Moose::...AndBUILD(3)

💖 NAME 💖

Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD — the one where we sneak into object construction using BUILDARGS and BUILD, bestie 💅

Like, this is Moose::...AndBUILD(3), from the User Contributed Perl Documentation, and it's literally a recipe from the Moose Cookbook, Basics chapter. Periodt.

---

✨ VERSION ✨

version 2.4000 🎀

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🦋 SYNOPSIS 🦋

package Person;

has 'ssn' => (
    is        => 'ro',
    isa       => 'Str',
    predicate => 'has_ssn',
);

has 'country_of_residence' => (
    is      => 'ro',
    isa     => 'Str',
    default => 'usa'
);

has 'first_name' => (
    is  => 'ro',
    isa => 'Str',
);

has 'last_name' => (
    is  => 'ro',
    isa => 'Str',
);

around BUILDARGS => sub {
    my $orig = shift;
    my $class = shift;

    if ( @_ == 1 && ! ref $_[0] ) {
        return $class->$orig(ssn => $_[0]);
    }
    else {
        return $class->$orig(@_);
    }
};

sub BUILD {
    my $self = shift;

    if ( $self->country_of_residence eq 'usa' ) {
        die 'Cannot create a Person who lives in the USA without an ssn.'
            unless $self->has_ssn;
    }
}

---

🌸 DESCRIPTION 🌸

Okay so this recipe is a whole demonstration of BUILDARGS and BUILD 🫶. By defining these two methods, you can hook yourself into the object construction process without overriding new. Not me avoiding new overrides like the plague, iconic behaviour honestly.

💜 BUILDARGS — the pre-construction era

The BUILDARGS method gets called before an object even exists 😭. It's called as a class method, and it receives all of the parameters that were passed to the new method. It's expected to do something with those arguments and return a hash reference. And listen — the keys of that hash must be attribute init_args. No cap.

The primary purpose of BUILDARGS is to let a class accept something other than named arguments. In the case of our Person class, we're letting it be called with a single argument: a social security number 🍓

my $person = Person->new('123-45-6789');

The key part of our BUILDARGS is this conditional right here:

      if ( @_ == 1 && ! ref $_[0] ) {
          return $class->$orig(ssn => $_[0]);
      }

By default, Moose constructors accept a list of key-value pairs, or a hash reference. So we need to make sure that $_[0] is not a reference before we go assuming it's a social security number. Vibe check first, always 💅

Then we call the original BUILDARGS method to handle all the other cases. And bestie, you should always do this in your own BUILDARGS methods, because Moose::Object provides its own BUILDARGS method that handles hash references and a list of key-value pairs. Don't skip it, fr fr.

🌈 BUILD — the post-construction era

The BUILD method is called after the object is constructed, but before it gets returned to the caller. It gives you an opportunity to check the object state as a whole 👑. This is the good place to put logic that just cannot be expressed as a type constraint on a single attribute.

In the Person class, we need to check the relationship between two attributes, ssn and country_of_residence. And if the object is not logically consistent? We throw an exception. Dramatic but necessary 😤

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🔥 MORE CONSIDERATIONS 🔥

This recipe is made significantly simpler because all of the attributes are read-only. If the country_of_residence attribute were settable, we would need to check that a Person had an ssn if the new country was usa. And that could be done with a before modifier. Slay.

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⭐ CONCLUSION ⭐

We have repeatedly discouraged overriding new in Moose classes. This recipe shows how you can use BUILDARGS and BUILD to hook into object construction without overriding new. Main character energy, honestly.

The BUILDARGS method lets us expand on Moose's built-in parameter handling for constructors. The BUILD method lets us implement logical constraints across the whole object after it is created. Two icons, one construction pipeline 💖

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👑 AUTHORS 👑

• Stevan Little <stevan@cpan.org>
• Dave Rolsky <autarch@urth.org>
• Jesse Luehrs <doy@cpan.org>
• Shawn M Moore <sartak@cpan.org>
• יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
• Karen Etheridge <ether@cpan.org>
• Florian Ragwitz <rafl@debian.org>
• Hans Dieter Pearcey <hdp@cpan.org>
• Chris Prather <chris@prather.org>
• Matt S Trout <mstrout@cpan.org>

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🎀 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. Free software, as it should be 💅✨

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And that's the whole page, bestie 🫶 — brought to you by perl v5.42.0, dated 2025-07-25, filed under Moose::...AndBUILD(3). Go build some objects, ily 💖