=head1 NAME
-Moose::Manual::Attribute - Object attributes with Moose
+Moose::Manual::Attributes - Object attributes with Moose
=head1 INTRODUCTION
-Attributes are the most feature-rich part of Moose, and also one of
-the most useful. It's easy to imagine a class without roles or method
-modifiers, but almost every class has attributes.
+Moose attributes have many properties, and attributes are probably the
+single most powerful and flexible part of Moose. You can create a
+powerful class simply by declaring attributes. In fact, it's possible
+to have classes that consist solely of attribute declarations.
-Attributes are properties that every member of a class has. For
-example, we might say that "every Person object has a first name and
-last name. Attributes can be optional, so that we can say "some Person
+An attribute is a property that every member of a class has. For
+example, we might say that "every C<Person> object has a first name and
+last name". Attributes can be optional, so that we can say "some C<Person>
objects have a social security number (and some don't)".
-At its simplest, an attribute can be thought of slot in hash that can
-be read and set. However, attributes, can also have things like
-default values, laziness, type constraints, delegation and much more.
+At its simplest, an attribute can be thought of as a named value (as
+in a hash) that can be read and set. However, attributes can also have
+defaults, type constraints, delegation and much more.
+
+In other languages, attributes are also referred to as slots or
+properties.
=head1 ATTRIBUTE OPTIONS
has 'first_name' => ( is => 'rw' );
-This says that all person objects have an optional "first_name"
-attribute that can be both read and set on the object.
+This says that all C<Person> objects have an optional read-write
+"first_name" attribute.
-=head2 Read-write Vs Read-only
+=head2 Read-write vs. read-only
-The code inside the parentheses defines the details of the
-attribute. There are a lot of options you can put here, but in the
-simplest form you just need to include C<is>, which can be either
-C<rw> (read-write) or C<ro> (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).
-(In fact, you could even omit C<is>, but that leaves you with an
-attribute that has no accessors, which is pointless unless you're
-doing some deep, dark magic).
+(In fact, you could even omit C<is>, but that gives you an attribute
+that has no accessors, which is pointless unless you're doing some
+deep, dark magic).
-=head2 Accessor Methods
+=head2 Accessor methods
Each attribute has one or more accessor methods. An accessor lets you
-read and write the value of the attribute for an object.
+read and write the value of that attribute for an object.
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
accessor. Simple.
-So with our Person example above, we now have a single C<first_name>
-accessor that can set or return a person object's first name.
+Given our C<Person> example above, we now have a single C<first_name>
+accessor that can read or write a C<Person> object's C<first_name>
+attribute's value.
If you want, you can also explicitly specify the method names to be
used for reading and writing an attribute's value. This is
-particularly handy when you'd like an attribute to be publically
+particularly handy when you'd like an attribute to be publicly
readable, but only privately settable. For example:
- has 'weight' =>
- ( is => 'rw',
- writer => '_set_weight',
- );
+ has 'weight' => (
+ is => 'ro',
+ writer => '_set_weight',
+ );
-This might be useful if weight is calculated based on other methods,
-for example every time the C<eat> method is called, we might adjust
+This might be useful if weight is calculated based on other methods.
+For example, every time the C<eat> method is called, we might adjust
weight. This lets us hide the implementation details of weight
changes, but still provide the weight value to users of the class.
Some people might prefer to have distinct methods for reading and
-writing, even when writing is a public method. In I<Perl Best
-Practices>, Damian Conway recommends that reader methods start with
-"get_" and writer methods start with "set_".
+writing. In I<Perl Best Practices>, Damian Conway recommends that
+reader methods start with "get_" and writer methods start with "set_".
We can do exactly that by providing names for both the C<reader> and
C<writer> methods:
- has 'weight' =>
- ( is => 'rw',
- reader => 'get_weight',
- writer => 'set_weight',
- );
+ has 'weight' => (
+ is => 'rw',
+ reader => 'get_weight',
+ writer => 'set_weight',
+ );
If you're thinking that doing this over and over would be insanely
tedious, you're right! Fortunately, Moose provides a powerful
-extension system that lets you do things like override the default
-accessor method conventions. See L<Moose::Manual::MooseX> for more
-details.
+extension system that lets you override the default naming
+conventions. See L<Moose::Manual::MooseX> for more details.
-=head2 Predicate and Clearer Methods
+=head2 Predicate and clearer methods
-Moose is able to explicitly distinguish between false or undefined
-values, and an attribute which is not set. If you want to be able to
-access and manipulate these states, you also need to define clearer
-and predicate methods for your attributes.
+Moose allows you to explicitly distinguish between a false or
+undefined attribute value and an attribute which has not been set. If
+you want to access this information, you must define clearer and
+predicate methods for an attribute.
-A predicate method can be used to determine whether or not a given
-attribute is currently set. Note that even if the attribute was
-explicitly set to undef or some other false value, the predicate will
-return true.
+A predicate method tells you whether or not a given attribute is
+currently set. Note that an attribute can be explicitly set to
+C<undef> or some other false value, but the predicate will return
+true.
The clearer method unsets the attribute. This is I<not> the
same as setting the value to C<undef>, but you can only distinguish
use Moose;
- has 'ssn' =>
- ( is => 'rw',
- clearer => 'clear_ssn',
- predicate => 'has_ssn',
- );
+ has 'ssn' => (
+ is => 'rw',
+ clearer => 'clear_ssn',
+ predicate => 'has_ssn',
+ );
...
my $person2 = Person->new( ssn => '111-22-3333');
$person2->has_ssn; # true
-Note that by default, Moose does not make a predicate or clearer for
-you. You have to explicitly provide a method name for the ones you
-want.
+By default, Moose does not make a predicate or clearer for you. You
+must explicitly provide names for them.
-=head2 Required or Not?
+=head2 Required or not?
-By default, all attributes are optional. That means that they do not
-need to be provided at object construction time. If you want to make
-an attribute required, simply set the C<required> option to true:
+By default, all attributes are optional, and do not need to be
+provided at object construction time. If you want to make an attribute
+required, simply set the C<required> option to true:
- has 'name' =>
- ( is => 'rw',
- required => 1,
- );
+ has 'name' => (
+ is => 'ro',
+ required => 1,
+ );
There are a couple caveats worth mentioning in regards to what
-required actually means.
+"required" actually means.
-Basically, all it says is that this attribute must be provided to the
-constructor. It does not say anything about its value, so it could be
-C<undef>.
+Basically, all it says is that this attribute (C<name>) must be provided to
+the constructor, or be lazy with either a default or a builder. It does not
+say anything about its value, so it could be C<undef>.
If you define a clearer method on a required attribute, the clearer
-I<will> work. So even though something is defined as required, you can
-remove it after object construction.
+I<will> work, so even a required attribute can be unset after object
+construction.
-So if you do make an attribute required, that probably means that
-providing a clearer doesn't make much sense. In some cases, it might
-be handy to have a I<private> C<clearer> and C<predicate> for a
-required attribute.
+This means that if you do make an attribute required, providing a
+clearer doesn't make much sense. In some cases, it might be handy to
+have a I<private> C<clearer> and C<predicate> for a required
+attribute.
-=head2 Default and Builder Methods
+=head2 Default and builder methods
-Attributes can have default values, and there are several ways to
-specify this.
+Attributes can have default values, and Moose provides two ways to
+specify that default.
In the simplest form, you simply provide a non-reference scalar value
for the C<default> option:
- has 'size' =>
- ( is => 'rw',
- default => 'medium',
- predicate => 'has_size',
- );
+ has 'size' => (
+ is => 'ro',
+ default => 'medium',
+ predicate => 'has_size',
+ );
If the size attribute is not provided to the constructor, then it ends
-up being set to "medium":
+up being set to C<medium>:
my $person = Person->new();
$person->size; # medium
$person->has_size; # true
You can also provide a subroutine reference for C<default>. This
-reference will be called a method on the object.
+reference will be called as a method on the object.
- has 'size' =>
- ( is => 'rw',
- default =>
- sub { ('small', 'medium', 'large')[ int( rand 3 ) ] },
- predicate => 'has_size',
- );
+ has 'size' => (
+ is => 'ro',
+ default =>
+ sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] },
+ predicate => 'has_size',
+ );
This is dumb example, but it illustrates the point that the subroutine
will be called for every new object created.
-Of course, if it's called during object construction, it may be before
-other attributes have been set. If your default is dependent on other
-parts of the object's state, you can make the default c<lazy>, which
-is covered in the next section.
+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',
+ default => sub {
+ my $self = shift;
+
+ return $self->height > 200 ? 'big' : 'average';
+ },
+ );
+
+When the C<default> is called during object construction, it may be
+called before other attributes have been set. If your default is
+dependent on other parts of the object's state, you can make the
+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:
- has 'mapping' =>
- ( is => 'rw',
- default => {}, # wrong!
- );
+ has 'mapping' => (
+ is => 'ro',
+ default => {}, # wrong!
+ );
+
+Moose will throw an error if you pass a bare non-subroutine reference
+as the default.
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 => 'rw',
- default => sub { {} }, # right!
- );
+ has 'mapping' => (
+ is => 'ro',
+ default => sub { {} }, # right!
+ );
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:
- has 'size' =>
- ( is => 'rw',
- builder => '_build_size',
- predicate => 'has_size',
- );
+ has 'size' => (
+ is => 'ro',
+ builder => '_build_size',
+ predicate => 'has_size',
+ );
sub _build_size {
- return ('small', 'medium', 'large')[ int( rand 3 ) ];
+ 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. Second, the C<_build_size> method can be overridden in
-subclasses.
+organization.
We strongly recommend that you use a C<builder> instead of a
C<default> for anything beyond the most trivial default.
-=head2 Laziness and lazy_build
+A C<builder>, just like a C<default>, is called as a method on the
+object with no additional parameters.
+
+=head3 Builders allow subclassing
+
+Because the C<builder> is called I<by name>, it goes through Perl's
+method resolution. This means that builder methods are both
+inheritable and overridable.
+
+If we subclass our C<Person> class, we can override C<_build_size>:
+
+ package Lilliputian;
+
+ use Moose;
+ extends 'Person';
+
+ sub _build_size { return 'small' }
+
+=head3 Builders can be composed from roles
+
+Because builders are called by name, they work well with roles. For
+example, a role could provide an attribute but require that the
+consuming class provide the C<builder>:
+
+ package HasSize;
+ use Moose::Role;
+
+ requires '_build_size';
+
+ has 'size' => (
+ is => 'ro',
+ lazy => 1,
+ builder => '_build_size',
+ );
+
+ package Lilliputian;
+ use Moose;
+
+ with 'HasSize';
+
+ sub _build_size { return 'small' }
+
+Roles are covered in L<Moose::Manual::Roles>.
+
+=head2 Laziness and C<lazy_build>
Moose lets you defer attribute population by making an attribute
C<lazy>:
- has 'size' =>
- ( is => 'rw',
- lazy => 1,
- builder => '_build_size',
- );
+ has 'size' => (
+ is => 'ro',
+ lazy => 1,
+ builder => '_build_size',
+ );
-When the C<lazy> option is true, the attribute is not populated until
-the reader method is called, rather than at object construction
-time. There are several reasons you might choose to do this.
+When C<lazy> is true, the default is not generated until the reader
+method is called, rather than at object construction time. There are
+several reasons you might choose to do this.
First, if the default value for this attribute depends on some other
attributes, then the attribute I<must> be C<lazy>. During object
-construction, default subroutine references are not called in any
-particular order, so you cannot count on other attribute being
-populated at that time.
+construction, defaults are not generated in a predictable order, so
+you cannot count on some other attribute being populated when
+generating a default.
-Second, there's often no reason to spend program time calculating a
-default before its needed. Making an attribute C<lazy> lets you defer
-the cost until the attribute is needed. If the attribute is I<never>
-needed, you save some CPU time.
+Second, there's often no reason to calculate a default before it's
+needed. Making an attribute C<lazy> lets you defer the cost until the
+attribute is needed. If the attribute is I<never> needed, you save
+some CPU time.
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 => 'rw',
- lazy_build => 1,
- );
+ has 'size' => (
+ is => 'ro',
+ lazy_build => 1,
+ );
This is the same as specifying all of these options:
- has 'size' =>
- ( is => 'rw',
- lazy => 1,
- builder => '_build_size',
- clearer => 'clear_size',
- predicate => 'has_size',
- );
+ has 'size' => (
+ is => 'ro',
+ lazy => 1,
+ builder => '_build_size',
+ clearer => 'clear_size',
+ predicate => 'has_size',
+ );
-If your attribute name starts with an underscore (_), then the clearer
+If your attribute name starts with an underscore (C<_>), then the clearer
and predicate will as well:
- has '_size' =>
- ( is => 'rw',
- lazy_build => 1,
- );
+ has '_size' => (
+ is => 'ro',
+ lazy_build => 1,
+ );
becomes:
- has '_size' =>
- ( is => 'rw',
- lazy => 1,
- builder => '_build__size',
- clearer => '_clear_size',
- predicate => '_has_size',
- );
+ 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
If you don't like the names that C<lazy_build> generates, you can
always provide your own:
- has 'size' =>
- ( is => 'rw',
- lazy_build => 1,
- clearer => '_has_size',
- );
+ 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 Weak References
+=head2 Constructor parameters (C<init_arg>)
+
+By default, each attribute can be passed by name to the class's
+constructor. On occasion, you may want to use a different name for
+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:
+
+ has 'bigness' => (
+ is => 'ro',
+ init_arg => 'size',
+ );
+
+Now we have an attribute named "bigness", but we pass C<size> to the
+constructor.
+
+Even more useful is the ability to disable setting an attribute via
+the constructor. This is particularly handy for private attributes:
+
+ has '_genetic_code' => (
+ is => 'ro',
+ lazy_build => 1,
+ init_arg => undef,
+ );
+
+By setting the C<init_arg> to C<undef>, we make it impossible to set
+this attribute when creating a new object.
+
+=head2 Weak references
Moose has built-in support for weak references. If you set the
-C<weak_ref> to a true value for the attribute, then it will call
+C<weak_ref> option to a true value, then it will call
C<Scalar::Util::weaken> whenever the attribute is set:
- has 'parent' =>
- ( is => 'rw',
- weak_ref => 1,
- );
+ has 'parent' => (
+ is => 'rw',
+ weak_ref => 1,
+ );
$node->parent($parent_node);
=head2 Triggers
-You can provide a C<trigger> option as a subroutine reference which
-will be called whenever the attribute is set:
+A C<trigger> is a subroutine that is called whenever the attribute is
+set:
- has 'size' =>
- ( is => 'rw',
- trigger => \&_size_set,
- );
+ has 'size' => (
+ is => 'rw',
+ trigger => \&_size_set,
+ );
sub _size_set {
- my ( $self, $size, $meta_attr ) = @_;
+ my ( $self, $size ) = @_;
- print $self->name, " size is now $size\n";
+ warn $self->name, " size is now $size\n";
}
-The trigger is called as a method, and receives the new value as well
-as the L<Moose::Meta::Attribute> object for the attribute.
+The trigger is called as a method, and receives the new value as its argument.
+The trigger is called I<after> the value is set.
+
+This differs from an after method modifier in two ways. First, a
+trigger is only called when the attribute is set, as opposed to
+whenever the accessor method is called (for reading or
+writing). Second, it is also called when an attribute's value is
+passed to the constructor.
-=head2 Attribute Types
+However, triggers are I<not> called when an attribute is populated
+from a C<default> or C<builder>
-Attributes can be restriced to only accept certain types:
+=head2 Attribute types
- has 'first_name' =>
- ( is => 'rw',
- isa => 'Str',
- );
+Attributes can be restricted to only accept certain types:
-This says that the first_name attribute must be a string.
+ has 'first_name' => (
+ is => 'ro',
+ isa => 'Str',
+ );
+
+This says that the C<first_name> attribute must be a string.
Moose also provides a shortcut for specifying that an attribute only
accepts objects that do a certain role:
- has 'weapon' =>
- ( is => 'rw',
- does => 'MyApp::Weapon',
- );
+ has 'weapon' => (
+ is => 'rw',
+ does => 'MyApp::Weapon',
+ );
See the L<Moose::Manual::Types> documentation for a complete
discussion of Moose's type system.
=head2 Delegation
-Attributes can define delegations to their values:
+Attributes can define methods which simply delegate to their values:
- has 'hair_color' =>
- ( is => 'rw',
- isa => 'Graphics::Color::RGB',
- handles => { hair_color_hex => 'as_hex_string' },
- );
+ has 'hair_color' => (
+ is => 'ro',
+ isa => 'Graphics::Color::RGB',
+ handles => { hair_color_hex => 'as_hex_string' },
+ );
-This adds a new method, C<hair_color_hex>. Internally, this just calls
-C<< $self->hair_color->as_hex_string >>.
+This adds a new method, C<hair_color_hex>. When someone calls
+C<hair_color_hex>, internally, the object just calls C<<
+$self->hair_color->as_hex_string >>.
-See L<Moose::Manual::Delegation> for more details on how to set up
+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 new metaclasses and metaclass traits.
+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:
use MooseX::AttributeHelpers;
- has 'mapping' =>
- ( metaclass => 'Collection::Hash',
- is => 'ro',
- default => sub { {} },
- );
+ has 'mapping' => (
+ metaclass => 'Collection::Hash',
+ is => 'ro',
+ default => sub { {} },
+ );
In this case, the metaclass C<Collection::Hash> really refers to
-C<MooseX::AttributeHelpers::Collection::Hash>.
+L<MooseX::AttributeHelpers::Collection::Hash>.
You can also apply one or more traits to an attribute:
-
use MooseX::MetaDescription;
- has 'size' =>
- ( is => 'rw',
- traits => [ 'MooseX::MetaDescription::Meta::Trait' ],
- description => { html_widget => 'text_input',
- serialize_as => 'element',
- },
- );
+ has 'size' => (
+ is => 'ro',
+ traits => ['MooseX::MetaDescription::Meta::Trait'],
+ description => {
+ html_widget => 'text_input',
+ serialize_as => 'element',
+ },
+ );
The advantage of traits is that you can mix more than one of them
together easily (in fact, a trait is just a role under the hood).
some examples. You can also write your own metaclasses and traits. See
the "Meta" and "Extending" recipes in L<Moose::Cookbook> for examples.
-=head2 Attribute Inheritance
+=head1 ATTRIBUTE INHERITANCE
By default, a child inherits all of its parent class(es)' attributes
-as-is. You can explicitly change some aspects of the inherited
-attribute in the child class.
+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:
=back
To override an attribute, you simply prepend its name with a plus sign
-(+):
+(C<+>):
package LazyPerson;
extends 'Person';
- has '+first_name' =>
- ( lazy => 1,
- default => 'Bill',
- );
+ has '+first_name' => (
+ lazy => 1,
+ default => 'Bill',
+ );
Now the C<first_name> attribute in C<LazyPerson> is lazy, and defaults
to C<'Bill'>.
We recommend that you exercise caution when changing the type (C<isa>)
-of an inherited attribute. It's best to only make the new type a
-subtype of the one accepted by the parent.
+of an inherited attribute.
+
+=head1 MORE ON ATTRIBUTES
+
+Moose attributes are a big topic, and this document glosses over a few
+aspects. We recommend that you read the L<Moose::Manual::Delegation>
+and L<Moose::Manual::Types> documents to get a more complete
+understanding of attribute features.
+
+=head1 A FEW MORE OPTIONS
+
+Moose has lots of attribute options. The ones listed below are
+superseded by some more modern features, but are covered for the sake
+of completeness.
=head2 The C<documentation> option
You can provide a piece of documentation as a string for an attribute:
- has 'first_name' =>
- ( is => 'rw',
- documentation => q{The person's first (personal) name},
- );
+ has 'first_name' => (
+ is => 'rw',
+ documentation => q{The person's first (personal) name},
+ );
Moose does absolutely nothing with this information other than store
it.
-As an alternative, you might want to look at the
-C<MooseX::MetaDescription> module, which lets you attach a
-"description" to each attribute. This description is a hashref that
-can include meta-information intended for use in other code, as well
-as documentation information.
-
-=head2 The C<auto_deref> Option
+=head2 The C<auto_deref> option
If your attribute is an array reference or hash reference, the
-C<auto_deref> option will make Moose de-reference the value when it is
+C<auto_deref> option will make Moose dereference the value when it is
returned from the reader method:
my %map = $object->mapping;
This option only works if your attribute is explicitly typed as an
-ArrayRef or HashRef.
+C<ArrayRef> or C<HashRef>.
-However, we recommend that you use C<MooseX::AttributeHelpers> for
+However, we recommend that you use L<MooseX::AttributeHelpers> for
these types of attributes, which gives you much more control over how
they are accessed and manipulated.
similar to C<builder>, except that it is I<only> called during object
construction.
-This option is inherited from C<Class::MOP>, but we recommend that you
+This option is inherited from L<Class::MOP>, but we recommend that you
use a C<builder> (which is Moose-only) instead.
-=head1 MORE ON ATTRIBUTES
+=head1 AUTHOR
-Moose attributes are a big topic, and this document glosses over a few
-topics. We recommend that you read the L<Moose::Manual::Delegation>
-and L<Moose::Manual::Types> documents to get a more complete
-understanding of attribute features.
+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.
+
+=cut
projects / gitmo/Moose.git / blobdiff