Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
in which classes are not present, and behavior reuse (known as inheritance in class-based languages) is performed via a process of cloning
Cloning (programming)
In computer science, cloning refers to the making of an exact copy of an object, frequently under the paradigm of instance-based programming, or object-oriented programming.-Shallow copies:...
A prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
s. This model can also be known as classless, prototype-oriented or instance-based programming. Delegation
Delegation (programming)
In object-oriented programming, there are two related notions of delegation.* Most commonly, it refers to a programming language feature making use of the method lookup rules for dispatching so-called self-calls as defined by Lieberman in his 1986 paper "Using Prototypical Objects to Implement...
is the language feature that supports prototype-based programming.
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
David Ungar, an American computer scientist, co-created the Self programming language with Randall Smith. The SELF development environment's animated user experience was described in the influential paper co-written with Bay-Wei Chang, which won a lasting impact award at the ACM Symposium on User...
and Randall Smith in the mid 1980s to research topics in object-oriented language design. Since the late 1990s, the classless paradigm has grown increasingly popular. Some current prototype-oriented languages are ECMAScript
ECMAScript
ECMAScript is the scripting language standardized by Ecma International in the ECMA-262 specification and ISO/IEC 16262. The language is widely used for client-side scripting on the web, in the form of several well-known dialects such as JavaScript, JScript, and ActionScript.- History :JavaScript...
JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
JScript is a scripting language based on the ECMAScript standard that is used in Microsoft's Internet Explorer.JScript is implemented as a Windows Script engine. This means that it can be "plugged in" to any application that supports Windows Script, such as Internet Explorer, Active Server Pages,...
ActionScript is an object-oriented language originally developed by Macromedia Inc. . It is a dialect of ECMAScript , and is used primarily for the development of websites and software targeting the Adobe Flash Player platform, used on Web pages in the form of...
NewtonScript is a prototype based programming language created to write programs for the Newton platform. It is heavily influenced by the Self computer language, but modified to be more suited to needs of mobile and embedded devices.- History :...
Io is a pure object-oriented programming language inspired by Smalltalk, Self, Lua, Lisp, Act1, and NewtonScript. Io has a prototype-based object model similar to the ones in Self and NewtonScript, eliminating the distinction between instance and class. Like Smalltalk, everything is an object and...
REBOL is a cross-platform data exchange language and a multi-paradigm dynamic programming language originally designed by Carl Sassenrath for network communications and distributed computing. The language and its official implementation, which is a proprietary freely redistributable software are...
Lisaac is a compiled object-oriented programming language based on prototype concepts, with system programming facilities and design by contract....
, and Lua.
Comparison with class-based models
With class-based languages, the structure of objects
Object (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
is specified in programmer-defined types called classes
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
. While classes define the type of data and functionality that objects will have, instances are "usable" objects based on the patterns of a particular class. In this model, classes act as collections of behavior (methods
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
) and structure that are the same for all instances, whereas instances carry the objects' data. The role distinction is thus primarily based on a distinction between structure and behavior on the one hand, and state
State (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
on the other.
Advocates of prototype-based programming often argue that class-based languages encourage a model of development that focuses first on the taxonomy and relationships between classes. In contrast, prototype-based programming is seen as encouraging the programmer to focus on the behavior of some set of examples and only later worry about classifying these objects into archetypal objects that are later used in a fashion similar to classes. As such, many prototype-based systems encourage the alteration of prototypes during run-time, whereas only very few class-based object-oriented systems (such as the dynamic object-oriented system, Common Lisp
Common Lisp
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
) allow classes to be altered during the execution of a program.
Almost all prototype-based systems are based on interpreted and dynamically typed languages. Systems based on statically typed languages are technically feasible, however. The Omega language discussed in Prototype-Based Programming is an example of such a system, though according to Omega's website even Omega is not exclusively static, but rather its "compiler may choose to use static binding where this is possible and may improve the efficiency of a program."
Object construction
In class-based languages, a new instance is constructed through a class-based constructor
Constructor (computer science)
In object-oriented programming, a constructor in a class is a special type of subroutine called at the creation of an object. It prepares the new object for use, often accepting parameters which the constructor uses to set any member variables required when the object is first created...
In computer programming, a parameter is a special kind of variable, used in a subroutine to refer to one of the pieces of data provided as input to the subroutine. These pieces of data are called arguments...
. The resulting instance is modeled on the layout and behavior dictated by the chosen class.
In prototype-based systems, there are two methods of constructing new objects: ex nihilo
Ex nihilo
Ex nihilo is a Latin phrase meaning "out of nothing". It often appears in conjunction with the concept of creation, as in creatio ex nihilo, meaning "creation out of nothing"—chiefly in philosophical or theological contexts, but also occurs in other fields.In theology, the common phrase creatio ex...
("from nothing") object creation or through cloning an existing object. While most systems support a variety of cloning, ex nihilo object creation is not as prominent.
Systems that support ex nihilo object creation allow new objects to be created from scratch without cloning from an existing prototype. Such systems provide a special syntax for specifying the properties and behaviors of new objects without referencing existing objects. In many prototype languages, there is often a basic Object prototype that carries commonly needed methods and is used as a master prototype for all other objects. One useful aspect of ex nihilo object creation is to ensure that a new object's slot names do not have namespace
Namespace (computer science)
A namespace is an abstract container or environment created to hold a logical grouping of unique identifiers or symbols . An identifier defined in a namespace is associated only with that namespace. The same identifier can be independently defined in multiple namespaces...
conflicts with the top-level Object object. (In the Mozilla JavaScript
JavaScript
JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
implementation, one can do this by setting a newly constructed object's __proto__ property to null.)
Cloning refers to a process whereby a new object is constructed by copying the behavior of an existing object (its prototype). The new object then carries all the qualities of the original. From this point on, the new object can be modified. In some systems the resulting child object maintains an explicit link (via delegation
Delegation (programming)
In object-oriented programming, there are two related notions of delegation.* Most commonly, it refers to a programming language feature making use of the method lookup rules for dispatching so-called self-calls as defined by Lieberman in his 1986 paper "Using Prototypical Objects to Implement...
or resemblance) to its prototype, and changes in the prototype cause corresponding changes to be apparent in its clone. Other systems, such as the Forth-like programming language Kevo, do not propagate change from the prototype in this fashion, and instead follow a more concatenative model where changes in cloned objects do not automatically propagate across descendants.
// Example of true prototypal inheritance style
// in JavaScript.
// "ex nihilo" object creation using the literal
// object notation {}.
var foo = {name: "foo", one: 1, two: 2};
// Another "ex nihilo" object.
var bar = {three: 3};
// Gecko and Webkit JavaScript engines can directly
// manipulate the internal prototype link.
// For the sake of simplicity, let us pretend
// that the following line works regardless of the
// engine used:
bar.__proto__ = foo; // foo is now the prototype of bar.
// If we try to access foo's properties from bar
// from now on, we'll succeed.
bar.one // Resolves to 1.
// The child object's properties are also accessible.
bar.three // Resolves to 3.
// Own properties shadow prototype properties
bar.name = "bar";
foo.name // unaffected, resolves to "foo"
bar.name // Resolves to "bar"
This example in JS 1.8.5 + ( see http://kangax.github.com/es5-compat-table/ )
var foo = {one: 1, two: 2};
// bar. prototype = foo
var bar = Object.create( foo );
bar.three = 3;
bar.one; // 1
bar.two; // 2
bar.three; //3
Delegation
In prototype-based languages that use delegation, the language runtime is capable of dispatching
Dynamic dispatch
In computer science, dynamic dispatch is the process of mapping a message to a specific sequence of code at runtime. This is done to support the cases where the appropriate method can't be determined at compile-time...
the correct method or finding the right piece of data simply by following a series of delegation pointers (from object to its prototype) until a match is found. All that is required to establish this behavior-sharing between objects is the delegation pointer. Unlike the relationship between class and instance in class-based object-oriented languages, the relationship between the prototype and its offshoots does not require that the child object have a memory or structural similarity to the prototype beyond this link. As such, the child object can continue to be modified and amended over time without rearranging the structure of its associated prototype as in class-based systems. It is also important to note that not only data but also methods can be added or changed. For this reason, most prototype-based languages refer to both data and methods as "slots".
Concatenation
Under pure prototyping, which is also referred to as concatenative prototypes, and is exemplified in the Kevo language, there are no visible pointers or links to the original prototype from which an object is cloned. The prototype object is copied exactly, but given a different name (or reference). Behavior and attributes are simply duplicated as-is.
One advantage of this approach is that object authors can alter the copy without worrying about side-effects across other children of the parent. Another advantage is that method lookup during dispatch is much cheaper computationally than with delegation, where an exhaustive search must be made of the entire delegation chain before failure to find a method or slot can be admitted.
Disadvantages to the concatenative approach include the organizational difficulty of propagating changes through the system; if a change occurs in a prototype, it is not immediately or automatically available on its clones. However, Kevo does provide additional primitives for publishing changes across sets of objects based on their similarity (so-called family resemblances) rather than through taxonomic origin, as is typical in the delegation model.
Another disadvantage is that, in the most naive implementations of this model, additional memory is wasted (versus the delegation model) on each clone for the parts that have stayed the same between prototype and clone. However, it is possible to provide concatenative behavior to the programming while sharing implementation and data behind-the-scenes; such an approach is indeed followed by Kevo.
An alternative quasi-solution to the problem of clones interfering with the behavior of the parent is to provide a means whereby the potential parent is flagged as being clonable or not. In MOO
MOO programming language
The MOO programming language is a relatively simple programming language used to support the MOO Server. It is dynamically typed and uses a prototype-based object-oriented system, with syntax roughly derived from the Algol school of programming languages....
, this is achieved with the "f" flag. Only objects with the "f" flag can be cloned. In practice, this leads to certain objects serving as surrogate classes; their properties are kept constant to serve as initial values for their children. These children then tend to have the "f" flag not set.
Criticism
Advocates of class-based object models who criticize prototype-based systems often have concerns that could be seen as similar to those concerns that proponents of static type systems for programming languages have of dynamic type systems (see datatype). Usually, such concerns involve: correctness, safety
Safety
Safety is the state of being "safe" , the condition of being protected against physical, social, spiritual, financial, political, emotional, occupational, psychological, educational or other types or consequences of failure, damage, error, accidents, harm or any other event which could be...
Predictability is the degree to which a correct prediction or forecast of a system's state can be made either qualitatively or quantitatively.-Predictability and Causality:...
In computer science, efficiency is used to describe properties of an algorithm relating to how much of various types of resources it consumes. Algorithmic efficiency can be thought of as analogous to engineering productivity for a repeating or continuous process, where the goal is to reduce...
and programmer unfamiliarity.
On the first three points, classes are often seen as analogous to types (in most statically typed object-oriented languages they serve that role) and are proposed to provide contractual guarantees to their instances, and to users of their instances, that they will behave in some given fashion.
Regarding efficiency, declaring classes simplifies many compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
optimizations that allow developing efficient method and instance variable lookup. For the Self language, much development time was spent on developing, compiling, and interpreting techniques to improve the performance of prototype-based systems versus class-based systems. For example, the Lisaac
Lisaac
Lisaac is a compiled object-oriented programming language based on prototype concepts, with system programming facilities and design by contract....
compiler produces code almost as fast as C. Tests have been run with an MPEG-2 codec written in Lisaac, copied from a C version. These show the Lisaac version is 1.9% slower than the C version with 37% fewer lines of code.
A common criticism made against prototype-based languages is that the community of software developer
Software developer
A software developer is a person concerned with facets of the software development process. Their work includes researching, designing, developing, and testing software. A software developer may take part in design, computer programming, or software project management...
s is unfamiliar with them, despite the popularity and market permeation of JavaScript
JavaScript
JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
. This knowledge level of prototype-based systems seems to be changing with the proliferation of JavaScript frameworks and increases in the complex use of JavaScript as "Web 2.0
Web 2.0
The term Web 2.0 is associated with web applications that facilitate participatory information sharing, interoperability, user-centered design, and collaboration on the World Wide Web...
Actor-Based Concurrent Language is a family of programming languages, developed in Japan in the 1980s and 1990s.-ABCL/1:ABCL/1 is a prototype-based concurrent programming language for the ABCL MIMD system, created in 1986 by Akinori Yonezawa, of the Department of Information Science at the...
ColdC is a programming language, originally designed for ColdMUD by Greg Hudson in 1993, but with some applications in fields unrelated to MUDs. It is a dynamically typed prototype-based object oriented system, with a syntax derived from the C programming language...
ECMAScript is the scripting language standardized by Ecma International in the ECMA-262 specification and ISO/IEC 16262. The language is widely used for client-side scripting on the web, in the form of several well-known dialects such as JavaScript, JScript, and ActionScript.- History :JavaScript...
ActionScript is an object-oriented language originally developed by Macromedia Inc. . It is a dialect of ECMAScript , and is used primarily for the development of websites and software targeting the Adobe Flash Player platform, used on Web pages in the form of...
Adobe Flash is a multimedia platform used to add animation, video, and interactivity to web pages. Flash is frequently used for advertisements, games and flash animations for broadcast...
Adobe Flex is a software development kit released by Adobe Systems for the development and deployment of cross-platform rich Internet applications based on the Adobe Flash platform...
ECMAScript for XML is a programming language extension that adds native XML support to ECMAScript . The goal is to provide an alternative to DOM interfaces that uses a simpler syntax for accessing XML documents. It also offers a new way of making XML visible...
JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
JScript is a scripting language based on the ECMAScript standard that is used in Microsoft's Internet Explorer.JScript is implemented as a Windows Script engine. This means that it can be "plugged in" to any application that supports Windows Script, such as Internet Explorer, Active Server Pages,...
Falcon is an open source, multi-paradigm programming language. Design and implementation is led by Giancarlo Niccolai, a native of Bologna, Italy and Information Technology graduate from Pistoia....
Io is a pure object-oriented programming language inspired by Smalltalk, Self, Lua, Lisp, Act1, and NewtonScript. Io has a prototype-based object model similar to the ones in Self and NewtonScript, eliminating the distinction between instance and class. Like Smalltalk, everything is an object and...
Ioke is a dynamic, strongly typed, prototype-based programming language targeting the Java Virtual Machine and the Common Language Runtime. It was designed by Ola Bini, a developer of JRuby. It has a very simple homoiconic syntax, somewhat similar to Io....
Logtalk is an object-oriented logic programming language that extends the Prolog language with a feature set suitable for programming in the large. It provides support for encapsulation and data hiding, separation of concerns and enhanced code reuse...
Neko is a high-level dynamically typed programming language developed by Nicolas Cannasse as part of R&D efforts at Motion-Twin.-Concept:Neko has a compiler and a virtual machine with garbage collection. The compiler converts a source .neko file into a bytecode .n file that can be executed with the...
NewtonScript is a prototype based programming language created to write programs for the Newton platform. It is heavily influenced by the Self computer language, but modified to be more suited to needs of mobile and embedded devices.- History :...
Object Lisp was a computer programming language, a dialect of the Lisp language. It was an object-oriented extension for the Lisp dialect Lisp Machine Lisp, designed by Lisp Machines, Inc...
OpenLaszlo is an open source platform for the development and delivery of rich Internet applications. It is released under the Open Source Initiative-certified Common Public License ....
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
R is a programming language and software environment for statistical computing and graphics. The R language is widely used among statisticians for developing statistical software, and R is widely used for statistical software development and data analysis....
REBOL is a cross-platform data exchange language and a multi-paradigm dynamic programming language originally designed by Carl Sassenrath for network communications and distributed computing. The language and its official implementation, which is a proprietary freely redistributable software are...
SmartFrog is an open-source software framework, written in Java, that manages the configuration, deployment and coordination of a software system broken into components. These components may be distributed across several network hosts....
Text Adventure Development System is a prototype-based domain-specific programming language and set of standard libraries for creating interactive fiction games.-History:...
Tcl is a scripting language created by John Ousterhout. Originally "born out of frustration", according to the author, with programmers devising their own languages intended to be embedded into applications, Tcl gained acceptance on its own...
Snit is an object-oriented extension to the Tcl programming language. Snit is a recursive acronym that stands for "Snit's Not Incr Tcl." Snit is a pure Tcl object and megawidget system. It is unique among Tcl object systems in that it is based not on inheritance but on delegation...
Class-based programming, or more commonly class-orientation, refers to the style of object-oriented programming in which inheritance is achieved by defining classes of objects, as opposed to the objects themselves .The most popular and developed model of OOP is a class-based model, as opposed to an...
A programming paradigm is a fundamental style of computer programming. Paradigms differ in the concepts and abstractions used to represent the elements of a program and the steps that compose a computation A programming paradigm is a fundamental style of computer programming. (Compare with a...
Differential Inheritance is a common inheritance model used by prototype-based programming languages such as JavaScript, Io and NewtonScript. It operates on the principle that most objects are derived from other, more general objects, and only differ in a few small aspects; while usually...
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