metaobject

A metaobject is an object whose primary purpose is to describe, control, or extend other objects at the meta level. In languages that support metaobject protocols or similar reflection facilities, metaobjects expose metadata about objects or implement language mechanisms such as class creation, attribute access, or method dispatch. By manipulating metaobjects, programmers can alter how ordinary objects behave without changing their source code.

In Common Lisp, the metaobject protocol (MOP) provides a formal interface for customizing the object system. Implementations can supply metaobjects that govern class creation, method combination, and slot layout. In Python, the metaclass concept treats classes as objects; a metaclass defines how classes behave and how instances are created, with __new__ and __init__ controlling construction. In Ruby and Smalltalk, similar ideas exist: classes and methods can be extended or modified at the meta level via singleton classes or meta-classes.

Uses of metaobjects include enabling frameworks and libraries to implement object-relational mapping, serialization, or validation, and supporting library-driven language extensions. They are also central to metaprogramming, reflection, and systems that aim to customize or extend a language’s semantics without altering existing code.

Considerations include added complexity and potential readability challenges, as well as possible performance overhead. Effective use typically requires clear design goals and careful documentation to avoid opaque or brittle behavior.

See also: metaprogramming, reflection, metaclass, metaobject protocol.