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.IX Title "Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild 3"
.TH Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild 3 "2021-11-07" "perl v5.26.3" "User Contributed Perl Documentation"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild \- Builder methods and lazy_build
.SH "VERSION"
.IX Header "VERSION"
version 2.2201
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\& package BinaryTree;
\& use Moose;
\&
\& has \*(Aqnode\*(Aq => (is => \*(Aqrw\*(Aq, isa => \*(AqAny\*(Aq);
\&
\& has \*(Aqparent\*(Aq => (
\& is => \*(Aqrw\*(Aq,
\& isa => \*(AqBinaryTree\*(Aq,
\& predicate => \*(Aqhas_parent\*(Aq,
\& weak_ref => 1,
\& );
\&
\& has \*(Aqleft\*(Aq => (
\& is => \*(Aqrw\*(Aq,
\& isa => \*(AqBinaryTree\*(Aq,
\& predicate => \*(Aqhas_left\*(Aq,
\& lazy => 1,
\& builder => \*(Aq_build_child_tree\*(Aq,
\& );
\&
\& has \*(Aqright\*(Aq => (
\& is => \*(Aqrw\*(Aq,
\& isa => \*(AqBinaryTree\*(Aq,
\& predicate => \*(Aqhas_right\*(Aq,
\& lazy => 1,
\& builder => \*(Aq_build_child_tree\*(Aq,
\& );
\&
\& before \*(Aqright\*(Aq, \*(Aqleft\*(Aq => sub {
\& my ($self, $tree) = @_;
\& $tree\->parent($self) if defined $tree;
\& };
\&
\& sub _build_child_tree {
\& my $self = shift;
\&
\& return BinaryTree\->new( parent => $self );
\& }
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
If you've already read
Moose::Cookbook::Basics::BinaryTree_AttributeFeatures, then this example
should look very familiar. In fact, all we've done here is replace the
attribute's \f(CW\*(C`default\*(C'\fR parameter with a \f(CW\*(C`builder\*(C'\fR.
.PP
In this particular case, the \f(CW\*(C`default\*(C'\fR and \f(CW\*(C`builder\*(C'\fR options act in
exactly the same way. When the \f(CW\*(C`left\*(C'\fR or \f(CW\*(C`right\*(C'\fR attribute is read,
Moose calls the builder method to initialize the attribute.
.PP
Note that Moose calls the builder method \fIon the object which has the
attribute\fR. Here's an example:
.PP
.Vb 1
\& my $tree = BinaryTree\->new();
\&
\& my $left = $tree\->left();
.Ve
.PP
When \f(CW\*(C`$tree\->left()\*(C'\fR is called, Moose calls \f(CW\*(C`$tree\->_build_child_tree()\*(C'\fR in order to populate the \f(CW\*(C`left\*(C'\fR
attribute. If we had passed \f(CW\*(C`left\*(C'\fR to the original constructor, the
builder would not be called.
.PP
There are some differences between \f(CW\*(C`default\*(C'\fR and \f(CW\*(C`builder\*(C'\fR. Notably,
a builder is subclassable, and can be composed from a role. See
Moose::Manual::Attributes for more details.
.SS "The lazy_build shortcut"
.IX Subsection "The lazy_build shortcut"
The \f(CW\*(C`lazy_build\*(C'\fR attribute option can be used as sugar to specify
a whole set of attribute options at once:
.PP
.Vb 5
\& has \*(Aqanimal\*(Aq => (
\& is => \*(Aqro\*(Aq,
\& isa => \*(AqAnimal\*(Aq,
\& lazy_build => 1,
\& );
.Ve
.PP
This is a shorthand for:
.PP
.Vb 9
\& has \*(Aqanimal\*(Aq => (
\& is => \*(Aqro\*(Aq,
\& isa => \*(AqAnimal\*(Aq,
\& required => 1,
\& lazy => 1,
\& builder => \*(Aq_build_animal\*(Aq,
\& predicate => \*(Aqhas_animal\*(Aq,
\& clearer => \*(Aqclear_animal\*(Aq,
\& );
.Ve
.PP
If your attribute starts with an underscore, Moose is smart and will
do the right thing with the \f(CW\*(C`predicate\*(C'\fR and \f(CW\*(C`clearer\*(C'\fR, making them
both start with an underscore. The \f(CW\*(C`builder\*(C'\fR method \fIalways\fR starts
with an underscore.
.PP
You can read more about \f(CW\*(C`lazy_build\*(C'\fR in Moose::Meta::Attribute
.SH "CONCLUSION"
.IX Header "CONCLUSION"
The \f(CW\*(C`builder\*(C'\fR option is a more OO-friendly version of the \f(CW\*(C`default\*(C'\fR
functionality. It also separates the default-generating code into a
well-defined method. Sprinkling your attribute definitions with
anonymous subroutines can be quite ugly and hard to follow.
.SH "AUTHORS"
.IX Header "AUTHORS"
.IP "\(bu" 4
Stevan Little <stevan@cpan.org>
.IP "\(bu" 4
Dave Rolsky <autarch@urth.org>
.IP "\(bu" 4
Jesse Luehrs <doy@cpan.org>
.IP "\(bu" 4
Shawn M Moore <sartak@cpan.org>
.IP "\(bu" 4
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
.IP "\(bu" 4
Karen Etheridge <ether@cpan.org>
.IP "\(bu" 4
Florian Ragwitz <rafl@debian.org>
.IP "\(bu" 4
Hans Dieter Pearcey <hdp@cpan.org>
.IP "\(bu" 4
Chris Prather <chris@prather.org>
.IP "\(bu" 4
Matt S Trout <mstrout@cpan.org>
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is copyright (c) 2006 by Infinity Interactive, Inc.
.PP
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.