Diab Jerius

NAME

Hash::Wrap - create on-the-fly objects from hashes

VERSION

version 0.08

SYNOPSIS

  use Hash::Wrap;

  my $result = wrap_hash( { a => 1 } );
  print $result->a;  # prints
  print $result->b;  # throws

  # import two constructors, <cloned> and <copied> with different behaviors.
  use Hash::Wrap
    { -as => 'cloned', clone => 1},
    { -as => 'copied', copy => 1 };

  my $cloned = cloned( { a => 1 } );
  print $cloned->a;

  my $copied = copied( { a => 1 } );
  print $copied->a;

DESCRIPTION

Hash::Wrap creates objects from hashes, providing accessors for hash elements. The objects are hashes, and may be modified using the standard Perl hash operations and the object's accessors will behave accordingly.

Why use this class? Sometimes a hash is created on the fly and it's too much of a hassle to build a class to encapsulate it.

  sub foo () { ... ; return { a => 1 }; }

With Hash::Wrap:

  use Hash::Wrap;

  sub foo () { ... ; return wrap_hash( { a => 1 ); }

  my $obj = foo ();
  print $obj->a;

Elements can be added or removed to the object and accessors will track them. If the object should be immutable, use the lock routines in Hash::Util on it.

There are many similar modules on CPAN (see "SEE ALSO" for comparisons).

What sets Hash::Wrap apart is that it's possible to customize object construction and accessor behavior:

  • It's possible to use the passed hash directly, or make shallow or deep copies of it.

  • Accessors can be customized so that accessing a non-existent element can throw an exception or return the undefined value.

  • On recent enough versions of Perl, accessors can be lvalues, e.g.

       $obj->existing_key = $value;

USAGE

Simple Usage

use'ing Hash::Wrap without options imports a subroutine called wrap_hash which takes a hash, blesses it into a wrapper class and returns the hash:

  use Hash::Wrap;

  my $h = wrap_hash { a => 1 };
  print $h->a, "\n";             # prints 1

The wrapper class has no constructor method, so the only way to create an object is via the wrap_hash subroutine. (See "WRAPPER CLASSES" for more about wrapper classes)

Advanced Usage

wrap_hash is an awful name for the constructor subroutine

So rename it:

  use Hash::Wrap { -as => "a_much_better_name_for_wrap_hash" };

  $obj = a_much_better_name_for_wrap_hash( { a => 1 } );

The Wrapper Class name matters

If the class name matters, but it'll never be instantiated except via the imported constructor subroutine:

  use Hash::Wrap { -class => 'My::Class' };

  my $h = wrap_hash { a => 1 };
  print $h->a, "\n";             # prints 1
  $h->isa( 'My::Class' );        # returns true

Again, the wrapper class has no constructor method, so the only way to create an object is via the wrap_hash subroutine.

The Wrapper Class needs its own class constructor method

To generate a wrapper class which can be instantiated via its own constructor method:

  use Hash::Wrap { -class => 'My::Class', -new => 1 };

The default wrap_hash constructor subroutine is still exported, so

  $h = My::Class->new( { a => 1 } );

and

  $h = wrap_hash( { a => 1 } );

do the same thing.

To give the constructor method a different name:

  use Hash::Wrap { -class => 'My::Class',  -new => '_my_new' };

To prevent the constructor subroutine from being imported:

  use Hash::Wrap { -as => undef, -class => 'My::Class', -new => 1 };

A stand alone Wrapper Class

To create a stand alone wrapper class,

   package My::Class;

   use Hash::Wrap { -base => 1 };

   1;

And later...

   use My::Class;

   $obj = My::Class->new( \%hash );

It's possible to modify the constructor and accessors:

   package My::Class;

   use Hash::Wrap { -base => 1, -new => 'new_from_hash', -undef => 1 };

   1;

OPTIONS

Hash::Wrap works at compile time. To modify its behavior pass it options when it is use'd:

  use Hash::Wrap { %options1 }, { %options2 }, ... ;

Multiple options hashes may be passed; each hash specifies options for a separate constructor or class.

For example,

  use Hash::Wrap
    { -as => 'cloned', clone => 1},
    { -as => 'copied', copy => 1 };

creates two constructors, cloned and copied with different behaviors.

Constructor

-as => subroutine name

Import the constructor subroutine with the given name. It defaults to wrap_hash.

-copy => boolean

If true, the object will store the data in a shallow copy of the hash. By default, the object uses the hash directly.

-clone => boolean | coderef

Store the data in a deep copy of the hash. if true, "dclone" in Storable is used. If a coderef, it will be called as

   $clone = coderef->( $hash )

By default, the object uses the hash directly.

Accessors

-undef => boolean

Normally an attempt to use an accessor for an non-existent key will result in an exception. This option causes the accessor to return undef instead. It does not create an element in the hash for the key.

-lvalue => flag

If non-zero, the accessors will be lvalue routines, e.g. they can change the underlying hash value by assigning to them:

   $obj->attr = 3;

The hash entry must already exist or this will throw an exception.

lvalue subroutines are only available on Perl version 5.16 and later.

If -lvalue = 1 this option will silently be ignored on earlier versions of Perl.

If -lvalue = -1 this option will cause an exception on earlier versions of Perl.

Class

-base => boolean

If true, the enclosing package is converted into a proxy wrapper class. This should not be used in conjunction with -class. See "A stand alone Wrapper Class".

-class => class name

A class with the given name will be created and new objects will be blessed into the specified class by the constructor subroutine. The new class will not have a constructor method.

If not specified, the class name will be constructed based upon the options. Do not rely upon this name to determine if an object is wrapped by Hash::Wrap.

-new => boolean | Perl Identifier

Add a class constructor method.

If -new is a true boolean value, the method will be called new. Otherwise -new specifies the name of the method.

Extra Class Methods

-defined => boolean | Perl Identifier

Add a method which returns true if the passed hash key is defined or does not exist. If -defined is a true boolean value, the method will be called defined. Otherwise it specifies the name of the method. For example,

   use Hash::Wrap { -defined => 1 };
   $obj = wrap_hash( { a => 1, b => undef } );

   $obj->defined( 'a' ); # TRUE
   $obj->defined( 'b' ); # FALSE
   $obj->defined( 'c' ); # FALSE

or

   use Hash::Wrap { -defined => 'is_defined' };
   $obj = wrap_hash( { a => 1 } );
   $obj->is_defined( 'a' );
-exists => boolean | Perl Identifier

Add a method which returns true if the passed hash key exists. If -exists is a boolean, the method will be called exists. Otherwise it specifies the name of the method. For example,

   use Hash::Wrap { -exists => 1 };
   $obj = wrap_hash( { a => 1 } );
   $obj->exists( 'a' );

or

   use Hash::Wrap { -exists => 'is_present' };
   $obj = wrap_hash( { a => 1 } );
   $obj->is_present( 'a' );

WRAPPER CLASSES

A wrapper class has the following characteristics.

  • It has the methods DESTROY, AUTOLOAD and can.

  • It will have other methods if the -undef and -exists options are specified. It may have other methods if it is a stand alone class.

  • It will have a constructor if either of -base or -new is specified.

Wrapper Class Limitations

  • Wrapper classes have DESTROY, can method, and AUTOLOAD methods, which will mask hash keys with the same names.

  • Classes which are generated without the -base or -new options do not have a class constructor method, e.g Class->new() will not return a new object. The only way to instantiate them is via the constructor subroutine generated via Hash::Wrap. This allows the underlying hash to have a new attribute which would otherwise be masked by the constructor.

LIMITATIONS

Lvalue accessors

Lvalue accessors are available only on Perl 5.16 and later.

Accessors for deleted hash elements

Accessors for deleted elements are not removed. The class's can method will return undef for them, but they are still available in the class's stash.

SEE ALSO

Here's a comparison of this module and others on CPAN.

Hash::Wrap (this module)
  • core dependencies only

  • only applies object paradigm to top level hash

  • accessors may be lvalue subroutines

  • accessing a non-existing element via an accessor throws by default, but can optionally return undef

  • can use custom package

  • can copy/clone existing hash. clone may be customized

Object::Result

As you might expect from a DCONWAY module, this does just about everything you'd like. It has a very heavy set of dependencies.

Hash::AsObject
  • core dependencies only

  • applies object paradigm recursively

  • accessing a non-existing element via an accessor creates it

Data::AsObject
  • moderate dependency chain (no XS?)

  • applies object paradigm recursively

  • accessing a non-existing element throws

Class::Hash
  • core dependencies only

  • only applies object paradigm to top level hash

  • can add generic accessor, mutator, and element management methods

  • accessing a non-existing element via an accessor creates it (not documented, but code implies it)

  • can() doesn't work

Hash::Inflator
  • core dependencies only

  • accessing a non-existing element via an accessor returns undef

  • applies object paradigm recursively

Hash::AutoHash
  • moderate dependency chain. Requires XS, tied hashes

  • applies object paradigm recursively

  • accessing a non-existing element via an accessor creates it

Hash::Objectify
  • light dependency chain. Requires XS.

  • only applies object paradigm to top level hash

  • accessing a non-existing element throws, but if an existing element is accessed, then deleted, accessor returns undef rather than throwing

  • can use custom package

Data::OpenStruct::Deep
  • uses source filters

  • applies object paradigm recursively

Object::AutoAccessor
  • light dependency chain

  • applies object paradigm recursively

  • accessing a non-existing element via an accessor creates it

Data::Object::Autowrap
  • core dependencies only

  • no documentation

Object::Accessor
  • core dependencies only

  • only applies object paradigm to top level hash

  • accessors may be lvalue subroutines

  • accessing a non-existing element via an accessor returns undef by default, but can optionally throw. Changing behavior is done globally, so all objects are affected.

  • accessors must be explicitly added.

  • accessors may have aliases

  • values may be validated

  • invoking an accessor may trigger a callback

BUGS

Please report any bugs or feature requests on the bugtracker website https://rt.cpan.org/Public/Dist/Display.html?Name=Hash-Wrap or by email to bug-Hash-Wrap@rt.cpan.org.

When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature.

SOURCE

The development version is on github at https://github.com/djerius/hash-wrap and may be cloned from git://github.com/djerius/hash-wrap.git

AUTHOR

Diab Jerius <djerius@cpan.org>

COPYRIGHT AND LICENSE

This software is Copyright (c) 2017 by Smithsonian Astrophysical Observatory.

This is free software, licensed under:

  The GNU General Public License, Version 3, June 2007