Programming Perl, 4th Edition

Inheritable Constructors

Like all methods, a constructor is just a subroutine, but we don’t call it as a subroutine. We always invoke it as a method—a class method, in this particular case, because the invocant is a package name. Method invocations differ from regular subroutine calls in two ways. First, they get the extra argument we discussed earlier. Second, they obey inheritance, allowing one class to use another’s methods.

We’ll describe the underlying mechanics of inheritance more rigorously in the next section, but, for now, some simple examples of its effects should help you design your constructors. For instance, suppose we have a Spider class that inherits methods from the Critter class. In particular, suppose the Spider class doesn’t have its own spawn method. The correspondences shown in Table 12-1 apply:

Table 12-1. Mapping methods to subroutines

Method Call	Resulting Subroutine Call
Critter–>spawn()	Critter::spawn("Critter")
Spider–>spawn()	Critter::spawn("Spider")

The subroutine called is the same in both cases, but the argument differs. Note that our spawn constructor above completely ignored its argument, which means our Spider object was incorrectly blessed into class Critter. A better constructor would provide the package name (passed in as the first argument) to bless:

sub spawn {
    my $class =  shift;       # Store the package name
    my $self  =  { };
    bless($self, $class);     # Bless the reference into that package
    return $self;
}

Now you could use the same subroutine for both these cases: ...