NAME

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

VERSION

Version 0.46

SYNOPSIS

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" },
                          copykey => 1,
                          op_info  => VMG_OP_INFO_NAME;

         my %h = (default => 0, apple => 2);
         cast %h, $wiz, 'default';
         print $h{banana}, "\n"; # "0", because the 'banana' key doesn't exist in %h
         $h{pear} = 1;           # "key pear stored in helem"
        }

DESCRIPTION

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's 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 iff "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.

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). At the C level, magic tokens owned by magic created by this module have their "mg->mg_private" field set to 0x3891 or 0x3892, so please don't use these magic (sic) numbers in other extensions.

FUNCTIONS
"wizard" 

        wizard 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]) = @; ... },
               copykey => $bool,
               op_info  => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ]

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 : 

        All the callbacks are expected to return an integer, which is passed
        straight to the perl magic API. However, only the return value of
        the "len" callback currently holds a meaning.

Each callback can be specified as a code or a string reference, in which case the function denoted by the string will be used as the callback.

Note that "free" callbacks are never called during global destruction, as there's no way to ensure that the wizard and the "free" callback weren't destroyed before the variable.

Here's 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"

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 croaks on error. All extra arguments specified after $wiz are passed to the private data constructor in @_[1 .. @_-1]. If the variable isn't a hash, any "uvar" callback of the wizard is safely ignored. 

        # Casts $wiz onto $x, and pass '1' to the data constructor.
        my $x;
        cast $x, $wiz, 1;

The "var" argument can be an array or hash value. Magic for those behaves like for any other scalar, 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 overcome the possible deletion of the 'TZ' entry, you have no choice but to rely on "store" uvar magic.

"getdata"

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

This accessor fetches the private data associated with the magic $wiz in the variable. It croaks when $wiz do 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 attached data, or undef if the wizard does not attach any.
        my $data = getdata $x, $wiz;

"dispell"

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;

CONSTANTS
"MGf_COPY"
Evaluates to true iff the 'copy' magic is available.

"MGf_DUP"
Evaluates to true iff the 'dup' magic is available.

"MGf_LOCAL"
Evaluates to true iff the 'local' magic is available.

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

"VMG_COMPAT_ARRAY_PUSH_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.

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

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

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

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

"VMG_COMPAT_GLOB_GET"
True for perls that call 'get' magic for operations on globs.

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

"VMG_THREADSAFE"
True iff this module could have been built with thread-safety features enabled.

"VMG_FORKSAFE"
True iff this module could have been built with fork-safety features enabled. This will always be true except on Windows where it's false for perl 5.10.0 and below .

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

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

COOKBOOK
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";
          }
          $$data;
         }
        }

        {
         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 will print 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".

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.

EXPORT

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'.

CAVEATS

If you store a magic object in the private data slot, the magic won't be accessible by "getdata" since it's not copied by assignment. The only way to address this would be to return a reference.

If you define a wizard with a "free" callback and cast it on itself, this destructor won't be called because the wizard will be destroyed first.

In order to define magic on hash members, you need at least perl 5.10.0 (see "VMG_UVAR")

DEPENDENCIES

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

Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash (since 5.002).

Some uvar tests need Hash::Util::FieldHash (standard since perl 5.009004).

Glob tests need Symbol (standard since perl 5.002).

Threads tests need threads and threads::shared.

SEE ALSO

perlguts and perlapi for internal information about magic.

perltie and overload for other ways of enhancing objects.

AUTHOR

Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.

You can contact me by mail or on "irc.perl.org" (vincent).

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

Tests code coverage report is available at <http://www.profvince.com/perl/cover/Variable-Magic>.

COPYRIGHT & LICENSE

Copyright 2007,2008,2009,2010,2011 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.