-=pod
+package Moose::Manual::Attributes;
+
+# ABSTRACT: Object attributes with Moose
-=head1 NAME
+__END__
-Moose::Manual::Attribute - Object attributes with Moose
+=pod
=head1 INTRODUCTION
to have classes that consist solely of attribute declarations.
An attribute is a property 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
+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 as a named value (as
has 'first_name' => ( is => 'rw' );
-This says that all person objects have an optional read-write
+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 options passed to C<has> define the properties of the
-attribute. There are a 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).
+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 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 accessor. This can be useful with other attribute options,
+such as C<handles>. However, if your attribute generates I<no>
+accessors, Moose will issue a warning, because that usually means the
+programmer forgot to say the attribute is read-only or read-write. If
+you really mean to have no accessors, you can silence this warning by
+setting C<is> to C<bare>.
-=head2 Accessor Methods
+=head2 Accessor methods
Each attribute has one or more accessor methods. An accessor lets you
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
+read-only. If you declared it as C<rw>, you get a read-write
accessor. Simple.
-Given our Person example above, we now have a single C<first_name>
-accessor that can read or write 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',
+ 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.
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 override the default naming
+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 allows you to explicitly distinguish between a false or
undefined attribute value and an attribute which has not been set. If
predicate methods for an attribute.
A predicate method tells you whether or not a given attribute is
-currently set. Note an attribute can be explicitly set to undef or
-some other false value, but the predicate will return true.
+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
my $person2 = Person->new( ssn => '111-22-3333');
$person2->has_ssn; # true
-By default, Moose does not make a predicate or clearer for you. You
-must explicitly provide names for them.
+By default, Moose does not make a predicate or clearer for you. You must
+explicitly provide names for them, and then Moose will create the methods
+for you.
-=head2 Required or Not?
+=head2 Required or not?
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',
+ 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 (name) 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 a required attribute can be unset after object
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 Moose provides two ways to
specify that default.
for the C<default> option:
has 'size' => (
- is => 'rw',
+ 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
reference will be called as a method on the object.
has 'size' => (
- is => 'rw',
+ 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
+This is a trivial 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 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.
+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 ? 'large' : '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:
+must return it from a subroutine.
has 'mapping' => (
- is => 'rw',
+ 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',
default => {}, # wrong!
);
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!
- );
+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 => 'rw',
+ is => 'ro',
builder => '_build_size',
predicate => 'has_size',
);
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.
+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.
-=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 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
+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
Moose lets you defer attribute population by making an attribute
C<lazy>:
has 'size' => (
- is => 'rw',
+ is => 'ro',
lazy => 1,
builder => '_build_size',
);
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,
- );
-
-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',
- );
-
-If your attribute name starts with an underscore (_), then the clearer
-and predicate will as well:
-
- has '_size' => (
- is => 'rw',
- lazy_build => 1,
- );
-
-becomes:
-
- has '_size' => (
- is => 'rw',
- 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 => 'rw',
- lazy_build => 1,
- clearer => '_clear_size',
- );
-
-Options that you explicitly provide are always used in favor of
-Moose's internal defaults.
-
-=head2 Constructor Parameters (init_arg)
+=head2 Constructor parameters (C<init_arg>)
By default, each attribute can be passed by name to the class's
-constructor. On occassion, you may want to use a different name for
+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:
+You can do either of these things with the C<init_arg> option:
has 'bigness' => (
- is => 'rw',
+ is => 'ro',
init_arg => 'size',
);
-Now we have an attribute named bigness, but we pass C<size> to the
+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 => 'rw',
- 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 attribute when creating a new object.
-=head2 Weak References
+=head2 Weak references
Moose has built-in support for weak references. If you set the
C<weak_ref> option to a true value, then it will call
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
);
sub _size_set {
- my ( $self, $size, $meta_attr ) = @_;
+ my ( $self, $size, $old_size ) = @_;
+
+ my $msg = $self->name;
- warn $self->name, " size is now $size\n";
+ if ( @_ > 2 ) {
+ $msg .= " - old size was $old_size";
+ }
+
+ $msg .= " - size is now $size";
+ warn $msg;
}
-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 I<after> the value is set.
+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 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
+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>.
+
+=head2 Attribute types
Attributes can be restricted to only accept certain types:
has 'first_name' => (
- is => 'rw',
+ is => 'ro',
isa => 'Str',
);
-This says that the first_name attribute must be a string.
+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:
=head2 Delegation
-Attributes can define methods which simple delegate to their values:
+An attribute can define methods which simply delegate to its value:
has 'hair_color' => (
- is => 'rw',
+ is => 'ro',
isa => 'Graphics::Color::RGB',
handles => { hair_color_hex => 'as_hex_string' },
);
See L<Moose::Manual::Delegation> for documentation on how to set up
delegation methods.
-=head2 Metaclass and traits
+=head2 Attribute traits and metaclasses
-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.
+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.
-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 { {} },
- );
-
-In this case, the metaclass C<Collection::Hash> really refers to
-C<MooseX::AttributeHelpers::Collection::Hash>.
-
-You can also apply one or more traits to an attribute:
+You can apply one or more traits to an attribute:
use MooseX::MetaDescription;
has 'size' => (
- is => 'rw',
+ is => 'ro',
traits => ['MooseX::MetaDescription::Meta::Trait'],
description => {
html_widget => 'text_input',
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
-
-=item * coerce
-
-=item * required
-
-=item * documentation
-
-=item * lazy
-
-=item * isa
-
-=item * handles
-
-=item * builder
+=head2 Native Delegations
+
+Native delegations allow you to delegate to standard Perl data structures as
+if they were objects.
+
+For example, we can pretend that an array reference has methods like
+C<push()>, C<shift()>, C<map()>, C<count()>, and more.
+
+ 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<+>):
package LazyPerson;
We recommend that you exercise caution when changing the type (C<isa>)
of an inherited attribute.
+=head1 MULTIPLE ATTRIBUTE SHORTCUTS
+
+If you have a number of attributes that differ only by name, you can declare
+them all at once:
+
+ package Point;
+
+ use Moose;
+
+ has [ 'x', 'y' ] => ( is => 'ro', isa => 'Int' );
+
+Also, because C<has> is just a function call, you can call it in a loop:
+
+ for my $name ( qw( x y ) ) {
+ my $builder = '_build_' . $name;
+ has $name => ( is => 'ro', isa => 'Int', builder => $builder );
+ }
+
=head1 MORE ON ATTRIBUTES
Moose attributes are a big topic, and this document glosses over a few
-aspects of their 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.
+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
-superceded by some more modern features, but are covered for the sake
+superseded by some more modern features, but are covered for the sake
of completeness.
=head2 The C<documentation> option
Moose does absolutely nothing with this information other than store
it.
-=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
-returned from the reader method:
+C<auto_deref> option will make Moose dereference the value when it is
+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
-ArrayRef or 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 C<MooseX::AttributeHelpers> for
-these types of attributes, which gives you much more control over how
-they are accessed and manipulated.
+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. 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 C<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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