computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as anal ...

, a generic function is a function defined for polymorphism.

In statically typed languages

In statically typed languages (such as

C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...

and

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mos ...

), the term ''generic functions'' refers to a mechanism for ''compile-time polymorphism'' ( static dispatch), specifically

parametric polymorphism In programming languages and type theory, parametric polymorphism allows a single piece of code to be given a "generic" type, using variables in place of actual types, and then instantiated with particular types as needed. Parametrically polymorph ...

. These are functions defined with TypeParameters, intended to be resolved with

compile time In computer science, compile time (or compile-time) describes the time window during which a computer program is compiled. The term is used as an adjective to describe concepts related to the context of program compilation, as opposed to concep ...

type information. The compiler uses these types to instantiate suitable versions, resolving any

function overloading In some programming languages, function overloading or method overloading is the ability to create multiple functions of the same name with different implementations. Calls to an overloaded function will run a specific implementation of that f ...

appropriately.

In Common Lisp Object System

In some systems for

object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...

such as the

Common Lisp Object System 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 ...

(CLOS) and Dylan, a ''generic function'' is an entity made up of all methods having the same name. Typically a ''generic function'' is an instance of a class that inherits both from ''function'' and ''standard-object''. Thus generic functions are both functions (that can be called with and applied to arguments) and ordinary objects. The book ''

The Art of the Metaobject Protocol ''The Art of the Metaobject Protocol'' (AMOP) is a 1991 book by Gregor Kiczales, Jim des Rivieres, and Daniel G. Bobrow (all three working for Xerox PARC) on the subject of metaobject protocol. Overview The book contains an explanation of what ...

'' explains the implementation and use of CLOS generic functions in detail. One of the early object-oriented programming extensions to Lisp is Flavors. It used the usual message sending paradigm influenced by Smalltalk. The Flavors syntax to send a message is: (send object :message) With New Flavors, it was decided the message should be a real function and the usual function calling syntax should be used: (message object) ''message'' now is a ''generic function'', an object and function in its own right. Individual implementations of the ''message'' are called ''methods''. The same idea was implemented in CommonLoops. New Flavors and CommonLoops were the main influence for the Common Lisp Object System.

Example

Common Lisp

Define a generic function with two parameters object-1 and object-2. The name of the generic function is ''collide''. (defgeneric collide (object-1 object-2)) Methods belonging to the generic function are defined outside of classes. Here we define a method for the generic function ''collide'' which is specialized for the classes asteroid (first parameter object-1) and spaceship (second parameter object-2). The parameters are used as normal variables inside the method body. There is no special namespace that has access to class slots. (defmethod collide ((object-1 asteroid) (object-2 spaceship)) (format t "asteroid ~a collides with spaceship ~a" object-1 object-2)) Calling the generic function: ? (collide (make-instance 'asteroid) (make-instance 'spaceship)) asteroid # collides with spaceship # Common Lisp can also retrieve individual methods from the generic function. FIND-METHOD finds the method from the generic function ''collide'' specialized for the classes ''asteroid'' and ''spaceship''. ? (find-method #'collide nil (list (find-class 'asteroid) (find-class 'spaceship))) #

Comparison to other languages

Generic functions correspond roughly to what Smalltalk terms methods, with the notable exception that, in Smalltalk, the receiver's class is the sole determinant of which body of code is called: the types or values of the arguments are irrelevant ( single dispatch). In a programming language with

multiple dispatch Multiple dispatch or multimethods is a feature of some programming languages in which a function or method can be dynamically dispatched based on the run-time (dynamic) type or, in the more general case, some other attribute of more than one of ...

when a generic function is called, method dispatch occurs on the basis of all arguments, not just one which is privileged. New Flavors also provided generic functions, but only single dispatch.

References

{{DEFAULTSORT:Generic Function Method (computer programming)