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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 ...
, a class is an extensible program-code-template for creating
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
s, providing initial values for state (
member variable In object-oriented programming, a member variable (sometimes called a member field) is a variable that is associated with a specific object, and accessible for all its methods (''member functions''). In class-based programming languages, these ...
s) and implementations of behavior (member functions or
methods Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
). In many languages, the class name is used as the name for the class (the template itself), the name for the default constructor of the class (a subroutine that creates objects), and as the type of objects generated by instantiating the class; these distinct concepts are easily conflated. Although, to the point of conflation, one could argue that is a feature inherent in a language because of its polymorphic nature and why these languages are so powerful, dynamic and adaptable for use compared to languages without polymorphism present. Thus they can model dynamic systems (i.e. the real world, machine learning, AI) more easily. When an object is created by a constructor of the class, the resulting object is called an instance of the class, and the member variables specific to the object are called
instance variable In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e. a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has similariti ...
s, to contrast with the
class variable In class-based, object-oriented programming, a class variable is a variable defined in a class of which a single copy exists, regardless of how many instances of the class exist. A class variable is not an instance variable. It is a special ...
s shared across the class. In certain languages, classes are, as a matter of fact, only a compile-time feature (new classes cannot be declared at run-time), while in other languages classes are
first-class citizen In programming language design, a first-class citizen (also type, object, entity, or value) in a given programming language is an entity which supports all the operations generally available to other entities. These operations typically include ...
s, and are generally themselves objects (typically of type or similar). In these languages, a class that creates classes within itself is called a
metaclass 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-oriente ...
.


Class vs. type

In its most casual usage, people often refer to the "class" of an object, but narrowly speaking objects have ''type'': the interface, namely the types of member variables, the signatures of
member function A method in object-oriented programming (OOP) is a procedure associated with a message and an object. An object consists of ''state data'' and ''behavior''; these compose an ''interface'', which specifies how the object may be utilized by any of ...
s (methods), and properties these satisfy. At the same time, a class has an implementation (specifically the implementation of the methods), and can create objects of a given type, with a given implementation. In the terms of type theory, a class is an implementationa ''concrete'' data structure and collection of subroutineswhile a type is an
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
. Different (concrete) classes can produce objects of the same (abstract) type (depending on type system); for example, the type might be implemented with two classes (fast for small stacks, but scales poorly) and (scales well but high overhead for small stacks). Similarly, a given class may have several different constructors. Class types generally represent nouns, such as a person, place or thing, or something nominalized, and a class represents an implementation of these. For example, a type might represent the properties and functionality of bananas in general, while the and classes would represent ways of producing bananas (say, banana suppliers or data structures and functions to represent and draw bananas in a video game). The class could then produce particular bananas: instances of the class would be objects of type . Often only a single implementation of a type is given, in which case the class name is often identical with the type name.


Design and implementation

Classes are composed from structural and behavioral constituents. Programming languages that include classes as a programming construct offer support, for various class-related features, and the syntax required to use these features varies greatly from one programming language to another.


Structure

A class contains
data In the pursuit of knowledge, data (; ) is a collection of discrete Value_(semiotics), values that convey information, describing quantity, qualitative property, quality, fact, statistics, other basic units of meaning, or simply sequences of sy ...
field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grass ...
descriptions (or ''
properties Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property. Property may also refer to: Mathematics * Property (mathematics) Philosophy and science * Property (philosophy), in philosophy an ...
'', ''
fields Fields may refer to: Music * Fields (band), an indie rock band formed in 2006 * Fields (progressive rock band), a progressive rock band formed in 1971 * ''Fields'' (album), an LP by Swedish-based indie rock band Junip (2010) * "Fields", a song b ...
'', ''data
members Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...
'', or ''
attributes Attribute may refer to: * Attribute (philosophy), an extrinsic property of an object * Attribute (research), a characteristic of an object * Grammatical modifier, in natural languages * Attribute (computing), a specification that defines a prope ...
''). These are usually field types and names that will be associated with state variables at program run time; these state variables either belong to the class or specific instances of the class. In most languages, the structure defined by the class determines the layout of the memory used by its instances. Other implementations are possible: for example, objects in
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
use associative key-value containers. Some programming languages such as Eiffel support specification of invariants as part of the definition of the class, and enforce them through the type system. Encapsulation of state is necessary for being able to enforce the invariants of the class.


Behavior

The behavior of class or its instances is defined using
methods Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
. Methods are subroutines with the ability to operate on objects or classes. These operations may alter the state of an object or simply provide ways of accessing it. Many kinds of methods exist, but support for them varies across languages. Some types of methods are created and called by programmer code, while other special methods—such as constructors, destructors, and conversion operators—are created and called by compiler-generated code. A language may also allow the programmer to define and call these special methods.


The concept of class interface

Every class ''implements'' (or ''realizes'') an interface by providing structure and behavior. Structure consists of data and state, and behavior consists of code that specifies how methods are implemented. There is a distinction between the definition of an interface and the implementation of that interface; however, this line is blurred in many programming languages because class declarations both define and implement an interface. Some languages, however, provide features that separate interface and implementation. For example, an
abstract class In programming languages, an abstract type is a type in a nominative type system that cannot be instantiated directly; a type that is not abstract – which ''can'' be instantiated – is called a ''concrete type''. Every instance of an abstra ...
can define an interface without providing implementation. Languages that support class inheritance also allow classes to inherit interfaces from the classes that they are derived from. For example, if "class A" inherits from "class B" and if "class B" implements the interface "interface B" then "class A" also inherits the functionality(constants and methods declaration) provided by "interface B". In languages that support
access specifiers Access modifiers (or access specifiers) are keywords in object-oriented languages that set the accessibility of classes, methods, and other members. Access modifiers are a specific part of programming language syntax used to facilitate the enc ...
, the interface of a class is considered to be the set of public members of the class, including both methods and attributes (via implicit getter and setter methods); any private members or internal data structures are not intended to be depended on by external code and thus are not part of the interface. Object-oriented programming methodology dictates that the operations of any interface of a class are to be independent of each other. It results in a layered design where clients of an interface use the methods declared in the interface. An interface places no requirements for clients to invoke the operations of one interface in any particular order. This approach has the benefit that client code can assume that the operations of an interface are available for use whenever the client has access to the object.


Example

The buttons on the front of your television set are the interface between you and the electrical wiring on the other side of its plastic casing. You press the "power" button to toggle the television on and off. In this example, your particular television is the instance, each method is represented by a button, and all the buttons together compose the interface (other television sets that are the same model as yours would have the same interface). In its most common form, an interface is a specification of a group of related methods without any associated implementation of the methods. A television set also has a myriad of ''attributes'', such as size and whether it supports colour, which together comprise its structure. A class represents the full description of a television, including its attributes (structure) and buttons (interface). Getting the total number of televisions manufactured could be a ''static method'' of the television class. This method is clearly associated with the class, yet is outside the domain of each individual instance of the class. A static method that finds a particular instance out of the set of all television objects is another example.


Member accessibility

The following is a common set of
access specifiers Access modifiers (or access specifiers) are keywords in object-oriented languages that set the accessibility of classes, methods, and other members. Access modifiers are a specific part of programming language syntax used to facilitate the enc ...
: * ''Private'' (or ''class-private'') restricts the access to the class itself. Only methods that are part of the same class can access private members. * ''Protected'' (or ''class-protected'') allows the class itself and all its subclasses to access the member. * ''Public'' means that any code can access the member by its name. Although many object-oriented languages support the above access specifiers, their semantics may differ. Object-oriented design uses the access specifiers in conjunction with careful design of public method implementations to enforce class invariants—constraints on the state of the objects. A common usage of access specifiers is to separate the internal data of a class from its interface: the internal structure is made private, while public
accessor method In computer science, a mutator method is a method used to control changes to a variable. They are also widely known as setter methods. Often a setter is accompanied by a getter (together also known as accessors), which returns the value of the priva ...
s can be used to inspect or alter such private data. Access specifiers do not necessarily control ''visibility'', in that even private members may be visible to client external code. In some languages, an inaccessible but visible member may be referred to at run-time (for example, by a pointer returned from a member function), but an attempt to use it by referring to the name of the member from client code will be prevented by the type checker. The various object-oriented programming languages enforce member accessibility and visibility to various degrees, and depending on the language's
type system In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer progr ...
and compilation policies, enforced at either
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 concept ...
or run-time. For example, the
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 ...
language does not allow client code that accesses the private data of a class to compile. In the
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 ...
language, private methods are visible, but not accessible in the interface; however, they may be made invisible by explicitly declaring fully abstract classes that represent the interfaces of the class. Some languages feature other accessibility schemes: * ''Instance vs. class accessibility'':
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
supports ''instance-private'' and ''instance-protected'' access specifiers in lieu of class-private and class-protected, respectively. They differ in that they restrict access based on the instance itself, rather than the instance's class. * ''Friend'': C++ supports a mechanism where a function explicitly declared as a
friend function In object-oriented programming, a friend function, that is a "friend" of a given class, is a function that is given the same access as methods to private and protected data. A friend function is declared by the class that is granting access, so ...
of the class may access the members designated as private or protected. * ''Path-based'': Java supports restricting access to a member within a
Java package A Java package organizes Java classes into namespaces, providing a unique namespace for each type it contains. Classes in the same package can access each other's package-private and protected members. In general, a package can contain the follo ...
, which is the logical path of the file. However, it is a common practice when extending a Java framework to implement classes in the same package as a framework class in order to access protected members. The source file may exist in a completely different location, and may be deployed to a different .jar file, yet still be in the same logical path as far as the JVM is concerned.


Inter-class relationships

In addition to the design of standalone classes, programming languages may support more advanced class design based upon relationships between classes. The inter-class relationship design capabilities commonly provided are ''compositional'' and ''hierarchical''.


Compositional

Classes can be composed of other classes, thereby establishing a compositional relationship between the enclosing class and its embedded classes. Compositional relationship between classes is also commonly known as a ''
has-a In database design, object-oriented programming and design (see object oriented program architecture), has-a (has_a or has a) is a composition relationship where one object (often called the constituted object, or part/constituent/member object) " ...
'' relationship. For example, a class "Car" could be composed of and contain a class "Engine". Therefore, a Car ''has an'' Engine. One aspect of composition is containment, which is the enclosure of component instances by the instance that has them. If an enclosing object contains component instances by value, the components and their enclosing object have a similar lifetime. If the components are contained by reference, they may not have a similar lifetime. For example, in Objective-C 2.0: @interface Car : NSObject @property NSString *name; @property Engine *engine @property NSArray *tires; @end This class ''has'' an instance of (a string object), , and (an array object).


Hierarchical

Classes can be ''derived'' from one or more existing classes, thereby establishing a hierarchical relationship between the derived-from classes (''base classes'', ''parent classes'' or ') and the derived class (''child class'' or ''subclass'') . The relationship of the derived class to the derived-from classes is commonly known as an
is-a In knowledge representation, object-oriented programming and design (see object-oriented program architecture), is-a (is_a or is a) is a subsumption relationship between abstractions (e.g. types, classes), wherein one class ''A'' is a subclass ...
relationship. For example, a class 'Button' could be derived from a class 'Control'. Therefore, a Button is a Control. Structural and behavioral members of the parent classes are ''inherited'' by the child class. Derived classes can define additional structural members (data fields) and behavioral members (methods) in addition to those that they ''inherit'' and are therefore ''specializations'' of their superclasses. Also, derived classes can
override Override may refer to: * Dr. Gregory Herd, a Marvel Comics character formerly named Override * Manual override, a function where an automated system is placed under manual control * Method overriding, a subclassing feature in Object Oriented progr ...
inherited methods if the language allows. Not all languages support multiple inheritance. For example, Java allows a class to implement multiple interfaces, but only inherit from one class. If multiple inheritance is allowed, the hierarchy is a
directed acyclic graph In mathematics, particularly graph theory, and computer science, a directed acyclic graph (DAG) is a directed graph with no directed cycles. That is, it consists of vertices and edges (also called ''arcs''), with each edge directed from one v ...
(or DAG for short), otherwise it is a
tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
. The hierarchy has classes as nodes and inheritance relationships as links. Classes in the same level are more likely to be
associated Associated may refer to: *Associated, former name of Avon, Contra Costa County, California * Associated Hebrew Schools of Toronto, a school in Canada *Associated Newspapers, former name of DMG Media, a British publishing company See also *Associati ...
than classes in different levels. The levels of this hierarchy are called
layers Layer or layered may refer to: Arts, entertainment, and media * ''Layers'' (Kungs album) * ''Layers'' (Les McCann album) * ''Layers'' (Royce da 5'9" album) *"Layers", the title track of Royce da 5'9"'s sixth studio album *Layer, a female Maveric ...
or levels of abstraction. Example (Simplified Objective-C 2.0 code, from iPhone SDK): @interface UIResponder : NSObject //... @interface UIView : UIResponder //... @interface UIScrollView : UIView //... @interface UITableView : UIScrollView //... In this example, a UITableView is a UIScrollView is a UIView is a UIResponder is an NSObject.


Definitions of subclass

Conceptually, a superclass is a
superset In mathematics, set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset of ...
of its subclasses. For example, a common class hierarchy would involve as a superclass of and , while would be a subclass of . These are all subset relations in set theory as well, i.e., all squares are rectangles but not all rectangles are squares. A common conceptual error is to mistake a ''part of'' relation with a subclass. For example, a car and truck are both kinds of vehicles and it would be appropriate to model them as subclasses of a vehicle class. However, it would be an error to model the component parts of the car as subclass relations. For example, a car is composed of an engine and body, but it would not be appropriate to model engine or body as a subclass of car. In
object-oriented modeling Object-oriented modeling (OOM) is an approach to modeling an application that is used at the beginning of the software life cycle when using an object-oriented approach to software development. The software life cycle is typically divided up into ...
these kinds of relations are typically modeled as object properties. In this example, the class would have a property called . would be typed to hold a collection of objects, such as instances of , , , etc. Object modeling languages such as
UML The Unified Modeling Language (UML) is a general-purpose, developmental modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a system. The creation of UML was originally m ...
include capabilities to model various aspects of "part of" and other kinds of relations – data such as the cardinality of the objects, constraints on input and output values, etc. This information can be utilized by developer tools to generate additional code beside the basic data definitions for the objects, such as error checking on get and set methods. One important question when modeling and implementing a system of object classes is whether a class can have one or more superclasses. In the real world with actual sets it would be rare to find sets that didn't intersect with more than one other set. However, while some systems such as Flavors and CLOS provide a capability for more than one parent to do so at run time introduces complexity that many in the object-oriented community consider antithetical to the goals of using object classes in the first place. Understanding which class will be responsible for handling a message can get complex when dealing with more than one superclass. If used carelessly this feature can introduce some of the same system complexity and ambiguity classes were designed to avoid. Most modern object-oriented languages such as Smalltalk and Java require single inheritance at run time. For these languages, multiple inheritance may be useful for modeling but not for an implementation. However, semantic web application objects do have multiple superclasses. The volatility of the Internet requires this level of flexibility and the technology standards such as the Web Ontology Language (OWL) are designed to support it. A similar issue is whether or not the class hierarchy can be modified at run time. Languages such as Flavors, CLOS, and Smalltalk all support this feature as part of their
meta-object protocol 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 inc ...
s. Since classes are themselves first-class objects, it is possible to have them dynamically alter their structure by sending them the appropriate messages. Other languages that focus more on strong typing such as Java and C++ do not allow the class hierarchy to be modified at run time. Semantic web objects have the capability for run time changes to classes. The rational is similar to the justification for allowing multiple superclasses, that the Internet is so dynamic and flexible that dynamic changes to the hierarchy are required to manage this volatility.


Orthogonality of the class concept and inheritance

Although class-based languages are commonly assumed to support inheritance, inheritance is not an intrinsic aspect of the concept of classes. Some languages, often referred to as " object-based languages", support classes yet do not support inheritance. Examples of object-based languages include earlier versions of
Visual Basic Visual Basic is a name for a family of programming languages from Microsoft. It may refer to: * Visual Basic .NET (now simply referred to as "Visual Basic"), the current version of Visual Basic launched in 2002 which runs on .NET * Visual Basic ( ...
.


Within object-oriented analysis

In object-oriented analysis and in
UML The Unified Modeling Language (UML) is a general-purpose, developmental modeling language in the field of software engineering that is intended to provide a standard way to visualize the design of a system. The creation of UML was originally m ...
, an association between two classes represents a collaboration between the classes or their corresponding instances. Associations have direction; for example, a bi-directional association between two classes indicates that both of the classes are aware of their relationship. Associations may be labeled according to their name or purpose. An association role is given end of an association and describes the role of the corresponding class. For example, a "subscriber" role describes the way instances of the class "Person" participate in a "subscribes-to" association with the class "Magazine". Also, a "Magazine" has the "subscribed magazine" role in the same association. Association role multiplicity describes how many instances correspond to each instance of the other class of the association. Common multiplicities are "0..1", "1..1", "1..*" and "0..*", where the "*" specifies any number of instances.


Taxonomy of classes

There are many categories of classes, some of which overlap.


Abstract and concrete

In a language that supports inheritance, an abstract class, or abstract base class (ABC), is a class that cannot be instantiated because it is either labeled as abstract or it simply specifies
abstract method A method in object-oriented programming (OOP) is a procedure associated with a message and an object. An object consists of ''state data'' and ''behavior''; these compose an ''interface'', which specifies how the object may be utilized by any of ...
s (or ''
virtual method In object-oriented programming, in languages such as C++, and Object Pascal, a virtual function or virtual method is an inheritable and overridable function or method for which dynamic dispatch is facilitated. This concept is an important part ...
s''). An abstract class may provide implementations of some methods, and may also specify virtual methods via
signatures A signature (; from la, signare, "to sign") is a handwritten (and often stylized) depiction of someone's name, nickname, or even a simple "X" or other mark that a person writes on documents as a proof of identity and intent. The writer of a ...
that are to be implemented by direct or indirect descendants of the abstract class. Before a class derived from an abstract class can be instantiated, all abstract methods of its parent classes must be implemented by some class in the derivation chain. Most object-oriented programming languages allow the programmer to specify which classes are considered abstract and will not allow these to be instantiated. For example, in
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 ...
, C# and
PHP PHP is a general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by The PHP Group. ...
, the keyword ''abstract'' is used. In
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 ...
, an abstract class is a class having at least one abstract method given by the appropriate syntax in that language (a pure virtual function in C++ parlance). A class consisting of only virtual methods is called a Pure Abstract Base Class (or ''Pure ABC'') in C++ and is also known as an ''interface'' by users of the language. Other languages, notably Java and C#, support a variant of abstract classes called an
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
via a keyword in the language. In these languages,
multiple inheritance Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object or ...
is not allowed, but a class can implement multiple interfaces. Such a class can only contain abstract publicly accessible methods. A concrete class is a class that can be instantiated, as opposed to abstract classes, which cannot.


Local and inner

In some languages, classes can be declared in scopes other than the global scope. There are various types of such classes. An
inner class In object-oriented programming (OOP), an inner class or nested class is a class declared entirely within the body of another class or interface. It is distinguished from a subclass. Overview An instance of a normal or top-level class can exist on ...
is a class defined within another class. The relationship between an inner class and its containing class can also be treated as another type of class association. An inner class is typically neither associated with instances of the enclosing class nor instantiated along with its enclosing class. Depending on language, it may or may not be possible to refer to the class from outside the enclosing class. A related concept is ''inner types'', also known as ''inner data type'' or ''nested type'', which is a generalization of the concept of inner classes.
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 ...
is an example of a language that supports both inner classes and inner types (via ''
typedef typedef is a reserved keyword in the programming languages C, C++, and Objective-C. It is used to create an additional name (''alias'') for another data type, but does not create a new type, except in the obscure case of a qualified typedef of ...
'' declarations). Another type is a local class, which is a class defined within a procedure or function. This limits references to the class name to within the scope where the class is declared. Depending on the semantic rules of the language, there may be additional restrictions on local classes compared to non-local ones. One common restriction is to disallow local class methods to access local variables of the enclosing function. For example, in C++, a local class may refer to
static variable In computer programming, a static variable is a variable that has been allocated "statically", meaning that its lifetime (or "extent") is the entire run of the program. This is in contrast to shorter-lived automatic variables, whose storage is ...
s declared within its enclosing function, but may not access the function's automatic variables.


Metaclasses

Metaclasses are classes whose instances are classes. A metaclass describes a common structure of a collection of classes and can implement a
design pattern A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The "Gang of Four" boo ...
or describe particular kinds of classes. Metaclasses are often used to describe frameworks. In some languages, such as
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
,
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
or Smalltalk, a class is also an object; thus each class is an instance of a unique metaclass that is built into the language. 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) provides metaobject protocols (MOPs) to implement those classes and metaclasses.


Non-subclassable

Non-subclassable classes allow programmers to design classes and hierarchies of classes where at some level in the hierarchy, further derivation is prohibited (a stand-alone class may be also designated as non-subclassable, preventing the formation of any hierarchy). Contrast this to ''abstract'' classes, which imply, encourage, and require derivation in order to be used at all. A non-subclassable class is implicitly ''concrete''. A non-subclassable class is created by declaring the class as in C# or as in Java or PHP. For example, Java's class is designated as ''final''. Non-subclassable classes may allow a compiler (in compiled languages) to perform optimizations that are not available for subclassable classes.


Open class

An open class is one that can be changed. Typically, an
executable program In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a data file ...
cannot be changed by customers. Developers can often change some classes, but typically cannot change standard or built-in ones. In
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
, all classes are open. In
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
, classes can be created at runtime, and all can be modified afterwards. Objective-C categories permit the programmer to add methods to an existing class without the need to recompile that class or even have access to its source code.


Mixins

Some languages have special support for
mixin In object-oriented programming languages, a mixin (or mix-in) 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 depen ...
s, though in any language with multiple inheritance a mixin is simply a class that does not represent an is-a-type-of relationship. Mixins are typically used to add the same methods to multiple classes; for example, a class might provide a method called when included in classes and that do not share a common parent.


Partial

In languages supporting the feature, a partial class is a class whose definition may be split into multiple pieces, within a single
source-code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the w ...
file or across multiple files. The pieces are merged at compile-time, making compiler output the same as for a non-partial class. The primary motivation for introduction of partial classes is to facilitate the implementation of code generators, such as visual designers. It is otherwise a challenge or compromise to develop code generators that can manage the generated code when it is interleaved within developer-written code. Using partial classes, a code generator can process a separate file or coarse-grained partial class within a file, and is thus alleviated from intricately interjecting generated code via extensive parsing, increasing compiler efficiency and eliminating the potential risk of corrupting developer code. In a simple implementation of partial classes, the compiler can perform a phase of precompilation where it "unifies" all the parts of a partial class. Then, compilation can proceed as usual. Other benefits and effects of the partial class feature include: * Enables separation of a class's interface and implementation code in a unique way. * Eases navigation through large classes within an editor. * Enables
separation of concerns In computer science, separation of concerns is a design principle for separating a computer program into distinct sections. Each section addresses a separate '' concern'', a set of information that affects the code of a computer program. A concern ...
, in a way similar to
aspect-oriented programming 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 behavior to existing code (an advice) ''without'' modifying th ...
but without using any extra tools. * Enables multiple developers to work on a single class concurrently without the need to merge individual code into one file at a later time. Partial classes have existed in Smalltalk under the name of ''Class Extensions'' for considerable time. With the arrival of the .NET framework 2,
Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washin ...
introduced partial classes, supported in both C# 2.0 and Visual Basic 2005.
WinRT Windows Runtime (WinRT) is a platform-agnostic component and application architecture first introduced in Windows 8 and Windows Server 2012 in 2012. It is implemented in C++ and officially supports development in C++ (via C++/WinRT, C++/CX or W ...
also supports partial classes.


Example in VB.NET

This simple example, written in
Visual Basic .NET Visual Basic, originally called Visual Basic .NET (VB.NET), is a multi-paradigm, object-oriented programming language, implemented on .NET, Mono, and the .NET Framework. Microsoft launched VB.NET in 2002 as the successor to its original Visua ...
, shows how parts of the same class are defined in two different files. ;file1.vb: Partial Class MyClass Private _name As String End Class ;file2.vb: Partial Class MyClass Public Readonly Property Name() As String Get Return _name End Get End Property End Class When compiled, the result is the same as if the two files were written as one, like this: Class MyClass Private _name As String Public Readonly Property Name() As String Get Return _name End Get End Property End Class


Example in Objective-C

In
Objective-C Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXT ...
, partial classes, also known as categories, may even spread over multiple libraries and executables, like the following example. But a key difference is that Objective-C's categories can overwrite definitions in another interface declaration, and that categories aren't equal to original class definition (the first requires the last). Instead, .NET partial class can't have conflicting definitions, and all partial definitions are equal to the others. In Foundation, header file NSData.h: @interface NSData : NSObject - (id)initWithContentsOfURL:(NSURL *)URL; //... @end In user-supplied library, a separate binary from Foundation framework, header file NSData+base64.h: #import @interface NSData (base64) - (NSString *)base64String; - (id)initWithBase64String:(NSString *)base64String; @end And in an app, yet another separate binary file, source code file main.m: #import #import "NSData+base64.h" int main(int argc, char *argv[]) The dispatcher will find both methods called over the NSData instance and invoke both of them correctly.


Uninstantiable

Uninstantiable classes allow programmers to group together per-class fields and methods that are accessible at runtime without an instance of the class. Indeed, instantiation is prohibited for this kind of class. For example, in C#, a class marked "static" can not be instantiated, can only have static members (fields, methods, other), may not have ''instance constructors'', and is ''sealed''.


Unnamed

An unnamed class or anonymous class is a class that is not bound to a name or identifier upon definition. This is analogous to named versus unnamed functions.


Benefits

The benefits of organizing software into object classes fall into three categories: * Rapid development * Ease of maintenance * Reuse of code and designs Object classes facilitate rapid development because they lessen the semantic gap between the code and the users. System analysts can talk to both developers and users using essentially the same vocabulary, talking about accounts, customers, bills, etc. Object classes often facilitate rapid development because most object-oriented environments come with powerful debugging and testing tools. Instances of classes can be inspected at run time to verify that the system is performing as expected. Also, rather than get dumps of core memory, most object-oriented environments have interpreted debugging capabilities so that the developer can analyze exactly where in the program the error occurred and can see which methods were called to which arguments and with what arguments. Object classes facilitate ease of maintenance via encapsulation. When developers need to change the behavior of an object they can localize the change to just that object and its component parts. This reduces the potential for unwanted side effects from maintenance enhancements. Software re-use is also a major benefit of using Object classes. Classes facilitate re-use via inheritance and interfaces. When a new behavior is required it can often be achieved by creating a new class and having that class inherit the default behaviors and data of its superclass and then tailor some aspect of the behavior or data accordingly. Re-use via interfaces (also known as methods) occurs when another object wants to invoke (rather than create a new kind of) some object class. This method for re-use removes many of the common errors that can make their way into software when one program re-uses code from another.


Run-time representation

As a data type, a class is usually considered as a compile-time construct. A language or library may also support prototype or
factory A factory, manufacturing plant or a production plant is an industrial facility, often a complex consisting of several buildings filled with machinery, where workers manufacture items or operate machines which process each item into another. ...
metaobject 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 incl ...
s that represent run-time information about classes, or even represent metadata that provides access to
reflection Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...
facilities and ability to manipulate data structure formats at run-time. Many languages distinguish this kind of
run-time type information In computer programming, run-time type information or run-time type identification (RTTI) is a feature of some programming languages (such as C++, Object Pascal, and Ada) that exposes information about an object's data type at runtime. Run-time typ ...
about classes from a class on the basis that the information is not needed at run-time. Some dynamic languages do not make strict distinctions between run-time and compile-time constructs, and therefore may not distinguish between metaobjects and classes. For example, if Human is a
metaobject 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 incl ...
representing the class Person, then instances of class Person can be created by using the facilities of the Human
metaobject 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 incl ...
.


See also

*
Class-based programming Class-based programming, or more commonly class-orientation, is a style of object-oriented programming (OOP) in which inheritance occurs via defining ''classes'' of objects, instead of inheritance occurring via the objects alone (compare prototy ...
*
Class diagram In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the rela ...
(UML) * List of object-oriented programming languages *
Mixin In object-oriented programming languages, a mixin (or mix-in) 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 depen ...
*
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 ...
*
Prototype-based programming Prototype-based programming is a style of object-oriented programming in which behaviour reuse (known as inheritance) is performed via a process of reusing existing objects that serve as prototypes. This model can also be known as ''prototypa ...
*
Trait (computer programming) In computer programming, a trait is a concept used in object-oriented programming which represents a set of methods that can be used to extend the functionality of a class. Rationale In object-oriented programming, behavior is sometimes share ...


Notes


References

* * *


Further reading


Abadi; Cardelli: A Theory of Objects

ISO/IEC 14882:2003 Programming Language C++, International standard


by Brian Foote * Meyer, B.: "Object-oriented software construction", 2nd edition, Prentice Hall, 1997, * Rumbaugh et al.: "Object-oriented modeling and design", Prentice Hall, 1991, {{DEFAULTSORT:Class (Computer Programming) Programming constructs Programming language topics