Programming Perl, 4th Edition

Method Autoloading

Normally, when you call an undefined subroutine in a package that defines an AUTOLOAD subroutine, the AUTOLOAD subroutine is called in lieu of raising an exception (see the section Autoloading in Chapter 10). With methods, this works a little differently. If the regular method search (through the class, its ancestors, and finally UNIVERSAL) fails to find a match, the same sequence is run again, this time looking for an AUTOLOAD subroutine. If found, this subroutine is called as a method, with the package’s $AUTOLOAD variable set to the fully qualified name of the subroutine on whose behalf AUTOLOAD was called.

You need to be a bit cautious when autoloading methods. First, the AUTOLOAD subroutine should return immediately if it’s being called on behalf of a method named DESTROY, unless your goal was to simulate DESTROY, which has a special meaning to Perl (see the section Instance Destructors later in this chapter).

sub AUTOLOAD {
    return if our $AUTOLOAD =~ /::DESTROY$/;
    ...
}

Second, if the class is providing an AUTOLOAD safety net, you won’t be able to use UNIVERSAL::can on a method name to check whether it’s safe to invoke. You have to check for AUTOLOAD separately:

if ($obj–>can("methname") || $obj–>can("AUTOLOAD")) {
    $obj–>methname();
}

Finally, under multiple inheritance, if a class inherits from two or more classes—each of which has an AUTOLOAD—only the leftmost will ever be triggered, since Perl stops as soon as it finds the first AUTOLOAD.

The last two quirks ...