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;
}