Vincent Pit > Variable-Magic > Variable::Magic



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


Version 0.60


    use Variable::Magic qw<wizard cast VMG_OP_INFO_NAME>;

    { # A variable tracer
     my $wiz = wizard(
      set  => sub { print "now set to ${$_[0]}!\n" },
      free => sub { print "destroyed!\n" },

     my $a = 1;
     cast $a, $wiz;
     $a = 2;        # "now set to 2!"
    }               # "destroyed!"

    { # A hash with a default value
     my $wiz = wizard(
      data     => sub { $_[1] },
      fetch    => sub { $_[2] = $_[1] unless exists $_[0]->{$_[2]}; () },
      store    => sub { print "key $_[2] stored in $_[-1]\n" },
      copy_key => 1,
      op_info  => VMG_OP_INFO_NAME,

     my %h = (_default => 0, apple => 2);
     cast %h, $wiz, '_default';
     print $h{banana}, "\n"; # "0" (there is no 'banana' key in %h)
     $h{pear} = 1;           # "key pear stored in helem"


Magic is Perl's way of enhancing variables. This mechanism lets the user add extra data to any variable and hook syntactical operations (such as access, assignment or destruction) that can be applied to it. With this module, you can add your own magic to any variable without having to write a single line of XS.

You'll realize that these magic variables look a lot like tied variables. It is not surprising, as tied variables are implemented as a special kind of magic, just like any 'irregular' Perl variable : scalars like $!, $( or $^W, the %ENV and %SIG hashes, the @ISA array, vec() and substr() lvalues, threads::shared variables... They all share the same underlying C API, and this module gives you direct access to it.

Still, the magic made available by this module differs from tieing and overloading in several ways :

The operations that can be overloaded are :

The following actions only apply to hashes and are available if and only if "VMG_UVAR" is true. They are referred to as uvar magics.

You can refer to the tests to have more insight of where the different magics are invoked.



     data     => sub { ... },
     get      => sub { my ($ref, $data [, $op]) = @_; ... },
     set      => sub { my ($ref, $data [, $op]) = @_; ... },
     len      => sub {
      my ($ref, $data, $len [, $op]) = @_; ... ; return $newlen
     clear    => sub { my ($ref, $data [, $op]) = @_; ... },
     free     => sub { my ($ref, $data [, $op]) = @_, ... },
     copy     => sub { my ($ref, $data, $key, $elt [, $op]) = @_; ... },
     local    => sub { my ($ref, $data [, $op]) = @_; ... },
     fetch    => sub { my ($ref, $data, $key [, $op]) = @_; ... },
     store    => sub { my ($ref, $data, $key [, $op]) = @_; ... },
     exists   => sub { my ($ref, $data, $key [, $op]) = @_; ... },
     delete   => sub { my ($ref, $data, $key [, $op]) = @_; ... },
     copy_key => $bool,
     op_info  => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],

This function creates a 'wizard', an opaque object that holds the magic information. It takes a list of keys / values as argument, whose keys can be :

Each callback can be specified as :

Note that free magic is never called during global destruction, as there is no way to ensure that the wizard object and the callback were not destroyed before the variable.

Here is a simple usage example :

    # 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" },


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

This function associates $wiz magic to the supplied variable, without overwriting any other kind of magic. It returns true on success or when $wiz magic is already attached, and croaks on error. When $wiz provides a data constructor, it is called just before magic is cast onto the variable, and it receives a reference to the target variable in $_[0] and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.

    # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
    my $x;
    cast $x, $wiz, 1;

The var argument can be an array or hash value. Magic for these scalars behaves like for any other, except that it is dispelled when the entry is deleted from the container. For example, if you want to call POSIX::tzset each time the 'TZ' environment variable is changed in %ENV, you can use :

    use POSIX;
    cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };

If you want to handle the possible deletion of the 'TZ' entry, you must also specify store magic.


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

This accessor fetches the private data associated with the magic $wiz in the variable. It croaks when $wiz does not represent a valid magic object, and returns an empty list if no such magic is attached to the variable or when the wizard has no data constructor.

    # Get the data attached to $wiz in $x, or undef if $wiz
    # did not attach any.
    my $data = getdata $x, $wiz;


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

The exact opposite of "cast" : it dissociates $wiz magic from the variable. This function returns true on success, 0 when no magic represented by $wiz could be found in the variable, and croaks if the supplied wizard is invalid.

    # Dispell now.
    die 'no such magic in $x' unless dispell $x, $wiz;



Evaluates to true if and only if the copy magic is available. This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.


Evaluates to true if and only if the dup magic is available. This is the case for perl 5.7.3 and greater, which is ensured by the requirements of this module.


Evaluates to true if and only if the local magic is available. This is the case for perl 5.9.3 and greater.


When this constant is true, you can use the fetch, store, exists and delete magics on hashes. Initial "VMG_UVAR" capability was introduced in perl 5.9.5, with a fully functional implementation shipped with perl 5.10.0.


True for perls that don't call len magic when taking the length of a magical scalar.


True for perls that don't call len magic on scalars. Implies "VMG_COMPAT_SCALAR_LENGTH_NOLEN".


True for perls that don't call len magic when you push an element in a magical array. Starting from perl 5.11.0, this only refers to pushes in non-void context and hence is false.


True for perls that don't call len magic when you push in void context an element in a magical array.


True for perls that don't call len magic when you unshift in void context an element in a magical array.


True for perls that call clear magic when undefining magical arrays.


True for perls that don't call delete magic when you delete an element from a hash in void context.


True for perls that call copy magic when a magical closure prototype is cloned.


True for perls that call get magic for operations on globs.


The perl patchlevel this module was built with, or 0 for non-debugging perls.


True if and only if this module could have been built with thread-safety features enabled.


True if and only if this module could have been built with fork-safety features enabled. This is always true except on Windows where it is false for perl 5.10.0 and below.


Value to pass with op_info to get the current op name in the magic callbacks.


Value to pass with op_info to get a B::OP object representing the current op in the magic callbacks.


Associate an object to any perl variable

This technique can be useful for passing user data through limited APIs. It is similar to using inside-out objects, but without the drawback of having to implement a complex destructor.

     package Magical::UserData;

     use Variable::Magic qw<wizard cast getdata>;

     my $wiz = wizard data => sub { \$_[1] };

     sub ud (\[$@%*&]) : lvalue {
      my ($var) = @_;
      my $data = &getdata($var, $wiz);
      unless (defined $data) {
       $data = \(my $slot);
       &cast($var, $wiz, $slot)
                 or die "Couldn't cast UserData magic onto the variable";

     BEGIN { *ud = \&Magical::UserData::ud }

     my $cb;
     $cb = sub { print 'Hello, ', ud(&$cb), "!\n" };

     ud(&$cb) = 'world';
     $cb->(); # Hello, world!

Recursively cast magic on datastructures

cast can be called from any magical callback, and in particular from data. This allows you to recursively cast magic on datastructures :

    my $wiz;
    $wiz = wizard data => sub {
     my ($var, $depth) = @_;
     $depth ||= 0;
     my $r = ref $var;
     if ($r eq 'ARRAY') {
      &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for @$var;
     } elsif ($r eq 'HASH') {
      &cast((ref() ? $_ : \$_), $wiz, $depth + 1) for values %$var;
     return $depth;
    free => sub {
     my ($var, $depth) = @_;
     my $r = ref $var;
     print "free $r at depth $depth\n";

     my %h = (
      a => [ 1, 2 ],
      b => { c => 3 }
     cast %h, $wiz;

When %h goes out of scope, this prints something among the lines of :

    free HASH at depth 0
    free HASH at depth 1
    free SCALAR at depth 2
    free ARRAY at depth 1
    free SCALAR at depth 3
    free SCALAR at depth 3

Of course, this example does nothing with the values that are added after the cast.

Delayed magic actions

Starting with Variable::Magic 0.58, the return value of the magic callbacks can be used to delay the action until after the original action takes place :

    my $delayed;
    my $delayed_aux = wizard(
     data => sub { $_[1] },
     free => sub {
      my ($target) = $_[1];
      my $target_data = &getdata($target, $delayed);
      local $target_data->{guard} = 1;
      if (ref $target eq 'SCALAR') {
       my $orig = $$target;
       $$target = $target_data->{mangler}->($orig);
    $delayed = wizard(
     data => sub {
      return +{ guard => 0, mangler => $_[1] };
     set  => sub {
      return if $_[1]->{guard};
      my $token;
      cast $token, $delayed_aux, $_[0];
      return \$token;
    my $x = 1;
    cast $x, $delayed => sub { $_[0] * 2 };
    $x = 2;
    # $x is now 4
    # But note that the delayed action only takes place at the end of the
    # current statement :
    my @y = ($x = 5, $x);
    # $x is now 10, but @y is (5, 5)


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


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

All the constants are also only exported on request, either individually or by the tags ':consts' and ':all'.


In order to hook hash operations with magic, you need at least perl 5.10.0 (see "VMG_UVAR").

If you want to store a magic object in the private data slot, you will not be able to recover the magic with "getdata", since magic is not copied by assignment. You can work around this gotcha by storing a reference to the magic object instead.

If you define a wizard with free magic and cast it on itself, it results in a memory cycle, so this destructor will not be called when the wizard is freed.


perl 5.8.

A C compiler. This module may happen to build with a C++ compiler as well, but don't rely on it, as no guarantee is made in this regard.

Carp (core since perl 5), XSLoader (since 5.6.0).


perlguts and perlapi for internal information about magic.

perltie and overload for other ways of enhancing objects.


Vincent Pit, <perl at>,

You can contact me by mail or on (vincent).


Please report any bugs or feature requests to bug-variable-magic at, or through the web interface at I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.


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

    perldoc Variable::Magic


Copyright 2007,2008,2009,2010,2011,2012,2013,2014,2015,2016 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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