View on
MetaCPAN is shutting down
For details read Perl NOC. After June 25th this page will redirect to
Ian Brayshaw > Class-Declare-Attributes-0.09 > Class::Declare::Attributes



Annotate this POD


New  1
Open  0
View/Report Bugs
Module Version: 0.09   Source   Latest Release: Class-Declare-Attributes-0.12


Class::Declare::Attributes - Class::Declare method types using Perl attributes.


  package My::Class;

  use 5.006;
  use strict;
  use warnings;

  use base qw( Class::Declare::Attributes );

  # declare the class/instance attributes
  __PACKAGE__->declare( ... );

  # declare class/static/restricted/etc methods of this package

  sub my_abstract   : abstract   { ... }
  sub my_class      : class      { ... }
  sub my_static     : static     { ... }
  sub my_restricted : restricted { ... }
  sub my_public     : public     { ... }
  sub my_private    : private    { ... }
  sub my_protected  : protected  { ... }


Class::Declare::Attributes extends Class::Declare by adding support for Perl attributes for specifying class method types. This extension was inspired by Damian Conway's Attribute::Handlers module, and Tatsuhiko Miyagawa's Attribute::Protected module. The original implementation used Attribute::Handlers, but now simply refers to attributes.

The addition of Perl attribute support (not to be confused with object attributes, which are entirely different, and also supported by Class::Declare) greatly simplifies the specification of Class::Declare-derived class and instance methods. This should aid in the porting of existing code (Perl, Java and C++) to a Class::Declare framework, as well as simplify the development of new modules.

With the addition of Perl attributes, Class::Declare methods can now be written as

  sub method : public
    my $self = shift;

instead of

  sub method
    my $self = __PACKAGE__->public( shift );


Class::Declare::Attributes defines six method or subroutine attributes that correspond to the six method and object- and class-attribute types of Class::Declare:


abstract methods are merely placeholders and must be defined in subclasses. If called, an abstract method will throw an error through die().


class methods are accessible from anywhere, and may be called through the class, a derived class, or any instance derived from the defining class. This is the class equivalent of public methods.


static methods may only be accessed within the defining class and instances of that class. This is the class equivalent of private methods.


restricted methods may only be accessed from within the defining class and all classes and objects that inherit from it. This is the class equivalent of protected methods.


public methods are accessible from anywhere, but only through object instances derived from the defining class.


private methods are only accessible from within the defining class and instances of that class, and only through instances of the defining class.


protected methods are only accessible from within the defining class and all classes and objects derived from the defining class. As an instance method it may only be accessed via an object instance.

The attributes defined by Class::Declare::Attributes are not to be confused with the object and class attributes defined by Class::Declare::declare(). The clash in terminology is unfortunate, but as long as you remember the context of your attributes, i.e. are they Perl-attributes, or class-/object-attributes, the distinction should be clear.

Attribute Modifiers

Class::Declare::Attributes supports the use of the class and instance attribute modifiers defined by Class::Declare. These modifiers may be imported into the current namespace by either explicitly listing the modifier (rw and ro) or using one of the predefined tags: :read-write, :read-only and :modifiers. For example:

  use Class::Declare::Attributes qw( :read-only );

Note: The "magic" of Class::Declare::Attributes that defines the method attributes is performed during the compilation of the module it is used in. To access the attribute modifiers, the use base approach should be replaced with the more traditional:

  use Class::Declare::Attributes qw( :modifiers );
  use vars qw( @ISA );
  @ISA = qw( Class::Declare::Attributes );

However, because Class::Declare::Attributes (or more precisely Attribute::Handlers) operates before the execution phase, the assignment to @ISA will occur too late to take effect (resulting in an invalid attribute error). To prevent this error, and to bring the assignment to @ISA forward in the module compilation/execution phase, the assignment should be wrapped in a BEGIN {} block.

  BEGIN { @ISA = qw( Class::Declare::Attributes ); }

For more information on class and instance attribute modifiers, please refer to Class::Declare.


require( class )

In the event that a Class::Declare::Attributes-derived class needs to be loaded dynamically, the require() method should be used to ensure correct handling of the Class::Declare::Attributes-style attributes. require() is a class method of Class::Declare::Attributes and should therefore be called along the lines of the following:

  package My::Class;

  use strict;
  use warnings;

  use bae qw( Class::Declare::Attributes );


      my $class   = 'My::Class::To::Load';
         __PACKAGE__->require( $class )    or die;

$class can be either a class name (as above) or a string containing the definition of the class. require() will return true on success and undefined on failure, with $@ containing the error.


Class::Declare::Attributes is distributed as a separate module to Class::Declare as it requires Perl versions 5.6.0 and greater, while Class::Declare supports all object-aware versions of Perl (i.e. version 5.0 and above).

The interface Class::Declare::Attributes provides is not ideal. In fact, some might suggest that it's 'illegal'. In some ways, yes, it is illegal, because it has hijacked some lowercase attribute names that Perl has marked down for possible future use. However, as of Perl 5.8.0, these attributes are not in use (:shared is, which is why Class::Declare changed this class of attributes and methods to restricted), and so we may as well take advantage of them.

This is an example of what can be done with Perl (especially if you're willing to bend the rules), and who knows, maybe it's a glimpse of the sort of capabilities we'll see in Perl 6.


Class::Declare, attributes, Attribute::Protected, Attribute::Handlers.


Ian Brayshaw, <>


Copyright 2003-2010 by Ian Brayshaw. All rights reserved.

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

syntax highlighting: