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Vincent Pit

NAME

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

VERSION

Version 0.53

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" },
      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"
    }

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 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 :

  • Magic is not copied on assignment.

    You attach it to variables, not values (as for blessed references).

  • Magic does not replace the original semantics.

    Magic callbacks usually get triggered before the original action takes place, and cannot prevent it from happening. This also makes catching individual events easier than with tie, where you have to provide fallbacks methods for all actions by usually inheriting from the correct Tie::Std* class and overriding individual methods in your own class.

  • Magic is multivalued.

    You can safely apply different kinds of magics to the same variable, and each of them will be invoked successively.

  • Magic is type-agnostic.

    The same magic can be applied on scalars, arrays, hashes, subs or globs. But the same hook (see below for a list) may trigger differently depending on the type of the variable.

  • Magic is invisible at Perl level.

    Magical and non-magical variables cannot be distinguished with ref, tied or another trick.

  • Magic is notably faster.

    Mainly because perl's way of handling magic is lighter by nature, and because there is no need for any method resolution. Also, since you don't have to reimplement all the variable semantics, you only pay for what you actually use.

The operations that can be overloaded are :

  • get

    This magic is invoked when the variable is evaluated. It is never called for arrays and hashes.

  • set

    This magic is called each time the value of the variable changes. It is called for array subscripts and slices, but never for hashes.

  • len

    This magic only applies to arrays (though it used to also apply to scalars), and is triggered when the 'size' or the 'length' of the variable has to be known by Perl. This is typically the magic involved when an array is evaluated in scalar context, but also on array assignment and loops (for, map or grep). The length is returned from the callback as an integer.

    Starting from perl 5.12, this magic is no longer called by the length keyword, and starting from perl 5.17.4 it is also no longer called for scalars in any situation, making this magic only meaningful on arrays. You can use the constants "VMG_COMPAT_SCALAR_LENGTH_NOLEN" and "VMG_COMPAT_SCALAR_NOLEN" to see if this magic is available for scalars or not.

  • 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 magic is called when a variable is destroyed as the result of going out of scope (but not when it is undefined). It behaves roughly like Perl object destructors (i.e. DESTROY methods), except that exceptions thrown from inside a free callback will always be propagated to the surrounding code.

  • copy

    This magic only applies to tied arrays and hashes, and fires when you try to access or change their elements.

  • dup

    This magic is invoked when the variable is cloned across threads. It is currently not available.

  • local

    When this magic is set on a variable, all subsequent localizations of the variable will trigger the callback. It is available on your perl if and only if MGf_LOCAL is true.

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.

  • fetch

    This magic is invoked each time an element is fetched from the hash.

  • store

    This one is called when an element is stored into the hash.

  • exists

    This magic fires when a key is tested for existence in the hash.

  • delete

    This magic is triggered when a key is deleted in the hash, regardless of whether the key actually exists in it.

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

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]) = @_; ... },
     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 :

  • data

    A code (or string) reference to a private data constructor. It is called in scalar context each time the magic is cast onto a variable, with $_[0] being a reference to this variable and @_[1 .. @_-1] being all extra arguments that were passed to "cast". The scalar returned from this call is then attached to the variable and can be retrieved later with "getdata".

  • get, set, len, clear, free, copy, local, fetch, store, exists and delete

    Code (or string) references to the respective magic callbacks. You don't have to specify all of them : the magic corresponding to undefined entries will simply not be hooked.

    When those callbacks are executed, $_[0] is a reference to the magic variable and $_[1] is the associated private data (or undef when no private data constructor is supplied with the wizard). Other arguments depend on which kind of magic is involved :

    • len

      $_[2] contains the natural, non-magical length of the variable (which can only be a scalar or an array as len magic is only relevant for these types). The callback is expected to return the new scalar or array length to use, or undef to default to the normal length.

    • copy

      $_[2] is a either an alias or a copy of the current key, and $_[3] is an alias to the current element (i.e. the value). Because $_[2] might be a copy, it is useless to try to change it or cast magic on it.

    • fetch, store, exists and delete

      $_[2] is an alias to the current key. Note that $_[2] may rightfully be readonly if the key comes from a bareword, and as such it is unsafe to assign to it. You can ask for a copy instead by passing copy_key => 1 to "wizard" which, at the price of a small performance hit, allows you to safely assign to $_[2] in order to e.g. redirect the action to another key.

    Finally, if op_info => $num is also passed to wizard, then one extra element is appended to @_. Its nature depends on the value of $num :

    • VMG_OP_INFO_NAME

      $_[-1] is the current op name.

    • VMG_OP_INFO_OBJECT

      $_[-1] is the B::OP object for the current op.

    Both result in a small performance hit, but just getting the name is lighter than getting the op object.

    These callbacks are executed in scalar context and are expected to return an integer, which is then passed straight to the perl magic API. However, only the return value of the len magic callback currently holds a meaning.

Each callback can be specified as :

  • a code reference, which will be called as a subroutine.

  • a string reference, where the string denotes which subroutine is to be called when magic is triggered. If the subroutine name is not fully qualified, then the current package at the time the magic is invoked will be used instead.

  • a reference to undef, in which case a no-op magic callback is installed instead of the default one. This may especially be helpful for local magic, where an empty callback prevents magic from being copied during localization.

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

    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

    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

    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 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.

MGf_DUP

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.

MGf_LOCAL

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

VMG_UVAR

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.

VMG_COMPAT_SCALAR_LENGTH_NOLEN

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

VMG_COMPAT_SCALAR_NOLEN

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

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_HASH_DELETE_NOUVAR_VOID

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

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

VMG_FORKSAFE

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.

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 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.

PERL MAGIC HISTORY

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

  • 5.6.x

    p14416 : copy and dup magic.

  • 5.8.9

    p28160 : Integration of p25854 (see below).

    p32542 : Integration of p31473 (see below).

  • 5.9.3

    p25854 : len magic is no longer called when pushing an element into a magic array.

    p26569 : local magic.

  • 5.9.5

    p31064 : Meaningful uvar magic.

    p31473 : clear magic was not invoked when undefining an array. The bug is fixed as of this version.

  • 5.10.0

    Since PERL_MAGIC_uvar is uppercased, hv_magic_check() triggers copy magic on hash stores for (non-tied) hashes that also have uvar magic.

  • 5.11.x

    p32969 : len magic is no longer invoked when calling length with a magical scalar.

    p34908 : len magic is no longer called when pushing / unshifting an element into a magical array in void context. The push part was already covered by p25854.

    g9cdcb38b : len magic is called again when pushing into a magical array in non-void context.

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

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.

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

Copy tests need Tie::Array (core since perl 5.005) and Tie::Hash (since 5.002). Some uvar tests need Hash::Util::FieldHash (since 5.9.4). Glob tests need Symbol (since 5.002). Threads tests need threads and threads::shared (both since 5.7.3).

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 2007,2008,2009,2010,2011,2012,2013 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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