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"
130 "VMG_COMPAT_ARRAY_PUSH_NOLEN"
131 True for perls that don't call 'len' magic when you push an element in a
134 "VMG_COMPAT_ARRAY_UNDEF_CLEAR"
135 True for perls that call 'clear' magic when undefining magical arrays.
137 "VMG_COMPAT_HASH_LISTASSIGN_COPY"
138 True for perls that call 'copy' magic on list assignments. Implies that
145 get => sub { my ($ref, $data) = @_; ... },
146 set => sub { my ($ref, $data) = @_; ... },
147 len => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
148 clear => sub { my ($ref, $data) = @_; ... },
149 free => sub { my ($ref, $data) = @_, ... },
150 copy => sub { my ($ref, $data, $elt) = @_; ... },
151 local => sub { my ($ref, $data) = @_; ... },
152 fetch => sub { my ($ref, $data, $key) = @_; ... },
153 store => sub { my ($ref, $data, $key) = @_; ... },
154 exists => sub { my ($ref, $data, $key) = @_; ... },
155 delete => sub { my ($ref, $data, $key) = @_; ... }
157 This function creates a 'wizard', an opaque type that holds the magic
158 information. It takes a list of keys / values as argument, whose keys
162 The numerical signature. If not specified or undefined, a random
163 signature is generated. If the signature matches an already defined
164 magic, then the existant magic object is returned.
167 A code reference to a private data constructor. It is called each
168 time this magic is cast on a variable, and the scalar returned is
169 used as private data storage for it. $_[0] is a reference to the
170 magic object and @_[1 .. @_-1] are all extra arguments that were
173 "get", "set", "len", "clear", "free", "copy", "local", "fetch", "store",
174 "exists" and "delete"
175 Code references to corresponding magic callbacks. You don't have to
176 specify all of them : the magic associated with undefined entries
177 simply won't be hooked. In those callbacks, $_[0] is always a
178 reference to the magic object and $_[1] is always the private data
179 (or "undef" when no private data constructor was supplied). In the
180 special case of "len" magic and when the variable is an array, $_[2]
181 contains its normal length. "copy" magic receives the current
182 element (i.e. the value) in $_[2]. $_[2] is also the current key in
183 "fetch", "store", "exists" and "delete" callbacks.
185 # A simple scalar tracer
186 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
187 set => sub { print STDERR "set to ${$_[0]}\n" },
188 free => sub { print STDERR "${$_[0]} was deleted\n" }
191 With this tool, you can manually generate random magic signature between
192 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
193 no signature is supplied.
195 # Generate a signature
201 This accessor returns the magic signature of this wizard.
204 my $sig = getsig $wiz;
207 cast [$@%&*]var, [$wiz|$sig], ...
209 This function associates $wiz magic to the variable supplied, without
210 overwriting any other kind of magic. You can also supply the numeric
211 signature $sig instead of $wiz. It returns true on success or when $wiz
212 magic is already present, 0 on error, and "undef" when no magic
213 corresponds to the given signature (in case $sig was supplied). All
214 extra arguments specified after $wiz are passed to the private data
215 constructor. If the variable isn't a hash, any "uvar" callback of the
216 wizard is safely ignored.
218 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
220 die 'error' unless cast $x, $wiz;
223 getdata [$@%&*]var, [$wiz|$sig]
225 This accessor fetches the private data associated with the magic $wiz
226 (or the signature $sig) in the variable. "undef" is returned when no
227 such magic or data is found, or when $sig does not represent a current
230 # Get the attached data.
231 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
234 dispell [$@%&*]variable, [$wiz|$sig]
236 The exact opposite of "cast" : it dissociates $wiz magic from the
237 variable. You can also pass the magic signature $sig as the second
238 argument. True is returned on success, 0 on error or when no magic
239 represented by $wiz could be found in the variable, and "undef" when no
240 magic corresponds to the given signature (in case $sig was supplied).
242 # Dispell now. If $wiz isn't a signature, undef can't be returned.
243 die 'no such magic or error' unless dispell $x, $wiz;
246 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
247 "dispell" are only exported on request. All of them are exported by the
248 tags ':funcs' and ':all'.
250 The constants "SIG_MIN", "SIG_MAX", "SIG_NBR", "MGf_COPY", "MGf_DUP",
251 "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are
252 all exported by the tags ':consts' and ':all'.
255 If you store a magic object in the private data slot, the magic won't be
256 accessible by "getdata" since it's not copied by assignation. The only
257 way to address this would be to return a reference.
259 If you define a wizard with a "free" callback and cast it on itself,
260 this destructor won't be called because the wizard will be destroyed
266 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
268 Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
271 Some uvar tests need Hash::Util::FieldHash (standard since perl
274 Glob tests need Symbol (standard since perl 5.002).
277 perlguts and perlapi for internal information about magic.
279 perltie and overload for other ways of enhancing objects.
282 Vincent Pit, "<perl at profvince.com>"
284 You can contact me by mail or on #perl @ FreeNode (Prof_Vince).
287 Please report any bugs or feature requests to "bug-variable-magic at
288 rt.cpan.org", or through the web interface at
289 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
290 be notified, and then you'll automatically be notified of progress on
291 your bug as I make changes.
294 You can find documentation for this module with the perldoc command.
296 perldoc Variable::Magic
299 Copyright 2007-2008 Vincent Pit, all rights reserved.
301 This program is free software; you can redistribute it and/or modify it
302 under the same terms as Perl itself.