Метаобъектілер және олардың қасиеттері

Object that manipulates, creates, describes, and implements other objects
In computer science, a metaobject is an object that manipulates, creates, describes, or implements objects (including itself). The object that the metaobject pertains to is called the base object. Some information that a metaobject might define includes the base object's type, interface, class, methods, attributes, parse tree, etc. Metaobjects are examples of the computer science concept of reflection, where a system has access (usually at run time) to its own internal structure. Reflection enables a system to essentially rewrite itself on the fly, to alter its own implementation as it executes.

When compilation is not available at run time there are additional complications for the implementation of metaobject protocol. For example, it is possible to change the type hierarchy with such a protocol but doing so may cause problems for code compiled with an alternative class model definition. Some environments have found innovative solutions for this, e. g., by handling metaobject issues at compile time. A good example of this is OpenC++. The Semantic Web object oriented model is more dynamic than most standard object systems, and is consistent with runtime metaobject protocols. For example, in the Semantic Web model classes are expected to change their relations to each other and there is a special inference engine known as a classifier that can validate and analyze evolving class models.

The first metaobject protocol was in the Smalltalk object oriented programming language developed at Xerox PARC. The Common Lisp Object System (CLOS) came later and was influenced by the Smalltalk protocol as well as by Brian C. Smith's original studies on 3 Lisp as an infinite tower of evaluators. The CLOS model, unlike the Smalltalk model, allows a class to have more than one superclass; this raises additional complexity in issues such as resolving the lineage of the class hierarchy on some object instance. CLOS also allows for dynamic multimethod dispatch, which is handled via generic functions rather than message passing like in Smalltalk's single dispatch. The most influential book describing the semantics and implementation of the metaobject protocol in Common Lisp is The Art of the Metaobject Protocol by Gregor Kiczales et al. Metaobject protocols are also extensively used in software engineering applications. In virtually all commercial CASE, re engineering, and Integrated Development Environments there is some form of metaobject protocol to represent and manipulate the design artifacts. A metaobject protocol is one way to implement aspect oriented programming. Many of the early founders of MOPs, including Gregor Kiczales, have since moved on to be the primary advocates for aspect oriented programming. Kiczales et al. of PARC were hired to design AspectJ for Java, a language which does not possess a native metaobject protocol.