5 Moose::Manual::Attributes - Object attributes with Moose
9 Moose attributes have many properties, and attributes are probably the
10 single most powerful and flexible part of Moose. You can create a
11 powerful class simply by declaring attributes. In fact, it's possible
12 to have classes that consist solely of attribute declarations.
14 An attribute is a property that every member of a class has. For
15 example, we might say that "every C<Person> object has a first name and
16 last name". Attributes can be optional, so that we can say "some C<Person>
17 objects have a social security number (and some don't)".
19 At its simplest, an attribute can be thought of as a named value (as
20 in a hash) that can be read and set. However, attributes can also have
21 defaults, type constraints, delegation and much more.
23 In other languages, attributes are also referred to as slots or
26 =head1 ATTRIBUTE OPTIONS
28 Use the C<has> function to declare an attribute:
34 has 'first_name' => ( is => 'rw' );
36 This says that all C<Person> objects have an optional read-write
37 "first_name" attribute.
39 =head2 Read-write Vs read-only
41 The options passed to C<has> define the properties of the
42 attribute. There are a many options, but in the simplest form you just
43 need to set C<is>, which can be either C<rw> (read-write) or C<ro>
46 (In fact, you could even omit C<is>, but that gives you an attribute
47 that has no accessors, which is pointless unless you're doing some
50 =head2 Accessor methods
52 Each attribute has one or more accessor methods. An accessor lets you
53 read and write the value of that attribute for an object.
55 By default, the accessor method has the same name as the attribute. If
56 you declared your attribute as C<ro> then your accessor will be
57 read-only. If you declared it read-write, you get a read-write
60 Given our C<Person> example above, we now have a single C<first_name>
61 accessor that can read or write a C<Person> object's C<first_name>
64 If you want, you can also explicitly specify the method names to be
65 used for reading and writing an attribute's value. This is
66 particularly handy when you'd like an attribute to be publicly
67 readable, but only privately settable. For example:
71 writer => '_set_weight',
74 This might be useful if weight is calculated based on other methods,
75 for example every time the C<eat> method is called, we might adjust
76 weight. This lets us hide the implementation details of weight
77 changes, but still provide the weight value to users of the class.
79 Some people might prefer to have distinct methods for reading and
80 writing. In I<Perl Best Practices>, Damian Conway recommends that
81 reader methods start with "get_" and writer methods start with "set_".
83 We can do exactly that by providing names for both the C<reader> and
88 reader => 'get_weight',
89 writer => 'set_weight',
92 If you're thinking that doing this over and over would be insanely
93 tedious, you're right! Fortunately, Moose provides a powerful
94 extension system that lets override the default naming
95 conventions. See L<Moose::Manual::MooseX> for more details.
97 =head2 Predicate and clearer methods
99 Moose allows you to explicitly distinguish between a false or
100 undefined attribute value and an attribute which has not been set. If
101 you want to access this information, you must define clearer and
102 predicate methods for an attribute.
104 A predicate method tells you whether or not a given attribute is
105 currently set. Note an attribute can be explicitly set to C<undef> or
106 some other false value, but the predicate will return true.
108 The clearer method unsets the attribute. This is I<not> the
109 same as setting the value to C<undef>, but you can only distinguish
110 between them if you define a predicate method!
112 Here's some code to illustrate the relationship between an accessor,
113 predicate, and clearer method.
121 clearer => 'clear_ssn',
122 predicate => 'has_ssn',
127 my $person = Person->new();
128 $person->has_ssn; # false
131 $person->ssn; # returns undef
132 $person->has_ssn; # true
135 $person->ssn; # returns undef
136 $person->has_ssn; # false
138 $person->ssn('123-45-6789');
139 $person->ssn; # returns '123-45-6789'
140 $person->has_ssn; # true
142 my $person2 = Person->new( ssn => '111-22-3333');
143 $person2->has_ssn; # true
145 By default, Moose does not make a predicate or clearer for you. You
146 must explicitly provide names for them.
148 =head2 Required or not?
150 By default, all attributes are optional, and do not need to be
151 provided at object construction time. If you want to make an attribute
152 required, simply set the C<required> option to true:
159 There are a couple caveats worth mentioning in regards to what
160 "required" actually means.
162 Basically, all it says is that this attribute (C<name>) must be provided
163 to the constructor. It does not say anything about its value, so it
166 If you define a clearer method on a required attribute, the clearer
167 I<will> work, so even a required attribute can be unset after object
170 This means that if you do make an attribute required, providing a
171 clearer doesn't make much sense. In some cases, it might be handy to
172 have a I<private> C<clearer> and C<predicate> for a required
175 =head2 Default and builder methods
177 Attributes can have default values, and Moose provides two ways to
178 specify that default.
180 In the simplest form, you simply provide a non-reference scalar value
181 for the C<default> option:
186 predicate => 'has_size',
189 If the size attribute is not provided to the constructor, then it ends
190 up being set to C<medium>:
192 my $person = Person->new();
193 $person->size; # medium
194 $person->has_size; # true
196 You can also provide a subroutine reference for C<default>. This
197 reference will be called as a method on the object.
202 sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] },
203 predicate => 'has_size',
206 This is dumb example, but it illustrates the point that the subroutine
207 will be called for every new object created.
209 Of course, if it's called during object construction, it may be called
210 before other attributes have been set. If your default is dependent on
211 other parts of the object's state, you can make the attribute
212 C<lazy>. Laziness is covered in the next section.
214 If you want to use a reference of any sort as the default value, you
215 must return it from a subroutine. This is necessary because otherwise
216 Perl would instantiate the reference exactly once, and it would be
217 shared by all objects:
221 default => {}, # wrong!
224 Moose will throw an error if you pass a bare non-subroutine reference
227 If Moose allowed this then the default mapping attribute could easily
228 end up shared across many objects. Instead, wrap it in a subroutine
233 default => sub { {} }, # right!
236 This is a bit awkward, but it's just the way Perl works.
238 As an alternative to using a subroutine reference, you can instead
239 supply a C<builder> method for your attribute:
243 builder => '_build_size',
244 predicate => 'has_size',
248 return ( 'small', 'medium', 'large' )[ int( rand 3 ) ];
251 This has several advantages. First, it moves a chunk of code to its
252 own named method, which improves readability and code
255 We strongly recommend that you use a C<builder> instead of a
256 C<default> for anything beyond the most trivial default.
258 =head3 Builders allow subclassing
260 Because the C<builder> is called I<by name>, it goes through Perl's
261 method resolution. This means that builder methods are both
262 inheritable and overridable.
264 If we subclass our C<Person> class, we can override C<_build_size>:
271 sub _build_size { return 'small' }
273 =head3 Builders can be composed from roles
275 Because builders are called by name, they work well with roles. For
276 example, a role could provide an attribute but require that the
277 consuming class provide the C<builder>:
282 requires '_build_size';
287 builder => '_build_animal',
295 sub _build_size { return 'small' }
297 Roles are covered in L<Moose::Manual::Roles>.
299 =head2 Laziness and C<lazy_build>
301 Moose lets you defer attribute population by making an attribute
307 builder => '_build_size',
310 When C<lazy> is true, the default is not generated until the reader
311 method is called, rather than at object construction time. There are
312 several reasons you might choose to do this.
314 First, if the default value for this attribute depends on some other
315 attributes, then the attribute I<must> be C<lazy>. During object
316 construction, defaults are not generated in a predictable order, so
317 you cannot count on some other attribute being populated when
318 generating a default.
320 Second, there's often no reason to calculate a default before it's
321 needed. Making an attribute C<lazy> lets you defer the cost until the
322 attribute is needed. If the attribute is I<never> needed, you save
325 We recommend that you make any attribute with a builder or non-trivial
326 default C<lazy> as a matter of course.
328 To facilitate this, you can simply specify the C<lazy_build> attribute
329 option. This bundles up a number of options together:
336 This is the same as specifying all of these options:
341 builder => '_build_size',
342 clearer => 'clear_size',
343 predicate => 'has_size',
346 If your attribute name starts with an underscore (C<_>), then the clearer
347 and predicate will as well:
359 builder => '_build__size',
360 clearer => '_clear_size',
361 predicate => '_has_size',
364 Note the doubled underscore in the builder name. Internally, Moose
365 simply prepends the attribute name with "_build_" to come up with the
368 If you don't like the names that C<lazy_build> generates, you can
369 always provide your own:
374 clearer => '_clear_size',
377 Options that you explicitly provide are always used in favor of
378 Moose's internal defaults.
380 =head2 Constructor parameters (C<init_arg>)
382 By default, each attribute can be passed by name to the class's
383 constructor. On occasion, you may want to use a different name for
384 the constructor parameter. You may also want to make an attribute
385 unsettable via the constructor.
387 Both of these goals can be accomplished with the C<init_arg> option:
394 Now we have an attribute named "bigness", but we pass C<size> to the
397 Even more useful is the ability to disable setting an attribute via
398 the constructor. This is particularly handy for private attributes:
400 has '_genetic_code' => (
406 By setting the C<init_arg> to C<undef>, we make it impossible to set
407 this attribute when creating a new object.
409 =head2 Weak references
411 Moose has built-in support for weak references. If you set the
412 C<weak_ref> option to a true value, then it will call
413 C<Scalar::Util::weaken> whenever the attribute is set:
420 $node->parent($parent_node);
422 This is very useful when you're building objects that may contain
427 A C<trigger> is a subroutine that is called whenever the attribute is
432 trigger => \&_size_set,
436 my ( $self, $size, $meta_attr ) = @_;
438 warn $self->name, " size is now $size\n";
441 The trigger is called as a method, and receives the new value as well
442 as the L<Moose::Meta::Attribute> object for the attribute. The trigger
443 is called I<after> the value is set.
445 This differs from an after method modifier in two ways. First, a
446 trigger is only called when the attribute is set, as opposed to
447 whenever the accessor is called. Second, it is also called if the
448 attribute is set via a lazy default or builder.
450 =head2 Attribute types
452 Attributes can be restricted to only accept certain types:
454 has 'first_name' => (
459 This says that the C<first_name> attribute must be a string.
461 Moose also provides a shortcut for specifying that an attribute only
462 accepts objects that do a certain role:
466 does => 'MyApp::Weapon',
469 See the L<Moose::Manual::Types> documentation for a complete
470 discussion of Moose's type system.
474 Attributes can define methods which simply delegate to their values:
476 has 'hair_color' => (
478 isa => 'Graphics::Color::RGB',
479 handles => { hair_color_hex => 'as_hex_string' },
482 This adds a new method, C<hair_color_hex>. When someone calls
483 C<hair_color_hex>, internally, the object just calls C<<
484 $self->hair_color->as_hex_string >>.
486 See L<Moose::Manual::Delegation> for documentation on how to set up
489 =head2 Metaclass and traits
491 One of Moose's best features is that it can be extended in all sorts
492 of ways through the use of custom metaclasses and metaclass traits.
494 When declaring an attribute, you can declare a metaclass or a set of
495 traits for the attribute:
497 use MooseX::AttributeHelpers;
500 metaclass => 'Collection::Hash',
502 default => sub { {} },
505 In this case, the metaclass C<Collection::Hash> really refers to
506 L<MooseX::AttributeHelpers::Collection::Hash>.
508 You can also apply one or more traits to an attribute:
510 use MooseX::MetaDescription;
514 traits => ['MooseX::MetaDescription::Meta::Trait'],
516 html_widget => 'text_input',
517 serialize_as => 'element',
521 The advantage of traits is that you can mix more than one of them
522 together easily (in fact, a trait is just a role under the hood).
524 There are a number of MooseX modules on CPAN which provide useful
525 attribute metaclasses and traits. See L<Moose::Manual::MooseX> for
526 some examples. You can also write your own metaclasses and traits. See
527 the "Meta" and "Extending" recipes in L<Moose::Cookbook> for examples.
529 =head1 ATTRIBUTE INHERITANCE
531 By default, a child inherits all of its parent class(es)' attributes
532 as-is. However, you can explicitly change some aspects of the
533 inherited attribute in the child class.
535 The options that can be overridden in a subclass are:
545 =item * documentation
561 To override an attribute, you simply prepend its name with a plus sign
570 has '+first_name' => (
575 Now the C<first_name> attribute in C<LazyPerson> is lazy, and defaults
578 We recommend that you exercise caution when changing the type (C<isa>)
579 of an inherited attribute.
581 =head1 MORE ON ATTRIBUTES
583 Moose attributes are a big topic, and this document glosses over a few
584 aspects of their aspects. We recommend that you read the
585 L<Moose::Manual::Delegation> and L<Moose::Manual::Types> documents to
586 get a more complete understanding of attribute features.
588 =head1 A FEW MORE OPTIONS
590 Moose has lots of attribute options. The ones listed below are
591 superseded by some more modern features, but are covered for the sake
594 =head2 The C<documentation> option
596 You can provide a piece of documentation as a string for an attribute:
598 has 'first_name' => (
600 documentation => q{The person's first (personal) name},
603 Moose does absolutely nothing with this information other than store
606 =head2 The C<auto_deref> option
608 If your attribute is an array reference or hash reference, the
609 C<auto_deref> option will make Moose dereference the value when it is
610 returned from the reader method:
612 my %map = $object->mapping;
614 This option only works if your attribute is explicitly typed as an
615 C<ArrayRef> or C<HashRef>.
617 However, we recommend that you use L<MooseX::AttributeHelpers> for
618 these types of attributes, which gives you much more control over how
619 they are accessed and manipulated.
623 Moose provides an attribute option called C<initializer>. This is
624 similar to C<builder>, except that it is I<only> called during object
627 This option is inherited from L<Class::MOP>, but we recommend that you
628 use a C<builder> (which is Moose-only) instead.
632 Dave Rolsky E<lt>autarch@urth.orgE<gt>
634 =head1 COPYRIGHT AND LICENSE
636 Copyright 2009 by Infinity Interactive, Inc.
638 L<http://www.iinteractive.com>
640 This library is free software; you can redistribute it and/or modify
641 it under the same terms as Perl itself.