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.

