1 package Variable::Magic;
10 Variable::Magic - Associate user-defined magic to variables from Perl.
18 our $VERSION = '0.05';
22 use Variable::Magic qw/wizard cast dispell/;
24 my $wiz = wizard set => sub { print STDERR "now set to ${$_[0]}!\n" };
27 $a = 2; # "now set to 2!"
33 Magic is Perl way of enhancing objects. This mechanism let the user add extra data to any variable and overload syntaxical operations (such as access, assignation or destruction) that can be applied to it. With this module, you can add your own magic to any variable without the pain of the C API.
35 The operations that can be overloaded are :
41 This magic is invoked when the variable is evaluated (does not include array/hash subscripts and slices).
45 This one is triggered each time the value of the variable changes (includes array/hash subscripts and slices).
49 This magic is a little special : it is called when the 'size' or the 'length' of the variable has to be known by Perl. Typically, it's the magic involved when an array is evaluated in scalar context, but also on array assignation and loops (C<for>, C<map> or C<grep>). The callback has then to return the length as an integer.
53 This magic is invoked when the variable is reset, such as when an array is emptied. Please note that this is different from undefining the variable, even though the magic is called when the clearing is a result of the undefine (e.g. for an array, but actually a bug prevent it to work before perl 5.9.5 - see the L<history|/PERL MAGIC HISTORY>).
57 This last one can be considered as an object destructor. It happens when the variable goes out of scope (with the exception of global scope), but not when it is undefined.
61 To prevent any clash between different magics defined with this module, an unique numerical signature is attached to each kind of magic (i.e. each set of callbacks for magic operations).
63 =head1 PERL MAGIC HISTORY
65 The places where magic is invoked have changed a bit through perl history. Here's a little list of the most recent ones.
71 =item 'len' magic is no longer called when pushing an element into a magic array.
79 =item 'clear' magic wasn't invoked when undefining an array. The bug is fixed as of this version.
87 The minimum integer used as a signature for user-defined magic.
91 The maximum integer used as a signature for user-defined magic.
95 SIG_NBR = SIG_MAX - SIG_MIN + 1
103 XSLoader::load __PACKAGE__, $VERSION;
107 wizard sig => .., data => ..., get => .., set => .., len => .., clear => .., free => ..
109 This function creates a 'wizard', an opaque type that holds the magic information. It takes a list of keys / values as argument, whose keys can be :
115 The numerical signature. If not specified or undefined, a random signature is generated. If the signature matches an already defined magic, then the existant magic object is returned.
119 A code reference to a private data constructor. It is called each time this magic is cast on a variable, and the scalar returned is used as private data storage for it. C<$_[0]> is a reference to the magic object and C<@_[1 .. @_-1]> are all extra arguments that were passed to L</cast>.
121 =item C<'get'>, C<'set'>, C<'len'>, C<'clear'> and C<'free'>
123 Code references to corresponding magic callbacks. You don't have to specify all of them : the magic associated with undefined entries simply won't be hooked. In those callbacks, C<$_[0]> is a reference to the magic object and C<$_[1]> is the private data (or C<undef> when no private data constructor was supplied). In the special case of C<len> magic and when the variable is an array, C<$_[2]> contains its normal length.
127 # A simple scalar tracer
128 my $wiz = wizard get => sub { print STDERR "got ${$_[0]}\n" },
129 set => sub { print STDERR "set to ${$_[0]}\n" },
130 free => sub { print STDERR "${$_[0]} was deleted\n" }
135 croak 'Wrong number of arguments for wizard()' if @_ % 2;
137 return _wizard(map { $opts{$_} } qw/sig get set len clear free data/);
142 With this tool, you can manually generate random magic signature between SIG_MIN and SIG_MAX inclusive. That's the way L</wizard> creates them when no signature is supplied.
144 # Generate a signature
151 This accessor returns the magic signature of this wizard.
154 my $sig = getsig $wiz;
158 cast [$@%&*]var, [$wiz|$sig], ...
160 This function associates C<$wiz> magic to the variable supplied, without overwriting any other kind of magic. You can also supply the numeric signature C<$sig> instead of C<$wiz>. It returns true on success or when C<$wiz> magic is already present, C<0> on error, and C<undef> when no magic corresponds to the given signature (in case C<$sig> was supplied). All extra arguments specified after C<$wiz> are passed to the private data constructor.
162 # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.
164 die 'error' unless cast $x, $wiz;
168 getdata [$@%&*]var, [$wiz|$sig]
170 This accessor fetches the private data associated with the magic C<$wiz> (or the signature C<$sig>) in the variable. C<undef> is returned when no such magic or data is found, or when C<$sig> does not represent a current valid magic object.
172 # Get the attached data.
173 my $data = getdata $x, $wiz or die 'no such magic or magic has no data';
177 dispell [$@%&*]variable, [$wiz|$sig]
179 The exact opposite of L</cast> : it dissociates C<$wiz> magic from the variable. You can also pass the magic signature C<$sig> as the second argument. True is returned on success, C<0> on error or when no magic represented by C<$wiz> could be found in the variable, and C<undef> when no magic corresponds to the given signature (in case C<$sig> was supplied).
181 # Dispell now. If $wiz isn't a signature, undef can't be returned.
182 die 'no such magic or error' unless dispell $x, $wiz;
186 The functions L</wizard>, L</gensig>, L</getsig>, L</cast>, L</getdata> and L</dispell> are only exported on request. All of them are exported by the tags C<':funcs'> and C<':all'>.
188 The constants L</SIG_MIN>, L</SIG_MAX> and L</SIG_NBR> are also only exported on request. They are all exported by the tags C<':consts'> and C<':all'>.
192 use base qw/Exporter/;
196 'funcs' => [ qw/wizard gensig getsig cast getdata dispell/ ],
197 'consts' => [ qw/SIG_MIN SIG_MAX SIG_NBR/ ]
199 our @EXPORT_OK = map { @$_ } values %EXPORT_TAGS;
200 $EXPORT_TAGS{'all'} = \@EXPORT_OK;
204 L<Carp> (standard since perl 5), L<XSLoader> (standard since perl 5.006).
206 Glob tests need L<Symbol> (standard since perl 5.002).
210 L<perlguts> and L<perlapi> for internal information about magic.
214 Vincent Pit, C<< <perl at profvince.com> >>
216 You can contact me by mail or on #perl @ FreeNode (Prof_Vince).
220 Please report any bugs or feature requests to
221 C<bug-variable-magic at rt.cpan.org>, or through the web interface at
222 L<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic>.
223 I will be notified, and then you'll automatically be notified of progress on
224 your bug as I make changes.
228 You can find documentation for this module with the perldoc command.
230 perldoc Variable::Magic
232 =head1 COPYRIGHT & LICENSE
234 Copyright 2007 Vincent Pit, all rights reserved.
236 This program is free software; you can redistribute it and/or modify it
237 under the same terms as Perl itself.
241 1; # End of Variable::Magic