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 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 you 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 that an attribute can be explicitly set to
106 C<undef> or some other false value, but the predicate will return
109 The clearer method unsets the attribute. This is I<not> the
110 same as setting the value to C<undef>, but you can only distinguish
111 between them if you define a predicate method!
113 Here's some code to illustrate the relationship between an accessor,
114 predicate, and clearer method.
122 clearer => 'clear_ssn',
123 predicate => 'has_ssn',
128 my $person = Person->new();
129 $person->has_ssn; # false
132 $person->ssn; # returns undef
133 $person->has_ssn; # true
136 $person->ssn; # returns undef
137 $person->has_ssn; # false
139 $person->ssn('123-45-6789');
140 $person->ssn; # returns '123-45-6789'
141 $person->has_ssn; # true
143 my $person2 = Person->new( ssn => '111-22-3333');
144 $person2->has_ssn; # true
146 By default, Moose does not make a predicate or clearer for you. You must
147 explicitly provide names for them, and then Moose will create the methods
150 =head2 Required or not?
152 By default, all attributes are optional, and do not need to be
153 provided at object construction time. If you want to make an attribute
154 required, simply set the C<required> option to true:
161 There are a couple caveats worth mentioning in regards to what
162 "required" actually means.
164 Basically, all it says is that this attribute (C<name>) must be provided to
165 the constructor, or be lazy with either a default or a builder. It does not
166 say anything about its value, so it could be C<undef>.
168 If you define a clearer method on a required attribute, the clearer
169 I<will> work, so even a required attribute can be unset after object
172 This means that if you do make an attribute required, providing a
173 clearer doesn't make much sense. In some cases, it might be handy to
174 have a I<private> C<clearer> and C<predicate> for a required
177 =head2 Default and builder methods
179 Attributes can have default values, and Moose provides two ways to
180 specify that default.
182 In the simplest form, you simply provide a non-reference scalar value
183 for the C<default> option:
188 predicate => 'has_size',
191 If the size attribute is not provided to the constructor, then it ends
192 up being set to C<medium>:
194 my $person = Person->new();
195 $person->size; # medium
196 $person->has_size; # true
198 You can also provide a subroutine reference for C<default>. This
199 reference will be called as a method on the object.
204 sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] },
205 predicate => 'has_size',
208 This is dumb example, but it illustrates the point that the subroutine
209 will be called for every new object created.
211 When you provide a C<default> subroutine reference, it is called as a
212 method on the object, with no additional parameters:
219 return $self->height > 200 ? 'big' : 'average';
223 When the C<default> is called during object construction, it may be
224 called before other attributes have been set. If your default is
225 dependent on other parts of the object's state, you can make the
226 attribute C<lazy>. Laziness is covered in the next section.
228 If you want to use a reference of any sort as the default value, you
229 must return it from a subroutine. This is necessary because otherwise
230 Perl would instantiate the reference exactly once, and it would be
231 shared by all objects:
235 default => {}, # wrong!
238 Moose will throw an error if you pass a bare non-subroutine reference
241 If Moose allowed this then the default mapping attribute could easily
242 end up shared across many objects. Instead, wrap it in a subroutine
247 default => sub { {} }, # right!
250 This is a bit awkward, but it's just the way Perl works.
252 As an alternative to using a subroutine reference, you can instead
253 supply a C<builder> method for your attribute:
257 builder => '_build_size',
258 predicate => 'has_size',
262 return ( 'small', 'medium', 'large' )[ int( rand 3 ) ];
265 This has several advantages. First, it moves a chunk of code to its
266 own named method, which improves readability and code
269 We strongly recommend that you use a C<builder> instead of a
270 C<default> for anything beyond the most trivial default.
272 A C<builder>, just like a C<default>, is called as a method on the
273 object with no additional parameters.
275 =head3 Builders allow subclassing
277 Because the C<builder> is called I<by name>, it goes through Perl's
278 method resolution. This means that builder methods are both
279 inheritable and overridable.
281 If we subclass our C<Person> class, we can override C<_build_size>:
288 sub _build_size { return 'small' }
290 =head3 Builders can be composed from roles
292 Because builders are called by name, they work well with roles. For
293 example, a role could provide an attribute but require that the
294 consuming class provide the C<builder>:
299 requires '_build_size';
304 builder => '_build_size',
312 sub _build_size { return 'small' }
314 Roles are covered in L<Moose::Manual::Roles>.
316 =head2 Laziness and C<lazy_build>
318 Moose lets you defer attribute population by making an attribute
324 builder => '_build_size',
327 When C<lazy> is true, the default is not generated until the reader
328 method is called, rather than at object construction time. There are
329 several reasons you might choose to do this.
331 First, if the default value for this attribute depends on some other
332 attributes, then the attribute I<must> be C<lazy>. During object
333 construction, defaults are not generated in a predictable order, so
334 you cannot count on some other attribute being populated when
335 generating a default.
337 Second, there's often no reason to calculate a default before it's
338 needed. Making an attribute C<lazy> lets you defer the cost until the
339 attribute is needed. If the attribute is I<never> needed, you save
342 We recommend that you make any attribute with a builder or non-trivial
343 default C<lazy> as a matter of course.
345 To facilitate this, you can simply specify the C<lazy_build> attribute
346 option. This bundles up a number of options together:
353 This is the same as specifying all of these options:
358 builder => '_build_size',
359 clearer => 'clear_size',
360 predicate => 'has_size',
363 If your attribute name starts with an underscore (C<_>), then the clearer
364 and predicate will as well:
376 builder => '_build__size',
377 clearer => '_clear_size',
378 predicate => '_has_size',
381 Note the doubled underscore in the builder name. Internally, Moose
382 simply prepends the attribute name with "_build_" to come up with the
385 If you don't like the names that C<lazy_build> generates, you can
386 always provide your own:
391 clearer => '_clear_size',
394 Options that you explicitly provide are always used in favor of
395 Moose's internal defaults.
397 =head2 Constructor parameters (C<init_arg>)
399 By default, each attribute can be passed by name to the class's
400 constructor. On occasion, you may want to use a different name for
401 the constructor parameter. You may also want to make an attribute
402 unsettable via the constructor.
404 Both of these goals can be accomplished with the C<init_arg> option:
411 Now we have an attribute named "bigness", but we pass C<size> to the
414 Even more useful is the ability to disable setting an attribute via
415 the constructor. This is particularly handy for private attributes:
417 has '_genetic_code' => (
423 By setting the C<init_arg> to C<undef>, we make it impossible to set
424 this attribute when creating a new object.
426 =head2 Weak references
428 Moose has built-in support for weak references. If you set the
429 C<weak_ref> option to a true value, then it will call
430 C<Scalar::Util::weaken> whenever the attribute is set:
437 $node->parent($parent_node);
439 This is very useful when you're building objects that may contain
444 A C<trigger> is a subroutine that is called whenever the attribute is
449 trigger => \&_size_set,
453 my ( $self, $size ) = @_;
455 warn $self->name, " size is now $size\n";
458 The trigger is called as a method, and receives the new value as its argument.
459 The trigger is called I<after> the value is set.
461 This differs from an C<after> method modifier in two ways. First, a
462 trigger is only called when the attribute is set, as opposed to
463 whenever the accessor method is called (for reading or
464 writing). Second, it is also called when an attribute's value is
465 passed to the constructor.
467 However, triggers are I<not> called when an attribute is populated
468 from a C<default> or C<builder>
470 =head2 Attribute types
472 Attributes can be restricted to only accept certain types:
474 has 'first_name' => (
479 This says that the C<first_name> attribute must be a string.
481 Moose also provides a shortcut for specifying that an attribute only
482 accepts objects that do a certain role:
486 does => 'MyApp::Weapon',
489 See the L<Moose::Manual::Types> documentation for a complete
490 discussion of Moose's type system.
494 An attribute can define methods which simply delegate to its value:
496 has 'hair_color' => (
498 isa => 'Graphics::Color::RGB',
499 handles => { hair_color_hex => 'as_hex_string' },
502 This adds a new method, C<hair_color_hex>. When someone calls
503 C<hair_color_hex>, internally, the object just calls C<<
504 $self->hair_color->as_hex_string >>.
506 See L<Moose::Manual::Delegation> for documentation on how to set up
509 =head2 Metaclass and traits
511 One of Moose's best features is that it can be extended in all sorts
512 of ways through the use of custom metaclasses and metaclass traits.
514 When declaring an attribute, you can declare a metaclass or a set of
515 traits for the attribute:
517 use MooseX::AttributeHelpers;
520 metaclass => 'Collection::Hash',
522 default => sub { {} },
525 In this case, the metaclass C<Collection::Hash> really refers to
526 L<MooseX::AttributeHelpers::Collection::Hash>.
528 You can also apply one or more traits to an attribute:
530 use MooseX::MetaDescription;
534 traits => ['MooseX::MetaDescription::Meta::Trait'],
536 html_widget => 'text_input',
537 serialize_as => 'element',
541 The advantage of traits is that you can mix more than one of them
542 together easily (in fact, a trait is just a role under the hood).
544 There are a number of MooseX modules on CPAN which provide useful
545 attribute metaclasses and traits. See L<Moose::Manual::MooseX> for
546 some examples. You can also write your own metaclasses and traits. See
547 the "Meta" and "Extending" recipes in L<Moose::Cookbook> for examples.
549 =head1 ATTRIBUTE INHERITANCE
551 By default, a child inherits all of its parent class(es)' attributes
552 as-is. However, you can explicitly change some aspects of the
553 inherited attribute in the child class.
555 The options that can be overridden in a subclass are:
565 =item * documentation
581 To override an attribute, you simply prepend its name with a plus sign
590 has '+first_name' => (
595 Now the C<first_name> attribute in C<LazyPerson> is lazy, and defaults
598 We recommend that you exercise caution when changing the type (C<isa>)
599 of an inherited attribute.
601 =head1 MORE ON ATTRIBUTES
603 Moose attributes are a big topic, and this document glosses over a few
604 aspects. We recommend that you read the L<Moose::Manual::Delegation>
605 and L<Moose::Manual::Types> documents to get a more complete
606 understanding of attribute features.
608 =head1 A FEW MORE OPTIONS
610 Moose has lots of attribute options. The ones listed below are
611 superseded by some more modern features, but are covered for the sake
614 =head2 The C<documentation> option
616 You can provide a piece of documentation as a string for an attribute:
618 has 'first_name' => (
620 documentation => q{The person's first (personal) name},
623 Moose does absolutely nothing with this information other than store
626 =head2 The C<auto_deref> option
628 If your attribute is an array reference or hash reference, the
629 C<auto_deref> option will make Moose dereference the value when it is
630 returned from the reader method:
632 my %map = $object->mapping;
634 This option only works if your attribute is explicitly typed as an
635 C<ArrayRef> or C<HashRef>.
637 However, we recommend that you use L<MooseX::AttributeHelpers> for
638 these types of attributes, which gives you much more control over how
639 they are accessed and manipulated.
643 Moose provides an attribute option called C<initializer>. This is
644 similar to C<builder>, except that it is I<only> called during object
647 This option is inherited from L<Class::MOP>, but we recommend that you
648 use a C<builder> (which is Moose-only) instead.
652 Dave Rolsky E<lt>autarch@urth.orgE<gt>
654 =head1 COPYRIGHT AND LICENSE
656 Copyright 2009 by Infinity Interactive, Inc.
658 L<http://www.iinteractive.com>
660 This library is free software; you can redistribute it and/or modify
661 it under the same terms as Perl itself.