Moose::Manual::Attributes - phpMan

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.