Programming Perl, 4th Edition

UNIVERSAL: The Ultimate Ancestor Class

If no method definition with the right name is found after searching the invocant’s class and all its ancestor classes recursively, one more check for a method of that name is made in the special predefined class called UNIVERSAL. This package never appears in an @ISA, but it is always consulted when an @ISA check fails. You can think of UNIVERSAL as the ultimate ancestor from which all classes implicitly derive, making it work like class Object does in Java or class object in Python’s new-style classes.

The following predefined methods are available in class UNIVERSAL, and thus in all classes. These all work regardless of whether they are invoked as class methods or object methods.

INVOCANT–>isa(CLASS)

The isa method returns true if INVOCANT’s class is CLASS or any class inheriting from CLASS. Instead of a package name, CLASS may also be one of the built-in types, such as “HASH” or “ARRAY”. (Checking for an exact type does not bode well for encapsulation or polymorphism, though. You should be relying on method dispatch to give you the right method.)

use IO::Handle;
if (IO::Handle–>isa("Exporter")) {
    print "IO::Handle is an Exporter.\n";
}

$fh = IO::Handle–>new();
if ($fh–>isa("IO::Handle")) {
    print "\$fh is some sort of IOish object.\n";
}
if ($fh–>isa("GLOB")) {
    print "\$fh is really a GLOB reference.\n";
}

INVOCANT–>DOES(ROLE)

Perl v5.10 added the idea of roles, a way that a class can include external methods without necessarily inheriting them, ...