-=pod
+package Moose::Manual::Attributes;
+
+# ABSTRACT: Object attributes with Moose
-=head1 NAME
+__END__
-Moose::Manual::Attributes - Object attributes with Moose
+=pod
=head1 INTRODUCTION
=head2 Read-write vs. read-only
-The options passed to C<has> define the properties of the
-attribute. There are many options, but in the simplest form you just
-need to set C<is>, which can be either C<rw> (read-write) or C<ro>
-(read-only). When an attribute is C<rw>, you can change it by passing
-a value to its accessor. When an attribute is C<ro>, you may only read
-the current value of the attribute.
+The options passed to C<has> define the properties of the attribute. There are
+many options, but in the simplest form you just need to set C<is>, which can
+be either C<ro> (read-only) or C<rw> (read-write). When an attribute is C<rw>,
+you can change it by passing a value to its accessor. When an attribute is
+C<ro>, you may only read the current value of the attribute.
In fact, you could even omit C<is>, but that gives you an attribute
that has no accessor. This can be useful with other attribute options,
By default, the accessor method has the same name as the attribute. If
you declared your attribute as C<ro> then your accessor will be
-read-only. If you declared it read-write, you get a read-write
+read-only. If you declared it as C<rw>, you get a read-write
accessor. Simple.
Given our C<Person> example above, we now have a single C<first_name>
predicate => 'has_size',
);
-This is a dumb example, but it illustrates the point that the subroutine
+This is a trivial example, but it illustrates the point that the subroutine
will be called for every new object created.
When you provide a C<default> subroutine reference, it is called as a
method on the object, with no additional parameters:
has 'size' => (
- is => 'ro',
+ is => 'ro',
default => sub {
my $self = shift;
- return $self->height > 200 ? 'big' : 'average';
+ return $self->height > 200 ? 'large' : 'average';
},
);
attribute C<lazy>. Laziness is covered in the next section.
If you want to use a reference of any sort as the default value, you
-must return it from a subroutine. This is necessary because otherwise
-Perl would instantiate the reference exactly once, and it would be
-shared by all objects:
+must return it from a subroutine.
+
+ has 'mapping' => (
+ is => 'ro',
+ default => sub { {} },
+ );
+
+This is necessary because otherwise Perl would instantiate the reference
+exactly once, and it would be shared by all objects:
has 'mapping' => (
is => 'ro',
If Moose allowed this then the default mapping attribute could easily
end up shared across many objects. Instead, wrap it in a subroutine
-reference:
-
- has 'mapping' => (
- is => 'ro',
- default => sub { {} }, # right!
- );
+reference as we saw above.
This is a bit awkward, but it's just the way Perl works.
-As an alternative to using a subroutine reference, you can instead
-supply a C<builder> method for your attribute:
+As an alternative to using a subroutine reference, you can supply a C<builder>
+method for your attribute:
has 'size' => (
is => 'ro',
return ( 'small', 'medium', 'large' )[ int( rand 3 ) ];
}
-This has several advantages. First, it moves a chunk of code to its
-own named method, which improves readability and code
-organization.
+This has several advantages. First, it moves a chunk of code to its own named
+method, which improves readability and code organization. Second, because this
+is a I<named> method, it can be subclassed or provided by a role.
We strongly recommend that you use a C<builder> instead of a
C<default> for anything beyond the most trivial default.
sub _build_size { return 'small' }
-=head3 Builders can be composed from roles
+=head3 Builders work well with roles
Because builders are called by name, they work well with roles. For
example, a role could provide an attribute but require that the
Roles are covered in L<Moose::Manual::Roles>.
-=head2 Laziness and C<lazy_build>
+=head2 Laziness
Moose lets you defer attribute population by making an attribute
C<lazy>:
We recommend that you make any attribute with a builder or non-trivial
default C<lazy> as a matter of course.
-To facilitate this, you can simply specify the C<lazy_build> attribute
-option. This bundles up a number of options together:
-
- has 'size' => (
- is => 'ro',
- lazy_build => 1,
- );
-
-This is the same as specifying all of these options:
-
- has 'size' => (
- is => 'ro',
- lazy => 1,
- builder => '_build_size',
- clearer => 'clear_size',
- predicate => 'has_size',
- );
-
-If your attribute name starts with an underscore (C<_>), then the clearer
-and predicate will as well:
-
- has '_size' => (
- is => 'ro',
- lazy_build => 1,
- );
-
-becomes:
-
- has '_size' => (
- is => 'ro',
- lazy => 1,
- builder => '_build__size',
- clearer => '_clear_size',
- predicate => '_has_size',
- );
-
-Note the doubled underscore in the builder name. Internally, Moose
-simply prepends the attribute name with "_build_" to come up with the
-builder name.
-
-If you don't like the names that C<lazy_build> generates, you can
-always provide your own:
-
- has 'size' => (
- is => 'ro',
- lazy_build => 1,
- clearer => '_clear_size',
- );
-
-Options that you explicitly provide are always used in favor of
-Moose's internal defaults.
-
=head2 Constructor parameters (C<init_arg>)
By default, each attribute can be passed by name to the class's
the constructor parameter. You may also want to make an attribute
unsettable via the constructor.
-Both of these goals can be accomplished with the C<init_arg> option:
+You can do either of these things with the C<init_arg> option:
has 'bigness' => (
is => 'ro',
the constructor. This is particularly handy for private attributes:
has '_genetic_code' => (
- is => 'ro',
- lazy_build => 1,
- init_arg => undef,
+ is => 'ro',
+ lazy => 1,
+ builder => '_build_genetic_code',
+ init_arg => undef,
);
By setting the C<init_arg> to C<undef>, we make it impossible to set
This is very useful when you're building objects that may contain
circular references.
+When the object in a weak references goes out of scope, the attribute's value
+will become C<undef> "behind the scenes". This is done by the Perl interpreter
+directly, so Moose does not see this change. This means that triggers don't
+fire, coercions aren't applied, etc.
+
+The attribute is not cleared, so a predicate method for that attribute will
+still return true. Similarly, when the attribute is next accessed, a default
+value will not be generated.
+
=head2 Triggers
A C<trigger> is a subroutine that is called whenever the attribute is
}
$msg .= " - size is now $size";
- warn $msg.
+ warn $msg;
}
The trigger is called I<after> an attribute's value is set. It is
called as a method on the object, and receives the new and old values as
its arguments. If the attribute had not previously been set at all,
then only the new value is passed. This lets you distinguish between
-the case where the attribute had no value versus when it was C<undef>.
+the case where the attribute had no value versus when the old value was C<undef>.
This differs from an C<after> method modifier in two ways. First, a
trigger is only called when the attribute is set, as opposed to
passed to the constructor.
However, triggers are I<not> called when an attribute is populated
-from a C<default> or C<builder>
+from a C<default> or C<builder>.
=head2 Attribute types
See L<Moose::Manual::Delegation> for documentation on how to set up
delegation methods.
-=head2 Metaclass and traits
-
-One of Moose's best features is that it can be extended in all sorts
-of ways through the use of custom metaclasses and metaclass traits.
-
-When declaring an attribute, you can declare a metaclass or a set of
-traits for the attribute:
-
- has 'mapping' => (
- metaclass => 'Hash',
- is => 'ro',
- default => sub { {} },
- );
+=head2 Attribute traits and metaclasses
-In this case, the metaclass C<Hash> really refers to
-L<Moose::Meta::Attribute::Native::Trait::Hash>. Moose also provides
-native traits for L<Number|Moose::Meta::Attribute::Native::Trait::Number>,
-L<Counter|Moose::Meta::Attribute::Native::Trait::Counter>,
-L<String|Moose::Meta::Attribute::Native::Trait::String>,
-L<Bool|Moose::Meta::Attribute::Native::Trait::Bool>, and
-L<Array|Moose::Meta::Attribute::Native::Trait::Array>.
+One of Moose's best features is that it can be extended in all sorts of ways
+through the use of metaclass traits and custom metaclasses.
-You can also apply one or more traits to an attribute:
+You can apply one or more traits to an attribute:
use MooseX::MetaDescription;
some examples. You can also write your own metaclasses and traits. See
the "Meta" and "Extending" recipes in L<Moose::Cookbook> for examples.
-=head1 ATTRIBUTE INHERITANCE
-
-By default, a child inherits all of its parent class(es)' attributes
-as-is. However, you can explicitly change some aspects of the
-inherited attribute in the child class.
-
-The options that can be overridden in a subclass are:
-
-=over 4
-
-=item * default
+=head2 Native Delegations
-=item * coerce
+Native delegations allow you to delegate to standard Perl data structures as
+if they were objects.
-=item * required
+For example, we can pretend that an array reference has methods like
+C<push()>, C<shift()>, C<map()>, C<count()>, and more.
-=item * documentation
-
-=item * lazy
-
-=item * isa
-
-=item * handles
-
-=item * builder
+ has 'options' => (
+ traits => ['Array'],
+ is => 'ro',
+ isa => 'ArrayRef[Str]',
+ default => sub { [] },
+ handles => {
+ all_options => 'elements',
+ add_option => 'push',
+ map_options => 'map',
+ option_count => 'count',
+ sorted_options => 'sort',
+ },
+ );
-=item * metaclass
+See L<Moose::Manual::Delegation> for more details.
-=item * traits
+=head1 ATTRIBUTE INHERITANCE
-=back
+By default, a child inherits all of its parent class(es)' attributes
+as-is. However, you can change most aspects of the inherited attribute in the
+child class. You cannot change any of its associated method names (reader,
+writer, predicate, etc).
To override an attribute, you simply prepend its name with a plus sign
(C<+>):
If your attribute is an array reference or hash reference, the
C<auto_deref> option will make Moose dereference the value when it is
-returned from the reader method:
+returned from the reader method I<in list context>:
my %map = $object->mapping;
This option only works if your attribute is explicitly typed as an
-C<ArrayRef> or C<HashRef>.
+C<ArrayRef> or C<HashRef>. When the reader is called in I<scalar> context,
+the reference itself is returned.
However, we recommend that you use L<Moose::Meta::Attribute::Native> traits
for these types of attributes, which gives you much more control over how
-they are accessed and manipulated.
+they are accessed and manipulated. See also
+L<Moose::Manual::BestPractices#Use_Moose::Meta::Attribute::Native_traits_instead_of_auto_deref>.
=head2 Initializer
-Moose provides an attribute option called C<initializer>. This is
-similar to C<builder>, except that it is I<only> called during object
-construction.
-
-This option is inherited from L<Class::MOP>, but we recommend that you
-use a C<builder> (which is Moose-only) instead.
-
-=head1 AUTHOR
-
-Dave Rolsky E<lt>autarch@urth.orgE<gt>
-
-=head1 COPYRIGHT AND LICENSE
-
-Copyright 2009 by Infinity Interactive, Inc.
-
-L<http://www.iinteractive.com>
-
-This library is free software; you can redistribute it and/or modify
-it under the same terms as Perl itself.
+Moose provides an attribute option called C<initializer>. This is called when
+the attribute's value is being set in the constructor, and lets you change the
+value before it is set.
=cut
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