Chapter 2

  if (ref($other) and $other >isa( Even )) {

     # another Even object will always be even, so the addition

     # can always be done

     $result = $$self + $$other;

  } else {

     # make sure it s even

     $other += 1 if $other % 2;

     $result = $$self + $other;

  }

  # return a new object in the same class as $self

  return ref($self) >new($result);

}

This method will work with other objects that are either of the Even class or inherit 

from it. It also uses an inheritance safe method for creating new objects by calling the 

new() method (implemented earlier in this section) on the result of calling ref() on the 

object. This means that the new() method will be called on whatever class $self

belongs to, even if that's a child class of the class where add() is implemented.

Auto Generation

As you can see from the preceding example, it takes a lot of work to write a safe 

overload method. Fortunately, it's usually not necessary to create methods for all 

the possible overloadable operations. This is because the overload module can 

auto generate many overload methods from existing overloaded methods. Table 

2 4 contains the rules for method auto generation.

Table 2 4. Method Auto Generation

Method(s)

Auto Generation Description

Assignment forms of math  Can be auto generated from the nonassignment forms 

operators

(+= can be auto generated from +, for example)

Conversion operators

Any conversion operator can be auto generated from 

any other

++,   

Auto generated from += and  =

abs()

Auto generated from < and binary subtraction

Unary  

Auto generated from subtraction

Negation

Auto generated from Boolean conversion

Concatenation

Auto generated from string conversion

54