Add a variable to a class instance at runtime: Difference between revisions

=={{header|Perl 6}}==

You can add variables/methods to a class at runtime by composing in a role. The role only affects that instance, though it is inheritable. An object created from an existing object will inherit any roles composed in with values set to those at the time the role was created. If you want to keep changed values in the new object, clone it instead.

<lang perl6>class Bar { } # an empty class

my $object = Bar.new; # new instance

role a_role { # role to add a variable: foo,

has $.foo is rw = 2; # with an initial value of 2

}

$object does a_role; # compose in the role

say $object.foo; # prints: 2

$object.foo = 5; # change the variable

say $object.foo; # prints: 5

my $ohno = Bar.new; # new Bar object

#say $ohno.foo; # runtime error, base Bar class doesn't have the variable foo

my $this = $object.new; # instantiate a new Bar derived from $object

say $this.foo; # prints: 2 - original role value

my $that = $object.clone; # instantiate a new Bar derived from $object copying any variables

say $that.foo; # 5 - value from the cloned object</lang>