In computer science, a metaobject or meta-object is any entity that manipulates, creates, describes, or implements other objects. The object that the metaobject is about is called the base object. Some information that a metaobject might store is the base object's type, interface, class, methods, attributes, variables, functions, control structures, etc.
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A metaobject protocol (MOP) is an interpreter of the semantics of a program that is open and extensible. Therefore, a MOP determines what a program means and what its behavior is, and it is extensible in that a programmer (or metaprogrammer) can alter program behavior by extending parts of the MOP. The MOP exposes some or all internal structure of the interpreter to the programmer. The MOP may manifest as a set of classes and methods that allow a program to inspect the state of the supporting system and alter its behaviour. MOPs are implemented as object-oriented programs where all objects are metaobjects.
MOPs may be run-time or compile-time. The metaobjects of runtime MOPs exist while a program is running, or executing. Instead, the metaobjects of compile-time MOPs exist only while a program is compiled. They may alter or extend the compiling process, but do not exist when the program is running.
One of the best-known runtime MOPs is the one described in the book The Art of the Metaobject Protocol (often referred to as AMOP); it applies to the Common Lisp Object System (CLOS) and allows the mechanisms of inheritance, method dispatching, class instantiation and so on to be manipulated. At Xerox PARC, one of the authors of AMOP, Gregor Kiczales, developed a small but powerful MOP-based runtime object system in Scheme called TinyCLOS.[1] which has been ported to many Scheme implementations.
A good example of a compile-time MOP is OpenC++.
One example use of a MOP is to alter the implementation of multiple inheritance. A recurring issue is how to resolve conflicting slots and methods of the superclasses. Typically, language designers select one solution, and language users must live with it. Instead, a metaobject protocol makes it possible to change the rules of inheritance and choose a different solution for individual classes.
A metaobject protocol is one way to implement aspect-oriented programming languages. Many of the early founders of MOPs, including Gregor Kiczales have since moved on to be the primary advocates for aspect-oriented programming.