package Adres;
sub new
{
 my $prototype = shift;
 my $class = ref $prototype || shift;
 my $self = {
  '@persons' => [], 
    # [] = arrayref; {} = hashref; () = list
    # a list like this must be an arrayref
    # (reference to an array) because a list () cannot
    # be held in an hash directly.
  street => '',
 };
 bless $self, $class;
 return $self;
}
sub get_person_count
{
 my $self = shift;
 return (scalar @{ $self->{'@persons'} });
}
sub add_person
{
 my $self = shift;
 while (@_) # allow adding multiple people
 {
  die "Error! $_ is not a Person object." unless 
    (ref $_ && $_->isa('Person'));
  # note the magic clause "ref $x && $x->isa('Class')"
  # this catches child classes as well
  # and is similar in effect to the Java
  # expression "instanceof Class"
  push @{ $self->{'@persons'} }, $_;
 }
}