Moose::Manual::AttribuUser3Contributed Perl DocuMoose::Manual::Attributes(3pm)NAMEMoose::Manual::Attributes - Object attributes with Moose
VERSION
version 2.2009
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.little@iinteractive.com>
· Dave Rolsky <autarch@urth.org>
· Jesse Luehrs <doy@tozt.net>
· Shawn M Moore <code@sartak.org>
· יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
· Karen Etheridge <ether@cpan.org>
· Florian Ragwitz <rafl@debian.org>
· Hans Dieter Pearcey <hdp@weftsoar.net>
· Chris Prather <chris@prather.org>
· Matt S Trout <mst@shadowcat.co.uk>
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.
perl v5.26.1 2017-12-21 Moose::Manual::Attributes(3pm)
