NAME

Class::MOP::Attribute - Attribute Meta Object

VERSION

version 2.1605

SYNOPSIS

  Class::MOP::Attribute->new(
      foo => (
          accessor  => 'foo',           # dual purpose get/set accessor
          predicate => 'has_foo',       # predicate check for defined-ness
          init_arg  => '-foo',          # class->new will look for a -foo key
          default   => 'BAR IS BAZ!'    # if no -foo key is provided, use this
      )
  );
  
  Class::MOP::Attribute->new(
      bar => (
          reader    => 'bar',           # getter
          writer    => 'set_bar',       # setter
          predicate => 'has_bar',       # predicate check for defined-ness
          init_arg  => ':bar',          # class->new will look for a :bar key
                                        # no default value means it is undef
      )
  );

DESCRIPTION

The Attribute Protocol is almost entirely an invention of Class::MOP. Perl 5 does not have a consistent notion of attributes. There are so many ways in which this is done, and very few (if any) are easily discoverable by this module.

With that said, this module attempts to inject some order into this chaos, by introducing a consistent API which can be used to create object attributes.

METHODS

Creation

Class::MOP::Attribute->new($name, ?%options)

An attribute must (at the very least), have a $name. All other %options are added as key-value pairs.

The accessor, reader, writer, predicate and clearer options all accept the same parameters. You can provide the name of the method, in which case an appropriate default method will be generated for you. Or instead you can also provide hash reference containing exactly one key (the method name) and one value. The value should be a subroutine reference, which will be installed as the method itself.

$attr->clone(%options)

This clones the attribute. Any options you provide will override the settings of the original attribute. You can change the name of the new attribute by passing a name key in %options.

Informational

These are all basic read-only accessors for the values passed into the constructor.

$attr->name

Returns the attribute's name.

$attr->accessor
$attr->reader
$attr->writer
$attr->predicate
$attr->clearer

The accessor, reader, writer, predicate, and clearer methods all return exactly what was passed to the constructor, so it can be either a string containing a method name, or a hash reference.

$attr->initializer

Returns the initializer as passed to the constructor, so this may be either a method name or a subroutine reference.

$attr->init_arg
$attr->is_default_a_coderef
$attr->builder
$attr->default($instance)

The $instance argument is optional. If you don't pass it, the return value for this method is exactly what was passed to the constructor, either a simple scalar or a subroutine reference.

If you do pass an $instance and the default is a subroutine reference, then the reference is called as a method on the $instance and the generated value is returned.

$attr->slots

Return a list of slots required by the attribute. This is usually just one, the name of the attribute.

A slot is the name of the hash key used to store the attribute in an object instance.

$attr->get_read_method
$attr->get_write_method

Returns the name of a method suitable for reading or writing the value of the attribute in the associated class.

If an attribute is read- or write-only, then these methods can return undef as appropriate.

$attr->has_read_method
$attr->has_write_method

This returns a boolean indicating whether the attribute has a named read or write method.

$attr->get_read_method_ref
$attr->get_write_method_ref

Returns the subroutine reference of a method suitable for reading or writing the attribute's value in the associated class. These methods always return a subroutine reference, regardless of whether or not the attribute is read- or write-only.

$attr->insertion_order

If this attribute has been inserted into a class, this returns a zero based index regarding the order of insertion.

Informational predicates

These are all basic predicate methods for the values passed into new.

$attr->has_accessor
$attr->has_reader
$attr->has_writer
$attr->has_predicate
$attr->has_clearer
$attr->has_initializer
$attr->has_init_arg

This will be false if the init_arg was set to undef.

$attr->has_default

This will be false if the default was set to undef, since undef is the default default anyway.

$attr->has_builder
$attr->has_insertion_order

This will be false if this attribute has not be inserted into a class

Value management

These methods are basically "back doors" to the instance, and can be used to bypass the regular accessors, but still stay within the MOP.

These methods are not for general use, and should only be used if you really know what you are doing.

$attr->initialize_instance_slot($meta_instance, $instance, $params)

This method is used internally to initialize the attribute's slot in the object $instance.

The $params is a hash reference of the values passed to the object constructor.

It's unlikely that you'll need to call this method yourself.

$attr->set_value($instance, $value)

Sets the value without going through the accessor. Note that this works even with read-only attributes.

$attr->set_raw_value($instance, $value)

Sets the value with no side effects such as a trigger.

This doesn't actually apply to Class::MOP attributes, only to subclasses.

$attr->set_initial_value($instance, $value)

Sets the value without going through the accessor. This method is only called when the instance is first being initialized.

$attr->get_value($instance)

Returns the value without going through the accessor. Note that this works even with write-only accessors.

$attr->get_raw_value($instance)

Returns the value without any side effects such as lazy attributes.

Doesn't actually apply to Class::MOP attributes, only to subclasses.

$attr->has_value($instance)

Return a boolean indicating whether the attribute has been set in $instance. This how the default predicate method works.

$attr->clear_value($instance)

This will clear the attribute's value in $instance. This is what the default clearer calls.

Note that this works even if the attribute does not have any associated read, write or clear methods.

Class association

These methods allow you to manage the attributes association with the class that contains it. These methods should not be used lightly, nor are they very magical, they are mostly used internally and by metaclass instances.

$attr->associated_class

This returns the Class::MOP::Class with which this attribute is associated, if any.

$attr->attach_to_class($metaclass)

This method stores a weakened reference to the $metaclass object internally.

This method does not remove the attribute from its old class, nor does it create any accessors in the new class.

It is probably best to use the Class::MOP::Class add_attribute method instead.

$attr->detach_from_class

This method removes the associate metaclass object from the attribute it has one.

This method does not remove the attribute itself from the class, or remove its accessors.

It is probably best to use the Class::MOP::Class remove_attribute method instead.

Attribute Accessor generation

$attr->accessor_metaclass

Accessor methods are generated using an accessor metaclass. By default, this is Class::MOP::Method::Accessor. This method returns the name of the accessor metaclass that this attribute uses.

$attr->associate_method($method)

This associates a Class::MOP::Method object with the attribute. Typically, this is called internally when an attribute generates its accessors.

$attr->associated_methods

This returns the list of methods which have been associated with the attribute.

$attr->install_accessors

This method generates and installs code the attributes various accessors. It is typically called from the Class::MOP::Class add_attribute method.

$attr->remove_accessors

This method removes all of the accessors associated with the attribute.

This does not currently remove methods from the list returned by associated_methods.

$attr->inline_get
$attr->inline_set
$attr->inline_has
$attr->inline_clear

These methods return a code snippet suitable for inlining the relevant operation. They expect strings containing variable names to be used in the inlining, like '$self' or '$_[1]'.

Introspection

Class::MOP::Attribute->meta

This will return a Class::MOP::Class instance for this class.

It should also be noted that Class::MOP will actually bootstrap this module by installing a number of attribute meta-objects into its metaclass.

AUTHORS

COPYRIGHT AND LICENSE

This software is copyright (c) 2006 by Infinity Interactive, Inc.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.