phpman > perldoc > Moose::Manual::Attributes(3pm)

NAME
    Moose::Manual::Attributes - Object attributes with Moose

VERSION
    version 2.2200

INTRODUCTION
    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.

    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" objects have a social security number (and some don't)".

    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.

ATTRIBUTE OPTIONS
    Use the "has" function to declare an attribute:

      package Person;

      use Moose;

      has 'first_name' => ( is => 'rw' );

    This says that all "Person" objects have an optional read-write "first_name" attribute.

  Read-write vs. read-only
    The options passed to "has" define the properties of the attribute. There are many options, but
    in the simplest form you just need to set "is", which can be either "ro" (read-only) or "rw"
    (read-write). When an attribute is "rw", you can change it by passing a value to its accessor.
    When an attribute is "ro", you may only read the current value of the attribute through its
    accessor. You can, however, set the attribute when creating the object by passing it to the
    constructor.

    In fact, you could even omit "is", but that gives you an attribute that has no accessor. This
    can be useful with other attribute options, such as "handles". However, if your attribute
    generates *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 "is" to "bare".

  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 "ro" then your accessor will be read-only. If you declared it as "rw", you get a
    read-write accessor. Simple.

    Given our "Person" example above, we now have a single "first_name" accessor that can read or
    write a "Person" object's "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 publicly
    readable, but only privately settable. For example:

      has 'weight' => (
          is     => 'ro',
          writer => '_set_weight',
      );

    This might be useful if weight is calculated based on other methods. For example, every time the
    "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. In *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 "reader" and "writer" methods:

      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 override the default
    naming conventions. See Moose::Manual::MooseX for more details.

  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 you want to access this information, you must define
    clearer and predicate methods for an attribute.

    A predicate method tells you whether or not a given attribute is currently set. Note that an
    attribute can be explicitly set to "undef" or some other false value, but the predicate will
    return true.

    The clearer method unsets the attribute. This is *not* the same as setting the value to "undef",
    but you can only distinguish between them if you define a predicate method!

    Here's some code to illustrate the relationship between an accessor, predicate, and clearer
    method.

      package Person;

      use Moose;

      has 'ssn' => (
          is        => 'rw',
          clearer   => 'clear_ssn',
          predicate => 'has_ssn',
      );

      ...

      my $person = Person->new();
      $person->has_ssn; # false

      $person->ssn(undef);
      $person->ssn; # returns undef
      $person->has_ssn; # true

      $person->clear_ssn;
      $person->ssn; # returns undef
      $person->has_ssn; # false

      $person->ssn('123-45-6789');
      $person->ssn; # returns '123-45-6789'
      $person->has_ssn; # true

      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, and then Moose will create the methods for you.

  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 "required" option to true:

      has 'name' => (
          is       => 'ro',
          required => 1,
      );

    There are a couple caveats worth mentioning in regards to what "required" actually means.

    Basically, all it says is that this attribute ("name") must be provided to the constructor or it
    must have either a default or a builder. It does not say anything about its value, so it could
    be "undef".

    If you define a clearer method on a required attribute, the clearer *will* work, so even a
    required attribute can be unset after object construction.

    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 *private* "clearer" and "predicate" for a
    required attribute.

  Default and builder methods
    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 "default" option:

      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":

      my $person = Person->new();
      $person->size; # medium
      $person->has_size; # true

    You can also provide a subroutine reference for "default". This reference will be called as a
    method on the object.

      has 'size' => (
          is => 'ro',
          default =>
              sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] },
          predicate => 'has_size',
      );

    This is a trivial example, but it illustrates the point that the subroutine will be called for
    every new object created.

    When you provide a "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 "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 "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.

      has 'mapping' => (
          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!
      );

    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 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 supply a "builder" method for your
    attribute:

      has 'size' => (
          is        => 'ro',
          builder   => '_build_size',
          predicate => 'has_size',
      );

      sub _build_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, because this is a *named* method, it can be
    subclassed or provided by a role.

    We strongly recommend that you use a "builder" instead of a "default" for anything beyond the
    most trivial default.

    A "builder", just like a "default", is called as a method on the object with no additional
    parameters.

   Builders allow subclassing
    Because the "builder" is called *by name*, it goes through Perl's method resolution. This means
    that builder methods are both inheritable and overridable.

    If we subclass our "Person" class, we can override "_build_size":

      package Lilliputian;

      use Moose;
      extends 'Person';

      sub _build_size { return 'small' }

   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 "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 Moose::Manual::Roles.

  Laziness
    Moose lets you defer attribute population by making an attribute "lazy":

      has 'size' => (
          is      => 'ro',
          lazy    => 1,
          builder => '_build_size',
      );

    When "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 *must* be "lazy". During object 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 calculate a default before it's needed. Making an attribute
    "lazy" lets you defer the cost until the attribute is needed. If the attribute is *never*
    needed, you save some CPU time.

    We recommend that you make any attribute with a builder or non-trivial default "lazy" as a
    matter of course.

   Lazy defaults and $_
    Please note that a lazy default or builder can be called anywhere, even inside a "map" or
    "grep". This means that if your default sub or builder changes $_, something weird could happen.
    You can prevent this by adding "local $_" inside your default or builder.

  Constructor parameters ("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.

    You can do either of these things with the "init_arg" option:

      has 'bigness' => (
          is       => 'ro',
          init_arg => 'size',
      );

    Now we have an attribute named "bigness", but we pass "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     => 1,
          builder  => '_build_genetic_code',
          init_arg => undef,
      );

    By setting the "init_arg" to "undef", we make it impossible to set this attribute when creating
    a new object.

  Weak references
    Moose has built-in support for weak references. If you set the "weak_ref" option to a true
    value, then it will call "Scalar::Util::weaken" whenever the attribute is set:

      has 'parent' => (
          is       => 'rw',
          weak_ref => 1,
      );

      $node->parent($parent_node);

    This is very useful when you're building objects that may contain circular references.

    When the object in a weak reference goes out of scope, the attribute's value will become "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.

  Triggers
    A "trigger" is a subroutine that is called whenever the attribute is set:

      has 'size' => (
          is      => 'rw',
          trigger => \&_size_set,
      );

      sub _size_set {
          my ( $self, $size, $old_size ) = @_;

          my $msg = $self->name;

          if ( @_ > 2 ) {
              $msg .= " - old size was $old_size";
          }

          $msg .= " - size is now $size";
          warn $msg;
      }

    The trigger is called *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 "undef".

    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.

    However, triggers are *not* called when an attribute is populated from a "default" or "builder".

  Attribute types
    Attributes can be restricted to only accept certain types:

      has 'first_name' => (
          is  => 'ro',
          isa => 'Str',
      );

    This says that the "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',
      );

    See the Moose::Manual::Types documentation for a complete discussion of Moose's type system.

  Delegation
    An attribute can define methods which simply delegate to its value:

      has 'hair_color' => (
          is      => 'ro',
          isa     => 'Graphics::Color::RGB',
          handles => { hair_color_hex => 'as_hex_string' },
      );

    This adds a new method, "hair_color_hex". When someone calls "hair_color_hex", internally, the
    object just calls "$self->hair_color->as_hex_string".

    See Moose::Manual::Delegation for documentation on how to set up delegation methods.

  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
    metaclass traits and custom metaclasses.

    You can apply one or more traits to an attribute:

      use MooseX::MetaDescription;

      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).

    There are a number of MooseX modules on CPAN which provide useful attribute metaclasses and
    traits. See Moose::Manual::MooseX for some examples. You can also write your own metaclasses and
    traits. See the "Meta" and "Extending" recipes in Moose::Cookbook for examples.

  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 "push()", "shift()",
    "map()", "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',
          },
      );

    See Moose::Manual::Delegation for more details.

ATTRIBUTE INHERITANCE
    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 change some aspects of an attribute, you simply prepend a plus sign ("+") to its name:

      package LazyPerson;

      use Moose;

      extends 'Person';

      has '+first_name' => (
          lazy    => 1,
          default => 'Bill',
      );

    Now the "first_name" attribute in "LazyPerson" is lazy, and defaults to 'Bill'.

    We recommend that you exercise caution when changing the type ("isa") of an inherited attribute.

  Attribute Inheritance and Method Modifiers
    When an inherited attribute is defined, that creates an entirely new set of accessors for the
    attribute (reader, writer, predicate, etc.). This is necessary because these may be what was
    changed when inheriting the attribute.

    As a consequence, any method modifiers defined on the attribute's accessors in an ancestor class
    will effectively be ignored, because the new accessors live in the child class and do not see
    the modifiers from the parent class.

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 "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 );
      }

MORE ON ATTRIBUTES
    Moose attributes are a big topic, and this document glosses over a few aspects. We recommend
    that you read the Moose::Manual::Delegation and Moose::Manual::Types documents to get a more
    complete understanding of attribute features.

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.

  The "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},
      );

    Moose does absolutely nothing with this information other than store it.

  The "auto_deref" option
    If your attribute is an array reference or hash reference, the "auto_deref" option will make
    Moose dereference the value when it is returned from the reader method *in list context*:

      my %map = $object->mapping;

    This option only works if your attribute is explicitly typed as an "ArrayRef" or "HashRef". When
    the reader is called in *scalar* context, the reference itself is returned.

    However, we recommend that you use 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 "Use Moose::Meta::Attribute::Native traits instead of auto deref" in
    Moose::Manual::BestPractices.

  Initializer
    Moose provides an attribute option called "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.

AUTHORS
    *   Stevan Little <stevan AT cpan.org>

    *   Dave Rolsky <autarch AT urth.org>

    *   Jesse Luehrs <doy AT cpan.org>

    *   Shawn M Moore <sartak AT cpan.org>

    *   יובל קוג'מן (Yuval Kogman) <nothingmuch AT woobling.org>

    *   Karen Etheridge <ether AT cpan.org>

    *   Florian Ragwitz <rafl AT debian.org>

    *   Hans Dieter Pearcey <hdp AT cpan.org>

    *   Chris Prather <chris AT prather.org>

    *   Matt S Trout <mstrout AT cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2006 by Infinity Interactive, Inc.

    This is free software; you can redistribute it and/or modify it under the same terms as the Perl
    5 programming language system itself.