Moose class attribute that acts like Class::Data::Inheritable - perl

Question

I've got a class attribute in a Moose class, but I'd like it to work like Class::Data::Inheritable in terms of a subclass overriding the parent's value. That is, the subclass inherits the parent's value until the setter is called on the subclass, at which point the values become distinct. e.g.
#!/usr/bin/perl
use warnings;
use strict;
{
package Foo;
use Moose;
use MooseX::ClassAttribute;
class_has Item => ( is => 'rw' );
}
{
package Bar;
use Moose;
extends 'Foo';
}
Foo->Item(4);
# This prints "4, 4" as expected
#
print join( ", ", Foo->Item(), Bar->Item() ) . "\n";
Bar->Item(5);
# Would like this to print "4, 5", but it prints "5, 5"
#
print join( ", ", Foo->Item(), Bar->Item() ) . "\n";
What's the best way to get this effect, with MooseX::ClassAttribute or otherwise? Seems like desirable behavior for any class w/class-data that expects to be inherited from.

Answer 1

I know you asked for inheritance but may be roles would help you to get your problem solved in a different way.
Try it with a simple example:
#!/usr/local/bin/perl
use strict;
use feature 'say';
{
package Bomb;
use Moose::Role;
sub fuse { say "Bomb explode" }
sub explode { say "Bomb fuse"}
}
{
package Spouse;
use Moose::Role;
sub fuse { say "Spouse explode"}
sub explode { say "Spouse fuse"}
}
{
package PracticalJoke;
use Moose;
with 'Bomb' => { excludes => 'explode' },
'Spouse' => { excludes => 'fuse' };
}
my $joke = PracticalJoke->new();
$joke->fuse();
$joke->explode();
And with the 'excludes' you can exactly control what should happen.
Have a look at why roles are awsome
and the slides from Ovid about inheritance versus roles.

Answer 2

I had the same problem and found this page when looking for a solution. It's almost a decade after the original post but, as I have now worked out a solution, it might help the next person.
The solution was to add MooseX::ClassAttribute to Bar and a cloned version of class_has Item ... (note the leading + on Item).
{
package Bar;
use Moose;
use MooseX::ClassAttribute;
extends 'Foo';
class_has '+Item' => ();
}
I can now change the subclass Item without it affecting the superclass Item.