use strict;
use warnings;
use Data::Dumper;


package Datum;
use Class::Struct;

struct (Datum => {
    id => '$',
    new_id => '$',
});

sub Datum::new_id {
    my $self = shift();
    if (@_) {
       $self->{new_id} = $_[0] + 1;
    }
    return $self->{new_id};
}

package Dataset;
use Class::Struct;

struct (Dataset => {
    member => '$',
});

sub Dataset::dump_new_id {
    my $self = shift();
    print "\n\n new_id is undefined\n\n" unless defined ($self->{new_id});
}

package main;
my $dataset = Dataset->new;
my $datum = Datum->new;
$datum->id(3);
$datum->new_id(3);

$dataset->member($datum);
$dataset->dump_new_id;


die Dumper($dataset);