How to Build a Horse (Learning Perl Objects, References & Modules)

9.4. How to Build a Horse

If you constructed all your horses by hand, you'd most likely make mistakes from time to time. Making the "inside guts" of a Horse visible also violates one of the principles of OOP. That's good if you're a veterinarian but not if you just like to own horses. Let the Horse class build a new horse:

{ package Horse;
  @ISA = qw(Animal);
  sub sound { "neigh" }
  sub name {
    my $self = shift;
    $$self;
  }
  sub named {
    my $class = shift;
    my $name = shift;
    bless \$name, $class;
  }
}

Now with the new named method, build a Horse:

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

You're back to a class method, so the two arguments to Horse::named are "Horse" and "Mr. Ed". The bless operator not only blesses $name, it also returns the reference to $name, so that's fine as a return value. And that's how to build a horse.

You called the constructor named here so it quickly denotes the constructor's argument as the name for this particular Horse. You can use different constructors with different names for different ways of "giving birth" to the object (such as recording its pedigree or date of birth). However, you'll find that most people coming to Perl from less-flexible languages (such as Java or C++) use a single constructor named new, with various ways of interpreting the arguments to new. Either style is fine, as long as you document your particular way of giving birth to an object. Most core and CPAN modules use new, with notable exceptions, such as DBI's DBI->connect( ). It's really up to the author. It all works, as long as it's documented.


9.3. Accessing the Instance Data		9.5. Inheriting the Constructor