
251 tags ':funcs' and ':all'.
253 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
254 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
255 all exported by the tags ':consts' and ':all'.
258 If you store a magic object in the private data slot, the magic won't be
259 accessible by "getdata" since it's not copied by assignation. The only
260 way to address this would be to return a reference.
262 If you define a wizard with a "free" callback and cast it on itself,
263 this destructor won't be called because the wizard will be destroyed
269 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
271 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
274 Some uvar tests need Hash::Util::FieldHash (standard since perl
277 Glob tests need Symbol (standard since perl 5.002).
280 perlguts and perlapi for internal information about magic.
282 perltie and overload for other ways of enhancing objects.
285 Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
287 You can contact me by mail or on #perl @ FreeNode (vincent or
291 Please report any bugs or feature requests to "bug-variable-magic at
292 rt.cpan.org", or through the web interface at
293 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
294 be notified, and then you'll automatically be notified of progress on
295 your bug as I make changes.
298 You can find documentation for this module with the perldoc command.
300 perldoc Variable::Magic
302 Tests code coverage report is available at
303 <http://www.profvince.com/perl/cover/Variable-Magic>.
306 Copyright 2007-2008 Vincent Pit, all rights reserved.
308 This program is free software; you can redistribute it and/or modify it
309 under the same terms as Perl itself.