package My::UNIVERSAL;

use strict;
use warnings;
use Scalar::Util 'reftype';
use Carp 'croak';

use base qw(Class::Data::Inheritable);
__PACKAGE__->mk_classdata('__data_key');

sub mk_accessors {
    my ( $class, @accessors ) = @_;
    foreach my $accessor (@accessors) {
        no strict 'refs';
        my $key  = "$class\::$accessor";
        *$key = sub {
            my $self = shift;
            return $self->{$key} unless @_;
            $self->{$key} = shift;
            return $self;
        };
    }
}

my $check_keys = sub {
    my $self = shift;
    my $data = delete $self->{ $self->__data_key };
    if ( my @keys = keys %$data ) {
        local $" = ', ';
        croak("Unknown keys to constructor:  (@keys)");
    }
};

sub new {
    my ( $class, $args ) = @_;
    my $key = " $class data ";
    $class->__data_key($key);
    $args ||= {};
    unless ( 'HASH' eq reftype $args ) {
        croak('Argument to new() must be a hashref');
    }
    my $self = bless { $key => $args } => $class;
    $self->_initialize;
    $self->$check_keys;
    return $self;
}

sub _initialize {
    my $class = ref($_[0]);
    croak "_initialize() must be overridden in a ($class)";
}

my $check_attribute = sub {
    my ( $self, $attribute ) = @_;
    my $value = delete $self->{ $self->__data_key }->{$attribute};
    if ( !defined wantarray ) {
        $self->$attribute($value);
    }
    else {
        return $value;
    }
};

sub _must_have {
    my ( $self, $attribute ) = @_;

    croak("Mandatory attribute ($attribute) not found")
      unless exists $self->{ $self->__data_key }->{$attribute};
    goto $check_attribute;
}

sub _may_have {
    my ( $self, $attribute ) = @_;
    return unless exists $self->{ $self->__data_key }->{$attribute};
    goto $check_attribute;
}

1;