-=pod
+package Moose::Manual::Construction;
+
+# ABSTRACT: Object construction (and destruction) with Moose
-=head1 NAME
+__END__
-Moose::Manual::Construction - Object construction (and destruction) with Moose
+=pod
=head1 WHERE'S THE CONSTRUCTOR?
-B<You do not need to define a C<new()> method for your classes!>
+B<Do not define a C<new()> method for your classes!>
-When you C<use Moose> in your class, you will become a subclass of
-L<Moose::Object>, which provides a C<new> method for you. If you
-follow our recommendations in L<Moose::Manual::BestPractices> and make
-your class immutable, then you actually get a class-specific C<new>
+When you C<use Moose> in your class, your class becomes a subclass of
+L<Moose::Object>. The L<Moose::Object> provides a C<new()> method for your
+class. If you follow our recommendations in L<Moose::Manual::BestPractices>
+and make your class immutable, then you actually get a class-specific C<new()>
method "inlined" in your class.
=head1 OBJECT CONSTRUCTION AND ATTRIBUTES
=head1 OBJECT CONSTRUCTION HOOKS
Moose lets you hook into object construction. You can validate an
-object's state, do logging, or maybe allow non-hash(ref) constructor
+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 C<BUILD> and/or C<BUILDARGS>
methods.
The C<BUILDARGS> method is called as a class method I<before> an
object is created. It will receive all of the arguments that were
-passed to C<new> I<as-is>, and is expected to return a hash
+passed to C<new()> I<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,
C<init_arg>s).
a hash or hash reference. We can use the C<BUILDARGS> method to
accommodate this calling style:
- sub BUILDARGS {
+ around BUILDARGS => sub {
+ my $orig = shift;
my $class = shift;
- if ( @_ == 1 && ! ref $_[0] ) {
- return { ssn => $_[0] };
+ if ( @_ == 1 && !ref $_[0] ) {
+ return $class->$orig( ssn => $_[0] );
}
else {
- return $class->SUPER::BUILDARGS(@_);
+ return $class->$orig(@_);
}
- }
+ };
-Note the call to C<SUPER::BUILDARGS>. This will call the default
-C<BUILDARGS> in L<Moose::Object>. This method handles distinguishing
-between a hash reference and a plain hash for you.
+Note the call to C<< $class->$orig >>. This will call the default C<BUILDARGS>
+in L<Moose::Object>. This method takes care of distinguishing between a hash
+reference and a plain hash for you.
=head2 BUILD
The C<BUILD> method is called I<after> an object is created. There are
-ways to use a C<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.
+several reasons to use a C<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;
debug( 'Made a new person - SSN = ', $self->ssn, );
}
+
+The C<BUILD> method is called with the hash reference of the parameters passed
+to the constructor (after munging by C<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},
+ )
+ );
+ }
+
=head3 BUILD and parent classes
-The interaction between multiple C<BUILD> methods in an inheritance
-hierarchy is different from normal Perl methods. B<You should never
-call C<< $self->SUPER::BUILD >>.>
+The interaction between multiple C<BUILD> methods in an inheritance hierarchy
+is different from normal Perl methods. B<You should never call C<<
+$self->SUPER::BUILD >>>, nor should you ever apply a method modifier to
+C<BUILD>.
Moose arranges to have all of the C<BUILD> methods in a hierarchy
called when an object is constructed, I<from parents to
The theory behind this is that C<BUILD> methods can only be used for
increasing specialization of a class's constraints, so it makes sense
-to call the least specific first (also, this is how Perl 6 does it).
+to call the least specific C<BUILD> method first. Also, this is how
+Perl 6 does it.
=head1 OBJECT DESTRUCTION
C<DEMOLISH> methods in your hierarchy to be called, from most to least
specific.
+Each C<DEMOLISH> method is called with a single argument.
+
In most cases, Perl's built-in garbage collection is sufficient, and
you won't need to provide a C<DEMOLISH> method.
-=head1 AUTHOR
-
-Dave Rolsky E<lt>autarch@urth.orgE<gt>
+=head2 Error Handling During Destruction
-=head1 COPYRIGHT AND LICENSE
+The interaction of object destruction and Perl's global C<$@> and C<$?>
+variables can be very confusing.
-Copyright 2009 by Infinity Interactive, Inc.
+Moose always localizes C<$?> when an object is being destroyed. This means
+that if you explicitly call C<exit>, that exit code will be preserved even if
+an object's destructor makes a system call.
-L<http://www.iinteractive.com>
+Moose also preserves C<$@> against any C<eval> calls that may happen during
+object destruction. However, if an object's C<DEMOLISH> method actually dies,
+Moose explicitly rethrows that error.
-This library is free software; you can redistribute it and/or modify
-it under the same terms as Perl itself.
+If you do not like this behavior, you will have to provide your own C<DESTROY>
+method and use that instead of the one provided by L<Moose::Object>. You can
+do this to preserve C<$@> I<and> capture any errors from object destruction by
+creating an error stack.
=cut
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