Moose::Manual::ConstruUsernContributed Perl DoMoose::Manual::Construction(3pm)NAMEMoose::Manual::Construction - Object construction (and destruction)
with Moose
VERSION
version 2.2009
WHERE'S THE CONSTRUCTOR?
Do not define a "new()" method for your classes!
When you "use Moose" in your class, your class becomes a subclass of
Moose::Object. The Moose::Object provides a "new()" method for your
class. If you follow our recommendations in
Moose::Manual::BestPractices and make your class immutable, then you
actually get a class-specific "new()" method "inlined" in your class.
OBJECT CONSTRUCTION AND ATTRIBUTES
The Moose-provided constructor accepts a hash or hash reference of
named parameters matching your attributes (actually, matching their
"init_arg"s). This is just another way in which Moose keeps you from
worrying how classes are implemented. Simply define a class and you're
ready to start creating objects!
OBJECT CONSTRUCTION HOOKS
Moose lets you hook into object construction. You can validate an
object's state, do logging, customize construction from parameters
which do not match your attributes, or maybe allow non-hash(ref)
constructor arguments. You can do this by creating "BUILD" and/or
"BUILDARGS" methods.
If these methods exist in your class, Moose will arrange for them to be
called as part of the object construction process.
BUILDARGS
The "BUILDARGS" method is called as a class method before an object is
created. It will receive all of the arguments that were passed to
"new()" as-is, and is expected to return a hash reference. This hash
reference will be used to construct the object, so it should contain
keys matching your attributes' names (well, "init_arg"s).
One common use for "BUILDARGS" is to accommodate a non-hash(ref)
calling style. For example, we might want to allow our Person class to
be called with a single argument of a social security number,
"Person->new($ssn)".
Without a "BUILDARGS" method, Moose will complain, because it expects a
hash or hash reference. We can use the "BUILDARGS" method to
accommodate this calling style:
around BUILDARGS => sub {
my $orig = shift;
my $class = shift;
if ( @_ == 1 && !ref $_[0] ) {
return $class->$orig( ssn => $_[0] );
}
else {
return $class->$orig(@_);
}
};
Note the call to "$class->$orig". This will call the default
"BUILDARGS" in Moose::Object. This method takes care of distinguishing
between a hash reference and a plain hash for you.
BUILD
The "BUILD" method is called after an object is created. There are
several reasons to use a "BUILD" method. One of the most common is to
check that the object state is valid. While we can validate individual
attributes through the use of types, we can't validate the state of a
whole object that way.
sub BUILD {
my $self = shift;
if ( $self->country_of_residence eq 'USA' ) {
die 'All US residents must have an SSN'
unless $self->has_ssn;
}
}
Another use of a "BUILD" method could be for logging or tracking object
creation.
sub BUILD {
my $self = shift;
debug( 'Made a new person - SSN = ', $self->ssn, );
}
The "BUILD" method is called with the hash reference of the parameters
passed to the constructor (after munging by "BUILDARGS"). This gives
you a chance to do something with parameters that do not represent
object attributes.
sub BUILD {
my $self = shift;
my $args = shift;
$self->add_friend(
My::User->new(
user_id => $args->{user_id},
)
);
}
BUILD and parent classes
The interaction between multiple "BUILD" methods in an inheritance
hierarchy is different from normal Perl methods. You should never call
"$self->SUPER::BUILD", nor should you ever apply a method modifier to
"BUILD".
Moose arranges to have all of the "BUILD" methods in a hierarchy called
when an object is constructed, from parents to children. This might be
surprising at first, because it reverses the normal order of method
inheritance.
The theory behind this is that "BUILD" methods can only be used for
increasing specialization of a class's constraints, so it makes sense
to call the least specific "BUILD" method first. Also, this is how Perl
6 does it.
OBJECT DESTRUCTION
Moose provides a hook for object destruction with the "DEMOLISH"
method. As with "BUILD", you should never explicitly call
"$self->SUPER::DEMOLISH". Moose will arrange for all of the "DEMOLISH"
methods in your hierarchy to be called, from most to least specific.
Each "DEMOLISH" method is called with a single argument. This is a
boolean value indicating whether or not this method was called as part
of the global destruction process (when the Perl interpreter exits).
In most cases, Perl's built-in garbage collection is sufficient, and
you won't need to provide a "DEMOLISH" method.
Error Handling During Destruction
The interaction of object destruction and Perl's global $@ and $?
variables can be very confusing.
Moose always localizes $? when an object is being destroyed. This means
that if you explicitly call "exit", that exit code will be preserved
even if an object's destructor makes a system call.
Moose also preserves $@ against any "eval" calls that may happen during
object destruction. However, if an object's "DEMOLISH" method actually
dies, Moose explicitly rethrows that error.
If you do not like this behavior, you will have to provide your own
"DESTROY" method and use that instead of the one provided by
Moose::Object. You can do this to preserve $@ and capture any errors
from object destruction by creating an error stack.
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::Construction(3pm)