Inheriting the Constructor (Learning Perl Objects, References & Modules)

start page | rating of books | rating of authors | reviews | copyrights

9.5. Inheriting the Constructor

Was there anything specific to Horse in that method? No. Therefore, it's also the same recipe for building anything else inherited from Animal, so let's put it there:

{ package Animal;
  sub speak {
    my $class = shift;
    print "a $class goes ", $class->sound, "!\n"
  }
  sub name {
    my $self = shift;
    $$self;
  }
  sub named {
    my $class = shift;
    my $name = shift;
    bless \$name, $class;
  }
}
{ package Horse;
  @ISA = qw(Animal);
  sub sound { "neigh" }
}

Ahh, but what happens if you invoke speak on an instance?

my $tv_horse = Horse->named("Mr. Ed");
$tv_horse->speak;

You get a debugging value:

a Horse=SCALAR(0xaca42ac) goes neigh!

Why? Because the Animal::speak routine expects a classname as its first parameter, not an instance. When the instance is passed in, you'll use a blessed scalar reference as a string, which shows up as you saw it just now—similar to a stringified reference, but with the class name in front.


9.4. How to Build a Horse		9.6. Making a Method Work with Either Classes or Instances