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.\" ========================================================================
.\"
.IX Title "Moose::Meta::Attribute 3pm"
.TH Moose::Meta::Attribute 3pm 2024-01-21 "perl v5.38.2" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
Moose::Meta::Attribute \- The Moose attribute metaclass
.SH VERSION
.IX Header "VERSION"
version 2.2207
.SH DESCRIPTION
.IX Header "DESCRIPTION"
This class is a subclass of Class::MOP::Attribute that provides
additional Moose-specific functionality.
.PP
To really understand this class, you will need to start with the
Class::MOP::Attribute documentation. This class can be understood
as a set of additional features on top of the basic feature provided
by that parent class.
.SH INHERITANCE
.IX Header "INHERITANCE"
\&\f(CW\*(C`Moose::Meta::Attribute\*(C'\fR is a subclass of Class::MOP::Attribute.
.SH METHODS
.IX Header "METHODS"
Many of the documented below override methods in
Class::MOP::Attribute and add Moose specific features.
.SS Creation
.IX Subsection "Creation"
.ie n .IP "\fBMoose::Meta::Attribute\->new($name, \fR\fB%options\fR\fB)\fR" 4
.el .IP "\fBMoose::Meta::Attribute\->new($name, \fR\f(CB%options\fR\fB)\fR" 4
.IX Item "Moose::Meta::Attribute->new($name, %options)"
This method overrides the Class::MOP::Attribute constructor.
.Sp
Many of the options below are described in more detail in the
Moose::Manual::Attributes document.
.Sp
It adds the following options to the constructor:
.RS 4
.IP \(bu 8
is => 'ro', 'rw', 'bare'
.Sp
This provides a shorthand for specifying the \f(CW\*(C`reader\*(C'\fR, \f(CW\*(C`writer\*(C'\fR, or
\&\f(CW\*(C`accessor\*(C'\fR names. If the attribute is read-only ('ro') then it will
have a \f(CW\*(C`reader\*(C'\fR method with the same attribute as the name.
.Sp
If it is read-write ('rw') then it will have an \f(CW\*(C`accessor\*(C'\fR method
with the same name. If you provide an explicit \f(CW\*(C`writer\*(C'\fR for a
read-write attribute, then you will have a \f(CW\*(C`reader\*(C'\fR with the same
name as the attribute, and a \f(CW\*(C`writer\*(C'\fR with the name you provided.
.Sp
Use 'bare' when you are deliberately not installing any methods
(accessor, reader, etc.) associated with this attribute; otherwise,
Moose will issue a warning when this attribute is added to a
metaclass.
.IP \(bu 8
isa => \f(CW$type\fR
.Sp
This option accepts a type. The type can be a string, which should be
a type name. If the type name is unknown, it is assumed to be a class
name.
.Sp
This option can also accept a Moose::Meta::TypeConstraint object.
.Sp
If you \fIalso\fR provide a \f(CW\*(C`does\*(C'\fR option, then your \f(CW\*(C`isa\*(C'\fR option must
be a class name, and that class must do the role specified with
\&\f(CW\*(C`does\*(C'\fR.
.IP \(bu 8
does => \f(CW$role\fR
.Sp
This is short-hand for saying that the attribute's type must be an
object which does the named role.
.IP \(bu 8
coerce => \f(CW$bool\fR
.Sp
This option is only valid for objects with a type constraint
(\f(CW\*(C`isa\*(C'\fR) that defined a coercion. If this is true, then coercions will be applied whenever
this attribute is set.
.Sp
You cannot make both this and the \f(CW\*(C`weak_ref\*(C'\fR option true.
.IP \(bu 8
trigger => \f(CW$sub\fR
.Sp
This option accepts a subroutine reference, which will be called after
the attribute is set.
.IP \(bu 8
required => \f(CW$bool\fR
.Sp
An attribute which is required must be provided to the constructor. An
attribute which is required can also have a \f(CW\*(C`default\*(C'\fR or \f(CW\*(C`builder\*(C'\fR,
which will satisfy its required-ness.
.Sp
A required attribute must have a \f(CW\*(C`default\*(C'\fR, \f(CW\*(C`builder\*(C'\fR or a
non\-\f(CW\*(C`undef\*(C'\fR \f(CW\*(C`init_arg\*(C'\fR
.IP \(bu 8
lazy => \f(CW$bool\fR
.Sp
A lazy attribute must have a \f(CW\*(C`default\*(C'\fR or \f(CW\*(C`builder\*(C'\fR. When an
attribute is lazy, the default value will not be calculated until the
attribute is read.
.IP \(bu 8
weak_ref => \f(CW$bool\fR
.Sp
If this is true, the attribute's value will be stored as a weak
reference.
.IP \(bu 8
documentation
.Sp
An arbitrary string that can be retrieved later by calling \f(CW\*(C`$attr\->documentation\*(C'\fR.
.IP \(bu 8
auto_deref => \f(CW$bool\fR
.Sp
\&\fBNote that in cases where you want this feature you are often better served
by using a Moose::Meta::Attribute::Native trait instead\fR.
.Sp
If this is true, then the reader will dereference the value when it is
called. The attribute must have a type constraint which defines the
attribute as an array or hash reference.
.IP \(bu 8
lazy_build => \f(CW$bool\fR
.Sp
\&\fBNote that use of this feature is strongly discouraged.\fR Some documentation
used to encourage use of this feature as a best practice, but we have changed
our minds.
.Sp
Setting this to true makes the attribute lazy and provides a number of
default methods.
.Sp
.Vb 4
\&  has \*(Aqsize\*(Aq => (
\&      is         => \*(Aqro\*(Aq,
\&      lazy_build => 1,
\&  );
.Ve
.Sp
is equivalent to this:
.Sp
.Vb 7
\&  has \*(Aqsize\*(Aq => (
\&      is        => \*(Aqro\*(Aq,
\&      lazy      => 1,
\&      builder   => \*(Aq_build_size\*(Aq,
\&      clearer   => \*(Aqclear_size\*(Aq,
\&      predicate => \*(Aqhas_size\*(Aq,
\&  );
.Ve
.Sp
If your attribute name starts with an underscore (\f(CW\*(C`_\*(C'\fR), then the clearer
and predicate will as well:
.Sp
.Vb 4
\&  has \*(Aq_size\*(Aq => (
\&      is         => \*(Aqro\*(Aq,
\&      lazy_build => 1,
\&  );
.Ve
.Sp
becomes:
.Sp
.Vb 7
\&  has \*(Aq_size\*(Aq => (
\&      is        => \*(Aqro\*(Aq,
\&      lazy      => 1,
\&      builder   => \*(Aq_build_\|_size\*(Aq,
\&      clearer   => \*(Aq_clear_size\*(Aq,
\&      predicate => \*(Aq_has_size\*(Aq,
\&  );
.Ve
.Sp
Note the doubled underscore in the builder name. Internally, Moose
simply prepends the attribute name with "_build_" to come up with the
builder name.
.IP \(bu 8
role_attribute => \f(CW$role_attribute\fR
.Sp
If provided, this should be a Moose::Meta::Role::Attribute object.
.RE
.RS 4
.RE
.ie n .IP \fR\fB$attr\fR\fB\->clone(%options)\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->clone(%options)\fR 4
.IX Item "$attr->clone(%options)"
This creates a new attribute based on attribute being cloned. You must
supply a \f(CW\*(C`name\*(C'\fR option to provide a new name for the attribute.
.Sp
The \f(CW%options\fR can only specify options handled by
Class::MOP::Attribute.
.SS "Value management"
.IX Subsection "Value management"
.ie n .IP "\fR\fB$attr\fR\fB\->initialize_instance_slot($meta_instance, \fR\fB$instance\fR\fB, \fR\fB$params\fR\fB)\fR" 4
.el .IP "\fR\f(CB$attr\fR\fB\->initialize_instance_slot($meta_instance, \fR\f(CB$instance\fR\fB, \fR\f(CB$params\fR\fB)\fR" 4
.IX Item "$attr->initialize_instance_slot($meta_instance, $instance, $params)"
This method is used internally to initialize the attribute's slot in
the object \f(CW$instance\fR.
.Sp
This overrides the Class::MOP::Attribute method to handle lazy
attributes, weak references, and type constraints.
.IP \fBget_value\fR 4
.IX Item "get_value"
.PD 0
.IP \fBset_value\fR 4
.IX Item "set_value"
.PD
.Vb 4
\&  eval { $point\->meta\->get_attribute(\*(Aqx\*(Aq)\->set_value($point, \*(Aqforty\-two\*(Aq) };
\&  if($@) {
\&    print "Oops: $@\en";
\&  }
.Ve
.Sp
\&\fIAttribute (x) does not pass the type constraint (Int) with 'forty\-two'\fR
.Sp
Before setting the value, a check is made on the type constraint of
the attribute, if it has one, to see if the value passes it. If the
value fails to pass, the set operation dies.
.Sp
Any coercion to convert values is done before checking the type constraint.
.Sp
To check a value against a type constraint before setting it, fetch the
attribute instance using "find_attribute_by_name" in Class::MOP::Class,
fetch the type_constraint from the attribute using "type_constraint" in Moose::Meta::Attribute
and call "check" in Moose::Meta::TypeConstraint. See Moose::Cookbook::Basics::Company_Subtypes
for an example.
.SS "Attribute Accessor generation"
.IX Subsection "Attribute Accessor generation"
.ie n .IP \fR\fB$attr\fR\fB\->install_accessors\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->install_accessors\fR 4
.IX Item "$attr->install_accessors"
This method overrides the parent to also install delegation methods.
.Sp
If, after installing all methods, the attribute object has no associated
methods, it throws an error unless \f(CW\*(C`is => \*(Aqbare\*(Aq\*(C'\fR was passed to the
attribute constructor.  (Trying to add an attribute that has no associated
methods is almost always an error.)
.ie n .IP \fR\fB$attr\fR\fB\->remove_accessors\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->remove_accessors\fR 4
.IX Item "$attr->remove_accessors"
This method overrides the parent to also remove delegation methods.
.ie n .IP "\fR\fB$attr\fR\fB\->inline_set($instance_var, \fR\fB$value_var\fR\fB)\fR" 4
.el .IP "\fR\f(CB$attr\fR\fB\->inline_set($instance_var, \fR\f(CB$value_var\fR\fB)\fR" 4
.IX Item "$attr->inline_set($instance_var, $value_var)"
This method return a code snippet suitable for inlining the relevant
operation. It expect strings containing variable names to be used in the
inlining, like \f(CW\*(Aq$self\*(Aq\fR or \f(CW\*(Aq$_[1]\*(Aq\fR.
.ie n .IP \fR\fB$attr\fR\fB\->install_delegation\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->install_delegation\fR 4
.IX Item "$attr->install_delegation"
This method adds its delegation methods to the attribute's associated
class, if it has any to add.
.ie n .IP \fR\fB$attr\fR\fB\->remove_delegation\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->remove_delegation\fR 4
.IX Item "$attr->remove_delegation"
This method remove its delegation methods from the attribute's
associated class.
.ie n .IP \fR\fB$attr\fR\fB\->accessor_metaclass\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->accessor_metaclass\fR 4
.IX Item "$attr->accessor_metaclass"
Returns the accessor metaclass name, which defaults to
Moose::Meta::Method::Accessor.
.ie n .IP \fR\fB$attr\fR\fB\->delegation_metaclass\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->delegation_metaclass\fR 4
.IX Item "$attr->delegation_metaclass"
Returns the delegation metaclass name, which defaults to
Moose::Meta::Method::Delegation.
.SS "Additional Moose features"
.IX Subsection "Additional Moose features"
These methods are not found in the superclass. They support features
provided by Moose.
.ie n .IP \fR\fB$attr\fR\fB\->does($role)\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->does($role)\fR 4
.IX Item "$attr->does($role)"
This indicates whether the \fIattribute itself\fR does the given
role. The role can be given as a full class name, or as a resolvable
trait name.
.Sp
Note that this checks the attribute itself, not its type constraint,
so it is checking the attribute's metaclass and any traits applied to
the attribute.
.ie n .IP "\fBMoose::Meta::Class\->interpolate_class_and_new($name, \fR\fB%options\fR\fB)\fR" 4
.el .IP "\fBMoose::Meta::Class\->interpolate_class_and_new($name, \fR\f(CB%options\fR\fB)\fR" 4
.IX Item "Moose::Meta::Class->interpolate_class_and_new($name, %options)"
This is an alternate constructor that handles the \f(CW\*(C`metaclass\*(C'\fR and
\&\f(CW\*(C`traits\*(C'\fR options.
.Sp
Effectively, this method is a factory that finds or creates the
appropriate class for the given \f(CW\*(C`metaclass\*(C'\fR and/or \f(CW\*(C`traits\*(C'\fR.
.Sp
Once it has the appropriate class, it will call \f(CW\*(C`$class\->new($name,
%options)\*(C'\fR on that class.
.ie n .IP \fR\fB$attr\fR\fB\->clone_and_inherit_options(%options)\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->clone_and_inherit_options(%options)\fR 4
.IX Item "$attr->clone_and_inherit_options(%options)"
This method supports the \f(CW\*(C`has \*(Aq+foo\*(Aq\*(C'\fR feature. It does various bits
of processing on the supplied \f(CW%options\fR before ultimately calling
the \f(CW\*(C`clone\*(C'\fR method.
.Sp
One of its main tasks is to make sure that the \f(CW%options\fR provided
does not include the options returned by the
\&\f(CW\*(C`illegal_options_for_inheritance\*(C'\fR method.
.ie n .IP \fR\fB$attr\fR\fB\->illegal_options_for_inheritance\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->illegal_options_for_inheritance\fR 4
.IX Item "$attr->illegal_options_for_inheritance"
This returns a blacklist of options that can not be overridden in a
subclass's attribute definition.
.Sp
This exists to allow a custom metaclass to change or add to the list
of options which can not be changed.
.ie n .IP \fR\fB$attr\fR\fB\->type_constraint\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->type_constraint\fR 4
.IX Item "$attr->type_constraint"
Returns the Moose::Meta::TypeConstraint object for this attribute,
if it has one.
.ie n .IP \fR\fB$attr\fR\fB\->has_type_constraint\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->has_type_constraint\fR 4
.IX Item "$attr->has_type_constraint"
Returns true if this attribute has a type constraint.
.ie n .IP \fR\fB$attr\fR\fB\->verify_against_type_constraint($value)\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->verify_against_type_constraint($value)\fR 4
.IX Item "$attr->verify_against_type_constraint($value)"
Given a value, this method returns true if the value is valid for the
attribute's type constraint. If the value is not valid, it throws an
error.
.ie n .IP \fR\fB$attr\fR\fB\->handles\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->handles\fR 4
.IX Item "$attr->handles"
This returns the value of the \f(CW\*(C`handles\*(C'\fR option passed to the
constructor.
.ie n .IP \fR\fB$attr\fR\fB\->has_handles\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->has_handles\fR 4
.IX Item "$attr->has_handles"
Returns true if this attribute performs delegation.
.ie n .IP \fR\fB$attr\fR\fB\->is_weak_ref\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->is_weak_ref\fR 4
.IX Item "$attr->is_weak_ref"
Returns true if this attribute stores its value as a weak reference.
.ie n .IP \fR\fB$attr\fR\fB\->is_required\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->is_required\fR 4
.IX Item "$attr->is_required"
Returns true if this attribute is required to have a value.
.ie n .IP \fR\fB$attr\fR\fB\->is_lazy\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->is_lazy\fR 4
.IX Item "$attr->is_lazy"
Returns true if this attribute is lazy.
.ie n .IP \fR\fB$attr\fR\fB\->is_lazy_build\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->is_lazy_build\fR 4
.IX Item "$attr->is_lazy_build"
Returns true if the \f(CW\*(C`lazy_build\*(C'\fR option was true when passed to the
constructor.
.ie n .IP \fR\fB$attr\fR\fB\->should_coerce\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->should_coerce\fR 4
.IX Item "$attr->should_coerce"
Returns true if the \f(CW\*(C`coerce\*(C'\fR option passed to the constructor was
true.
.ie n .IP \fR\fB$attr\fR\fB\->should_auto_deref\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->should_auto_deref\fR 4
.IX Item "$attr->should_auto_deref"
Returns true if the \f(CW\*(C`auto_deref\*(C'\fR option passed to the constructor was
true.
.ie n .IP \fR\fB$attr\fR\fB\->trigger\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->trigger\fR 4
.IX Item "$attr->trigger"
This is the subroutine reference that was in the \f(CW\*(C`trigger\*(C'\fR option
passed to the constructor, if any.
.ie n .IP \fR\fB$attr\fR\fB\->has_trigger\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->has_trigger\fR 4
.IX Item "$attr->has_trigger"
Returns true if this attribute has a trigger set.
.ie n .IP \fR\fB$attr\fR\fB\->documentation\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->documentation\fR 4
.IX Item "$attr->documentation"
Returns the value that was in the \f(CW\*(C`documentation\*(C'\fR option passed to
the constructor, if any.
.ie n .IP \fR\fB$attr\fR\fB\->has_documentation\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->has_documentation\fR 4
.IX Item "$attr->has_documentation"
Returns true if this attribute has any documentation.
.ie n .IP \fR\fB$attr\fR\fB\->role_attribute\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->role_attribute\fR 4
.IX Item "$attr->role_attribute"
Returns the Moose::Meta::Role::Attribute object from which this attribute
was created, if any. This may return \f(CW\*(C`undef\*(C'\fR.
.ie n .IP \fR\fB$attr\fR\fB\->has_role_attribute\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->has_role_attribute\fR 4
.IX Item "$attr->has_role_attribute"
Returns true if this attribute has an associated role attribute.
.ie n .IP \fR\fB$attr\fR\fB\->applied_traits\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->applied_traits\fR 4
.IX Item "$attr->applied_traits"
This returns an array reference of all the traits which were applied
to this attribute. If none were applied, this returns \f(CW\*(C`undef\*(C'\fR.
.ie n .IP \fR\fB$attr\fR\fB\->has_applied_traits\fR 4
.el .IP \fR\f(CB$attr\fR\fB\->has_applied_traits\fR 4
.IX Item "$attr->has_applied_traits"
Returns true if this attribute has any traits applied.
.SH BUGS
.IX Header "BUGS"
See "BUGS" in Moose for details on reporting bugs.
.SH AUTHORS
.IX Header "AUTHORS"
.IP \(bu 4
Stevan Little <stevan@cpan.org>
.IP \(bu 4
Dave Rolsky <autarch@urth.org>
.IP \(bu 4
Jesse Luehrs <doy@cpan.org>
.IP \(bu 4
Shawn M Moore <sartak@cpan.org>
.IP \(bu 4
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
.IP \(bu 4
Karen Etheridge <ether@cpan.org>
.IP \(bu 4
Florian Ragwitz <rafl@debian.org>
.IP \(bu 4
Hans Dieter Pearcey <hdp@cpan.org>
.IP \(bu 4
Chris Prather <chris@prather.org>
.IP \(bu 4
Matt S Trout <mstrout@cpan.org>
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is copyright (c) 2006 by Infinity Interactive, Inc.
.PP
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.