NAME

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

VERSION

Version 0.17

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 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.
copy: This magic only applies to tied arrays and hashes. It fires when you try to access or change their elements. It is available on your perl iff MGf_COPY is true.
dup: Invoked when the variable is cloned across threads. 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 iff MGf_LOCAL is true.

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

fetch: This magic happens 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 last one triggers 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.

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.

5.6.x

p14416 : 'copy' and 'dup' magic.

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 wasn't 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.

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

`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.

`VMG_COMPAT_ARRAY_PUSH_NOLEN`

True for perls that don't call 'len' magic when you push 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.

FUNCTIONS

`wizard`

    wizard sig    => ...,
           data   => sub { ... },
           get    => sub { my ($ref, $data) = @_; ... },
           set    => sub { my ($ref, $data) = @_; ... },
           len    => sub { my ($ref, $data, $len) = @_; ... ; return $newlen; },
           clear  => sub { my ($ref, $data) = @_; ... },
           free   => sub { my ($ref, $data) = @_, ... },
           copy   => sub { my ($ref, $data, $key, $elt) = @_; ... },
           local  => sub { my ($ref, $data) = @_; ... },
           fetch  => sub { my ($ref, $data, $key) = @_; ... },
           store  => sub { my ($ref, $data, $key) = @_; ... },
           exists => sub { my ($ref, $data, $key) = @_; ... },
           delete => sub { my ($ref, $data, $key) = @_; ... }

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, free, copy, local, fetch, store, exists and delete: 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 always a reference to the magic object and $_[1] is always 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. $_[2] is the current key in copy, fetch, store, exists and delete callbacks, although for copy it may just be a copy of the actual key so it's useless to (for example) cast magic on it. copy magic also receives the current element (i.e. the value) in $_[3].

    # 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. If the variable isn't a hash, any uvar callback of the wizard is safely ignored.

    # 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", "SIG_NBR", "MGf_COPY", "MGf_DUP", "MGf_LOCAL" and "VMG_UVAR" are also only exported on request. They are all exported 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 assignation. 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.

DEPENDENCIES

perl 5.7.3.

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).

AUTHOR

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

You can contact me by mail or on #perl @ FreeNode (vincent or 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?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

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

To install Variable::Magic, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Variable::Magic

CPAN shell

perl -MCPAN -e shell
install Variable::Magic

For more information on module installation, please visit the detailed CPAN module installation guide.

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NAME

VERSION

SYNOPSIS

DESCRIPTION

PERL MAGIC HISTORY

5.6.x

5.9.3

5.9.5

5.10.0

5.11.x

CONSTANTS

SIG_MIN

SIG_MAX

SIG_NBR

MGf_COPY

MGf_DUP

MGf_LOCAL

VMG_UVAR

VMG_COMPAT_ARRAY_PUSH_NOLEN

VMG_COMPAT_ARRAY_UNDEF_CLEAR

VMG_COMPAT_SCALAR_LENGTH_NOLEN