The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.
 / 
Class-Storage-0.03
River stage zero No dependents
/ Class::Storage

NAME

Class::Storage - pack objects by removing blessing so they can be unpacked back into objects again later.

Handles blessed HASHes and ARRAYs

VERSION

Version 0.03

SYNOPSIS

This module came into existence out of the need to be able to send objects over JSON. JSON does not allow any blessed references to be sent by default and if sent, provides no generic way to resurrect these objects again after decoding. This can now all be done like this:

    use JSON;
    use Class::Storage qw(packObjects unpackObjects);

    my $object = bless { a => 1 }, 'MyModule';
    my $packed = packObjects( $object );

    # $packed is now { __class__ => 'MyModule', a => 1 }

    print $writeHandle encode_json($packed), "\n";

    # And on the other "side":

    my $jsonString = <$readHandle>;
    my $packed = decode_json($jsonString);
    my $unpackedObject = unpackObjects($packed);

    # $unpacked is now bless { a => 1 }, 'MyModule'
    # Which is_deeply the same as $object that we started with

However, there is no JSON-specific functionality in this module whatsoever, only a way to cleanly remove the bless-ing in a way that reliably can be re-introduced later.

DESCRIPTION

Using a magic string

As you can see from the "SYNOPSIS", we use a magic string (__class__ by default) to store the class information for HASHes and ARRAYs.

So packObjects turns:

    bless { key => "value" }, "ModuleA";
    bless [ "val1", "val2" ], "ModuleB";

into:

    { __class__ => 'ModuleA', key => "value" }
    [ "__class__", 'ModuleB', "val1", "val2" ]

unpackObjects converts any hashes with the magic string as a key and any arrays with the magic string as the first element back to blessed references

This "magic string" can be given as an option (see "OPTIONS"), but if you cannot live with a magic string, you can also provide magicString => undef. But then you won't be able to unpack that data and turn it back into objects. If this is your itch, you may actually want Data::Structure::Util instead.

Returns packed/unpacked data + modifies input argument

The valid data is returned. However, for speed, we also modify and re-use data from the input value. So don't rely on being able to reuse the $data input for packObjects and unpackObjects after they've been called and don't modify them either.

If you don't want your input modified:

    use Storable qw(dclone);
    my $pristineData = somesub();
    my $packed = packObjects(dclone($pristineData));

Inspiration

Class::Storage is inspired by MooseX::Storage but this is a generic implementation that works on all plain perl classes that are implemented as blessed references to HASHes and ARRAYs (only hashes and arrays).

NOTE: MooseX::Storage uses __CLASS__ as its magic string and we use __class__ to make sure they're not the same.

TO_PACKED and FROM_PACKED

If you want to control how internal state gets represeted when packed, then provide a TO_PACKED instance method. It will be called like:

    my $packed = $object->TO_PACKED();

This $packed data will be used by packObjects instead of the guts of $object.

Similarly, during unpackObjects, if a module has a FROM_PACKED static method it will be called like this:

    my $object = $module->FROM_PACKED($packed);

As you can see, TO_PACKED and FROM_PACKED go together as pairs.

You can also modify the names of these methods with the toPackedMethodName and fromPackedMethodName options. See ""OPTIONS".

NOTE ABOUT KINDS OF BLESSED OBJECTS

perlobj says:

"... it's possible to bless any type of data structure or referent, including scalars, globs, and subroutines. You may see this sort of thing when looking at code in the wild."

In particular I've seen several XS modules create instances where the internal state is not visible to Perl, and hence cannot be handled properly by this module. Here is an example with JSON:

    use Data::Dumper;
    use JSON;
    print Dumper(JSON->new()->pretty(1));
    # prints
    # $VAR1 = bless( do{\(my $o = '')}, 'JSON' );

Clearly a JSON object has internal state and other data. This is an example of a blessed reference, but not a blessed HASH or ARRAY that Class::Storage can handle. If you try packObjects-ing such a JSON instance, Class::Storage will just leave the JSON object altogether untouched.

EXPORT

    our @EXPORT_OK = qw(packObjects unpackObjects);

SUBROUTINES/METHODS

Both packObjects and unpackObjects share the same %options. See "OPTIONS" below.

packObjects

    my $packed = packObjects($blessed, %options);

unpackObjects

    my $unpacked = unpackObjects($unbessed, %options);

OPTIONS

These options are common to packObjects and unpackObjects:

  • toPackedMethodName

    This option lets you change the name of the TO_UNBLESSED method to something else. Hint: TO_JSON could be a good idea here!

  • fromPackedMethodName

    This option lets you change the name of the TO_BLESSED method to something else. Hint: FROM_JSON could be a good idea here, even though JSON doesn't have such a method. Which is actually the entire Raison d'Etre of this module!

  • magicString

    Change the magic string used to store the class name to something else than __class__.

    If this is false, don't store class information at all, in which case packObjects becomes analogous to Data::Structure::Util::packObjects.

AUTHOR

Peter Valdemar Mørch, <peter@morch.com>

BUGS

Please report any bugs or feature requests to https://github.com/pmorch/perl-Class-Storage/issues. 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 Class::Storage

You can also look for information at:

ACKNOWLEDGEMENTS

This has been inspired by many sources, but checkout:

LICENSE AND COPYRIGHT

Copyright 2015 Peter Valdemar Mørch.

This program is free software; you can redistribute it and/or modify it under the terms of the the Artistic License (2.0). You may obtain a copy of the full license at:

http://www.perlfoundation.org/artistic_license_2_0

Any use, modification, and distribution of the Standard or Modified Versions is governed by this Artistic License. By using, modifying or distributing the Package, you accept this license. Do not use, modify, or distribute the Package, if you do not accept this license.

If your Modified Version has been derived from a Modified Version made by someone other than you, you are nevertheless required to ensure that your Modified Version complies with the requirements of this license.

This license does not grant you the right to use any trademark, service mark, tradename, or logo of the Copyright Holder.

This license includes the non-exclusive, worldwide, free-of-charge patent license to make, have made, use, offer to sell, sell, import and otherwise transfer the Package with respect to any patent claims licensable by the Copyright Holder that are necessarily infringed by the Package. If you institute patent litigation (including a cross-claim or counterclaim) against any party alleging that the Package constitutes direct or contributory patent infringement, then this Artistic License to you shall terminate on the date that such litigation is filed.

Disclaimer of Warranty: THE PACKAGE IS PROVIDED BY THE COPYRIGHT HOLDER AND CONTRIBUTORS "AS IS' AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES. THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT ARE DISCLAIMED TO THE EXTENT PERMITTED BY YOUR LOCAL LAW. UNLESS REQUIRED BY LAW, NO COPYRIGHT HOLDER OR CONTRIBUTOR WILL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING IN ANY WAY OUT OF THE USE OF THE PACKAGE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.