=head1 NAME
-Moose::Manual::Classes - Object construction (and destruction) with Moose
+Moose::Manual::Construction - Object construction (and destruction) with Moose
=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
-C<Moose::Object>, which provides a C<new> method for you. And if you
+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>
-method genreated in your class.
+method "inlined" in your class.
+
+=head1 OBJECT CONSTRUCTION AND ATTRIBUTES
The Moose-provided constructor accepts a hash or hash reference of
named parameters matching your attributes (actually, matching their
=head1 OBJECT CONSTRUCTION HOOKS
-Sometimes you need to hook into object construction. Some common needs
-are validating an object's state, logging, and allowing non-hash(ref)
-constructor arguments. Moose provides hooks for these needs with the
-C<BUILD> and C<BUILDARGS> methods.
+Moose lets you hook into object construction. You can validate an
+object's state, do logging, or maybe allow non-hash(ref) constructor
+arguments. You can do this by creating C<BUILD> and/or C<BUILDARGS>
+methods.
-If these are defined in your class, then Moose will arrange for them
-to be called as part of the object construction process.
+If these methods exist in your class, Moose will arrange for them to
+be called as part of the object construction process.
=head2 BUILDARGS
-The C<BUILDARGS> method is called I<before> an object is created, and
-is therefore called as a class method. It will receive all of the
-arguments that were passed to C<new> I<as-is>. Your C<BUILDARGS>
-method must then 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).
+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
+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).
-One common use for C<BUILDARGS> is to accomodate a non-hash(ref)
+One common use for C<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, C<<
Person->new($ssn) >>.
Without a C<BUILDARGS> method, Moose will complain, because it expects
a hash or hash reference. We can use the C<BUILDARGS> method to
-accomodate this calling style:
+accommodate this calling style:
- sub BUILDARGS {
+ around BUILDARGS => sub {
+ my $orig = shift;
my $class = shift;
if ( @_ == 1 && ! ref $_[0] ) {
- return { ssn => $_[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 C<Moose::Object>. This method handles distinguishing
-between a hash reference and a plain hash, so you don't have to.
+Note the call to C<< $class->$orig >>. 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.
=head2 BUILD
The C<BUILD> method is called I<after> an object is created. There are
-many potential uses for a C<BUILD> method. One of the most common is
-to check that the object state makes sense. While we can validate
-individual attributes through the use of types, we can't validate the
-state of a whole object that way.
+several 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.
sub BUILD {
my $self = shift;
debug( 'Made a new person - SSN = ', $self->ssn, );
}
-=head3 BUILD and Parent Classes
+Note that while it is not shown here, the C<BUILD> method receives
+not only the created object, but also a hashref of the original
+arguments passed to new (or the results of your C<BUILDARGS>,
+if you have overridden the default C<BUILDARGS>.) This can be
+useful if you need to venture beyond what the default
+initialization behavior and coercions can accomplish.
+
+=head3 BUILD and parent classes
The interaction between multiple C<BUILD> methods in an inheritance
hierarchy is different from normal Perl methods. B<You should never
The theory behind this is that C<BUILD> methods can only be used for
increasing specialization of a class's constraints, so it makes sense
-to call the least specific first (also, this is how Perl 6 does it).
+to call the least specific C<BUILD> method first. Also, this is how
+Perl 6 does it.
=head1 OBJECT DESTRUCTION
C<DEMOLISH> methods in your hierarchy to be called, from most to least
specific.
+Each C<DEMOLISH> method is called with a single argument.
+
In most cases, Perl's built-in garbage collection is sufficient, and
-you won't need ot provide a C<DEMOLISH> method.
+you won't need to provide a C<DEMOLISH> method.
+
+=head2 Error Handling During Destruction
+
+The interaction of object destruction and Perl's global C<$@> and C<$?>
+variables can be very confusing.
+
+Moose always localizes C<$?> 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.
+
+Moose also preserves C<$@> against any C<eval> calls that may happen during
+object destruction. However, if an object's C<DEMOLISH> method actually dies,
+Moose explicitly rethrows that error.
+
+If you do not like this behavior, you will have to provide your own C<DESTROY>
+method and use that instead of the one provided by L<Moose::Object>. You can
+do this to preserve C<$@> I<and> capture any errors from object destruction by
+creating an error stack.
=head1 AUTHOR
=head1 COPYRIGHT AND LICENSE
-Copyright 2008 by Infinity Interactive, Inc.
+Copyright 2009 by Infinity Interactive, Inc.
L<http://www.iinteractive.com>
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