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 last one can be considered as an object destructor. It happens
50 when the variable goes out of scope (with the exception of global
51 scope), but not when it is undefined.
53 To prevent any clash between different magics defined with this module,
54 an unique numerical signature is attached to each kind of magic (i.e.
55 each set of callbacks for magic operations).
58 The places where magic is invoked have changed a bit through perl
59 history. Here's a little list of the most recent ones.
62 'len' magic is no longer called when pushing an element into a magic
66 'clear' magic wasn't invoked when undefining an array. The bug is fixed
71 The minimum integer used as a signature for user-defined magic.
74 The maximum integer used as a signature for user-defined magic.
77 SIG_NBR = SIG_MAX - SIG_MIN + 1
81 wizard sig => .., data => ..., get => .., set => .., len => .., clear => .., free => ..
83 This function creates a 'wizard', an opaque type that holds the magic
84 information. It takes a list of keys / values as argument, whose keys
88 The numerical signature. If not specified or undefined, a random
89 signature is generated. If the signature matches an already defined
90 magic, then the existant magic object is returned.
93 A code reference to a private data constructor. It is called each
94 time this magic is cast on a variable, and the scalar returned is
95 used as private data storage for it. $_[0] is a reference to the
96 magic object and @_[1 .. @_-1] are all extra arguments that were
99 'get', 'set', 'len', 'clear' and 'free'
100 Code references to corresponding magic callbacks. You don't have to
101 specify all of them : the magic associated with undefined entries
102 simply won't be hooked. In those callbacks, $_[0] is a reference to
103 the magic object and $_[1] is the private data (or "undef" when no
104 private data constructor was supplied). In the special case of "len"
105 magic and when the variable is an array, $_[2] contains its normal
108 # A simple scalar tracer
109 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
110 set => sub { print STDERR "set to ${$_[0]}\n" },
111 free => sub { print STDERR "${$_[0]} was deleted\n" }
114 With this tool, you can manually generate random magic signature between
115 SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when
116 no signature is supplied.
118 # Generate a signature
124 This accessor returns the magic signature of this wizard.
127 my $sig = getsig $wiz;
130 cast [$@%&*]var, [$wiz|$sig], ...
132 This function associates $wiz magic to the variable supplied, without
133 overwriting any other kind of magic. You can also supply the numeric
134 signature $sig instead of $wiz. It returns true on success or when $wiz
135 magic is already present, 0 on error, and "undef" when no magic
136 corresponds to the given signature (in case $sig was supplied). All
137 extra arguments specified after $wiz are passed to the private data
140 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
142 die 'error' unless cast $x, $wiz;
145 getdata [$@%&*]var, [$wiz|$sig]
147 This accessor fetches the private data associated with the magic $wiz
148 (or the signature $sig) in the variable. "undef" is returned when no
149 such magic or data is found, or when $sig does not represent a current
152 # Get the attached data.
153 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
156 dispell [$@%&*]variable, [$wiz|$sig]
158 The exact opposite of "cast" : it dissociates $wiz magic from the
159 variable. You can also pass the magic signature $sig as the second
160 argument. True is returned on success, 0 on error or when no magic
161 represented by $wiz could be found in the variable, and "undef" when no
162 magic corresponds to the given signature (in case $sig was supplied).
164 # Dispell now. If $wiz isn't a signature, undef can't be returned.
165 die 'no such magic or error' unless dispell $x, $wiz;
168 The functions "wizard", "gensig", "getsig", "cast", "getdata" and
169 "dispell" are only exported on request. All of them are exported by the
170 tags ':funcs' and ':all'.
172 The constants "SIG_MIN", "SIG_MAX" and "SIG_NBR" are also only exported
173 on request. They are all exported by the tags ':consts' and ':all'.
176 Carp (standard since perl 5), XSLoader (standard since perl 5.006).
178 Glob tests need Symbol (standard since perl 5.002).
181 perlguts and perlapi for internal information about magic.
184 Vincent Pit, "<perl at profvince.com>"
186 You can contact me by mail or on #perl @ FreeNode (Prof_Vince).
189 Please report any bugs or feature requests to "bug-variable-magic at
190 rt.cpan.org", or through the web interface at
191 <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>. I will
192 be notified, and then you'll automatically be notified of progress on
193 your bug as I make changes.
196 You can find documentation for this module with the perldoc command.
198 perldoc Variable::Magic
201 Copyright 2007 Vincent Pit, all rights reserved.
203 This program is free software; you can redistribute it and/or modify it
204 under the same terms as Perl itself.