Definition and Application of Metaclasses

Metaclasses are classes whose instances are themselves classes. Metaclasses are generally used to define and query information relevant to the class level. The paper first analyzes the more general term meta and gives some examples of its use in various application domains. Then, it focuses on the description of metaclasses. To help better understand metaclasses, the paper suggests a set of criteria accounting for the variety of metaclass definitions existing in the literature. The paper finally presents the usage of metaclasses and discusses some questions raised about them.