Programming Perl, 4th Edition

The overload Pragma

The overload pragma implements operator overloading. You provide it with a key/value list of operators and their associated behaviors:

package MyClass;

use overload   "+" => \&myadd,            # coderef
               "<" => "less_than",        # named method
             "abs" => sub { return @_ };  # anonymous subroutine

Now when you try to add two MyClass objects, the myadd subroutine will be called to create the result.

When you try to compare two MyClass objects with the < operator, Perl notices that the behavior is specified as a string and interprets the string as a method name and not simply as a subroutine name. In the example above, the less_than method might be supplied by the MyClass package itself or inherited from a base class of MyClass, but the myadd subroutine must be supplied by the current package. The anonymous subroutine for abs supplies itself even more directly. However these routines are supplied, we’ll call them handlers.

For unary operators (those taking only one operand, like abs), the handler specified for the class is invoked whenever the operator is applied to an object of that class.

For binary operators like + or <, the handler is invoked whenever the first operand is an object of the class or when the second operand is an object of the class and the first operand has no overloading behavior. That’s so you can say either:

$object + 6

or:

6 + $object

without having to worry about the order of operands. (In the second case, the operands will be swapped when passed to the handler.) If our ...