=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 has many attribute-related features, and attributes are probably
+the single most useful aspect of Moose. You can do a lot in a class
+just by declaring attributes. In fact, it's quite possible to have
+classes that consist solely of attribute declarations.
-Attributes are properties that every member of a class has. For
+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
+last name". Attributes can be optional, so that we can say "some 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.
=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 person objects have an optional read-write
+"first_name" attribute.
=head2 Read-write Vs Read-only
-The code inside the parentheses defines the details of the
+The options passed to C<has> define 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).
=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 Person example above, we now have a single C<first_name>
+accessor that can read or write a person object's first name.
If you want, you can also explicitly specify the method names to be
used for reading and writing an attribute's value. This is
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:
=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 is not set. If you
+want to be able 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 even if the attribute was explicitly set to
+undef or some other false value, 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
-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 method names for these options if you want
+them.
=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',
There are a couple caveats worth mentioning in regards to what
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 (name) must be provided
+to the constructor. 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
-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:
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.
+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 default c<lazy>,
+which 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
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:
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 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.
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 in a non-lazy
+default subroutine.
-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.
has 'size' =>
( is => 'rw',
lazy_build => 1,
- clearer => '_has_size',
+ clearer => '_clear_size',
);
Options that you explicitly provide are always used in favor of
Moose's internal defaults.
+=head2 Constructor Parameters (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
+the constructor parameter. You may also want to make an attribute
+unsettable from the constructor.
+
+Both of these things can be done by providing a value for the
+C<init_arg> option:
+
+ has 'bigness' =>
+ ( is => 'rw',
+ init_arg => 'size',
+ );
+
+Now we have an attribute named bigness, but to set it during object
+construction we pass C<size> to the constructor.
+
+Even more useful is the ability to disable setting attribute. This is
+particularly handy for private attributes:
+
+ has '_genetic_code' =>
+ ( is => 'rw',
+ 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' =>
=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',
sub _size_set {
my ( $self, $size, $meta_attr ) = @_;
- 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.
+as the L<Moose::Meta::Attribute> object for the attribute. The trigger
+is called I<after> the value is set.
=head2 Attribute Types
-Attributes can be restriced to only accept certain types:
+Attributes can be restricted to only accept certain types:
has 'first_name' =>
( is => 'rw',
=head2 Delegation
-Attributes can define delegations to their values:
+Attributes can define methods which simple delegate to their values:
has 'hair_color' =>
( is => 'rw',
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
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:
You can also apply one or more traits to an attribute:
-
use MooseX::MetaDescription;
has 'size' =>
=head2 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:
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.
+
+=head1 AUTHOR
+
+Dave Rolsky E<lt>autarch@urth.orgE<gt>
+
+=head1 COPYRIGHT AND LICENSE
+
+Copyright 2008 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
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