Intermediate Perl

Chapter 14. Some Advanced Object Topics

You might wonder, “Do all objects inherit from a common class?” “What if a method is missing?” “What about multiple inheritance?” or “How can I tell what sort of object I have?” Well, wonder no more. This chapter covers these subjects and more.

UNIVERSAL Methods

As we define classes, we create inheritance hierarchies through the global @ISA variables in each package. To search for a method, Perl wanders through the @ISA tree until it finds a match or fails.

After the search fails, however, Perl always looks in one special class called UNIVERSAL and invokes a method from there, if found, just as if it had been located in any other class or superclass.

One way to look at this is that UNIVERSAL is the base class from which all objects derive. Any method we place here, such as:

sub UNIVERSAL::fandango {
  warn 'object ', shift, " can do the fandango!\n";
}

enables all objects of our program to be called as $some_object->fandango.

Generally, we should provide a fandango method for specific classes of interest and then provide a definition in UNIVERSAL::fandango as a backstop, in case Perl can’t find a more specific method. A practical example might be a data-dumping routine for debugging or maybe a marshaling strategy to dump all application objects to a file. We simply provide the general method in UNIVERSAL and override it in the specific classes for unusual objects.

Obviously, we should use UNIVERSAL sparingly, because there’s only one universe of objects, ...