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.
A metaobject protocol (MOP) provides the vocabulary (protocol) to access and manipulate the structure and behaviour of systems of objects. Typical functions of a metaobject protocol include:
Metaobject protocol is contrary to Bertrand Meyer's open/closed principle, which holds that software object systems should be open for extension but closed for modification. This principle effectively draws a distinction between extending an object by adding to it, and modifying an object by redefining it, proposing that the former is a desirable quality ("objects should be extensible to meet the requirements of future use cases"), while the latter is undesirable ("objects should provide a stable interface not subject to summary revision"). Metaobject protocol, by contrast, transparently exposes the internal composition of objects and the entire object system in terms of the system itself. In practice, this means that programmers may use objects to redefine themselves, possibly in quite complex ways.
Furthermore, metaobject protocol is not merely an interface to an "underlying" implementation; rather, through metaobject protocol the object system is recursively implemented in terms of a meta-object system, which itself is theoretically implemented in terms of a meta-metaobject system, and so on until an arbitrary base case (a consistent state of the object system) is determined, with the protocol as such being the recursive functional relationship between these implementation levels.
Implementing object systems in such a way opens the possibility for radical discretionary redesign, providing deep flexibility but introducing possibly complex or difficult-to-understand metastability issues (for instance, the object system must not destructively update its own metaobject protocol - its internal self-representation - but the potential destructiveness of some updates is non-trivial to predict and may be hard to reason about), depending on the recursive depth to which the desired modifications are propagated.For this reason, metaobject protocol, when present in a language, is usually used sparingly and for specialised purposes such as software that transforms other software or itself in sophisticated ways, for example in reverse engineering.
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.
In object-oriented programming, a class is an extensible program-code-template for creating objects, providing initial values for state and implementations of behavior. In many languages, the class name is used as the name for the class, the name for the default constructor of the class, and as the type of objects generated by instantiating the class; these distinct concepts are easily conflated.
Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit characteristics and features from more than one parent object or parent class. It is distinct from single inheritance, where an object or class may only inherit from one particular object or class.
Smalltalk is an object-oriented, dynamically typed reflective programming language. Smalltalk was created as the language underpinning the "new world" of computing exemplified by "human–computer symbiosis". It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by Alan Kay, Dan Ingalls, Adele Goldberg, Ted Kaehler, Diana Merry, Scott Wallace, and others during the 1970s.
In computing, serialization or serialisation is the process of translating a data structure or object state into a format that can be stored or transmitted and reconstructed later. When the resulting series of bits is reread according to the serialization format, it can be used to create a semantically identical clone of the original object. For many complex objects, such as those that make extensive use of references, this process is not straightforward. Serialization of object-oriented objects does not include any of their associated methods with which they were previously linked.
In computing, aspect-oriented programming (AOP) is a programming paradigm that aims to increase modularity by allowing the separation of cross-cutting concerns. It does so by adding additional behavior to existing code without modifying the code itself, instead separately specifying which code is modified via a "pointcut" specification, such as "log all function calls when the function's name begins with 'set'". This allows behaviors that are not central to the business logic to be added to a program without cluttering the code, core to the functionality. AOP forms a basis for aspect-oriented software development.
In software engineering, a software design pattern is a general, reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into source or machine code. Rather, it is a description or template for how to solve a problem that can be used in many different situations. Design patterns are formalized best practices that the programmer can use to solve common problems when designing an application or system.
In computer science, an object can be a variable, a data structure, a function, or a method, and as such, is a value in memory referenced by an identifier.
The Common Lisp Object System (CLOS) is the facility for object-oriented programming which is part of ANSI Common Lisp. CLOS is a powerful dynamic object system which differs radically from the OOP facilities found in more static languages such as C++ or Java. CLOS was inspired by earlier Lisp object systems such as MIT Flavors and CommonLoops, although it is more general than either. Originally proposed as an add-on, CLOS was adopted as part of the ANSI standard for Common Lisp and has been adapted into other Lisp dialects such as EuLisp or Emacs Lisp.
In computer programming, a generic function is a function defined for polymorphism.
In object-oriented programming languages, a mixin is a class that contains methods for use by other classes without having to be the parent class of those other classes. How those other classes gain access to the mixin's methods depends on the language. Mixins are sometimes described as being "included" rather than "inherited".
Metaprogramming is a programming technique in which computer programs have the ability to treat other programs as their data. It means that a program can be designed to read, generate, analyze or transform other programs, and even modify itself while running. In some cases, this allows programmers to minimize the number of lines of code to express a solution, in turn reducing development time. It also allows programs greater flexibility to efficiently handle new situations without recompilation.
In object-oriented programming, a metaclass is a class whose instances are classes. Just as an ordinary class defines the behavior of certain objects, a metaclass defines the behavior of certain classes and their instances. Not all object-oriented programming languages support metaclasses. Among those that do, the extent to which metaclasses can override any given aspect of class behavior varies. Metaclasses can be implemented by having classes be first-class citizen, in which case a metaclass is simply an object that constructs classes. Each language has its own metaobject protocol, a set of rules that govern how objects, classes, and metaclasses interact.
Gregor Kiczales is an American computer scientist. He is currently a full time professor of computer science at the University of British Columbia in Vancouver, British Columbia, Canada. He is best known for developing the concept of aspect-oriented programming, and the AspectJ extension to the Java programming language, both of which he designed while working at Xerox PARC. He is also one of the co-authors of the specification for the Common Lisp Object System, and is the author of the book The Art of the Metaobject Protocol, along with Jim Des Rivières and Daniel G. Bobrow.
ObjVlisp is a 1984 object-oriented extension of Vlisp–Vincennes LISP, a LISP dialect developed since 1971 at the University of Paris VIII – Vincennes. It is noteworthy as one of the earliest implementations of the concept of metaclasses, and in particular explicit metaclasses. In the ObjVlisp model, "each entity is an instance of a single class. Classes are instances of other classes, called metaclasses. This model allows for extension of the static part of OOL, i.e. the structural aspects of objects considered as implementation of abstract data types"
In computer science, object composition is a way to combine objects or data types into more complex ones. Common kinds of compositions are objects used in object-oriented programming, tagged unions, sets, sequences, and various graph structures. Object compositions relate to, but are not the same as, data structures.
Programming language theory (PLT) is a branch of computer science that deals with the design, implementation, analysis, characterization, and classification of programming languages and of their individual features. It falls within the discipline of computer science, both depending on and affecting mathematics, software engineering, linguistics and even cognitive science. It has become a well-recognized branch of computer science, and an active research area, with results published in numerous journals dedicated to PLT, as well as in general computer science and engineering publications.
CommonLoops is an early programming language which extended Common Lisp to include Object-oriented programming functionality and is a dynamic object system which differs from the OOP facilities found in static languages such as C++ or Java. Like New Flavors, CommonLoops supported multiple inheritance, generic functions and method combination. CommonLoops also supported multi-methods and made use of metaobjects. CommonLoops and New Flavors were the primary ancestors of CLOS. CommonLoops was supported by a portable implementation known as Portable CommonLoops (PCL) which ran on all Common Lisp implementations of the day.
The Art of the Metaobject Protocol (AMOP) is a 1991 book by Gregor Kiczales, Jim des Rivieres, and Daniel G. Bobrow on the subject of metaobject protocol.
Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code: data in the form of fields, and code, in the form of procedures.