Abstract classes provide definitions for instance methods and class methods but are not intended to create instances. Abstract classes often define the message interfaces that subclasses should implement.

You create an abstract class by specifying the ABSTRACT subkeyword on the ::CLASS directive. Trying to create an instance from an abstract class will result in an error.

It is possible to create abstract methods or attributes on a class. Abstract methods or attributes are placeholders that subclasses are expected to override. Failing to provide a real method or attribute implementation will result in an error when the abstract version is called.