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

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


NAME

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


VERSION

Version 0.06


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 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 history).

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).


PERL MAGIC HISTORY

The places where magic is invoked have changed a bit through perl history. Here's a little list of the most recent ones.

5.9.3

'len' magic is no longer called when pushing an element into a magic array.

5.9.5

'clear' magic wasn't invoked when undefining an array. The bug is fixed as of this version.


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. If the signature matches an already defined magic, then the existant magic object is returned.

'data'
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. $_[0] is a reference to the magic object and @_[1 .. @_-1] are all extra arguments that were passed to cast.

'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. In those callbacks, $_[0] is a reference to the magic object and $_[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|$sig], ...

This function associates $wiz magic to the variable supplied, without overwriting any other kind of magic. You can also supply the numeric signature $sig instead of $wiz. It returns true on success or when $wiz magic is already present, 0 on error, and undef when no magic corresponds to the given signature (in case $sig was supplied). All extra arguments specified after $wiz are passed to the private data constructor.


    # Casts $wiz onto $x. If $wiz isn't a signature, undef can't be returned.

    my $x;

    die 'error' unless cast $x, $wiz;

getdata


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

This accessor fetches the private data associated with the magic $wiz (or the signature $sig) in the variable. undef is returned when no such magic or data is found, or when $sig does not represent a current valid magic object.


    # Get the attached data.

    my $data = getdata $x, $wiz or die 'no such magic or magic has no data';

dispell


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

The exact opposite of cast : it dissociates $wiz magic from the variable. You can also pass the magic signature $sig as the second argument. True is returned on success, 0 on error or when no magic represented by $wiz could be found in the variable, and undef when no magic corresponds to the given signature (in case $sig was supplied).


    # Dispell now. If $wiz isn't a signature, undef can't be returned.

    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

perl 5.7.3.

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

Glob tests need Symbol (standard since perl 5.002).


SEE ALSO

perlguts and perlapi for internal information about magic.


AUTHOR

Vincent Pit, <perl at profvince.com>

You can contact me by mail or on #perl @ FreeNode (Prof_Vince).


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. 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.

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