/ 
Variable-Magic-0.01
River stage three • 29 direct dependents • 119 total dependents
/ Variable::Magic

NAME

Variable::Magic - Associate user-defined magic to variables from Perl.

VERSION

Version 0.01

SYNOPSIS

use Variable::Magic qw/wizard cast dispell/;

my $wiz = wizard set => sub { print STDERR "now set to $_[0]!\n" };
my $a = 1;
cast $a, $wiz;
$a = 2;          # "now set to 2!"
dispell $a, $wiz;
$a = 3           # (nothing)

DESCRIPTION

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.

The operations that can be overloaded are :

get

This magic is invoked when the variable is evaluated (does not include array/hash subscripts and slices).

set

This one is triggered each time the value of the variable changes (includes array/hash subscripts and slices).

len

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 (for, map or grep). The callback has then to return the length as an integer.

clear

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 reset is a result of the undefine (e.g. for an array).

free

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.

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).

CONSTANTS

SIG_MIN

The minimum integer used as a signature for user-defined magic.

SIG_MAX

The maximum integer used as a signature for user-defined magic.

SIG_NBR

SIG_NBR = SIG_MAX - SIG_MIN + 1

FUNCTIONS

wizard

wizard sig => .., data => ..., get => .., set => .., len => .., clear => .., free => ..

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 :

'sig'

The numerical signature. If not specified or undefined, a random signature is generated.

'data'

A code reference to a private data constructor. It will be called each time this magic is cast on a variable, and the scalar returned will be used as private data storage for it.

'get', 'set', 'len', 'clear' and 'free'

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. When the magic variable is an array or a hash, $_[0] is a reference to it, but directly references it otherwise. $_[1] is the private data (or undef when no private data constructor was supplied). In the special case of len magic and when the variable is an array, $_[2] contains its normal length.

# A simple scalar tracer
my $wiz = wizard get  => sub { print STDERR "got $_[0]\n" },
                 set  => sub { print STDERR "set to $_[0]\n" },
                 free => sub { print STDERR "$_[0] was deleted\n" }

gensig

With this tool, you can manually generate random magic signature between SIG_MIN and SIG_MAX inclusive. That's the way "wizard" creates them when no signature is supplied.

# Generate a signature
my $sig = gensig;

getsig

getsig $wiz

This accessor returns the magic signature of this wizard.

# Get $wiz signature
my $sig = getsig $wiz;

cast

cast [$@%&*]var, $wiz

This function associates $wiz magic to the variable supplied, without overwriting any other kind of magic. It returns true on success or when $wiz magic is already present, and false on error.

# Casts $wiz to $x
my $x;
die 'error' unless cast $x, $wiz;

getdata

getdata [$@%&*]var, $wiz

This accessor fetches the private data associated with the magic $wiz in the variable. undef is returned when no such magic or data is found.

dispell

dispell [$@%&*]variable, $wiz
dispell [$@%&*]variable, $sig

The exact opposite of "cast" : it dissociates $wiz magic from the variable. You can also pass the magic signature as the second argument. True is returned on success, and false on error or when no magic represented by $wiz could be found in the variable.

# Dispell now
die 'no such magic or error' unless dispell $x, $wiz;

EXPORT

The functions "wizard", "gensig", "getsig", "cast", "getdata" and "dispell" are only exported on request. All of them are exported by the tags ':funcs' and ':all'.

The constants "SIG_MIN", "SIG_MAX" and "SIG_NBR" are also only exported on request. They are all exported by the tags ':consts' and ':all'.

DEPENDENCIES

Carp (standard since perl 5), XSLoader (standard since perl 5.006).

Tests use Symbol (standard since perl 5.002).

SEE ALSO

perlguts and perlapi for internal information about magic.

AUTHOR

Vincent Pit, <perl at profvince.com>

BUGS

Please report any bugs or feature requests to bug-variable-magic at rt.cpan.org, or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Variable-Magic. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

SUPPORT

You can find documentation for this module with the perldoc command.

perldoc Variable::Magic

COPYRIGHT & LICENSE

Copyright 2007 Vincent Pit, all rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.