Chapter 2

Listing 2 1. Overloading Unary Operations in the Even Class

package Even;

use overload

    ++  => "incr",

        => "decr",

    +0  => "numify",

sub new {

   my ($pkg, $num) = @_;

   croak("Even requires an even number to start with!") if $num % 2;

   return bless(\$num, $pkg);   

}

sub incr {

   my $self = shift;

   $$self += 2;

   return $self;

}

sub decr {

   my $self = shift;

   $$self  = 2;

   return $self;

}

sub numify {

   my $self = shift;

   return $$self;

}

1;

This module also serves as a demonstration of an idea briefly discussed 

earlier: Objects need not be based on hashes but can be based on any reference. In 

this case objects in the Even class are implemented using a scalar as the under 

lying type. When new() creates a object, it simply blesses a reference to a scalar:

return bless(\$num, $pkg);

Then when object methods need access to the underlying scalar, they simply use a 

scalar dereference:

$$self  = 2;

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