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.IX Title "Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild 3"
.TH Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild 3 "2012-05-07" "perl v5.10.1" "User Contributed Perl Documentation"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.SH "NAME"
Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild \- Builder methods and lazy_build
.SH "VERSION"
.IX Header "VERSION"
version 2.0602
.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 "AUTHOR"
.IX Header "AUTHOR"
Moose is maintained by the Moose Cabal, along with the help of many contributors. See \*(L"\s-1CABAL\s0\*(R" in Moose and \*(L"\s-1CONTRIBUTORS\s0\*(R" in Moose for details.
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is copyright (c) 2012 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.