For.example

Smalltalk objects are ordered by classes hierarchies. But still, an object may change its class membership! Objects are able to move between classes and hierarchies at runtime.

In this post I shall show how to transform a rabbit into a light house!

First let us create two classes Rabbit and Lighthouse with the same format. We use Pharo’s public API to do so.

Object subclass: #Rabbit instanceVariableNames: 'age size'.
Object subclass: #Lighthouse instanceVariableNames: 'latitude longitude'.

Lets check that both classes are of the same format, but neither is a subclass of the other. (Please refer to Phexample for more information on expectation matchers.)

Rabbit format should = Lighthouse format.
Rabbit should not beKindOf: Lighthouse.
Lighthouse should not beKindOf: Rabbit.

Now, lets create a rabbit and transform it into an instance of a light house.

r := Rabbit new.
r class should beKindOf: Rabbit.
r primitiveChangeClassTo: Lighthouse new.
r class should beKindOf: Lighthouse.

And, abra cadabra, we’ve turned r from a Rabbit into a Lighthouse!

To do so we use the #primitiveChangeClassTo: method. This method expects an instance of the target class as argument. However this instance is only used to determine the target class of the receiver. (In other Smalltalk dialects, as for example Cincom Smalltalk, #changeClassTo: expects a class rather than an instance. We can only guess why Pharo and its sibling Squeak require an instance of the target class. My guess is that this required as proof that the target class is a valid class, since otherwise it would not have been possible to create an instance of itself.)

Hackety hacking!

Post scriptum: please note that it is not possible to change the class of a point to that of an associations even though both got two instance variables. This is because both are so called “compact” classes with a smaller header. We’ll cover that in another superpowers issue.