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Class::MOP::Attribute(User Contributed Perl DocumentatClass::MOP::Attribute(3)

NAME
       Class::MOP::Attribute - Attribute Meta Object

VERSION
       version 2.0604

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.

	   ·	   init_arg

		   This is a string value representing the expected key in an
		   initialization hash. For instance, if we have an "init_arg"
		   value of "-foo", then the following code will Just Work.

		     MyClass->meta->new_object( -foo => 'Hello There' );

		   If an init_arg is not assigned, it will automatically use
		   the attribute's name. If "init_arg" is explicitly set to
		   "undef", the attribute cannot be specified during
		   initialization.

	   ·	   builder

		   This provides the name of a method that will be called to
		   initialize the attribute. This method will be called on the
		   object after it is constructed. It is expected to return a
		   valid value for the attribute.

	   ·	   default

		   This can be used to provide an explicit default for
		   initializing the attribute. If the default you provide is a
		   subroutine reference, then this reference will be called as
		   a method on the object.

		   If the value is a simple scalar (string or number), then it
		   can be just passed as is. However, if you wish to
		   initialize it with a HASH or ARRAY ref, then you need to
		   wrap that inside a subroutine reference:

		     Class::MOP::Attribute->new(
			 'foo' => (
			     default => sub { [] },
			 )
		     );

		     # or ...

		     Class::MOP::Attribute->new(
			 'foo' => (
			     default => sub { {} },
			 )
		     );

		   If you wish to initialize an attribute with a subroutine
		   reference itself, then you need to wrap that in a
		   subroutine as well:

		     Class::MOP::Attribute->new(
			 'foo' => (
			     default => sub {
				 sub { print "Hello World" }
			     },
			 )
		     );

		   And lastly, if the value of your attribute is dependent
		   upon some other aspect of the instance structure, then you
		   can take advantage of the fact that when the "default"
		   value is called as a method:

		     Class::MOP::Attribute->new(
			 'object_identity' => (
			     default => sub { Scalar::Util::refaddr( $_[0] ) },
			 )
		     );

		   Note that there is no guarantee that attributes are
		   initialized in any particular order, so you cannot rely on
		   the value of some other attribute when generating the
		   default.

	   ·	   initializer

		   This option can be either a method name or a subroutine
		   reference. This method will be called when setting the
		   attribute's value in the constructor. Unlike "default" and
		   "builder", the initializer is only called when a value is
		   provided to the constructor. The initializer allows you to
		   munge this value during object construction.

		   The initializer is called as a method with three arguments.
		   The first is the value that was passed to the constructor.
		   The second is a subroutine reference that can be called to
		   actually set the attribute's value, and the last is the
		   associated "Class::MOP::Attribute" object.

		   This contrived example shows an initializer that sets the
		   attribute to twice the given value.

		     Class::MOP::Attribute->new(
			 'doubled' => (
			     initializer => sub {
				 my ( $self, $value, $set, $attr ) = @_;
				 $set->( $value * 2 );
			     },
			 )
		     );

		   Since an initializer can be a method name, you can easily
		   make attribute initialization use the writer:

		     Class::MOP::Attribute->new(
			 'some_attr' => (
			     writer	 => 'some_attr',
			     initializer => 'some_attr',
			 )
		     );

		   Your writer (actually, a wrapper around the writer, using
		   method modifications) will need to examine @_ and determine
		   under which context it is being called:

		     around 'some_attr' => sub {
			 my $orig = shift;
			 my $self = shift;
			 # $value is not defined if being called as a reader
			 # $setter and $attr are only defined if being called as an initializer
			 my ($value, $setter, $attr) = @_;

			 # the reader behaves normally
			 return $self->$orig if not @_;

			 # mutate $value as desired
			 # $value = <something($value);

			 # if called as an initializer, set the value and we're done
			 return $setter->($row) if $setter;

			 # otherwise, call the real writer with the new value
			 $self->$orig($row);
		     };

	   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.

	   ·	   accessor

		   An "accessor" is a standard Perl-style read/write accessor.
		   It will return the value of the attribute, and if a value
		   is passed as an argument, it will assign that value to the
		   attribute.

		   Note that "undef" is a legitimate value, so this will work:

		     $object->set_something(undef);

	   ·	   reader

		   This is a basic read-only accessor. It returns the value of
		   the attribute.

	   ·	   writer

		   This is a basic write accessor, it accepts a single
		   argument, and assigns that value to the attribute.

		   Note that "undef" is a legitimate value, so this will work:

		     $object->set_something(undef);

	   ·	   predicate

		   The predicate method returns a boolean indicating whether
		   or not the attribute has been explicitly set.

		   Note that the predicate returns true even if the attribute
		   was set to a false value (0 or "undef").

	   ·	   clearer

		   This method will uninitialize the attribute. After an
		   attribute is cleared, its "predicate" will return false.

	   ·	   definition_context

		   Mostly, this exists as a hook for the benefit of Moose.

		   This option should be a hash reference containing several
		   keys which will be used when inlining the attribute's
		   accessors. The keys should include "line", the line number
		   where the attribute was created, and either "file" or
		   "description".

		   This information will ultimately be used when eval'ing
		   inlined accessor code so that error messages report a
		   useful line and file name.

       $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.

AUTHOR
       Moose is maintained by the Moose Cabal, along with the help of many
       contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for
       details.

COPYRIGHT AND LICENSE
       This software is copyright (c) 2012 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.

perl v5.16.2			  2012-09-19	      Class::MOP::Attribute(3)
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