package Base_class;

##  new (args:hashref) : Object_ref
#
#   create a new object, then initialize it from class-specific 
#   defaults and/or argument values.
#
sub new {
    my $O = bless {}, shift;
    
    my $args     = shift;
    my $defaults = $O->_defaults;

##  iterate over the keys in %$defaults, because those are all
##  we really care about:

    for $k (keys %$defaults) {
        $O->{ $k } = (defined $args->{$k}) ?
            $args->{ $k } : $defaults->{ $k };
    }
    return ($O);
}


package Subclass_1;
@ISA = qw( Base_class );

##  _defaults (nil) : hashref
#
#   return a hash of default values for objects of this class
#
sub _defaults {

##  this is basically a static variable for the class.   i 
##  define such things at runtime, but that's just a personal 
##  taste.

    if ( ! defined $DEFAULTS ) {
        $DEFAULTS = {
            'attr1' => 'val1',
            'attr2' => 'val2',
            'attr3' => 'val3',
        };
    }
    return ($DEFAULTS);
}