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Moose::Manual::Attributes(3pm) User Contributed Perl Documentation 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
o Stevan Little <stevan AT cpan.org>
o Dave Rolsky <autarch AT urth.org>
o Jesse Luehrs <doy AT cpan.org>
o Shawn M Moore <sartak AT cpan.org>
o ' (Yuval Kogman) <nothingmuch AT woobling.org>
o Karen Etheridge <ether AT cpan.org>
o Florian Ragwitz <rafl AT debian.org>
o Hans Dieter Pearcey <hdp AT cpan.org>
o Chris Prather <chris AT prather.org>
o 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.
perl v5.34.0 2022-02-06 Moose::Manual::Attributes(3pm)
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On Apache
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